polymers Article
Multi-Responsive Behaviors of Copolymers Bearing N-Isopropylacrylamide with or without Phenylboronic Acid in Aqueous Solution Jiaxing Li 1 , Lei Yang 1, * 1
2
*
ID
, Xiaoguang Fan 2, *, Fei Wang 1 , Jing Zhang 1 and Zhanyong Wang 1
ID
School of Environmental and Biological Engineering, Liaoning Shihua University, Fushun 113001, China;
[email protected] (J.L.);
[email protected] (F.W.);
[email protected] (J.Z.);
[email protected] (Z.W.) College of Engineering, Shenyang Agricultural University, Shenyang 110866, China Correspondence:
[email protected] (L.Y.);
[email protected] (X.F.); Tel.: +86-024-5686-1705 (L.Y.); +86-024-8848-7119 (X.F.)
Received: 30 January 2018; Accepted: 6 March 2018; Published: 9 March 2018
Abstract: Continuing efforts to develop novel smart materials are anticipated to upgrade the quality of life of humans. Thermo-responsive poly(N-isopropylacrylamide) and glucose-responsive phenylboronic acid—typical representatives—are often integrated as multi-stimuli-sensitive materials, but few are available for side-by-side comparisons with their properties. In this study, both copolymers bearing N-isopropylacrylamide (NIPAAm), with or without 3-acrylamidophenylboronic acid (AAPBA), were synthesized by free radical polymerization, and characterized by Fourier transform infrared spectrometry, nuclear magnetic resonance hydrogen spectroscopy and gel permeation chromatography. Dynamic light scattering was used to analyze and compare the responsive behaviors of the copolymers in different aqueous solutions. Atomic force microscopy was also employed to investigate the apparent morphology changes with particle sizes. The results demonstrated that the introduction of NIPAAm endowed the composite materials with thermosensitivity, whereas the addition of AAPBA lowered the molecular weight of the copolymers, intensified the intermolecular aggregation of the nanoparticles, reduced the lower critical solution temperature (LCST) of the composites, and accordingly allowed the copolymers to respond to glucose. It was also concluded that the responding of smart copolymers to operating parameters can be activated only under special conditions, and copolymer dimension and conformation were affected by inter/intramolecular interactions. Keywords: poly(N-isopropylacrylamide); phenylboronic acid; smart responsive behavior; hydrodynamic diameter; dynamic light scattering
1. Introduction Poly(N-isopropylacrylamide) (PNIPAAm)-comprising hydrophilic amide bonds and hydrophobic isopropyl groups, empowers hydration–dehydration changes in polymer chains with ambient temperature in aqueous solutions [1]. The products with the PNIPAAm component exhibit lower critical solution temperature (LCST), and often appears as sharp decrease or increase in hydrodynamic diameter (DH ) of polymers. Therefore, the changing trend of DH along with temperature is usually employed to infer the temperature sensitivity of PNIPAAm-containing materials. Given that the overall structure and surface property of the end products can be changed by temperature control with appropriate design and simplified operation, PNIPAAm and its derivatives are widely used for drug delivery [2], biomacromolecule immobilization [3], intelligent cell culture substrate [4], artificial extracellular matrix [5] and other biomedical fields. Phenylboronic acid (PBA) can ionize in aqueous Polymers 2018, 10, 293; doi:10.3390/polym10030293
www.mdpi.com/journal/polymers
Polymers 2018, 10, 293
2 of 16
solutions to give uncharged hydrophobic forms and charged hydrophilic forms. The charged forms generate reversible covalent complexes with cis-diol compounds, thus a response to the increasing population of negatively charged PBA will result in the formation of more hydrophilic structure [6]. Glucose addition moderately changes water solubility of PBA-containing materials, which gives them attractive glucose responsiveness. This unique characteristic can be utilized to design a drug-controlled release system [7] and develop a glucose sensor [8]. It is very valuable that the elaborate combination of stimuli-responsive polymers with such advanced structure can produce novel materials with virtues from both moieties [9]. More attempts have been made to create multiresponsive materials, especially for dual responses to temperature and glucose. Dynamic light scattering (DLS) is a technique in physics that can be applied to determine the size distribution profile of small particles in suspension or polymers in solution [10]. Because it is easy to change physical conditions of working solutions, DLS technique is often used to examine DH of PNIPAAm-mediated materials changed in response to the changes of environmental parameters, in order to illustrate the stimulus responses of intelligent composites [11–13]. Recently, scholars had also conducted prominent works in the synthesis of smart materials carrying NIPAAm and PBA, and further investigated their dual responses to temperature and glucose by means of the DLS technique [14–18]. It can be seen that PBA moieties are supposed to be modified on PNIPAAm backbone, making it easier to upgrade the thermosensitivity of PNIPAAm to multiple responses to temperature, pH and glucose, so their effective applications can be further expanded. In our previous study, we had prepared PNIPAAm-containing copolymers and determined their temperature responses in various aqueous solutions with different temperature, pH value, species and concentrations of salts by DLS [19]. Our results demonstrated that the intervention of hydrophilic/hydrophobic reactants, salt substances and pH reagents could change the type or strength of inter/intramolecular acting forces between polymers, so the conformation changes of polymer molecules were different from what occurred in ultrapure water. If PBA is designedly introduced into the molecular segments of PNIPAAm-based copolymers, it is bound to bring inestimable impacts on the physical properties of temperature-responsive polymers, particularly for intelligent responses. The research on performance comparison between the copolymers bearing NIPAAm with or without PBA is limited, but it is the point of this study with particular emphasis on the influence of measurement temperature, pH and carbohydrate on the stimulus responses of both copolymers. Simultaneously, particle size, morphology as well as LCST are also essential to understand the interplay between smart behavior, polymer structure and driving force [20]. This is also the focus of our study. In this study, the novel P(NIPAAm-co-AAPBA-co-HPM-co-TMSPM) copolymers were synthesized by free radical polymerization of N-isopropylacrylamide (NIPAAm), 3-acrylamidophenylboronic acid (AAPBA), hydroxypropyl methacrylate (HPM) and 3-trimethoxysilylpropyl methacrylate (TMSPM), while P(NIPAAm-co-HPM-co-TMSPM) copolymers prepared from the same synthesis conditions were used as opponents. The following measurements were performed in succession to analyze and compare the copolymers bearing NIPAAm with or without AAPBA. Fourier transform infrared (FT-IR) spectrometry, nuclear magnetic resonance hydrogen (1 H-NMR) spectroscopy and gel permeation chromatography (GPC) were employed to identify the chemical structure, molecular composition and molecular weight of the end products. Importantly, DLS technique was applied to determine the DH of both copolymers in ultrapure water and aqueous solutions with different temperature, pH value and glucose concentration, so as to analyze the influences of inter/intramolecular interactions on copolymer size and conformation. Atomic force microscopy (AFM) were used to display visually the particle sizes of the copolymers deposited on glass surfaces, which can offer more auxiliary information for smart responsive behaviors of the copolymers.
Polymers 2018, 10, 293
3 of 16
2. Materials and Methods 2.1. Materials NIPAAm, HPM, TMSPM and 2,2-azobisisobutyronitrile (AIBN) were purchased from Aladdin (Shanghai, China). AAPBA was supplied by J&K Scientific (Beijing, China). NIPAAm, AAPBA and AIBN were fully recrystallized respectively followed by freeze-drying. The others including hydrochloric acid (HCl), sodium hydroxide (NaOH), absolute ethanol and glucose were provided by Aldrich. UHQ water was derived from Millipore-Q ultrapure water purification system (Millipore Corporation, Molsheim, France). 2.2. Copolymer Synthesis and Identification The synthesis and identification of the copolymers bearing NIPAAm with or without AAPBA were conducted according to the reported protocols with some modifications [19,21,22]. Briefly, P(NIPAAm-co-AAPBA-co- HPM-co-TMSPM) (PNAHT for short) copolymers and P(NIPAAm-co-HPM-co-TMSPM) (PNHT for short) copolymers were synthesized by free radical polymerization of NIPAAm, AAPBA, HPM and TMSPM with the initial molar ratios of 20:1:1:1 and 20:0:1:1 respectively, initiated by AIBN (1% of total molar quantities for all reactants) at 60 ◦ C for 12 h under nitrogen, then precipitated and freeze-dried under vacuum for 24 h, and finally stored in a refrigerator for further use. The FT-IR spectra were used to analyze the functional groups of the copolymers via Nicolet Magna 750 FT-IR spectrometer (Nicolet Instrument Corporation, Madison, WI, USA) in the wavenumber range of 4000~500 cm−1 . The 1 H-NMR spectra were applied to identify the molecular compositions of the compounds through Bruker Avance 400 MHz NMR spectrometer (Bruker Corporation, Fallanden, Switzerland). The GPC (PL-GPC-50, Varian Inc., Palo Alto, CA, USA) was employed to determine the molecular weight distribution and polymerization degree of the composites. 2.3. Smart Responsive Behaviors of Copolymers PNAHT and PNHT copolymers were respectively dissolved at the final concentration of 1.0 mg·mL−1 in absolute ethanol, UHQ water and aqueous solutions with different pH values (1.0~12.0) and glucose concentrations (0~6.0 mg·mL−1 ), and then stirred at room temperature for 12 h. Prior to adding 1.0 mL sample into a cuvette, the copolymer solutions were filtrated and purified with Millipore filter (0.2 µm) to remove any impurities. The cuvette was then placed in the bath of Malvern DLS Nanosizer (NanoZS90, Malvern Instruments Ltd., Malvern, UK) with scattering angle fixed at 90◦ . The measurements were performed from 15 ◦ C to 40 ◦ C with lower temperature fluctuation (±0.1 ◦ C). The pH values or glucose concentrations of the solutions to be tested were modulated by changing the content of HCl or NaOH or glucose. To improve accuracy and comparability of experimental results, the data was collected after 10 min when the solutions had reached the setting parameters. Each correlation curve was accumulated 30~50 times to eliminate noise, and at least three replicates were carried out under the same conditions. The average value ± standard deviation was applied to present the DH of copolymers obtained by volume distribution under different conditions. DLS data were analyzed using Malvern Zetasizer Software v7.11. 2.4. Morphology Analysis of Glass Surfaces with Copolymer Deposition The glass coverslips were treated by constant velocity-dip coating of different aqueous solutions with the same copolymer concentration of 1.0 mg·mL−1 , which allowed for observing the particle sizes of PNAHT and PNHT copolymers under various working conditions. The fast-drying technique was also introduced to keep the copolymer particles deposited on glass surfaces with original states as much as possible in respective solutions in spite of some conformation changes caused by dehydration. The outlet air temperatures of dryer were in accordance with the temperature of working solutions as closely as possible. Then the morphology of coated coverslips were scanned by AFM (solve P47,
Polymers 2018, 10, 293
4 of 16
Polymers 2018, 10, x FOR PEER REVIEW 4 ofsample 16 NT-MDT Spectrum Instruments, Moscow, Russia). Three trials were randomly selected in each with different scanning sizes. The surface characteristic parameters of samples were obtained from were scanned by AFM (solve P47, NT-MDT Spectrum Instruments, Moscow, Russia). Three trials Nova 1.1.0.1918 Software.
were randomly selected in each sample with different scanning sizes. The surface characteristic parameters samples were obtained from Nova 1.1.0.1918 Software. 3. Results and of Discussion 3. Results and Discussion 3.1. Copolymer Synthesis and Identification Copolymer Identification 3.1.1.3.1. Synthesis ofSynthesis PNAHTand and PNHT Copolymers
PNAHT and PNHT copolymers been shown to have good responses to temperature or 3.1.1. Synthesis of PNAHT and PNHThave Copolymers glucose in aqueous solutions on the evidences of the follow-up measurements, but the intelligent PNAHT and PNHT copolymers have been shown to have good responses to temperature or copolymers their current forms of molecular will notmeasurements, be used for practical applications. glucose ininaqueous solutions on the evidences ofchains the follow-up but the intelligent However, both copolymers bearing siloxane bonds can couple with hydroxyl-contained substrates copolymers in their current forms of molecular chains will not be used for practical applications. withHowever, any shape andcopolymers dimensionbearing by heating or radiation Meanwhile, HPM can provide effective both siloxane bonds can[23]. couple with hydroxyl-contained substrates hydroxyl groups bonding of copolymers, so there are nanoparticles depositing with any shapefor andintermolecular dimension by heating or radiation [23]. Meanwhile, HPM can provide effective hydroxyl groups for intermolecular bonding of films copolymers, so therethickness. are nanoparticles depositing on twoor three-dimensional surfaces to form with certain It can be concluded on twoor three-dimensional surfaces to form films with certain thickness. It can be concluded that that synthesis of copolymers with smart responses is the premise and foundation for subsequent synthesis of copolymers with smart responses is the premise and foundation for subsequent preparation of functional films. Furthermore, copolymer performance in response to environmental preparation functional films.applications Furthermore,ofcopolymer performance in response to environmental factors directly of decides further copolymer films. Therefore, component kinds and factors directly decides further applications of copolymer films. Therefore, component kinds and compounding ratios of resultant composites are key elements for film preparation, performance tuning compounding ratios of resultant composites are key elements for film preparation, performance and scheduling optimization. NIPAAm, AAPBA, HPM and TMSPM selected in this study are all tuning and scheduling optimization. NIPAAm, AAPBA, HPM and TMSPM selected in this study unsaturated monomers with double bonds. The copolymers can be synthesized from three or four are all unsaturated monomers with double bonds. The copolymers can be synthesized from three or monomers by free radical where NIPAAm and AAPBA used asused primary reactants four monomers by freepolymerization, radical polymerization, where NIPAAm and AAPBA as primary will reactants provide their functional groups which enable them to have temperature and glucose responses. will provide their functional groups which enable them to have temperature and glucose The responses. recent studies that suggested larger PBAthat content to stronger glucose responsiveness The suggested recent studies largergave PBArise content gave rise to stronger glucose for the end products [6,17,24], excessive dosage couldbut restrict the thermo-sensitivity of copolymers. responsiveness for thebut end products [6,17,24], excessive dosage could restrict the thermo-sensitivity copolymers. Actually, we attempted various ratios to but Actually, we attemptedofvarious mole ratios of NIPAAm to AAPBA (suchmole as 20:1, 10:1,of5:1NIPAAm and so on), AAPBA (such as 20:1, 10:1, 5:1 and so on), but it was found that adding overmuch AAPBA to the it was found that adding overmuch AAPBA to the composites would result in interchain aggregation would result and in interchain aggregation even difficult at room to temperature, andMillipore the formed evencomposites at room temperature, the formed clusters were pass through filters. clusters were difficult to pass through Millipore filters. Thus, the mole ratio between NIPAAm and Thus, the mole ratio between NIPAAm and AAPBA was designed to 20:1 for PNAHT copolymers. AAPBA was designed to 20:1 for PNAHT copolymers. HPM and TMSPM can provide sufficient HPM and TMSPM can provide sufficient hydroxyl groups and siloxane bonds for the formation of hydroxyl groups and siloxane bonds for the formation of intelligent copolymer films. It is intelligent copolymer films. It is recommended to use 5% addition of NIPAAm for both HPM and recommended to use 5% addition of NIPAAm for both HPM and TMSPM [25]. So the initial molar TMSPM [25]. So the initial molar ratios of NIPAAm, AAPBA, HPM and TMSPM were determined to ratios of NIPAAm, AAPBA, HPM and TMSPM were determined to be 20:1:1:1 and 20:0:1:1 be 20:1:1:1 and 20:0:1:1 respectively forcopolymers PNAHT and copolymers in this The diagrams respectively for PNAHT and PNHT in PNHT this study. The diagrams forstudy. molecular structure for molecular structure of both copolymers were shown in Figure 1. of both copolymers were shown in Figure 1.
Figure Molecularstructure structureof of PNAHT PNAHT (A) copolymers. Figure 1. 1. Molecular (A)and andPNHT PNHT(B) (B) copolymers.
Polymers 2018, 10, 293
5 of 16
2018, 10, x FOR PEER REVIEW 3.1.2.Polymers Identification of PNAHT and PNHT Copolymers
5 of 16
3.1.2. Identification PNAHT and PNHT Copolymers Figure 2 displayedofFT-IR spectra for PNAHT and PNHT copolymers. As shown in Figure 2A, the stretching vibration absorption peak of amide bond I and the copolymers. bending vibration absorption Figure 2 displayed FT-IR spectra for PNAHT and PNHT As shown in Figure peak 2A, of − 1 − 1 amide II were detected at 1652peak cm ofand 1550 cmI and , belonging to vibration NIPAAmabsorption and AAPBA. thebond stretching vibration absorption amide bond the bending peak The −1 and −1, belonging symmetrical deformation vibration peaks bond discovered 1389 cmThe of amide bond II were detected at absorption 1652 cm−1 and 1550of cmisopropyl to NIPAAm andatAAPBA. − 1 − 1 1369symmetrical cm , weredeformation typical absorption of NIPAAm The peak 1340 cm atwas vibrationpeaks absorption peaks of[26]. isopropyl bondatdiscovered 1389characteristic cm−1 and − 1 −1 −1 1369 cmpeak , were absorption peaks NIPAAm [26].atThe 1340 the cm out-of-plane was characteristic absorption oftypical boric acid groups [27],ofand the peak 709peak cm at was bending −1 was the out-of-plane bending absorption peak ofpeak boricof acid groupsring, [27], which and thewere peakassigned at 709 cmto vibration absorption benzene AAPBA. The wider absorption absorption peak of benzene which were to AAPBA.ofThe absorption peakvibration appearing within 3100~3700 cm−1 ring, could be due to assigned the superposition the wider stretching vibration −1 could be due to the superposition of the stretching vibration peak appearing within 3100~3700 cm absorption peaks of hydroxyl group and amide bond II, indirectly proving the presence of HPM, absorption peaks of hydroxyl group and amide bond II, indirectly proving the presence of HPM, NIPAAm and AAPBA. The stretching vibration absorption peaks at 1270−1cm−1 , 1079 cm−1 and −1 and 799 NIPAAm and AAPBA. The stretching vibration absorption peaks at 1270 cm , 1079 cm 799 cm−1−1 indicated that siloxane bonds were integrated into the copolymers. Absorption peaks at cm indicated that siloxane were integrated into the copolymers. Absorption peaks at 1722 −1 werebonds 1722 cm−1−1 and 1173 cm attributed to the stretching vibration absorption peaks of ester groups cm and 1173 cm−1 were attributed to the stretching vibration absorption peaks of ester groups derived from HPM and TMSPM. thatthe thefunctional functional groups of four reactants derived from HPM and TMSPM.The Theresults results confirmed confirmed that groups of four reactants existed in PNAHT copolymers. absorptionpeaks peaksinin Figure showing a similar existed in PNAHT copolymers.The Thecharacteristic characteristic absorption Figure 2B2B showing a similar correlation to the results of Figure 2A except the absence of functional groups related to PBA, revealed correlation to the results of Figure 2A except the absence of functional groups related to PBA, that revealed PNHT copolymers were synthesized from NIPAAm, HPM and TMSPM. that PNHT copolymers were synthesized from NIPAAm, HPM and TMSPM.
Figure 2. FT-IR spectra of PNAHT (A) and PNHT (B) copolymers. Figure 2. FT-IR spectra of PNAHT (A) and PNHT (B) copolymers. Figure 3 illustrated 1H-NMR spectra (500 MHz, CDCl3) of PNAHT and PNHT copolymers
1 H-NMR spectra (500 MHz, CDCl ) of PNAHT and PNHT copolymers Figure 3 illustrated respectively. It was well-known that PNHT copolymers 3were not involved AAPBA during respectively. It was well-known copolymers were not involved synthetic process. There was a that peakPNHT appearing at 7.26 ppm in Figure 3B, but AAPBA the peak during intensitysynthetic was obviously than that of PNAHT group same 3B, chemical shift, sointensity it was most a process. Thereweaker was a peak appearing at 7.26 ppmatinthe Figure but the peak waslikely obviously solvent peak. integral area of solvent peak chemical must be excluded PNAHT group. weaker than thatThe of PNAHT group at the same shift, sowhen it wasanalyzing most likely a solvent peak. 1 H-NMR H-NMR area spectra of PNAHT copolymers was shown in Figure 3A, asPNAHT described below. δ (0.75~1.50 The 1integral of solvent peak must be excluded when analyzing group. spectra ppm), a (Si–CH 2–) and b (–CH3); δ (1.50~2.33 ppm), c (–CH2–) and d (–CH–); δ (3.49 ppm), e of PNAHT copolymers was shown in Figure 3A, as described below. δ (0.75~1.50 ppm), a (Si–CH2 –) 3); δ (3.71 ppm), f (–OCH2–); δ (3.78~4.22 ppm), g (N–CH–); δ (7.26 ppm), h (–C6H4–). The and (–OCH b (–CH 3 ); δ (1.50~2.33 ppm), c (–CH2 –) and d (–CH–); δ (3.49 ppm), e (–OCH3 ); δ (3.71 ppm), ratio of integral area of characteristic peaks was approximate to 128:71:9:6:20:4, therefore the molar f (–OCH2 –); δ (3.78~4.22 ppm), g (N–CH–); δ (7.26 ppm), h (–C6 H4 –). The ratio of integral area of ratio of NIPAAm, AAPBA, HPM and TMSPM in the end product was 20:1:1:1. For PNHT characteristic peaks was approximate to 128:71:9:6:20:4, therefore the molar ratio of NIPAAm, AAPBA, copolymers, peak distributions were as followed. δ (0.74~1.47 ppm), a (Si–CH2–) and b (–CH3); δ HPM and TMSPM in the end product was 20:1:1:1. For PNHT copolymers, peak distributions were as (1.47~2.49 ppm), c (–CH2–) and d (–CH–); δ (2.49~3.31 ppm), e (–OCH3); δ (3.46 ppm), f (–OCH2–); δ followed. δ (0.74~1.47 ppm), a (Si–CH and b (–CH ppm), c (–CH –) and d (–CH–); 2 –) of 3 ); δ (3.59~4.34 ppm), g (N–CH–). The ratio integral area of(1.47~2.49 characteristic peaks was 2approximately δ (2.49~3.31 ppm), e (–OCH ); δ (3.46 ppm), f (–OCH –); δ (3.59~4.34 ppm), g (N–CH–). The ratio 3 2 NIPAAm, HPM and TMSPM in PNHT equal to 128:68:9:6:20, and thus the molar ratio of of integral area was of characteristic peaks was approximately equal to 128:68:9:6:20, and thus the molar copolymers deduced as 20:1:1. Figure 3 indicated common traits as well as differences between ratiothe of two NIPAAm, TMSPM PNHT copolymers was deduced as 20:1:1. Figure 3 indicated spectra.HPM Both and displayed the in characteristic peaks of hydrogen derived from NIPAAm, HPM and TMSPM, increased chemical of characteristic and enhanced peak intensity common traits as however, well as differences between shift the two spectra. Bothpeaks displayed the characteristic peaks of hydrogen derived from NIPAAm, HPM and TMSPM, however, increased chemical shift of characteristic
Polymers 2018, 10, 293
6 of 16
Polymers 2018, 10, x FOR PEER REVIEW
6 of 16
peaks and enhanced peak intensity at 7.26 ppm proved the existence of AAPBA in PNAHT copolymers. 7.26 ppm proved existencewith of AAPBA in PNAHT copolymers. The analysis The at analysis results werethe consistent the principal objective of copolymer designs.results were consistent with the principal objective of copolymer designs. The molecular weights of PNAHT and PNHT copolymers were determined by GPC, and weights of were PNAHT and PNHT copolymers wereand determined by GPC, and of polymerThe unitmolecular of both copolymers deduced by 1 H-NMR spectra, accordingly the degree polymer unitwas of both copolymers by 1H-NMRin spectra, accordingly the degree of the polymerization calculated. Thewere datadeduced were summarized Table and 1. The results indicated that polymerization was calculated. The data were summarized in Table 1. The results indicated that the molecular weight distributions of both copolymers were relatively narrow and the molecular weights molecular weight distributions of both copolymers were relatively narrow and the molecular of the end products were relatively small. It was concluded that PNHT copolymers exceeded the weights of the end products were relatively small. It was concluded that PNHT copolymers opponents when comparing their molecular weights and polymerization degree. This was possibly exceeded the opponents when comparing their molecular weights and polymerization degree. This because the stereo-hindrance effect of AAPBA inhibited the polymerization of unsaturated monomers was possibly because the stereo-hindrance effect of AAPBA inhibited the polymerization of to some extent. monomers Furthermore, we also found that the molecular weights of PNAHT copolymers unsaturated to some extent. Furthermore, we also found that the molecular weights of got smaller withcopolymers the increase AAPBA content. The observation was also positive evidence PNAHT gotofsmaller with the increase of AAPBA content. Thea observation was alsofor a the above assumption. positive evidence for the above assumption.
1H-NMR spectra of PNAHT (A) and PNHT (B) copolymers. Figure Figure 3. 13.H-NMR spectra of PNAHT (A) and PNHT (B) copolymers.
Table 1. Molecularcomposition compositionof ofPNAHT PNAHT and Table 1. Molecular andPNHT PNHTcopolymers. copolymers. Molar Ratio Mn Mw Mu Mw/Mn Group DP Molar Ratio Mu −1) M M(g·mol −1) −1) n w (g·mol (g·mol TMSPM HPM AAPBA Group NIPAAm DP Mw /Mn (g·mol−1 ) (g·mol−1 ) (g·mol−1 ) NIPAAm TMSPM HPM AAPBA X Y Z P PNAHT 2846.76 15182 24061 1.584 6 20X 1 Y Z1 P1 PNAHT 2846.76 15182 24061 1.584 6 X′20 Y′ Z′ P′ 1 1 1 PNHT 2655.77 19475 29115 1.495 8 20X0 Y10 Z10 P00 PNHT 2655.77 19475 29115 1.495 8 20 1 1 of polymer 0 Note: Mu represents molecular weight unit; Mn represents number average molecular weight; w represents weight average molecular weight; Mw/M represents number polydispersity index; DP represents Note:MM molecular weight of polymer unit; Mnn represents average molecular weight; Mw u represents represents average molecular weight; Mw /Mn represents polydispersity index; DP represents degree of u. degreeweight of polymerization, DP = Mn/M polymerization, DP = Mn /Mu .
Polymers 2018, 10, 293
7 of 16
3.2. Smart Responsive Behaviors of Copolymers 3.2.1. Copolymers in Absolute Ethanol Absolute ethanol was good solvent for PNAHT and PNHT copolymers, so the trend of average hydrodynamic diameter (DAH ) of both copolymers changing with temperature in absolute ethanol was first determined. Throughout the temperature ranging from 15 to 40 ◦ C, DAH of PNAHT and PNHT copolymers showing little changes, were 36.95 ± 2.13 nm and 43.13 ± 1.54 nm respectively, indicating that the copolymers synthesized in this study were short-chain polymers. In absolute ethanol, both copolymers had unfolded conformation without detectable nanoparticle aggregation, therefore DLS data could relatively truly represent the sizes of single chains of the copolymers. Once added AAPBA, DAH of PNAHT copolymers decreased, owing to the case that the stereo-hindrance effect of PBA groups terminated the synthetic process of the copolymers in advance. 3.2.2. Copolymers in UHQ Water Figure 4 illustrated the representative DLS profiles plotted in volume distribution versus copolymer DH in UHQ water. At 15 ◦ C, DH of PNAHT copolymers distributed from 15.69 to 58.77 nm with the peak appearing at 28.21 nm and PDI of 0.189, while DH of PNHT copolymers distributed within 10.1~43.82 nm with the peak centered at 15.69 nm and PDI of 0.148. The results revealed that both copolymers were soluble in water without any obvious aggregation at lower temperature. But it was amazing that DH of PNAHT and PNHT copolymers in UHQ water were significantly smaller than those in absolute ethanol under the same environment, indicating that the copolymers had properly adjusted their configurations of molecular chains in UHQ water. The hydrogen bonds and hydrophobic interactions produced by hydrophilic and hydrophobic groups of the copolymers in water came to a dynamic balance, so the copolymer chains actually could not extend as they did in absolute ethanol. In addition, DH of PNHT copolymers were obviously lower than that of PNAHT copolymers, which would probably be the comparison between single molecular chains of PNHT copolymers and oligomers (aggregated from several molecular chains) of PNAHT copolymers. The pH of UHQ water was less than pKa of AAPBA, therefore PNAHT copolymers were mainly uncharged and hydrophobic in UHQ water, undoubtedly increasing the hydrophobic interactions between the copolymers. However, AAPBA content accounted for only 5% of molar quantity of NIPAAm, so the hydrophilic shells were also formed by hydrogen bonds between water molecules and hydrophilic groups of the copolymers, accordingly PNAHT copolymers could only aggregate to small clusters. As a result, DH of PNAHT copolymers were larger than that of control group in macroscopic manner. Obvious changes of DH for PNAHT and PNHT copolymers started at 23 and 27 ◦ C respectively, demonstrating that addition of hydrophobic AAPBA would significantly reduce LSCT of the resultant copolymers. At 30 ◦ C, DH peaks of PNAHT and PNHT copolymers shifted to 164.2 nm with distribution between 78.82 and 458.7 nm with PDI of 0.060 and 0.039 respectively, illustrating that water molecules bound with copolymers were gradually released. The continuously increased hydrophobic interactions and hydrogen bonds between the copolymers could enhance interchain aggregation, thus increasing the overall sizes of the agglomerates. As the temperature rose to 35 ◦ C, DH of PNHT copolymers continued to be expanding, with the peak at 255 nm and a wider distribution (105.7~531.2 nm) with PDI of 0.048. The volume distribution of PNHT copolymers at 40 ◦ C was nearly the same with that at 35 ◦ C. The results demonstrated that copolymer aggregation was constantly growing but did not unlimited, and it would be stopped once various kinds of inter/intramolecular forces kept a dynamic balance. Nevertheless, DH of PNAHT copolymers had no significant changes within the temperature between 27 and 40 ◦ C, indicating that there were only slight regulations with respect to the structure of molecular chains of PNAHT copolymers. In conclusion, when the temperature was lower than their LCST, both PNAHT and PNHT copolymers remained in hydrated state. Introduction of AAPBA obviously decreased the initial temperature for intermolecular aggregation of the constructed nanoparticles. When the temperature had risen to above LCST, the
Polymers 2018, 10, x293 FOR PEER REVIEW
88of of 16 16
molecular chains of the copolymers began to aggregate from various directions and stopped at unfolded molecular of the copolymers began to aggregate from various directions and force balance, wherechains large-size and widely-distributed nanometer agglomerations would be stopped formed, at force balance, where large-size and widely-distributed nanometer agglomerations would be formed, showing an increase in DH of copolymers. showing an increase in DHcharacters of copolymers. The thermosensitive of the copolymers were characterized by variation tendency of The thermosensitive characters of the copolymers characterized bylower variation of DAH with temperature, as displayed in Figure 5. When were the temperature was thantendency 23 °C, DAH ◦ C, D D with temperature, as displayed in Figure 5. When the temperature was lower than 23 AH ofAH PNAHT copolymers was around 30 nm, indicating that the nanoparticles were hydratable. The of PNAHT solution copolymers wasclear around nm, indicating thatpoint, the nanoparticles were hydratable. The copolymer looked and30 transparent. At this three kinds of hydrated hydrogen copolymer solution looked clear and and hydrophilic transparent.groups At thisincluding point, three kinds of hydrated hydrogen bonds between water molecules amide bonds, hydroxyl groups bonds between waterwere molecules and hydrophilic groups amide bonds, hydroxyl groups and siloxane bonds, considered as major forces to including prevent intermolecular aggregation and and siloxane bonds, were considered as major forces to prevent intermolecular aggregation and intramolecular collapse of the copolymers. However, introduction of AAPBA would undoubtedly intramolecular collapse of the copolymers. However, introduction of AAPBA would undoubtedly enhance hydrophobic interactions, leading to copolymer aggregation and LCST reduction. Across enhance hydrophobic interactions, leading copolymerand aggregation and LCST reduction. Across 23 °C, the scattered nanoparticles gatheredtoobviously become dehydrated. The transparent ◦ C, the scattered nanoparticles gathered obviously and become dehydrated. The transparent 23 aqueous solution began to turn opaque. It can be deduced that the LCST of PNAHT copolymers aqueous solution to turn opaque. It cancontinued be deduced thesix-fold LCST ofDPNAHT copolymers was was close to 23 began °C. As the temperature tothat rise, AH could be observed, ◦ close to 23 C. As continued to rise, copolymers six-fold DAHaggregated could be observed, demonstrating demonstrating thatthe thetemperature molecular chains of PNAHT dramatically due to the that the molecular chains of PNAHT copolymers aggregated dramatically due to the formation of formation of hydrophobic interactions caused by isopropyl groups and PBA groups as well as hydrophobic interactions isopropyl groups andstill PBAthree groups as well as hydrogen bonds hydrogen bonds betweencaused amide by bonds, but there were kinds of hydrated hydrogen between amide were still kinds of hydrated hydrogen wasthe to bonds. That wasbonds, to say,but thethere maximum DAHthree of the resultant copolymers were bonds. obtainedThat under say, the maximum D of the resultant copolymers were obtained under the actions of two kinds actions of two kinds AH of hydrophobic interactions and four kinds of hydrogen bonds. Figure 5 also of hydrophobic interactions and four kinds of 27 hydrogen bonds. Figure had 5 also when the revealed when the temperature dropped below °C, PNHT copolymers DAHrevealed of around 16 nm. ◦ C, PNHT copolymers had D temperature dropped below 27 of around 16 nm. The molecular The molecular chains of the copolymers started to aggregate atAH 27 °C, therefore the LCST of PNHT ◦ C, therefore the LCST of PNHT copolymers in chains of theincopolymers started at 27 copolymers UHQ water couldto beaggregate recognized as 27 °C. When the temperature was higher than UHQ water could be recognized as were 27 ◦ C.elevated When the was higher than (40 LCST, of PNHT LCST, DAH of PNHT copolymers totemperature 270 nm with PDI of 0.050 °C),DAH which were ◦ C), which were apparently larger than the copolymers were elevated to 270 nm with PDI of 0.050 (40 apparently larger than the results of PNAHT copolymers (183 nm, 0.053). This could be due to results PNAHT copolymers (183interactions nm, 0.053). derived This could be due the factled that hydrophobic fact thatofadditional hydrophobic from PBAtogroups toadditional the formation of more interactions derived from PBA groups led to the formation of more compact nanoaggregates. compact nanoaggregates.
Figure Typical D DH distributions distributions of of PNAHT PNAHT (A) and PNHT PNHT (B) Figure 4.4. Typical (A) and (B) copolymers copolymers in in UHQ UHQ water water at at H different temperatures. different temperatures. 3.2.3. 3.2.3. Copolymers Copolymers in in Acidic Acidic or or Alkaline Alkaline Solution Solution Pure pH and and temperature temperature Pure PNIPAAm PNIPAAm homopolymers homopolymers have have actually actually no no response response to to pH, pH, but but the the pH dual stimuli-responsive behaviors of PNIPAAm-based materials can via becopolymerization obtained via dual stimuli-responsive behaviors of PNIPAAm-based materials can be obtained copolymerization with hydrophilic or hydrophobic monomers [28]. At room temperature, the with hydrophilic or hydrophobic monomers [28]. At room temperature, the solution pH of PNAHT −1 were 6.2 and solution pH of PNAHT and PNHT copolymers with final concentration of 1.0 mg·mL and PNHT copolymers with final concentration of 1.0 mg·mL−1 were 6.2 and 6.6 respectively, showing 6.6 respectively, showingboth weak acidity. Therefore, not easily ionizedforms and weak acidity. Therefore, copolymers were not both easilycopolymers ionized andwere remained uncharged remained uncharged forms in acidic solution, whereas they could be ionized to produce charged in acidic solution, whereas they could be ionized to produce charged nanoparticles in neutral and nanoparticles in neutral and alkaline media. Since apparent pKa of PBA and derivatives were
Polymers 2018, 10, 293 Polymers 2018, 10, x FOR PEER REVIEW Polymers 2018, 10, x FOR PEER REVIEW
9 of 16 9 of 16 9 of 16
alkaline media. pKa of PBA derivatives approximate to 8.6 of [18], AAPBA approximate to 8.6Since [18], apparent AAPBA addition had and largely potentialwere to delay the ionization PNAHT approximate to 8.6 [18], AAPBA addition had largely potential to delay the ionization of PNAHT addition had largely potential to delay the ionization of PNAHT copolymers. copolymers. copolymers.
in DD AH of PNAHT and PNHT in UHQ water as plotted with temperature. Figure 5.5.Changes Figure Changesin in DAH AH of of PNAHT PNAHT and and PNHT PNHT in in UHQ UHQ water water as as plotted plotted with with temperature. temperature. Figure 5.Changes
Figure 6 showed that DAH of PNAHT copolymers decreased as pH increased at 20 and 30 °C, Figure 66showed showedthat thatDD AHof ofPNAHT PNAHTcopolymers copolymers decreased increased atand 20 and 30and °C, Figure decreased as as pHpH increased at 20 30 ◦ C, AH and two variation trend curves started to be basically overlapped from pH of 8.0. When pH exceeded 9.0, and two variation trend curves started to be basically overlapped from pHofof8.0. 8.0.When WhenpH pHexceeded exceeded9.0, 9.0, two variation trend curves started to be basically overlapped from pH DAH of the nanoparticles dropped to around 19 nm, which might be due to the enhanced AH of the thenanoparticles nanoparticles dropped to around 19which nm, might whichbemight to theelectrostatic enhanced DAH dropped to around 19 nm, due tobe thedue enhanced electrostatic repulsion and increased water solubility, proving that PNAHT copolymers were electrostatic repulsion increased water solubility, proving copolymers that PNAHT copolymers repulsion and increasedand water solubility, proving that PNAHT were hydrated were with hydrated with charged single chains at this moment, thus it could be deduced that pKa of the hydratedsingle with chains charged single chainsthus at this moment, thus itthat could thatcopolymers pKa of the charged at this moment, it could be deduced pKabeof deduced the resultant resultant copolymers was still within 8.0~9.0. At either 20 or 30 °C, DAH of the agglomerates would resultant copolymers within or 30 °C, Dwould AH of the was still within 8.0~9.0.was At still either 20 or 8.0~9.0. 30 ◦ C, DAt of the 20 agglomerates be agglomerates larger in morewould acidic AH either be larger in more acidic environment, owing to the fact that copolymer aggregation was heightened be larger in more acidic environment, owing to the fact that copolymer aggregation washydrophobic heightened environment, owing to the fact that copolymer aggregation was heightened by increasing by increasing hydrophobic interactions between PBA groups. When the ambient temperature was by increasing hydrophobic interactions PBAtemperature groups. When temperature was interactions between PBA groups. When between the ambient was the 30 ◦ambient C and pH was lower than 30 °C and pH was lower than pKa of AAPBA, DAH of PNAHT copolymers was significantly higher◦ 30 °C and pH was lower than pK a of AAPBA, D AH of PNAHT copolymers was significantly higher pKa of AAPBA, DAH of PNAHT copolymers was significantly higher than the results obtained at 20 C than the results obtained at 20 °C with the same pH. This was mainly because the molecular chains than the thesame results obtained 20 °C with thethe same pH. This was of mainly because the molecular chains with pH. This wasatmainly because molecular chains PNAHT copolymers were restricted of PNAHT copolymers were restricted hydrated hydrogen bonds at 20 °C, DAH of the ◦ C, D byof of hydrated PNAHT hydrogen copolymers were restricted by hydrated hydrogen bonds at 20 °C, D AH of the by bonds at 20 the agglomerates could not be infinitely expanded with AH agglomerates could not be infinitely expanded with the maximum value of 173 nm (PDI = 0.107). ◦ agglomerates could not be infinitely expanded with the maximum value of 173 nm (PDI = 0.107). the maximum value of 173 nm (PDI = 0.107). However at 30 C, the emerging nonhydrated hydrogen However at 30 °C, the emerging nonhydrated hydrogen bonds and hydrophobic interactions However 30 °C, the interactions emerging nonhydrated hydrogen bonds and bonds andat hydrophobic resulted in larger nanoparticles withhydrophobic the maximuminteractions of 716 nm resulted in larger nanoparticles with the maximum of 716 nm (PDI = 0.088). resulted in larger nanoparticles with the maximum of 716 nm (PDI = 0.088). (PDI = 0.088).
Figure 6. Changes in DAH of PNAHT and PNHT copolymers as plotted with pH value at 20 and 30 °C. Figure 6. of PNAHT and PNHT copolymers as plotted with pH value at 20 and 30 °C.◦ Figure 6. Changes Changesin inDDAH AH of PNAHT and PNHT copolymers as plotted with pH value at 20 and 30 C.
Polymers 2018, 10, 293
10 of 16
Figure 6 also indicated no obvious changes in DAH of PNHT copolymers along with pH at 20 ◦ C. No ascertainable aggregation or precipitation was detected throughout the entire pH range (1.0~12.0) at lower temperature. The hydrated hydrogen bonds developed by water molecules and hydrophilic groups of PNHT copolymers still played principal roles to effectively keep the dimension and structure of single chains of the copolymers. When the temperature rose to 30 ◦ C and pH was less than 6.6, DAH of PNHT copolymers would rapidly increase with the declining pH. Similarly to the situations in UHQ water, hydrophobic interactions between isopropyl groups and hydrogen bonds between amide bonds had gradually replaced the hydrated hydrogen bonds. The synergistic effect of inter/intramolecular forces triggered interchain aggregation and intrachain collapse. If in UHQ water, there were still massive hydrated hydrogen bonds, restricting further aggregation of PNHT copolymers, thus the sizes of the agglomerations could be increased only to a certain extent. However, intervention of HCl changed the action mode of solutions on copolymers [29]. By destroying hydrogen bonds between copolymers and water molecules, HCl enables hydrophobic interactions and nonhydrated hydrogen bonds to become dominant acting forces, accordingly intensifies copolymer aggregation at elevated temperatures. Higher temperature or more concentration of HCl (lower pH) gives rise to greater damage to hydrated hydrogen bonds, so larger aggregates will be observed. It was reasonably explained that DAH of the agglomerations in UHQ water only reached 270 nm (PDI = 0.050) whilst DAH was up to 3520 nm (PDI = 0.062) in HCl solution at higher temperature. Nevertheless, when the temperature was higher than 30 ◦ C and pH exceeded 6.6, PNHT copolymers ionized into negative ions via deprotonation. The increasing concentration of hydroxide ions accelerated the ionization of the copolymers, leading to a large number of charged nanoparticles in aqueous solution. The effect of electrostatic repulsion reduced copolymer aggregation, so as to reduce the sizes of particle clusters. From the above experimental results, it was clear that DAH of PNAHT copolymers kept changing over pH and was independent of temperature, whereas steep declines in DAH of PNHT copolymers with the increase of pH occurred only at the temperature above LCST. 3.2.4. Copolymers in Glucose Solution Considering that LCST for PNAHT and PNHT copolymers in UHQ water were around 23 and 27 ◦ C while pKa were within 8.0~9.0 and 6.0~7.0, the working aqueous solutions with temperatures of 20 and 30 ◦ C, pH of 4.0 and 10.0 and glucose concentration of 0~6.0 mg·mL−1 were selected to investigate the glucose responsiveness of the copolymers in this study. Figure 7 displayed DAH changes of both copolymers under different conditions of temperatures, pH values and glucose concentrations. It could be observed in Figure 7A, changes in pH value and glucose concentration had no significant impacts on DAH of PNHT copolymers at 20 ◦ C, and the copolymers always exhibited hydrated single chains. At 20 ◦ C with pH of 4.0, DAH of PNAHT copolymers had very low dependence of glucose concentration. PNAHT copolymers presented in the form of agglomeration at this moment, owing to the reason when pH was lower than pKa of AAPBA, the hydrophobic uncharged PBA groups were difficult to react with cis dihydroxy groups. Therefore, glucose addition would not contribute to the rise of copolymer sizes. However, at 20 ◦ C with pH of 10.0, DAH of PNAHT copolymers increased with the increasing glucose concentration, and the maximum size was twice as much as the initial value of DAH around 18 nm. As mentioned above, the combined impact of hydrophobic interactions and hydrogen bonds did not allow PNAHT copolymers to stretch themselves in UHQ water as they did in absolute ethanol, therefore copolymer sizes in UHQ water were lower than that in absolute ethanol. However, PBA groups would be converted into hydrophilic and negatively-charged phenyl borate esters in the presence of glucose. The electrostatic repulsion and hydrated hydrogen bonds enabled PNAHT copolymers with single chains to be more unfolded. Furthermore, higher glucose concentration gave rise to stronger acting forces, accordingly leading to more outstretched molecular chains of the copolymers, hence it was macroscopically reflected in the continuous expanding of copolymer sizes. The results implied that PNAHT copolymers had a glucose-stimulated response.
Polymers 2018, 10, 293 Polymers 2018, 10, x FOR PEER REVIEW
11 of 16 11 of 16
As shown in Figure 7B, DAH of PNHT copolymers in the aqueous solution with working pH of As shown in Figure 7B, DAH of PNHT copolymers in the aqueous solution with working pH of 4.0, was significantly larger than that in the solution with pH of 10.0 at 30 ◦ C, but addition of glucose 4.0, was significantly larger than that in the solution with pH of 10.0 at 30 °C, ◦but addition of with the increasing concentration did not cause the copolymers to respond. At 30 C with pH of 4.0, glucose with the increasing concentration did not cause the copolymers to respond. At 30 °C with DAH of PNAHT copolymers did not show clear changes with the variation of glucose concentration, pH of 4.0, DAH of PNAHT copolymers did not show clear changes with the variation of glucose but the acidic media withacidic the temperature than LCST facilitated theLCST copolymers to gather concentration, but the media withhigher the temperature higher than facilitated the ◦ together. At 30to C with pH of 10.0, trend PNAHTtrend copolymers with copolymers gather together. Atthe 30 variation °C with pH of for 10.0,DAH the of variation for DAH plotting of PNAHT ◦ C at pH 10.0, which glucose concentration was consistent with the results under the condition of 20 copolymers plotting with glucose concentration was consistent with the results under the condition also PNAHT hadthat a response glucose. For PNAHT copolymers, effective of confirmed 20 °C at pHthat 10.0, which copolymers also confirmed PNAHTtocopolymers had a response to glucose. For glucose response could only have occurred at the ambient pH above pK of AAPBA, and the ability a PNAHT copolymers, effective glucose response could only have occurred at the ambient pH aboveto respond withthe theability increase of glucose concentration, it did not associate with temperature pKa of increased AAPBA, and to respond increased with thebut increase of glucose concentration, but changes. Theassociate dimension andtemperature conformation of PNAHT PNHT copolymers under different working it did not with changes. The and dimension and conformation of PNAHT and conditions were summarized in Table 2. PNHT copolymers under different working conditions were summarized in Table 2.
Figure ChangesininDDAH of of PNAHT PNAHT and and PNHT atat Figure 7. 7.Changes PNHT copolymers copolymersas asplotted plottedwith withglucose glucoseconcentration concentration AH different temperature and pH. different temperature and pH.
3.2.5. Smart Responsive Behaviors of Copolymers 3.2.5. Smart Responsive Behaviors of Copolymers PNAHT copolymers contained temperature-responsive PNIPAAm and glucose-responsive PNAHT copolymers contained temperature-responsive PNIPAAm and glucose-responsive AAPBA, so they independently or simultaneously exhibited multiple responses to temperature, pH AAPBA, so they independently or simultaneously multiple responses to temperature, pH and glucose under suitable conditions. Regardlessexhibited of whether the ambient temperature exceeded and glucose under suitable conditions. Regardless of whether the ambient temperature exceeded LCST or not in the absence of glucose, copolymer DAH could shift with the changes of environmental LCST notPNAHT in the absence of glucose, copolymer DAH could shift withresponsiveness; the changes of environmental pH, or thus copolymers had shown temperature and pH-stimuli only if pH of pH, thus PNAHT copolymers temperature and pH-stimuli if pH working solutions was abovehad pKashown of AAPBA, the reversible covalent responsiveness; binding betweenonly charged of copolymers working solutions was above pK of AAPBA, the reversible covalent binding between charged a and glucose could be formed, indicating that PNAHT copolymers were temperature-, copolymers and glucose could be formed, indicating that temperature-, pHpH- and glucose-sensitive, whereas the copolymers werePNAHT hesitantcopolymers to bind withwere glucose at relatively and glucose-sensitive, copolymers were hesitant bind copolymers with glucosefell at sharply relatively lower pH, which hadwhereas limited the capability to response. DAH of to PNHT as lower pH pH, which had capability to response. DAH of PNHT fell had sharply pH grew grew when thelimited temperature was higher than LCST, revealing thatcopolymers the copolymers high as sensitivity to temperature and pH, whereas copolymers had failed respond to acidic or alkaline media atto when the temperature was higherthe than LCST, revealing thattothe copolymers had high sensitivity lower temperature. Furthermore, thermos-responsive polymers notordependent on glucose temperature and pH, whereas the copolymers had failed to respond were to acidic alkaline media at lower concentration in any case. It can be concluded that the responding of PNIPAAm PBA to temperature. Furthermore, thermos-responsive polymers were not dependent on glucoseand concentration pH be andconcluded glucose can bethe activated only under special conditions; there is no response if in temperature, any case. It can that responding of PNIPAAm and PBA to temperature, pH and the activation conditions are not satisfied. Therefore, it is necessary to adjust the environmental factors glucose can be activated only under special conditions; there is no response if the activation conditions thesatisfied. optimal conditions full play to their intelligent responsive performances applied. aretonot Therefore,toit give is necessary adjust the environmental factors to thewhen optimal conditions In summary, the major forces affecting temperature and glucose responsive behaviors of to give full play their intelligent responsive performances when applied. PNAHT and PNHT copolymers, include hydrated hydrogen bonds, non-hydrated hydrogen bonds, In summary, the major forces affecting temperature and glucose responsive behaviors of hydrophobic interactions, electrostatic interactions and so on.bonds, When non-hydrated the temperature is lower bonds, than PNAHT and PNHT copolymers, include hydrated hydrogen hydrogen LCST of the copolymers, hydrated hydrogen bonds formed by water molecules and hydrophilic hydrophobic interactions, electrostatic interactions and so on. When the temperature is lower than groups including amide bonds, hydroxyl groups and siloxane groups, maintain the structural LCST of the copolymers, hydrated hydrogen bonds formed by water molecules and hydrophilic groups organization of PNAHT and PNHT copolymers with single chains or oligomers, while hydrophobic
Polymers 2018, 10, 293
12 of 16
including amide bonds, hydroxyl groups and siloxane groups, maintain the structural organization Polymers 2018, 10, FOR PEER REVIEW 12 16 16 Polymers xx FOR REVIEW Polymers 2018,2018, 10, xx 10, FOR PEERPEER REVIEW 12 of of12 Polymers 2018, 10, FOR PEER REVIEW 1216of of 16 Polymers 2018, 10,single x FOR PEER REVIEW 12 of PNAHT and PNHT with chains or oligomers, while hydrophobic interactions Polymers 2018,2018, 10, x 10, FOR PEERPEER REVIEW 12 of1216of 16 Polymers xcopolymers FOR REVIEW Polymers 2018, 10, x FOR PEER REVIEW 12 of 16 16 Polymers 2018, 10, x FOR PEER REVIEW 12 of 16 Polymers 2018, 10, x10, FOR PEER REVIEW 12 of Polymers 2018, x FOR PEER REVIEW 12 16 of between PBA groups cause the intrachain coiling of copolymer segments to slightly decrease the interactions between PBA groups cause the intrachain coiling of copolymer segments to slightly interactions between PBA groups cause the intrachain coiling of copolymer segments to interactions between PBA groups cause the PBA intrachain coiling ofthe copolymer segments to copolymer slightly interactions between PBA groups cause the intrachain coiling ofintrachain copolymer segments to slightly slightlysegments to sli interactions between groups cause coiling of interactions between PBA groups cause the intrachain coiling of copolymer segments to slightly interactions between PBA groups cause the intrachain coiling of copolymer segments to slightly copolymer sizes. When the temperature is higher than the LCST of the copolymers, nonhydrated decrease the copolymer sizes. When the temperature is higher than the LCST of the copolymers, decrease copolymer sizes. When the temperature is higher than the LCST of interactions between PBA groups cause the intrachain coiling of copolymer segments to slightly interactions between PBA groups cause thethe intrachain coiling of copolymer segments tocopolymers, slightly decrease the the copolymer sizes. When the temperature is higher than the LCST of the copolymers, decrease the copolymer sizes. When the temperature is coiling higher than the LCST of the the copolymers, interactions between PBA groups cause the intrachain coiling of copolymer to slightly interactions between PBA groups cause intrachain of copolymer segments to decrease the copolymer sizes. When thethan temperature issegments higher than theslightly LCST of the copoly decrease the between copolymer sizes. When the temperature is higher the interactions LCST of the copolymers, decrease the copolymer sizes. When the temperature is higher than the LCST of the copolymers, hydrogen bonds amide bonds and hydrophobic interactions between isopropyl groups and nonhydrated hydrogen bonds between amide bonds and hydrophobic interactions between isopropyl nonhydrated hydrogen bonds between amide bonds and hydrophobic between isopropyl decrease the copolymer sizes. When the temperature is higher than the LCST of the copolymers, decrease the copolymer sizes. When the temperature is higher than the LCST of the copolymers, nonhydrated hydrogen bonds between amide bonds and hydrophobic interactions between isopropyl nonhydrated hydrogen bonds between amide bonds and between isopropyl between isop decrease thethe copolymer sizes. When thethe temperature is higher than theinteractions LCST of of the copolymers, decrease copolymer sizes. When temperature ishydrophobic higher than the LCST the copolymers, nonhydrated hydrogen bonds between amide bonds and hydrophobic interactions hydrogen bonds between amide bonds and and hydrophobic interactions between isopropyl nonhydrated hydrogen bonds between amide bonds hydrophobic interactions between isopropyl groups and PBA groups, trigger intermolecular aggregation and intramolecular collapse of copolymer PBA nonhydrated groups, trigger intermolecular aggregation and intramolecular collapse of copolymer segments, groups and PBA groups, trigger intermolecular aggregation and intramolecular collapse of copolymer nonhydrated hydrogen bonds between amide bonds and hydrophobic interactions between isopropyl nonhydrated hydrogen bonds between amide bonds andintermolecular hydrophobic interactions between isopropyl groups and PBA groups, trigger intermolecular aggregation and intramolecular collapse of copolymer groups and PBA groups, trigger intermolecular aggregation and intramolecular collapse of copolymer nonhydrated hydrogen bonds between amide bonds and hydrophobic interactions between isopropyl nonhydrated hydrogen bonds between amide bonds and hydrophobic interactions between isopropyl groups and PBA groups, trigger aggregation and intramolecular collapse of copol groups and PBA groups, trigger intermolecular aggregation and intramolecular collapse of copolymer groups and PBA groups, trigger intermolecular aggregation and intramolecular collapse of copolymer segments, resulting in the fact that D AH of the copolymers will be multiplied several times. When pH segments, resulting in the fact that D AH of the copolymers will be multiplied several times. When pH resulting in the fact that D of the copolymers will be multiplied several times. When pH is lower groups and PBA groups, trigger intermolecular aggregation and intramolecular collapse of copolymer groups and PBA groups, trigger intermolecular aggregation and intramolecular collapse ofmultiplied copolymer AH segments, resulting ingroups, the fact that Dintermolecular AH D ofAH the copolymers will be multiplied several times. When pH several segments, resulting in thetrigger fact that of the copolymers will beintramolecular multiplied several times. When pH groups and PBA groups, trigger intermolecular aggregation and intramolecular collapse of copolymer groups and PBA aggregation and collapse of copolymer segments, resulting in the fact that D AH AH of the copolymers will be times. Whe resulting inAAPBA, the fact that HCl D AH D ofAH the copolymers will will be multiplied several times. When pH pH segments, resulting in the fact that of the copolymers be multiplied several times. When is lower than pK a HCl of HCl addition severely destroys hydrated hydrogen bonds, eventually is than of AAPBA, addition severely destroys hydrated hydrogen bonds, eventually segments, resulting in the fact that D AH of the copolymers will be multiplied several times. When pH than segments, pK oflower AAPBA, addition severely destroys hydrated hydrogen bonds, eventually helping segments, resulting the fact that D AH of the copolymers will be multiplied several times. When pH is lower than pK a pK of ain AAPBA, HCl addition severely destroys hydrated hydrogen bonds, eventually asegments, is lower than pK aain of AAPBA, HCl addition severely destroys hydrated hydrogen bonds, eventually segments, resulting the fact that D AH of the copolymers will be multiplied several times. When pH resulting in the fact that D AH of the copolymers will be multiplied several times. When pH isAAPBA, lower than pK a of AAPBA, HCl addition severely destroys hydrated hydrogen bonds, event is lower thannon-hydrated pKa pK of aaAAPBA, HCl addition severely destroys hydrated hydrogen bonds, eventually is lower than of HCl addition severely destroys hydrated hydrogen bonds, eventually helping non-hydrated bonds and hydrophobic interactions become dominant driving helping hydrogen bonds and hydrophobic interactions become dominant driving is lower than ofhydrogen AAPBA, HCl addition severely destroys hydrated hydrogen bonds, eventually is lower lower than pKapK apK ofbonds AAPBA, HCl addition severely destroys hydrated hydrogen bonds, eventually non-hydrated hydrogen and hydrophobic interactions become dominant driving forces, even if helping non-hydrated hydrogen bonds andseverely hydrophobic interactions become dominant driving helping non-hydrated hydrogen bonds and hydrophobic interactions become dominant driving is than pK of HCl addition destroys hydrated hydrogen bonds, eventually is lower than a AAPBA, of AAPBA, HCl addition severely destroys hydrated hydrogen bonds, eventually helping non-hydrated hydrogen bonds and hydrophobic interactions become dominant dr helping non-hydrated hydrogen bonds and hydrophobic interactions become dominant driving helping non-hydrated hydrogen bonds and hydrophobic interactions become dominant driving forces, even if the temperature is lower than LCST, PNAHT copolymers will gather together to some forces, even if the temperature is lower than LCST, PNAHT copolymers will gather together to some helping non-hydrated hydrogen bonds and hydrophobic interactions become dominant driving helping non-hydrated hydrogen bonds and hydrophobic interactions become dominant driving the temperature is lower than LCST, PNAHT copolymers will gather together to some extent. When pH forces, even if the temperature is lower than LCST, PNAHT copolymers will gather together to some forces,non-hydrated even if the temperature is lower than LCST, PNAHT copolymers will gather together to some helping hydrogen bonds and hydrophobic interactions become dominant driving helping non-hydrated hydrogen bonds and hydrophobic interactions become dominant driving forces, even if the temperature is lower than LCST, PNAHT copolymers will gather together to forces, even if pH the temperature isthan lower than LCST, PNAHT copolymers will will gather together to some forces, even if the temperature is lower than LCST, PNAHT copolymers gather together to some extent. When is higher than of resultant copolymers, introduction of NaOH or glucose extent. When pH is higher athe of the resultant copolymers, introduction of NaOH or glucose forces, even ifthe the temperature isaa pK lower than LCST, PNAHT copolymers will gather together to someof NaOH or glu forces, even ifof the temperature ispK lower than LCST, PNAHT copolymers willwill gather together to some extent. When pH is higher than pK of resultant copolymers, introduction ofgather NaOH or glucose is higher than pK resultant copolymers, introduction of NaOH orresultant glucose changes the ionization extent. When pH is extent. higher than pK a of the resultant copolymers, introduction oftogether NaOH orsome glucose forces, even the temperature is lower than LCST, PNAHT will gather to forces, even if the temperature is lower than LCST, PNAHT copolymers together to some aif When pH is higher than pK athe a copolymers of the copolymers, introduction extent. When pHionization is higher thanthan pKaofpK ofthe resultant copolymers, introduction of aNaOH or glucose extent. When pH is higher athe of the resultant copolymers, introduction of or glucose changes the ionization correspondingly forming large amount of changes the equilibrium the copolymers, aaNaOH large extent. When pH isequilibrium higher than aof ofcopolymers, the resultant copolymers, introduction of NaOH or glucose extent. When pH is higher higher than pKof apK of the resultant copolymers, introduction ofa NaOH NaOH or amount glucose changes theWhen ionization equilibrium the copolymers, correspondingly forming large amount of of changes thepH ionization equilibrium the copolymers, correspondingly forming large amount of a large amou equilibrium of the copolymers, correspondingly forming a correspondingly large offorming charged hydrophilic extent. When is than pK apK of the resultant copolymers, introduction of or glucose extent. pH is higher than aof of the resultant copolymers, introduction of NaOH or glucose changes the ionization equilibrium of the amount copolymers, correspondingly forming changes the ionization equilibrium of the copolymers, correspondingly forming a large amount of of changes the ionization equilibrium of the copolymers, correspondingly forming a large amount charged hydrophilic nanoparticles. The emerging electrostatic repulsions and dramatically increased charged hydrophilic nanoparticles. The emerging electrostatic repulsions and dramatically increased changes the ionization equilibrium of the copolymers, correspondingly forming a large amount of changes the ionization equilibrium of the copolymers, correspondingly forming a large amount of charged hydrophilic nanoparticles. The emerging electrostatic repulsions and dramatically increased charged hydrophilic nanoparticles. The emerging electrostatic repulsions and dramatically increased changes the ionization equilibrium of the copolymers, correspondingly forming a large amount of changes the ionization equilibrium of the copolymers, correspondingly forming a large amount of nanoparticles. The emerging electrostatic repulsions and dramatically increased water solubility charged hydrophilic nanoparticles. Therepulsions emerging electrostatic repulsions and dramatically incre charged hydrophilic nanoparticles. The emerging electrostatic and dramatically increased charged hydrophilic nanoparticles. The emerging electrostatic repulsions and dramatically increased water solubility inhibit copolymer aggregation, accordingly cause the agglomerates to dissociate into water solubility inhibit copolymer aggregation, accordingly cause the agglomerates to dissociate charged hydrophilic nanoparticles. The emerging electrostatic repulsions and dramatically increased charged hydrophilic nanoparticles. The emerging electrostatic repulsions andand dramatically increased water solubility inhibit copolymer aggregation, accordingly cause the agglomerates to dissociate intointo water solubility inhibit copolymer aggregation, accordingly cause the agglomerates to the dissociate into to dissociate charged hydrophilic nanoparticles. The emerging electrostatic repulsions and dramatically increased charged hydrophilic nanoparticles. The emerging electrostatic repulsions dramatically increased water solubility inhibit copolymer aggregation, accordingly cause agglomerates inhibit copolymer aggregation, accordingly cause the agglomerates to dissociate into single-stranded water solubility inhibit copolymer aggregation, accordingly cause the agglomerates toconclusion, dissociate into water solubility inhibit copolymer aggregation, accordingly cause the agglomerates to dissociate into single-stranded or oligomeric forms and make their conformations more stretched. In the single-stranded or oligomeric forms and make their conformations more stretched. In the water solubility inhibit copolymer aggregation, accordingly cause the agglomerates toconclusion, dissociate into water solubility inhibit copolymer aggregation, accordingly cause the agglomerates to dissociate intointo single-stranded or oligomeric forms and make their conformations more stretched. Into conclusion, the single-stranded or oligomeric forms and make their conformations more stretched. In conclusion, the In conclusion water solubility inhibit copolymer aggregation, accordingly cause the agglomerates dissociate into water solubility inhibit copolymer aggregation, accordingly cause the agglomerates to dissociate single-stranded or oligomeric forms and make their conformations more and stretched. or oligomeric forms and make their conformations more stretched. Inmore conclusion, the dimension single-stranded or oligomeric forms and make their conformations stretched. In conclusion, the single-stranded or oligomeric forms and make their conformations more stretched. In conclusion, the dimension and configuration of temperature or glucose responsive copolymers are the results of dimension and configuration of temperature or glucose responsive copolymers are the results of single-stranded or oligomeric forms and make their conformations more stretched. In conclusion, the single-stranded or oligomeric forms and make their conformations more stretched. In conclusion, the dimension and configuration of temperature or glucose responsive copolymers are the results of dimension and configuration of temperature or glucose responsive copolymers are the results of are the resu single-stranded or oligomeric forms and make their conformations more stretched. In conclusion, the single-stranded or oligomeric forms and make their conformations more stretched. In conclusion, the dimension and configuration of temperature or glucose responsive copolymers configuration ofand temperature or glucose responsive copolymers are the results of synergistic effects of of dimension configuration ofinter/intramolecular temperature oracting glucose responsive copolymers are are the results of dimension and configuration of temperature or glucose responsive copolymers the results synergistic effects of various inter/intramolecular factors. synergistic effects of various acting factors. dimension and configuration of temperature or glucose responsive copolymers are the results of dimension and configuration of temperature or glucose responsive copolymers areare thethe results of of synergistic effects of various inter/intramolecular acting factors. synergistic effects of various inter/intramolecular acting factors. dimension and configuration of temperature or glucose responsive copolymers are the results of dimension and configuration of temperature or glucose responsive copolymers results synergistic effects of various inter/intramolecular acting factors. various inter/intramolecular acting factors. synergistic effects of various inter/intramolecular acting factors. synergistic effects of various inter/intramolecular acting factors. synergistic effects of various various inter/intramolecular acting factors. synergistic effects of various various inter/intramolecular acting factors. synergistic effects of inter/intramolecular acting factors. synergistic effects of inter/intramolecular acting factors. Table 2. and of and PNHT copolymers under different conditions. Table 2. Dimension and conformation of PNAHT and PNHT copolymers under different conditions. Table 2. Dimension Dimension and conformation conformation of PNAHT PNAHT and PNHT copolymers under different conditions. Table 2. Table 2. Dimension and conformation Dimension of PNAHT and conformation and PNHT of copolymers PNAHT and under PNHT different copolymers conditions. under different conditions. TableTable 2. Dimension and conformation of PNAHT and PNHT copolymers under different conditions. 2. Dimension and conformation of PNAHT and PNHT copolymers under different conditions. Table 2. Dimension and conformation of PNAHT and PNHT copolymers under different conditions. Table 2. Dimension Dimension and conformation of PNAHT PNAHT and PNHT copolymers under different conditions. Table 2. Dimension andand conformation of PNAHT PNAHT andand PNHT copolymers under different conditions. Table 2. Dimension and conformation of and PNHT copolymers under different conditions. Table 2. conformation of PNHT copolymers under different conditions. Item pH Glucose 30 Glucose 20 °C 30 ItemItem pH pH Glucose 20 °C °C 30 °C °C Item pH Item Glucose pH 20 20Glucose °C 30 °C °C 20 °C 30 °C Item pHpH Glucose 20 °C 30 Item pH Glucose 20 °C 30 ◦ ◦°C Item Glucose C 30 C Item pH Glucose 20 °C 30 °C °C Item pHpH Glucose Glucose 20 °C °C °C 30 °C °C Item pH 20 30 Item Glucose 20 30 °C Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence 4.0 4.0 4.0 4.0 4.04.04.0 4.0 4.04.0 4.0 PNAHT PNAHT PNAHT PNAHT PNAHT PNAHT PNAHT copolymers copolymers PNAHT PNAHT copolymers copolymers PNAHT PNAHT copolymers copolymers copolymers copolymers copolymers copolymers copolymers
Absence 4.0
Presence Presence Presence PNAHT Presence Presence Presence Presence copolymers Presence Presence Presence Presence
Presence
Absence Absence Absence Absence Absence Absence Absence Absence Absence 10.0 10.0 Absence Absence 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 Presence Presence Presence Presence Presence Presence Presence Presence Presence Presence Presence
Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence
PNHT PNHT PNHT PNHT PNHT copolymers PNHT PNHT copolymers copolymers PNHT PNHT copolymers copolymers PNHT PNHT copolymers copolymers copolymers copolymers copolymers copolymers
4.0 4.0 4.0 4.0 4.0 4.0 4.0 PNHT 4.0 4.0 4.04.0 copolymers
Absence
Presence
Absence
4.0
Presence Presence Presence Presence Presence Presence Presence Presence Presence Presence Presence
10.0 Absence 10.010.0 Absence 10.0 Absence Absence 10.0 10.010.0 Absence Absence 10.0 Absence Absence 10.0 Absence 10.0 10.0 Absence
Presence
Absence
3.3. Morphology and Size of Copolymer Particles DLS results showed that DAH of PNAHT and PNHT copolymers varied significantly in 4.0 4.0 different working solutions. For more clarity, the clean glass coverslips were dipping coated with PNHT PNHT the copolymer particles in diverse states, and then the coated surfaces were scanned by AFM. In copolymers copolymers Polymers 2018, 10, 293 13 of 16 order to eliminate experimental error, the same copolymer concentration and dipping speed were applied in coating processes. Furthermore, all matter became inert and incapable of reaction at the Presence Presence Table 2. state Cont. of the copolymers deposited on the glass temperature upper limit of 37 °C in the air. The surfaces would be somewhat different from those in aqueous solutions, but fast drying was ◦ Itemtheir originalpH 30 ◦ C employed to keep shapes as Glucose much as possible. 20 C Note that this project was not designed to detect the evenness of coating surfaces, so roughness 10.0 Absence 10.0 Absence Absence Polymers 2018, x FOR PEER REVIEW Polymers 10,It x 10, FOR PEER REVIEW 13 of1316of 16 made no 2018, sense. was limited to the discussion of peak heights which reflected the typical particle copolymersparticles sizes of PNHT the copolymer 10.0in this study. The height curve of X-cross section was randomly selected in each condition. The AFM images for glass surfaces with particle deposition of PNHT Presence Presence Presence copolymers in UHQ water at 20 °C (Figure 8A) and 37 °C (Figure 8B) were taken as examples, as shown in Figure 8. By comparison, Figure 8A illustrated the peak heights ranging from 0 to 100 nm with the peaks appeared at 30~50 nm, and relatively homogeneous tiny particles basically 3.3.most Morphology and of Copolymer Particles Morphology and Size of Copolymer Particles 3.3.3.3. Morphology and Size of Size Copolymer Particles covered with the glass substrate (dark regions), whereas Figure 8B displayed the peak heights DLS results showed Dof AH PNAHT of PNAHT PNHT copolymers varied DLS results showed that that DAH and and PNHT copolymers varied significantly in in DLS results that D of PNAHT PNHT varied insignificantly different AH distributing fromshowed 0 to 200 nm with the peakand centered atcopolymers 140~160 nm, andsignificantly white or almost white different working solutions. For more clarity, the clean coverslips dipping coated different working solutions. For the more clarity, the clean glassglass coverslips werewere dipping coated withwith working For more clarity, clean glass coverslips dipping coated with the copolymer particles solutions. were scattered within the dark regions, which leftwere behind greater exposed part of glass the copolymer particles in diverse states, and then the coated surfaces were scanned by AFM. the copolymer particles in diverse states, and then the coated surfaces were scanned by AFM. In In particles diverse then thethat coated scanned by AFM. In order to eliminate surfaces. in The AFMstates, resultsand indicated the surfaces particleswere of PNHT copolymers derived from UHQ order to eliminate experimental error, the same copolymer concentration and dipping speed order to eliminate experimental error, the same copolymer concentration and dipping speed were experimental same copolymer concentration and in dipping speed were applied in average coating were water at 20 °Cerror, werethe more plentiful in number and smaller size than those at 37 °C. The applied in coating processes. Furthermore, all incapable matter became inert incapable of reaction at the applied in coating processes. Furthermore, alland matter became and and incapable of reaction at the processes. Furthermore, all matter became inert ofinert reaction at the temperature upper particle sizes of PNHT copolymers at 37 °C were up to several times larger than those at 20 °C, temperature upper limit of 37 °C in the air. The state of the copolymers deposited on the glass temperature upper limit of 37 °C in the air. The state of the copolymers deposited on the glass ◦ limit of 37PNHT C in the air. The state of thereunited copolymers deposited on the glassabove surfaces would somewhat because copolymers became under the temperature LCST butbe hydrated at surfaces would be somewhat different from those in aqueous solutions, but fast drying surfaces would somewhat different those was in aqueous solutions, but original fast drying was was different from thosebe in aqueous solutions, butfrom fastagreement drying employed to keep their lower temperature. The phenomenon was in with DLS data. Similar findingsshapes were employed to keep original shapes as much as possible. employed to keep theirtheir original shapes as much as possible. as much asin possible. confirmed other groups of PNAHT and PNHT copolymers. project not designed to detect the evenness of coating surfaces, so roughness NoteNote that that this this project was was not designed to detect the evenness of coating surfaces, so roughness made no sense. It was limited to discussion the discussion of peak heights which reflected the typical particle made no sense. It was limited to the of peak heights which reflected the typical particle of copolymer the copolymer particles in this study. height curve of X-cross section randomly sizessizes of the particles in this study. The The height curve of X-cross section was was randomly selected in each condition. images for glass surfaces particle deposition of PNHT selected in each condition. The The AFMAFM images for glass surfaces withwith particle deposition of PNHT copolymers in UHQ water 20(Figure °C (Figure 37(Figure °C (Figure 8B) were taken as examples, copolymers in UHQ water at 20at°C 8A) 8A) and and 37 °C 8B) were taken as examples, as as shown in Figure By comparison, Figure 8A illustrated the peak heights ranging to nm 100 nm shown in Figure 8. By8.comparison, Figure 8A illustrated the peak heights ranging fromfrom 0 to 0100 peaks appeared at 30~50 relatively homogeneous particles basically withwith the the mostmost peaks appeared at 30~50 nm, nm, and and relatively homogeneous tiny tiny particles basically covered substrate (dark regions), whereas Figure 8B displayed heights covered withwith the the glassglass substrate (dark regions), whereas Figure 8B displayed the the peakpeak heights distributing to 200 the peak centered at 140~160 white or almost white distributing fromfrom 0 to 0200 nm nm withwith the peak centered at 140~160 nm, nm, and and white or almost white particles scattered within the dark regions, which behind greater exposed of glass particles werewere scattered within the dark regions, which left left behind greater exposed partpart of glass surfaces. results indicated particles of PNHT copolymers derived UHQ surfaces. The The AFMAFM results indicated that that the the particles of PNHT copolymers derived fromfrom UHQ water 20were °C were more plentiful in number smaller in size those 37 The °C. The average water at 20at°C more plentiful in number and and smaller in size thanthan those at 37at°C. average particle of PNHT copolymers 37 were °C were upseveral to several times larger those 20 °C, particle sizessizes of PNHT copolymers at 37at°C up to times larger thanthan those at 20at°C, because PNHT copolymers became reunited under the temperature above LCST hydrated because PNHT copolymers became reunited under the temperature above LCST but but hydrated at at lower temperature. The phenomenon was in agreement with DLS data. Similar findings lower temperature. The phenomenon was in agreement with DLS data. Similar findings werewere confirmed in other groups of PNAHT PNHT copolymers. confirmed in other groups of PNAHT and and PNHT copolymers.
Figure 8. AFM images for glass surfaces with particle deposition of PNHT copolymers in UHQ water Figure 8. AFM images for glass surfaces with particle deposition of PNHT copolymers in UHQ water at 20 °C (A) and 37 °C (B). at 20 ◦ C (A) and 37 ◦ C (B).
Note that this project was not designed to detect the evenness of coating surfaces, so roughness made no sense. It was limited to the discussion of peak heights which reflected the typical particle sizes of the copolymer particles in this study. The height curve of X-cross section was randomly selected in each condition. The AFM images for glass surfaces with particle deposition of PNHT copolymers in UHQ water at 20 ◦ C (Figure 8A) and 37 ◦ C (Figure 8B) were taken as examples, as shown in Figure 8. By comparison, Figure 8A illustrated the peak heights ranging from 0 to 100 nm with the most peaks appeared at 30~50 nm, and relatively homogeneous tiny particles basically covered with the glass substrate (dark regions), whereas Figure 8B displayed the peak heights distributing from 0 to 200 nm
Figure 8. AFM images for glass surfaces particle deposition of PNHT copolymers in UHQ water Figure 8. AFM images for glass surfaces withwith particle deposition of PNHT copolymers in UHQ water
Polymers 2018, 10, 293
14 of 16
with the peak centered at 140~160 nm, and white or almost white particles were scattered within the dark regions, which left behind greater exposed part of glass surfaces. The AFM results indicated that the particles of PNHT copolymers derived from UHQ water at 20 ◦ C were more plentiful in number and smaller in size than those at 37 ◦ C. The average particle sizes of PNHT copolymers at 37 ◦ C were up to several times larger than those at 20 ◦ C, because PNHT copolymers became reunited under the temperature above LCST but hydrated at lower temperature. The phenomenon was in agreement with DLS data. Similar findings were confirmed in other groups of PNAHT and PNHT copolymers. 4. Conclusions The P(NIPAAm-co-AAPBA-co-HPM-co-TMSPM) and P(NIPAAm-co-HPM-co-TMSPM) copolymers were synthesized by free radical polymerization. FTIR spectra showed the characteristic functional groups of the resultant copolymers, such as amide bonds, isopropyl groups, siloxane bonds, hydroxyl groups and ester groups for both copolymers, and phenylboronic acid groups only for PNAHT copolymers. The molar ratios of NIPAAm, AAPBA, HPM and TMSPM in the final products were confirmed by 1 H-NMR spectra to be 20:1:1:1 and 20:0:1:1 for PNAHT and PNHT copolymers. The GPC results indicated that the molecular weight distributions of both copolymers were relatively narrow, however, the molecular weight and polymerization degree of PNAHT copolymers were less than that of PNHT copolymers due to stereo-hindrance effect of AAPBA. The intelligent responsive behaviors of the copolymers in aqueous solutions with different temperature, pH value and glucose concentration were studied by DLS and AFM. The results demonstrated that PNHT copolymers were thermo- and pH-sensitive, whereas PNAHT copolymers had triple responses to temperature, pH and glucose under appropriate conditions. Furthermore, the introduction of AAPBA into PNIPAAm segments lowered the molecular weight of the resultant copolymers, intensified the interchain aggregation of the nanoparticles, and reduced the LSCT of the composites. It could be also concluded that the dimension, conformation and further LCST of the copolymers are affected by hydrous hydrogen bonds, nonhydrous hydrogen bonds, hydrophobic interactions and electrostatic interactions. Hydrated hydrogen bonds maintain the size and morphology of the nanoparticles with single chains or oligomers, nonhydrated hydrogen bonds and hydrophobic interactions lead to intermolecular agglomeration and intramolecular crimping, and electrostatic repulsions suppress intermolecular stacking and promote the extension of molecular segments. Acknowledgments: This work was supported by National Natural Science Foundation of China (21604034) and (31570097), Doctoral Scientific Research Foundation of Liaoning Province of China (20170520391), and General Scientific Research Program of Department of Education of Liaoning Province of China (LSNYB201619). Author Contributions: Lei Yang and Xiaoguang Fan conceived and designed the experiments; Jiaxing Li and Fei Wang performed the experiments; Jing Zhang analyzed the data; Zhanyong Wang contributed analysis tools; Jiaxing Li and Lei Yang wrote the paper. Conflicts of Interest: The authors declare no conflicts of interest.
References 1.
2.
3. 4.
Yang, H.W.; Lee, A.W.; Huang, C.H.; Chen, J.K. Characterization of poly(N-isopropylacrylamide)-nucleobase supramolecular complexes featuring bio-multiple hydrogen bonds. Soft Matter 2014, 10, 8330–8340. [CrossRef] [PubMed] Blanco, M.D.; Guerrero, S.; Benito, M.; Fernández, A.; Teijón, C.; Olmo, R.; Katime, I.; Teijón, J.M. In vitro and in vivo evaluation of a folate-targeted copolymeric submicrohydrogel based on N-isopropylacrylamide as 5-Fluorouracil delivery system. Polymers 2011, 3, 1107–1125. [CrossRef] Pandiyarajan, C.K.; Genzer, J. Effect of network density in surface-anchored poly(N-isopropylacrylamide) hydrogels on adsorption of fibrinogen. Langmuir 2017, 33, 1974–1983. [CrossRef] [PubMed] Zhang, J.; Peng, C.A. Poly(N-isopropylacrylamide) modified polydopamine as a temperature-responsive surface for cultivation and harvest of mesenchymal stem cells. Biomater. Sci. 2017, 5, 2310–2318. [CrossRef] [PubMed]
Polymers 2018, 10, 293
5. 6.
7. 8.
9.
10. 11. 12. 13. 14.
15.
16.
17. 18.
19.
20. 21. 22. 23. 24. 25.
26.
15 of 16
Barnes, A.L.; Genever, P.G.; Rimmer, S.; Coles, M.C. Collagen-poly(N-isopropylacrylamide) hydrogels with tunable properties. Biomacromolecules 2016, 17, 723–734. [CrossRef] [PubMed] Ma, R.J.; Yang, H.; Li, Z.; Liu, G.; Sun, X.C.; Liu, X.J.; An, Y.L.; Shi, L.Q. Phenylboronic acid-based complex micelles with enhanced glucose-responsiveness at physiological pH by complexation with glycopolymer. Biomacromolecules 2012, 13, 3409–3417. [CrossRef] [PubMed] Wang, X.; Wei, B.; Cheng, X.; Wang, J.; Tang, R. Phenylboronic acid-decorated gelatin nanoparticles for enhanced tumor targeting and penetration. Nanotechnology 2016, 27, 385101. [CrossRef] [PubMed] Bajgrowicz-Cieslak, M.; Alqurashi, Y.; Elshereif, M.I.; Yetisen, A.K.; Hassan, M.U.; Butt, H. Optical glucose sensors based on hexagonally-packed 2.5-dimensional photonic concavities imprinted in phenylboronic acid functionalized hydrogel films. RSC Adv. 2017, 7, 53916–53924. [CrossRef] [PubMed] Tu, X.Y.; Meng, C.; Liu, Z.; Sun, L.; Zhang, X.S.; Zhang, M.K.; Sun, M.R.; Ma, L.W.; Liu, M.Z.; Wei, H. Synthesis and phase transition of poly(N-isopropylacrylamide)-based thermo-sensitive cyclic brush polymer. Polymers 2017, 9, 301. [CrossRef] Berne, B.J.; Pecora, R. Dynamic Light Scattering: With Applications to Chemistry, Biology, and Physics; Courier Dover Publications: New York, NY, USA, 2000; ISBN 0-486-41155-9. Qiu, X.P.; Kwan, C.M.S.; Wu, C. Laser light scattering study of the formation and structure of poly(N-isopropylacrylamide-co-acrylic acid) nanoparticles. Macromolecules 1997, 30, 6090–6094. [CrossRef] Özyürek, Z.; Voit, B.; Krahl, F.; Arndt, K.-F. Thermoresponsive aggregation behavior of NiPAAm/glyco monomer block copolymers studied by dynamic light scattering. e-Polymers 2010, 10, 482–494. [CrossRef] ´ Sliwa, T.; Jarz˛ebski, M. Dynamic light scattering investigation of Pnipam-Co-Maa microgel solution. Curr. Top. Biophys. 2015, 37, 29–33. [CrossRef] Wang, D.; Liu, T.; Yin, J.; Liu, S.Y. Stimuli-responsive fluorescent poly(N-isopropylacrylamide) microgels labeled with phenylboronic acid moieties as multifunctional ratiometric probes for glucose and temperatures. Macromolecules 2011, 44, 2282–2290. [CrossRef] Farooqi, Z.H.; Wu, W.T.; Zhou, S.Q.; Siddiq, M. Engineering of phenylboronic acid based glucose-sensitive microgels with 4-Vinylpyridine for working at physiological pH and temperature. Macromol. Chem. Phys. 2011, 212, 1510–1514. [CrossRef] Farooqi, Z.H.; Khan, A.; Siddiq, M. Temperature-induced volume change and glucose sensitivity of poly[(N-isopropylacry-lamide)-co-acrylamide-co-(phenylboronic acid)] microgels. Polym. Int. 2011, 60, 1481–1486. [CrossRef] Wu, Q.; Wang, L.; Yu, H.J.; Chen, Z.F. The synthesis and responsive properties of novel glucose-responsive microgels. Polym. Sci. Ser. A 2012, 54, 209–213. [CrossRef] Tang, Y.C.; Wu, J.H.; Duan, J.X. The micellization and dissociation transitions of thermo-, pH- and sugar-sensitive block copolymer investigated by laser light scattering. eXPRESS Polym. Lett. 2012, 6, 647–656. [CrossRef] Yang, L.; Liu, T.Q.; Song, K.D.; Wu, S.; Fan, X. Effect of intermolecular and intramolecular forces on hydrodynamic diameters of poly(N-isopropylacrylamide) copolymers in aqueous solutions. J. Appl. Polym. Sci. 2013, 127, 4280–4287. [CrossRef] Wedel, B.; Hertle, Y.; Wrede, O.; Bookhold, J.; Hellweg, T. Smart homopolymer microgels: Influence of the monomer structure on the particle properties. Polymers 2016, 8, 162. [CrossRef] Uguzdo ˘ gan, ˘ E.; Denkba¸s, E.B.; Tuncel, A. RNA-sensitive N-isopropylacrylamide/vinylphenylboronic acid random copolymer. Macromol. Biosci. 2002, 2, 214–222. [CrossRef] Dinçer, S.; Köseli, V.; Kesim, H.; Rzaev, Z.M.O.; Pi¸skin, E. Radical copolymerization of N-isopropylacrylamide with anhydrides of maleic and citraconic acids. Eur. Polym. J. 2002, 38, 2143–2152. [CrossRef] Tang, Y.; Lu, J.R.; Lewis, A.L.; Vick, T.A.; Stratford, P.W. Swelling of zwitterionic polymer films characterized by spectroscopic ellipsometry. Macromolecules 2001, 34, 8768–8776. [CrossRef] Hoare, T.; Pelton, R. Engineering glucose swelling responses in poly(N-isopropylacrylamide)-based microgels. Macromolecules 2007, 40, 670–678. [CrossRef] Yang, L.; Pan, F.; Zhao, X.B.; Yaseen, M.; Padia, F.; Coffey, P.; Freund, A.; Yang, L.Y.; Liu, T.Q.; Ma, X.H.; et al. Thermoresponsive copolymer nanofilms for controlling cell adhesion, growth, and detachment. Langmuir 2010, 26, 17304–17314. [CrossRef] [PubMed] Tang, Z.; Guan, Y.; Zhang, Y.J. Contraction-type glucose-sensitive microgel functionalized with a 2-substituted phenylboronic acid ligand. Polym. Chem. 2014, 5, 1782–1790. [CrossRef]
Polymers 2018, 10, 293
27.
28.
29.
16 of 16
Brewer, S.H.; Allen, A.M.; Lappi, S.E.; Chasse, T.L.; Briggman, K.A.; Gorman, C.B.; Franzen, S. Infrared detection of a phenylboronic acid terminated alkane thiol monolayer on gold surfaces. Langmuir 2004, 20, 5512–5520. [CrossRef] [PubMed] Lai, E.P.; Wang, Y.X.; Wei, Y.; Li, G. Preparation of uniform-sized and dual stimuli-responsive microspheres of poly(N-isopropylacrylamide)/poly(acrylic acid) with semi-IPN structure by one-step method. Polymers 2016, 8, 90. [CrossRef] Khan, A.; Alhoshan, M. Preparation and characterization of pH-responsive and thermoresponsive hybrid microgel particles with gold nanorods. J. Polym. Sci. Part A Polym. Chem. 2013, 51, 39–46. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
