Structural Evolution and Formation Mechanism of the Soft Colloidal

Article pubs.acs.org/Langmuir

Structural Evolution and Formation Mechanism of the Soft Colloidal Arrays in the Core of PAAm Nanofibers by Electrospun Packing Qifeng Mu,† Qingsong Zhang,*,† Lu Gao,‡ Zhiyong Chu,‡ Zhongyu Cai,§ Xiaoyong Zhang,∥ Ke Wang,∥ and Yen Wei*,∥ †

State Key Laboratory of Separation Membranes and Membrane Processes, School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China ‡ School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China § Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States ∥ Department of Chemistry, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Electrospinning provides a facile and versatile method for generating nanofibers from a large variety of starting materials, including polymers, ceramic, composites, and micro-/nanocolloids. In particular, incorporating functional nanoparticles (NPs) with polymeric materials endows the electrospun fibers/sheets with novel or better performance. This work evaluates the spinnability of polyacrylamide (PAAm) solution containing thermoresponsive poly(N-isopropylacrylamide-co-tert-butyl acrylate) microgel nanospheres (PNTs) prepared by colloid electrospinning. In the presence of a suitable weight ratio (1:4) of PAAm and PNTs, the infiber arrangements of PNTs-electrospun fibers will evolve into chain-like arrays and beads-on-string structures by confining of PAAm nanofibers, and then the free amide groups of PAAm can bind amide moieties on the surfaces of PNTs, resulting in the assembling of PNTs in the cores of PAAm fibers. The present work serves as a reference in the fabrication of novel thermoresponsive hybrid fibers involving functional nanospheres via electrospun packing. The prepared nanofibers with chainlike and thermoresponsive colloid arrays in the cores are expected to have potential application in various fields.

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INTRODUCTION

properties, such as large specific surface area, controlled release, and stimuli responsivity. Among the various one-dimensional chain-like core−shell nanofibers production techniques, colloid electrospinning29 has been investigated most extensively due to its highly stable and continuous process under mild conditions as well as the flexibility in regulating the diameter and morphology of the fibers. On the other hand, coaxial electrospinning30,31 is proposed as a one-step process to fabricate core−shell nanofibers, which needs higher complexity for electrospinning devices. However, single-nozzle colloid electrospinning is more versatile than coaxial electrospinning, because easy scale-up and a simple process are essential requirements for continuous production on a massive scale. Recently, several groups26,28,32−35 have reported attempts of electrospinning colloidal spheres and polymer solution. In their studies, nanofibers were fabricated from a mixture of colloidal spheres and polymer solution via colloid electrospinning.

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One-dimensional (1D) nanostructures in the form of nanofibers,3,4 nanowires,5 nanotubes,6 nanorods,7 and nanobelts8 have been attracting significant attention lately due to their distinctive geometries, intriguing physical/chemical properties, and fascinating potential applications in topical drug or gene delivery,9−11 nanodevices,12 photonics,13 chemical and biological sensors,14−16 and catalytic supports.17,18 Many approaches were explored to fabricate one-dimensional nanostructures such as hydrothermal process,19 templatedirected method,20 vapor−liquid−solid (VLS) growth,21 solvothermal synthesis,22 chemical vapor deposition (CVD),23 and self-assembly.24 Within these materials, one-dimensional chain-like core−shell nanofibers compose an important class. Up to now, much effort has been devoted to the synthesis of diverse chain-like core−shell nanofibers such as polyacrylnitrile/TiO2,25 poly(vinyl alcohol)/SiO2,26 poly(vinyl alcohol)/ polystyrene (PS) colloidal spheres,27 and polyacrylamide (PAAm)/SiO2.28 Above all, chain-like core−shell nanofibers wrapped zero-dimensional colloidal arrays in the core have attracted a great deal of interest due to their challenging © 2017 American Chemical Society

Received: July 2, 2017 Revised: August 24, 2017 Published: September 6, 2017 10291

DOI: 10.1021/acs.langmuir.7b02275 Langmuir 2017, 33, 10291−10301

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Figure 1. Schematic illustration of the preparation process of PAAm/PNTs hybrid colloidal nanofibers.

Subsequently, new functional fibers such as water-stable fibers,35 necklace-like fibers,26 nanofibers containing colloidal arrays for programmable multiagent delivery,33 and colloidal fibers of structural color34 have been produced by the combination of the colloidal spheres and polymers via colloid electrospinning. However, the colloidal spheres applied in published works were rigid inorganic or lyophobic high polymer colloidal particles (such as SiO2 latex particles or PS spheres) and did not show thermoresponsivity in nanofibers. The soft and stimuli-responsive colloidal spheres36 provided us with a simple and efficient way to produce one-dimensional thermoresponsive nanofibers by colloid electrospinning, while enhancing the performances of electrospun hybrid fibers and expanding their applications. The combination of polymers and thermoresponsive colloidal spheres, the corresponding structural evolution of the electrospun hybrid fibers, and the force analysis of soft polymer spheres in a high-voltage field have been scarcely reported. Herein, we systematically investigated the interesting chainlike core−shell structure of PAAm with thermoresponsive PNTs via colloid electrospinning. The core was composed of thermoresponsive colloidal nanospheres, in which chemical cross-linking structures were synthesized by aqueous free radical precipitation polymerization.37,38 The effects of the PAAm:PNTs-1 mass ratio, the applied electric field, and the different colloidal spheres on 1D packing morphology and stability of spheres were investigated. The forming mechanism of electrospun PAAm/PNTs fibrous structures and structural evolution were proposed accordingly.

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Fengchuan Chemical Reagent Technologies Co. Ltd. Deionized (DI) water was used in all experiments. Preparation of Monodispersed PNTs Composite Nanospheres. Monodispersed latex nanospheres of poly(N-isopropylacrylamide-N,N′-methylenebis(acrylamide)-tert-butyl acrylate) (poly(NIPAm-co-tBA), PNTs) were prepared by aqueous free radical precipitation polymerization. Refer to Supporting Information Table S1 for monomer feed ratios for colloidal spheres PNTs 0−2. Briefly, NIPAm (12.4 mmol), MBA (0.7 mmol), tBA (1.6 mmol), and DI water (140 mL) were added sequentially to a three-necked flask equipped with a reflux condenser, an N2 inlet, and a mechanical stirrer at a stirring speed of 200 rpm. The reaction mixture was initially performed at 70 °C for at least 15 min under a nitrogen atmosphere. The synthesis was carried out at 70 °C for 6 h under continuous stirring, following the rapid addition of APS (0.45 mmol dissolved in 10 mL of DI water). The amount of tBA determined the size and the volume phase transition temperature (VPTT) of the nanospheres, while all other parameters were kept constant. The prepared latex spheres were purified two times by high-speed centrifugation and redispersed in DI water by ultrasonication. Synthesis of PAAm and Determination of Molecular Weight. A solution of AM (3.38 × 10−2 mol), APS (8.76 × 10−5 mol), and TEMED (20 μL) in a 20 mL mixture of DI water and ethanol (3:2 (v/ v)) was heated to 40 °C; polymerization was carried out at 40 °C for 12 h with vigorous stirring. Polymerization was stopped by cooling the reaction mixture to 25 °C, and the resulting transparent polymer solution was dialyzed against DI water for 24 h in order to remove the residual monomers. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (Mw/ Mn) were determined to be 1.17 × 105 g/mol, 1.46 × 105 g/mol, and 1.24, respectively, by means of gel-permeation chromatography (Viscotek 270 Max, Malvern Instruments Ltd., America) analysis under the following conditions: solvent, water and 0.1 M NaNO3; calibration standards, commercial polyether polyol; column set, CLM3017. Electrospinning of PAAm/PNTs-Electrospun Precursor Solutions. As shown in Figure 1, the electrospun precursor solution was prepared by blending 1 mL of PNTs colloidal latex with a high concentration of 40 wt % and a measured amount of PAAm solution with 16 wt % concentration. The weight ratio of PAAm to PNTs was variable (4:1, 3:2, 1:1, 2:3, and 1:4, respectively); when the weight ratio of PAAm and PNTs-1 was 1:4, the weight fraction of PAAm in the total weight of solution was 6.15 wt % (m/v), and the weight fraction of PNTs-1 in the total weight was 24.6 wt % (m/v). And the mixture was ultrasonically treated for at least 5 min and then stirred vigorously for 30 min to obtain a homogeneous solution. The resulting

EXPERIMENTAL SECTION

Chemicals and Materials. All chemicals are of analytical grade and used as purchased without further purification. Acrylamide (AM, 99%) and rhodamine B (99%) were purchased from Tianjin Guangfu Fine Chemical Research Institute. N-Isopropylacrylamide (NIPAm, 98%) was purchased from Tokyo Chemical Industry Co. Ltd. tertButyl acrylate (tBA, 99%) was purchased from Aladdin Reagent (Shanghai) Co. Ltd. N,N′-Methylenebis(acrylamide) (MBA, 98%) and ammonium persulfate (APS, 98%) were from Tianjin Kemi’ou Chemical Reagent Co. Ltd. N,N,N′,N′-Tetramethylethylenediamine (TEMED, 98%) was purchased from East China Normal University Chemical Factory. Ethanol (99%) was obtained from Tianjin 10292


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Figure 2. (a−c) Typical FE-SEM images of PNTs series latex colloidal spheres. (d) FTIR spectra of PNTs series colloidal spheres. (e−g) Optical images of PNTs-2 dispersion under different humidity conditions.

Figure 3. (a) Visible light transmission spectra at various temperatures of the latex spheres (the wavelength was 500 nm). (b) Optical images of PNTs-1 latex at different temperatures. Reproduced with permission. Copyright 2017, Tianjin Polytechnic University. (c) Hydrate particle size distributions of colloidal spheres at different temperatures analyzed by DLS (the wavelength was 660 nm, and temperature ranged from 25 to 50 °C). (d) Normal distribution curves of colloidal spheres hydrodynamic diameters at 25 °C. solution was transferred into a 10 mL plastic syringe fitted with a metallic needle of blunt tip and 0.5 mm inner diameter. The syringe was fixed horizontally on the syringe pump, and the solution was fed at a constant and controllable rate of 0.5 mL/h by using a syringe pump (LSP02-1B, Baoding Longer Precision Pump Co., Ltd., China). A high voltage of 10 kV was applied between the needle and collector, by using a direct current power supply (DW-P303, Tianjin Dongwen High Voltage Co., China). The white electrospun nanofibers were collected on a grounded rectangular metal collector covered by a piece of aluminum foil, with collecting distance of 15 cm. The complete electrospinning setup was enclosed in a fume hood, and the electrospinning was carried out in the surrounding environment; the temperature and relative humidity in all electrospinning processes were controlled at 20 °C and 30 ± 5%. Characterization. The morphologies of colloidal nanospheres, electrospun pure PAAm nanofibers, and colloidal spheres hybrid nanofibers were observed by a field emission scanning electron

microscope (FE-SEM, S-4800, Hitachi Ltd., Japan) and were sputtered with gold film before observation. The mean diameter and polydispersity of nanospheres and electrospun fibers were measured from FE-SEM images using analysis software (ImageJ 1.4.3p, National Institutes of Health, USA). The mean size and polydispersity of colloidal nanosphers in latex were measured by dynamic light scattering (DLS, Zetasizer Brookhaven, Brookhaven Instruments Ltd., USA) under different temperatures (ranging from 20 to 50 °C). Fourier transform infrared (FTIR, Spectrum 100, PerkinElmer Ltd., USA) spectra were recorded in the range of 500−4000 cm−1. The thermoresponsivity of colloidal nanospheres and PAAm/PNTs-1 fibers was studied by using an ultraviolet−visible spectrophotometer (UV, UV-1901, Purkinje General Instruments Ltd., China), at temperatures ranging from 20 to 50 °C. Contact angles were measured on an optical contact angle and interface tension meter (SL200 KB, Solon Tech Ltd., China). Thermal property analysis of the colloidal ectrospun nanofibers was carried out by using a differential 10293


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Figure 4. FE-SEM images of electrospun PAAm (16 wt %)/PNTs-1 (266 nm) fibers with different PAAm:PNTs-1 mass ratios: (a) 4:1, (b) 3:2, (c) 1:1, (d) 2:3, (e, f) 1:4. The panel f image was taken under higher magnification. (g, h) Curve of PAAm fibers and PNTs-1 aggregates mean diameters with the change of mass ratio of PAAm and PNTs-1, respectively. scanning calorimeter (204F1 Phoenix, Netzsch Instruments Ltd., Germany) and thermogravimetric analyzer (STA449F3, Netzsch Instruments Ltd., Germany). The viscosity of the spinning solution was measured by a rheometer (HAAKE Rheo Stress 6000, Thermo Scientific Inc., Germany). The water-soluble fluorescent dyes (rhodamine B) were embedded in PNT colloids so that the distribution of colloidal spheres confined into PAAm fibers could be optically monitored by confocal laser scanning microscopy (CLSM, ECS SP8, Leica Microsystems Ltd., Germany). The surface morphology and roughness of PAAm/PNTs-1 hybrid fibers were observed by atomic force microscopy (AFM, CSPM5500, Bing Nanoinstruments Ltd., China) and true color confocal microscopy (TCCM, CSM 700, Zeiss Ltd., Germany).

Large amounts of hydrophobic ester groups anchored in the structure of colloidal spheres, contributing to the decrease of VPTT of spheres compared to the pure PNTs-0 colloidal spheres. On the other hand, massive amide groups on the surface of colloidal spheres led to the formation of hydrogen bonds between spheres and PAAm during the subsequent mixture and electrospinning process. The PNTs-2 latex dispersions (10 wt %) appeared with structural color after being coated to the surface of black polyethylene plastic as shown in Figure 2e. The structural color was caused by a highly ordered and periodic structure of PNTs-2 aggregates, which was attributed to the photonic band gap effect40,41 and Mie scattering.42 The PNTs-2 became a white dry powder when the latex was completely dried (Figure 2f). Interestingly, the PNTs2 appeared in a different color again after being wetted by water. The representative thermoresponsivity analyses of PNTs colloidal spheres at the temperature ranging from 20 to 50 °C are shown in Figure 3. It was found that ambient temperature can dramatically affect the hydrophilic−hydrophobic property of colloidal spheres, contributing to changes in the transmittance of visible light (500 nm) shown in Figure 3a. When there was no tBA (hydrophobic moieties) in the structure of PNTs-0 colloidal spheres, the most significant changes in visible light transmission were aroused. The hydrophobic moieties tBA incorporated in the spheres such as PNTs-1 (molar ratio NIPAm:tBA = 7.75:1) and PNTs-2 (molar ratio NIPAm:tBA = 7.75:2), which had the weaker light transmission. When the molar ratio of tBA and NIPAm increased, the thermoresponsive

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RESULTS AND DISCUSSION The morphologies of colloidal spheres under dry condition are shown in Figure 2a−c. It can be seen clearly that the colloidal spheres with smooth surface were spherical in shape. Particle size distributions of PNTs-1, measured by the statistics of the FE-SEM images through ImageJ 1.4.3p and Origin Pro 8.5, were exceptionally uniform (Supporting Information Figure S1). According to previous research,39 massive hydrophobic ester groups were copolymerized in the structure of PNTs colloidal spheres. To verify the above results, the colloidal spheres were analyzed using FTIR spectra as shown in Figure 2d. In addition to the representative bands of the PNTs-0, the PNTs-1 and PNTs-2 colloidal spheres have the characteristic bands of ester carbonyl (CO) and (C−O−C) at 1727.0 and 1146.5 cm−1, which certified the existence of an ester group in the structure of the PNTs-1and PNTs-2 colloidal spheres. 10294


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the results we got were different from those of that published work. On the other hand, the PNTs-1 colloids were lyophilic and soft colloidal spheres; adjacent spheres tend to adhere to each other due to confinement of PAAm and hydrogen bonding between lyophilic spheres and PAAm fibers. And the soft spheres had been deformed by axial pulling force during the spinning flight (Figure 4f). However, the lyophobic and rigid PS colloids were unable to experience the abovementioned interactions and deformation. To evaluate the influence of voltages on the fibers and PNTs1, three different voltages were applied. As shown in Supporting Information Figure S3a, colloidal sphere aggregates (red circle marked), smaller aggregates (red arrow marked), and chain-like (red rectangle marked) structures were fabricated when 16 wt % PAAm solution was used and the voltage was 10 kV. PNTs-1 with a diameter of 266 nm (under dry condition) aligned along most fibers, with obvious distance between two neighboring PNTs-1. When the external electric field was increased, PNTs-1 tended to aggregate and the chain-like arrays tended to reduce, as shown in Figure S3b. PNTs-1 formed clusters of aggregates when the voltage was 20 kV (Figure S3c). Meanwhile, the chain-like arrays or beads-on-string structures almost disappeared. In previous research,46 it has been indicated that when the external voltage is increased, the flight speed of the polymer jet is improved and the jet whipping is more vigorous at the same time. Therefore, PNTs-1 suspended in PAAm solution tended to become aggregates instead of chain-like arrays along the fibers under faster flight speed. The smooth colloidal fibers were electrospun from a blend solution of latex spheres and PAAm solution; PAAm served to adhere and capsulate spheres. During the electrospinning process, the wrapped spheres were packed and notably deformed and thinned into the fibers, and their functionality remained in the final materials. To further explore the surface morphologies and roughness of PAAm and PAAm/PNTs-1 (1:4) hybrid fibrous mats, TCCM and AFM were performed on the final electrospun fibrous mats as shown in Figures 5 and 6, respectively. The typical three-dimensional surface morphologies of the PAAm/ PNTs-1 fibrous mat (Figure 5d) were rougher than those of the PAAm fibrous mat (Figure 5b), which was mainly due to the

properties of colloidal spheres faded away gradually due to out of balance hydrophobic isopropyl and tert-butyl moieties and hydrophilic amide groups. The hydrodynamic diameters (DH) of colloidal spheres decreased with increasing ambient temperature as shown in Figure 3c. It can be observed that the decrease in DH values of PNTs-0 and PNTs-1 colloidal spheres was evident; the VPTT of PNTs-0 and PNTs-1 spheres were around 32 °C37,43 and 30 °C, respectively. However, there was no evident change in the DH value of PNTs-2 spheres. While under the effect of hydrophobic tBA moieties, the DH value of the spheres decreased, and thermoresponsive properties of colloidal spheres faded away gradually with increasing molar ratio of tBA and NIPAm, which was the same as those of the Figure 3a results. Initially, three types colloidal spheres of a low concentration (0.2 wt %) were used to mix a constant amount of the PAAm aqueous solution (8 wt %) to prepare the blend solution for electrospinning. The spindle-like structures appeared due to the change of spinning solution viscosity in the presence of minor latex colloidal spheres (see Supporting Information Figure S2). Subsequently, PNTs-1 spheres were used to massively produce one-dimensional colloidal fibers via colloid electrospinning process in order to investigate the characteristic of structural evolution. Colloid electrospinning is one of the electrospinning methods but significantly differs from conventional electrospinning,27 as it incorporates organic/inorganic colloidal particles into the spinning solution.44,45 Figure 4 shows the structural evolution of the electrospun fibers that appear when the PAAm:PNTs-1 mass ratio is changed from 4:1 to 1:4, with a suitable concentration of PAAm solution (16 wt %) and the same process conditions (voltage,15 kV; distance, 15 cm; feed rate, 0.5 mL/h; relative humidity, 25%; temperature, 22 °C). The FE-SEM image in Figure 4a (PAAm:PNTs-1 = 4:1) indicates that PNTs-1 formed aggregates embedded in the PAAm fibers. In the fibers, PAAm was dominant in the mixture and the diameters of aggregate and fiber were 8.56 and 0.60 μm, respectively. When the amount of PNTs-1 in the mixture increased to 3:2 PAAm:PNTs-1, PNTs-1 formed into smaller aggregates packed by a layer of PAAm in Figure 4b. It can be seen in Figure 4c that a pile of PNTs-1 were bound and packed by PAAm, while the small aggregates transformed into beads-on-string structures, with a 1:1 PAAm:PNTs-1 weight ratio of the blend solution. With the further increase of PNTs-1 to obtain a mass ratio of 2:3, the emergence of smaller PNTs-1 aggregates and thicker PAAm fibers was observed, as shown in Figure 4d. To evaluate the influence of the mass ratio of PAAm and PNTs-1 on mean diameters of fibers and aggregates, the correlation curves were presented in Figure 4g,h, and the statistical information was shown in Supporting Information Table S2. When the mass ratio was 1:4 PAAm:PNTs-1, on the basis of wrapped PNTs-1, the chain-like arrays of zero-dimensional colloidal spheres in the cores of PAAm fibers were obtained by electrospinning, as shown in Figure 4e,f. In such conditions, PAAm acted as the binder layer to pack PNTs-1 into the cores of fibers, and there were obvious adhesions between PNTs-1 colloidal spheres. The distinction between panels a and f of Figure 4 indicates that the amount of PNTs-1 relative to PAAm is the key parameter for fabrication of chain-like arrays or beads-on-string structures. According to Yuan et al.,27 the PVA fibers confined the PS spheres into necklace-like structures, and the best necklace-like structures were obtained only when the weight ratio of PVA (11 wt %) and PS (473 nm) was 1:2, while

Figure 5. TCCM images of electrospun fibrous mats: (a) PAAm fibrous mat, two-dimensional optical image; (b) PAAm fibrous mat, three-dimensional scanning image; (c) PAAm/PNTs-1 fibrous mat, two-dimensional optical image; (d) PAAm/PNTs-1 fibrous mat, threedimensional scanning image. 10295


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which indicates that the colloidal spheres were evenly distributed and confined into PAAm nanofibers during the jet whipping and thinning process. To further investigate if the PNTs-1 colloidal spheres are stained in the hybrid fibers, a morphology comparison of wetted PAAm and PAAm/PNTs-1 (1:4) fibrous mats is shown in Figure 8, and blue dotted lines (Figure 8a,c) display the edges of water to soak. The clear structure of wetted PAAm fibers, shown in Figure 8b, demonstrates that the PAAm fibers were partially dissolved, and the overlapping of PAAm fibers (marked with red ellipse) collapsed and hardened. It should be noted that there are not many evident fibers in Figure 8c, because most of the fibrous structures were dissolved by water, and the upper right part of the blue dotted line shows many aggregates of spheres. A corresponding higher magnification FE-SEM image is shown in Figure 8d, and it should be noted that there are some residual necklace-like structures (marked with red rectangle); PNTs-1 colloidal spheres that were insoluble in water converged together to form aggregates (marked with red circle) due to the hydrogen bonding between spheres and the surface tension of water, when PAAm fibers were dissolved. Here, PAAm fibers acted as the confining template for the colloidal spheres. Compared to 1D continuous nanowires47,48 and 1D tubular nanostructures,49,50 confining colloidal spheres into PAAm nanofibers to obtain 1D colloidal chain-like structures was a relatively straightforward process. Based on the results mentioned above, the deformation mechanism of the chain-like arrays and colloidal spheres, as shown in Figure 9, is summarized in this section. In general, only if the viscosity of the polymer solution is optimal and a continuous jet will generated from the Taylor Cone adjacent to the needle tip. Once PAAm dominated the blend spinning solution, small amounts of PNTs-1 aggregated and were wrapped in the viscous polymer solution then confined into PAAm fibers during the jet whipping and thinning process (Figure 9a, case I). Meanwhile, the blackberry-like and ideal necklace-like structures were simultaneously prepared by colloid electrospinning, when PNTs-1 occupied a large portion of the solution (PAAm:PNTs-1 = 1:4), as shown in Figure 9b. The PNTs-1 aggregate marked with a red circle was called a blackberry-like structure, and blackberry-like structure evolved into the near necklace-like structure (marked with red arrows) during the spinning flight, as described in situation I. PNTs-1 aggregates deformed by sliding the soft and wet spheres along the axis of the jet to the lowest energy state. The PAAm thick layer acted as the confining template for the PNTs-1 under such conditions. It can be seen from Figure 9a, case II, that the PNTs-1 spheres were more feasible in the formation of necklace-like arrays under the confinement of PAAm layers. The beads-on-string marked with a red rectangle was called a necklace-like structure as shown in Figure 9b; PNTs-1 colloidal spheres encapsulated in the cores of PAAm fibers evolved into ellipsoids during the spinning flight. The PNTs-1 colloidal spheres consist of elastic cross-linked networks and fluid filling the interstitial spaces of the networks. Therefore, PNTs-1 colloidal spheres are wet and soft and capable of undergoing large deformation, as reported in the references.37,51 Here, PNTs-1 underwent deformation from sphere to ellipsoid or spindle under the interactions of four kinds of external force in the process of high-speed flight (Figure 9a, case III). Therefore, the chain-like arrays along the PAAm nanofibers were fabricated.

Figure 6. AFM images of electrospun fibers: (a) PAAm fibers, twodimensional scanning image; (b) PAAm fibers, three-dimensional scanning image; (c) PAAm/PNTs-1 fibers, two-dimensional scanning image; (d) PAAm/PNTs-1 fibers, three-dimensional scanning image.

aggregates produced by accumulation of PNTs-1 in the cores of the PAAm fibers. Figure 6 shows a comparison of the PAAm fibers and PAAm/PNTs-1 hybrid fibers; the PAAm nanofibers prepared using electrospinning had a mean diameter of 650 nm, exhibiting a smooth surface as shown in the areas marked with black arrows of Figure 6a,b. The PAAm/PNTs-1 hybrid nanofibers exhibited wrinkled surface morphologies, and it was clear that the PNTs-1 colloidal spheres were sequentially packed in chain-like arrays, as shown the areas marked with black rectangles in Figure 6c,d. The investigation of the array of polymeric colloidal spheres encapsulated in electrospun fibers can be, however, challenging due to contrast between polymeric colloidal spheres and polymer fibers usually being low in transmission electron microscopy. Confocal laser scanning microscopy is an appealing tool to circumvent the problem of low contrast between polymer phases. Since PNTs-1 colloids are stimuli-responsive polymeric colloidal spheres and demonstrate pronounced thermoresponsive properties and show VPTT around T = 30 °C in pure water (see Figure 3c), the colloidal spheres can be swollen and deswollen at the temperatures below and above the VPTT. Water-soluble fluorescent dye rhodamine B can be readily incorporated in PNTs-1 colloidal spheres by employing the swelling and deswelling characteristic in water on heating33 (see Figure 7a). And PNTs-1 spheres that internally contained rhodamine B were dialyzed in distilled water for 48 h, until rhodamine B was not detected in the dialysate. Panels b and c of Figure 7 show typical confocal laser scanning microscopy (CLSM) images of chain-like fibers consisting of an array of colloidal spheres core and PAAm sheath. The red dots arrangement in partial order indicate that the zero-dimensional PNTs-1 colloidal spheres lined up to form an array and chainlike structure in the central region of the fibers. The blue lines were added to represent the PAAm fibers, which connected PNTs-1 colloidal spheres together to form the necklace-like structures, as shown in Figure 7b. The diameter of isolated colloidal sphere was around 600 nm in the 400 nm thick fibers, 10296


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Figure 7. (a) Schematic illustration of incorporating rhodamine B in poly(NIPAm-co-tBA) colloidal spheres (PNTs) by employing the swelling and deswelling characteristic in water on heating. (b, c) CLSM images of the PAAm-based fibers containing PNTs-1 colloidal spheres arrays in the core. (b) Rhodamine B incorporation into the PNTs-1 colloidal fibers and two-dimensional scanning image. (c) Side view image with three-dimensional scanning.

To study the thermal behavior of PAAm/PNTs-1 nanofibers, thermogravimetric analysis (TGA) was performed on the obtained electrospun fibrous mat as shown in Supporting Information Figure S4. The onset mass loss of as-electrospun PAAm observed in this temperature range (from 50 to 180 °C) on the TGA curve was consistent with the volatilization of water, and the characteristic thermal gravimetric of pure PAAm fibers possessed three different stages in the upper 200 °C as reported in the literature.52,53 And the incorporation of PNTs-1 colloidal spheres decreased the initial decomposition temperature of the last stage, which was due to the fracture of an ester bond in the PNTs-1 molecular structure. The glass transition temperature (Tg) of pure PAAm fibers was 182.0 °C (see Supporting Information Figure S5). The PAAm/PNTs fibers appeared at two different Tg values, which indicated that PAAm and PNTs were not compatible completely. Figure 10a shows the enthalpy change of PNTs-1 wrapped by PAAm nanofibers in the chain-like arrays. Corresponding endothermic and exothermic peaks were measured by differential scanning calorimeter. A broad exothermic peak was found when the temperature was about 23−31 °C; the hydrogen bonding between the carbonyl group of PAAm and

Figure 8. (a, b) FE-SEM images of wetted electrospun PAAm fibrous mat. (c, d) FE-SEM images of wetted electrospun PAAm/PNTs-1 (1:4) fibrous mat.

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Figure 9. (a) Schematic representation of the deformation mechanism of diverse hybrid nanofibers under different conditions via colloid electrospinning. (b) FE-SEM image of PAAm/PNTs-1 fibers via colloid electrospinning. (c) Normal distribution curves of fibers, spheres, and aggregates of PAAm/PNTs-1 (1:4) fibers from panel b.

Figure 10. (a) Differential scanning calorimetry (DSC) curves of PAAm and PAAm/PNTs-1 fibrous mats in wet condition. (b) Schematic representation of macromolecule and hydrogen bonding between water and macromolecule. (c) UV absorption spectra of three samples of different compositions. (d) Intensity of UV absorption peak at various temperatures of three samples (the wavelength was 198 nm).

water would form simultaneously in this temperature range.54 The VPTT of PNTs-1 colloidal spheres was about 30 °C as in the above results (Figure 3), and the exothermic peak would move to higher temperature (above 30 °C) due to the rupture of hydrogen bonding between carbonyl and amino of PNTs-1 with water; in the meanwhile, the hydrogen bonding between

water and carbonyl of PAAm formed as shown in Figure 10b. A remarkable endothermic peak was found at about 26 °C; the hydrogen bounding interaction between PNTs-1 and PAAm would dissociate near this temperature. In order to demonstrate the thermoresponsivity of PAAm/PNTs-1 fibers more scientifically and neatly, the UV absorption spectra at different 10298


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wavelengths (190−600 nm) were tested, as shown in Figure 10c. It was found that the absorption intensity of three samples at 198 nm was the largest, where peaks were assigned to an amide group.55 And it can be seen from Figure 10d that the UV absorption (198 nm) intensity of PAAm/PNTs-1 fibers was related to the temperature, which was the same as thermoresponsive PNTs-1 colloidal spheres. According to Chen et al.,56 to discern if PAAm/PNTs-1 hybrid fibers are thermoresponsive, water contact angle (CA) measurements were performed (see Supporting Information Figure S6). The PAAm fibers showed superhydrophilicity at different temperatures, while the wettability of PAAm/PNTs-1 fibers displayed evident changes from good (CA = 35.67°) to poor (CA = 50.27°) with the temperature changes from 25 to 50 °C. And they indicated that the PNTs-1 colloidal spheres wrapped in cores of PAAm fibers still show thermoresponsivity. The use of thermoresponsive spheres to obtain smart nanofibers is effective for a wide variety of stimuli-responsive materials combinations. It should be emphasized that the programmable topical drug delivery applications of the prepared thermoresponsive nanofibrous mats will be investigated further.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b02275. PNTs colloidal spheres preparation recipes (Table S1); diameters statistical information (Table S2); particle size distribution histogram of PNTs-1 (Figure S1); SEM images of PAAm/PNTs hybrid nanofibrous mats (Figure S2); FE-SEM images of the PAAm/PNTs-1 nanofibers (Figure S3); thermogravimetric analysis results (Figure S4); typical DSC results (Figure S5); and water contact angles (Figure S6) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Q.Z.) E-mail: zqs8011@163.com. *(Y.W.) E-mail: weiyen@tsinghua.edu.cn. ORCID

Qingsong Zhang: 0000-0002-0603-387X Notes

The authors declare no competing financial interest.

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CONCLUSIONS

ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 21104058, 31200719, 21134004, and 21174103), the State Scholarship Fund of China Scholarship Council (Grant No. 201508120037), Applied Basic Research and Advanced Technology Programs of Science and Technology Commission Foundation of Tianjin (Grant Nos. 12JCQNJC01400 and 15JCYBJC18300), the Science and Technology Correspondent of Tianjin (Grant Nos. 14JCTPJC00502 and 15JCPJC62200), and the National Training Programs of Innovation and Entrepreneurship for Undergraduates (Grant Nos. 201510058005 and 201510058051).

In conclusion, it is demonstrated that a one-step production of 1D chain-like nanofibers may be obtained from PAAm and PNTs colloidal spheres. Tunable structures of electrospun colloidal fibers were obtained from a blended solution of PAAm and PNTs colloidal spheres. It was found that the PAAm fibers acted as a template to confine PNTs-1 into chain-like nanofibers, and PNTs-1 were not easy to disperse and were wrapped by PAAm solution in a PAAm-dominant blend solution (the two weight ratios of PAAm : PNTs-1, 4:1 and 3:2). In contrast, PNTs-1 spheres were packed by PAAm solution to form core−shell and chain-like structures in a PNTs-1-dominant blend solution (the weight ratio of PAAm and PNTs-1, 1:4). In such conditions, PAAm acted as the adhesive agent to adhere PNTs-1 to form several different structures, such as blackberry-like aggregates and beads-onstring and chain-like arrays. The structural evolution of electrospun hybrid colloidal fibers was mainly affected by the weight ratio of PAAm and PNTs-1 and the external electric field applied. The thermoresponsive PNTs colloidal spheres were successfully encapsulated for the first time into the cores of PAAm nanofibers in chain-like arrays by colloid electrospinning. The distribution and arrangement of PNTs-1 spheres as small as ∼200 nm embedded in PAAm nanofibers can be clearly observed via fluorescent tracking. It is worthy to note that (i) the VPTT of PNTs spheres can be turned by adjusting the molar ratio of hydrophobic tBA and thermoresponsive NIPAm and its composition and (ii) not only the morphologies of the PAAm/PNTs mat but also the hybrid fiber diameter can be controlled accurately by colloid electrospinning conditions. Additionally, the study will open up a new approach to prepare thermoresponsive nanofibers or fibrous mats, and these chainlike arrays containing thermoresponsive colloidal spheres present valuable potential applications such as in drug delivery systems or smart surfaces.

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REFERENCES

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