materials Communication
Cononsolvency Transition of Polymer Brushes: A Combined Experimental and Theoretical Study Huaisong Yong 1,2 , Sebastian Rauch 1 , Klaus-Jochen Eichhorn 1 , Petra Uhlmann 1 Andreas Fery 1,2, * and Jens-Uwe Sommer 1,3, * 1 2 3
*
ID
,
Leibniz-Institut für Polymerforschung Dresden e.V., 01069 Dresden, Germany;
[email protected] (H.Y.);
[email protected] (S.R.);
[email protected] (K.-J.E.);
[email protected] (P.U.) Institute of Physical Chemistry of Polymeric Materials, Technische Universität Dresden, 01062 Dresden, Germany Institute for Theoretical Physics, Technische Universität Dresden, 01062 Dresden, Germany Correspondence:
[email protected] (A.F.);
[email protected] (J.-U.S.); Tel.: +49-351-4658-225 (A.F.); +49-351-4658-750 (J.-U.S.)
Received: 30 April 2018; Accepted: 8 June 2018; Published: 11 June 2018
Abstract: In this study, the cononsolvency transition of poly(N-isopropylacrylamide) (PNiPAAm) brushes in aqueous ethanol mixtures was studied by using Vis-spectroscopic ellipsometry (SE) discussed in conjunction with the adsorption-attraction model. We proved that the cononsolvency transition of PNiPAAm brushes showed features of a volume phase transition, such as a sharp collapse, reaching a maximum decrease in thickness for a very narrow ethanol volume composition range of 15% to 17%. These observations are in agreement with the recently published preferential adsorption model of the cononsolvency effect. Keywords: cononsolvency; preferential adsorption; polymer brushes; PNiPAAm; ellipsometry
1. Introduction Cononsolvency occurs if a mixture of two good solvents causes the collapse and segregation of polymer solutions into a polymer-rich phase in a certain range of compositions of these two solvents [1,2]. A prominent example [3] is poly(N-isopropylacrylamide) (PNiPAAm) in a water and methanol mixture. Although both solvents are good solvents for PNiPAAm, in a certain range of compositions of methanol and water PNiPAAm segregates from the mixed solution. Besides segregation in solutions, the cononsolvencyinduced volume phase transition of PNiPAAm gels has received much attention since the first paper published by T. Amiya et al. [4]. It is notable that a discontinuous, or jump-like, volume phase transition as it is observed for gels in such solvent mixtures would not be expected for a transition from a good to poor solvent within the common Flory-Huggins scheme because of the absence of translational degrees of freedom of the chains here [5,6]. This indicates a new kind of transition due to the solvent–cosolvent–polymer interaction. Given these observations, it is interesting to explore the possibility of a jump-like collapse transition for polymers on surfaces, in particular for polymer brushes. Controlling the swelling of the polymer brushes and achieving a switching effect is quite important for the study of phase transitions [7–11] and applications of polymer brushes [12–14]; however, study of cononsolvency effects as a new switching effect in polymer brushes is still in its infancy stage. On the side of the experimental studies, previous studies mainly focused on how to use modern measurement techniques to monitor the cononsolvency transition of brushes. Liu and Zhang [15] used a quartz crystal microbalance with dissipation monitoring (QCM-D) to study the behavior of PNiPAAm brushes in water-methanol mixtures. They found a sharp cononsolvency transition, which they attributed to the formation of water-methanol complexes or clusters via hydrogen bonding. Later, Materials 2018, 11, 991; doi:10.3390/ma11060991
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S. Edmondson et al. [16] directly observed the volume change of polymer brushes by using ellipsometry to investigate the cononsolvency transition of poly(2-(methacryloyloxy)ethyl phosphorylcholine) brushes in alcohol-water mixtures. X. Sui et al. [17] observed morphological changes of PNiPAAm brushes by using surface probe microscopy in methanol-water mixtures. By using an extended surface forces apparatus, R. Espinosa-Marzal et al. [18] studied the impact of solvation on equilibrium conformation of dextran brushes in aqueous dimethyl sulfoxide mixtures. By combining the methods mentioned above, cononsolvency effects of polymer brushes have been related to applications for nanomaterials [19], tunable friction [20,21] and adhesive [22] properties of surfaces, as well as response of polymer brushes in liquid fluid media [23,24]. A possible molecular explanation for the new transition comes from atomistic simulations of single chains in the presence of a cosolution [25], and has been rationalized using the concept of preferential and non-specific adsorption of a cosolvent onto the polymer chain [26,27]. Based on the concept of preferential adsorption, one of the authors proposed a general model for a cononsolvency transition in polymer brushes [28], which has been extended recently to polymer solutions [29]. Chen et al. [30] also proposed a polymer lattice density functional theory (PLDFT) to investigate the cononsolvency phenomena related to polymer adsorption in a slit pore. What is missing up to now is a physical understanding of the cononsolvency effect on real polymer brushes, which explains the equilibrium behavior and nature of the phase transition. The aim of this work is an investigation on the cononsolvency transition behavior in hydrophilic polymer brush systems in comparison with the theoretical predictions. We especially focus on a systematical VIS-spectroscopic ellipsometry (SE) measurement to study the properties of PNiPAAm brushes grafted on flat silicon surfaces in a mixture of ethanol and water. 2. Materials and Methods Poly (glycidyl methacrylate) (PGMA, Mn = 10–20,000 g/mol) was purchased from Sigma-Aldrich (Darmstadt, Germany). Absolute ethanol (EtOH, 99.8%) was acquired from Merck (Darmstadt, Germany). Tetrahydrofuran (THF, 99.98%) and chloroform (CHCl3 , ≥99%) were purchased from Acros Organics (Darmstadt, Germany). Purified water (H2 O) was used from a Milli-Q Direct-8 system from Merck Millipore (Darmstadt, Germany). All chemicals were used as received, if not otherwise specifically noted. Highly polished single-crystal silicon wafers of {100} orientation (Si-Mat Silicon Materials, Kaufering, Germany) were used as a substrate. End-functionalized PNiPAAm polymers (Mn = 50,200 g/mol, Mw /Mn = 1.28, monomer number of every chain is about 443) with a terminal (tert-butyl protected) carboxy group were synthesized and characterized as described previously [31]. 2.1. Preperation of Polymer Brushes Silicon substrates (13 × 20 mm2 ) were treated with EtOH in an ultrasonic bath for 10 min, dried with a stream of nitrogen, and treated with oxygen plasma (440-G Plasma System, Technics Plasma GmbH, Wettenberg, Germany) for 1 min at 100 W. Next a thin layer of PGMA (2.5 nm) was deposited by spin coating (Spin 150, SPS Coating, Ingolstadt, Germany) a PGMA solution in CHCl3 (0.3 mg/mL) with subsequent annealing at 110 ◦ C in a vacuum oven for 20 min to react the silanol groups of the substrate with a fraction of the epoxy groups of PGMA, thus forming an anchoring layer equipped with the remaining epoxy groups for the following “grafting-to” process [32]. Afterwards, a filtered solution of end-functionalized PNiPAAm in THF (9.0 mg/mL) was spin coated onto the PGMA layer and subsequently annealed at 174 ◦ C in a vacuum oven for 18 h (Scheme 1). To remove non-covalently bonded polymers, the resulting films were immersed first in H2 O, then extracted in H2 O overnight, rinsed with EtOH, and dried in a stream of nitrogen. Here we point out the primary advantage of the “grafting-to” method is that it is a technically simple processing step consuming low quantities of an accurately characterized pre-formed polymer. This means molecular weight, polydispersity, chain–end functionality, and especially the architecture can be designed very precisely, and thus comprehensive studies like the present one can be carried out.
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Scheme Scheme 1. 1. Preparation Preparation of of PNiPAAm PNiPAAm brushes brushes on on silicon silicon substrates substrates using using the the“grafting-to” “grafting-to”approach. approach.
2.2. VIS-Spectroscopic Ellipsometry Measurement Measurement 2.2. A spectroscopic spectroscopic ellipsometer ellipsometer (alpha-SE, (alpha-SE, Woollam Woollam Co., Co., Inc., Inc., Lincoln, Lincoln, NE, NE, USA) USA) equipped equipped with with aa A rotatingcompensator compensatorwas wasused usedtoto measure relative phase (Δ) and the relative amplitude rotating measure thethe relative phase shiftshift (∆) and the relative amplitude ratio ratioΨ)(tan Ψ) polymer of the polymer brush films the dry state, as wellinaspurified in-situ H in2 O purified H2O and (tan of the brush films in the dryinstate, as well as in-situ and H2 O/EtOH H2O/EtOH mixtures batch cuvetteHellma, (TSL Spectrosil, Muellheim, at mixtures within a batchwithin cuvettea(TSL Spectrosil, Muellheim,Hellma, Germany) at constantGermany) temperature ◦ constant temperature of 25 °C [33].were All solvent mixtures were day prepared thebefore same day or one day of 25 C [33]. All solvent mixtures prepared in the same or oneinday measurements before measurements and in glass vials We at room temperature. Webrush always the same brush and sealed in glass vials at sealed room temperature. always use the same by use rinsing the solvent by rinsing the another solvent solvent and introduce another solvent with help of a syringe-pump and introduce with help of a syringe-pump apparatus. The cuvette apparatus. was flushedThe at cuvette was flushed least threesolvent times with the current solvent mixture right beforechanges measurement least three times withatthe current mixture right before measurement to avoid in the to avoid changes the composition of the ambient. All measurements performed composition of theinambient. All measurements were performed between were 400 and 800 nm atbetween an angle400 of ◦ and 800 nm an ,angle Φ0Brewster of 70°, which is silicon. close toTo theevaluate Brewster To incidence Φ0 at of 70 whichofisincidence close to the angle of theangle indexof ofsilicon. refraction evaluate the index refraction and of the brush films in dry (h) state and consisting in situ (H),ofa and thickness of theof brush films in drythickness (h) state and in situ (H), a multilayer-box-model multilayer-box-model consisting layer of silicon, silicon PGMA, polymer silicon, silicon dioxide, anchoring PGMA, and adioxide, polymeranchoring brush waslayer assumed [31].and Thearefractive brush was assumed [31]. The refractive of the environment (water and H2O/EtOH indices of the environment (water and Hindices mixtures) were measured using a digitalmixtures) multiple 2 O/EtOH were measured using a digital multiple wavelength refractometer + Haensch, wavelength refractometer (DSR-lambda, Schmidt + Haensch, Berlin,(DSR-lambda, Germany). AllSchmidt data was acquired Berlin, Germany). was acquired analyzed using the Complete EASE software package and analyzed usingAll thedata Complete EASE® and software package (version 4.46). For the ®brush sample used (version 4.46).we For the brush sample used in this work, measurements. we did three-time repeatable ellipsometry in this work, did three-time repeatable ellipsometry Result differences of these measurements.were Result differences ofdata these measurements were small, and as such, were data measurements small, and as such, reproducibility of our ellipsometry measurements reproducibility of our ellipsometry measurements were quite good. quite good. From the the determined determined film film thickness thickness (h), (h), polymer polymer brush brush parameters parameters like like grafting grafting density density (σ) (σ) and and From distance between between anchoring anchoring points points (S) (S) can can be becalculated calculatedby byusing usingEquation Equation(1): (1): distance σ = S–2 = NAρh/Mn, (1) σ = S−2 = NA ρh/Mn , (1) where Mn is the number-average molecular weight of the polymer, NA is the Avogadro’s number, ρ 3 for PNiPAAm where weight polymer, NA is the Avogadro’s number,For ρ is the n is the number-average is the M polymer’s melt density molecular [34]. Here, ρ is of 1.1theg/cm homo-polymer. the 3 for PNiPAAm homo-polymer. For the interpretation polymer’s melt density [34]. Here, ρ is 1.1 g/cm interpretation of our measurements we have to assume a lateral homogeneous polymer profile. This of measurements we have to of assume a lateral homogeneous profile.whether This is the for is our the case for the brush regime strongly overlapping chains.polymer To determine the case grafted the brush regime of strongly overlapping chains. To determine whether the grafted polymers are in the polymers are in the brush regime, the distance between grafting sites, S, should be small enough. For brush regime, the distance S, should be small enough. the case of achains good the case of a good solvent, between S shouldgrafting be muchsites, smaller than twice the radius of For gyration of the solvent, (RG): S should be much smaller than twice the radius of gyration of the chains (RG ): 0.588, S/2 σ**, the chains will form condition that chains collapse mostly, i.e., RP = αN1/3 and S/2 < RP, where the radius of gyration under stretched brushes. This condition gives the upper estimate for the grafting density, which is necessary poor solvent conditions is taken. We note that this estimate slightly underestimates the necessary for a homogeneous brush. One might also use the overlap radius of gyration of the chains (RP ) between grafting density. Basic physical parameters of PNiPAAm brushes used in this study are listed in the collapsed single chains as a rough lower estimate for a homogeneous brush under the condition that Table 1. chains collapse mostly, i.e., RP = αN1/3 and S/2 < RP , where the radius of gyration under poor solvent conditions is taken. We note that this estimate slightly underestimates the necessary grafting density. Table 1. Dry layer parameters (number-average molecular weight Mn, thickness h, grafting density σ, Basic physical parameters of PNiPAAm brushes used in this study are listed in the Table 1. grafting distance S, and corresponding brush criteria) of polymer films, which were used in this study.
2) 2) Table 1. DryM layer parameters (number-average molecular Mn , thickness h, grafting σ, P) n (g/mol) h (nm) σ (chains/nm σ**weight (chains/nm S (nm) S/(2RGdensity ) S/(2R Polymer PGMA 10–20,000 2.5 - used in- this study. grafting distance S, and corresponding brush- criteria) of polymer- films, which were PNiPAAm 50,200 14.9 0.20 0.084 2 2.3 0.1 0.5 2
Polymer
3.
Mn (g/mol)
PGMA 10–20,000 Results PNiPAAmand Discussion 50,200
h (nm)
σ (chains/nm )
σ** (chains/nm )
S (nm)
S/(2RG )
S/(2RP )
2.5 14.9
0.20
0.084
2.3
0.1
0.5
3.1. Ellipsometry Analysis of the Cononsolvency Transition in PNiPAAm Brushes 3. Results and Discussion Although previous experimental studies [15–24] claimed that the cononsolvency transition of 3.1. Ellipsometry Analysis of the Cononsolvency Transition in PNiPAAm Brushes polymer brushes show features of a phase transition, so far, direct experimental evidence is missing. In order to check whether the cononsolvency transition of that polymer brushes is an transition equilibrium Although previous experimental studies [15–24] claimed the cononsolvency of transition, we measured swollen brushtransition, thicknesssoas function of time.evidence Typical isreal-time polymer brushes show features of a phase far,adirect experimental missing. measurements V is-spectroscopic ellipsometry for of PNiPAAm are In order to checkof whether the cononsolvency transition of swollen polymer brush brushesthickness is an equilibrium transition, shown in Figure 1a–c. Except for the condition where the ethanol volume fraction is around 50%, it we measured swollen brush thickness as a function of time. Typical real-time measurements of V is clearly seen that after aboutfor 5 min of continuous measurement, the brush system in became is-spectroscopic ellipsometry swollen brush thickness of PNiPAAm are shown Figurestable. 1a–c. For thefor condition of thewhere ethanol fraction fraction around is 50%, in current measurement, the after time Except the condition the volume ethanol volume around 50%, it is clearly seen that needed to wait for the brush system to become stable was more than 20 min. Thus, the data shown about 5 min of continuous measurement, the brush system became stable. For the condition of the in Figure 1a–c supported the equilibrium nature of the cononsolvency transition that for wethe report in ethanol volume fraction around 50%, in current measurement, the time needed to wait brush this work. system to become stable was more than 20 min. Thus, the data shown in Figure 1a–c supported the Here, we pointofout these differences in waiting time to reach equilibrium state may depend equilibrium nature thethat cononsolvency transition that we report in this work. on the polymer characteristics itself and subsequent response. As the PNiPAAmstate polymer is quite Here, we point out that these differences in waiting time to reach equilibrium may depend thermally sensitive, even the enthalpy change via changing solvent composition can lead to changes on the polymer characteristics itself and subsequent response. As the PNiPAAm polymer is quite in the polymer’s [38]. We alsovia observed withcomposition an increasing fraction, thermally sensitive,configuration even the enthalpy change changingthat solvent canethanol lead to changes especially for the configuration regime of high ethanol concentration, scatter theincreasing real-time ethanol brush thickness in the polymer’s [38]. We also observed that withinan fraction, increased. for Thisthe might be explained due to lower optical contrast the ambient ethanol and especially regime of high ethanol concentration, scatter between in the real-time brush thickness the thin PNiPAAm Note that swollen thickness in theethanol following are increased. This mightfilm. be explained dueallto data lowerabout optical contrastbrush between the ambient and the averaged values of Note real-time ellipsometry in brush the stable regime, except whenarespecifically thin PNiPAAm film. that all data about data swollen thickness in the following averaged indicated. values of real-time ellipsometry data in the stable regime, except when specifically indicated.
Figure 1. Cont.
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Figure 1. Swollen brush thickness of a PNiPAAm brush (σ = 0.20 chains/nm222) in aqueous ethanol Figure Figure1.1.Swollen Swollenbrush brushthickness thicknessofofa aPNiPAAm PNiPAAmbrush brush(σ(σ= =0.20 0.20chains/nm chains/nm))in inaqueous aqueousethanol ethanol mixtures as a function of (a–c) measurement time and (d) ethanol fraction obtained via Vismixtures a function of (a–c) time and (d) ethanol obtained viaobtained Vis-spectroscopic mixturesasas a function of measurement (a–c) measurement time and (d)fraction ethanol fraction via VisspectroscopicNote ellipsometry. Note that lineisin Figure only used to guide ellipsometry. that the dashed linethe in dashed Figure 1d only used1d tois eyesight. The eyesight. error barsThe in spectroscopic ellipsometry. Note that the dashed line in Figure 1d isguide only used to guide eyesight. The error bars in Figure 1d are neglected because they are quite small (around 0.2 nm). Figure 1d are they are quite they smallare (around 0.2 nm). error bars in neglected Figure 1d because are neglected because quite small (around 0.2 nm).
Figure 1d summarises results for the PNiPAAm brush thickness (please see raw data in Figure 1d 1d summarises summarises results results for for the the PNiPAAm PNiPAAm brush brush thickness thickness (please (please see see raw raw data data in in Figure Appendix A.1) as a function of ethanol volume fraction (VE). We were able to qualitatively distinguish Appendix A.1) as a function of ethanol volume fraction (V E ). We were able to qualitatively distinguish Appendix A.1) as a function of ethanol volume fraction (VE ). We were able to qualitatively distinguish five solvent composition regimes: (i) the brush thickness of PNiPAAm was hardly affected by the fivesolvent solventcomposition compositionregimes: regimes: (i) (i) the the brush brush thickness thickness of of PNiPAAm PNiPAAmwas washardly hardlyaffected affectedby bythe the five change of VE when it was lower than 0.05; (ii) for the range of 0.05 < VE < 0.15, there was a sharp changeof ofVVEE when when ititwas waslower lowerthan than0.05; 0.05;(ii) (ii)for forthe therange rangeofof0.05 0.05
