Enlightening Freeze–Thaw Process of Physically Cross-Linked Poly

Apr 29, 2019 - ... ‡Medical Device Research Institute, College of Science and Engineering, Flinders University , Adelaide , South Australia 5042 , A...
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Enlightening Freeze−Thaw Process of Physically Cross-Linked Poly(vinyl alcohol) Hydrogels by Aggregation-Induced Emission Fluorogens Javad Tavakoli,†,‡ Jason Gascooke,† Ni Xie,§ Ben Zhong Tang,*,§ and Youhong Tang*,†,‡

ACS Appl. Polym. Mater. Downloaded from pubs.acs.org by UNIV OF SOUTHERN QUEENSLAND on 05/07/19. For personal use only.

†

Institute for NanoScale Science and Technology, College of Science and Engineering, and ‡Medical Device Research Institute, College of Science and Engineering, Flinders University, Adelaide, South Australia 5042, Australia § Department of Chemistry, The Hong Kong University of Science and Technology, Hong Kong, China S Supporting Information *

ABSTRACT: Crystallization is a conventional technique for fabricating physically cross-linked poly(vinyl alcohol)-based hydrogels (PVA) with desired properties. The creation of crystalline regions induced by the freeze−thaw process is attributed to the molecular organization that serves as the junction points containing folded chains. The current study employs an aggregation-induced emission fluorogen (AIEgen) to produce a new approach for studying the kinetics of the formation of crystalline regions induced by the freeze−thaw process. For the first time, this research directly links the effects of freeze−thaw cycles to the final physical and mechanical properties of PVA hydrogels by using AIEgen. The results of the study reveal an increase in the fluorescent properties during the freeze−thaw process that reflects the rise in the percentage of crystallinity. This approach can be employed effectively to demonstrate the crystallization process using significant color and brightness differences in crystalline regions compared to amorphous areas. This capability allows researchers to visualize the crystalline regions clearly in situ and to study their kinetics, formation, and transition in the solid state. The results of this study have implications for the broad class of gels derived through polymer crystallization from solution. KEYWORDS: aggregation-induced emission, poly(vinyl alcohol) hydrogel, freeze−thaw, mechanical properties, physical properties, fluorescent properties

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macromolecules in the crystalline regions.10 Freeze−thaw (FT) is a simple technique that contributes to densification of PVA macromolecules via formation of ice crystals (freezing) period and formation of ordered structure during thaw (heating) period. It is well-known that the FT technique, accompanying microstructural modifications in physically cross-linked PVA-based hydrogels, leads to significant changes in the hydrogel properties.11 The observed structural modification is attributed to the partial crystallization of chain segments into the microcrystalline structure.12 To elucidate the impact of FT on the formation of microcrystals and the subsequent material properties of physically cross-linked PVA-based hydrogels, the use of X-ray diffraction (XRD), differential scanning calorimetry (DSC), solid state nuclear magnetic resonance (NMR), and smallangle X-ray scattering (SAXS) has been proposed.13−16 By using the suggested methods, previous fundamental studies have revealed that the application of FT cycles results in alteration of the morphology in PVA hydrogels from a porous

INTRODUCTION Hydrogels with three-dimensional cross-linked network structures provide unique properties that suggest a wide range of applications in various engineering and medical fields.1−3 One of the most important hydrogels, poly(vinyl alcohol) (PVA), has attracted considerable attention for an extensive variety of applications including wound healing, drug delivery, tissue engineering, immune isolation, and nanofabrication.4−6 Among different methods for the preparation of PVA-based hydrogels, the physical cross-linking approach has stimulated great interest due to the preservation of the hydrogels’ intrinsic properties. Crystallization is a conventional technique used to fabricate physically cross-linked PVA-based hydrogels with desired mechanical and biological properties.7,8 Control of the crystallization process, which is easily achieved, can result in fine-tuning of the structure and optimization of the final physicomechanical and biological properties of hydrogels, without the addition of a cross-linking agent.9 From a structural viewpoint, the creation of crystalline regions is attributed to the molecular organization that serves as the junction points containing folded chains.7 In fact, the chain folding process develops a densification state for PVA © XXXX American Chemical Society

Received: February 22, 2019 Accepted: April 29, 2019 Published: April 29, 2019 A

DOI: 10.1021/acsapm.9b00173 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

Figure 1. (a) Chemical structure of TPE-2BA and the FL spectra of TPE-2BA aggregates in water−DMSO mixture for different volume fractions of water ranging from 90% to 10% upon excitation at 365 nm. (b) Comparison TPE-2BA FL spectra for different water volume fractions (99%) and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich, Australia. TPE-2BA was synthesized at Hong Kong University of Science and Technology, China.30 Sample Preparation. A stock solution of TPE-2BA in DMSO with a concentration of 10 mM was prepared by dissolving 0.105 g (250 μmol) of TPE-2BA in 25 mL of DMSO. The solution was stored in a refrigerator at 4 °C for further use. To investigate on the polymorphic transition of TPE-2BA in the absence of PVA, mixtures of stock solution (10 μM concentration) in distilled water with different water fractions (90%−10%) were prepared. To understand the fluorescence properties of TPE-2BA in aggregate states for wet samples, PVA solutions (3, 6, and 10% w/w) were prepared by mixing appropriate amounts of PVA powder in distilled water−DMSO solution (40/60% w/w) under constant stirring at 800 rpm at 70 °C for 6 h. PVA solutions were kept at room temperature in sealed containers overnight. The effect of FT cycles on the FL characteristics of PVA film containing TPE-2BA was studied by the following sample preparation. “PVA containing TPE-2BA” films were prepared by addition of 30 μL of TPE-2BA solution in DMSO (10 μM concentration) to 3 mL of PVA solutions (prepared with different concentration in distilled water) and shaken for a few minutes using high-speed shake. PVA films were prepared using a drop-casting technique on a glass sheet utilizing a 4 mL syringe. The samples were 450 ± 50 μm thick. FT cycles were performed at −20 and 80 °C for 1 h each. For microscope imaging, films were placed on the surface of microscope slides, mounted by a drop of mounting media (DPX) and covered by coverslips. To capture microscopic images from liquid, C

DOI: 10.1021/acsapm.9b00173 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials two drops of sample were poured in welled microscope slides and then covered by a coverslip. Characterization. Characterizations were performed for as prepared PVA films without swelling, unless otherwise stated (i.e., measuring the swelling properties). The FL spectra of films and solutions were collected by a spectrophotometer (Cary Eclipse, Agilent Technologies) at the excitation wavelength of 365 nm. When FL spectra were collected from “PVA containing TPE-2BA” solutions, samples were allowed to stand for 2 h before data collection. Fluorescent and polarized images were captured using an AX70 upright fluorescent microscope (Olympus, Japan), a Zeiss Axiocam camera, and Zen Blue Image capture software. FTIR (Bruker, Germany) spectra were measured over a wavelength range of 4000− 500 cm−1 and acquired through the accumulation of 64 scans with the resolution of 4 cm−1. For DSC experiment (TA Instruments, USA), sample were weighed and placed in an aluminum pan and heated at a scanning rate of 5 °C/min from 40 to 250 °C. A nitrogen purge through the sample chamber was implemented to obtain a more uniform, stable thermal environment. Swelling properties were measured using a gravimetric technique and tensile mechanical testing was performed on dry PVA films (1 cm × 4 cm) using an Instron machine at the strain rate of 1 cm s−1 until failure.

min (Figure 1c, inset). The normalized FL intensity represented in Figure 2i indicates the shift change to 420 nm as the WF decreased. The microstructural organization and FL characteristics for both “PVA containing TPE-2BA” and “TPE-2BA” solutions were similar in the DMSO-rich system. It is well understood that using a 60−70% volume fraction of DMSO in a DMSO− water mixture to prepare a PVA solution results in the formation of fibril-like crystals.17 That is due to the fact that DMSO is a better solvent for PVA than water. Therefore, PVA chains are considered to be more solvated in a DMSO-rich system. Interestingly, it was identified that TPE-2BA organized a fibril-like structure in a DMSO-rich mixture (WF = 60% or less; Figure 2). When vials filled with “PVA (6% w/w with WF 60% and 70%) containing TPE-2BA” solutions were stored for 48 h, their FL spectra were similar to those of the TPE-2BA in a water−DMSO mixture with the same WFs. A similar shift in FL spectra, to 420 nm, was observed for the “PVA (6% w/w with WF = 60%) containing TPE-2BA”, whereas the FL properties remained unchanged for the “PVA (6% w/w with WF = 70%) containing TPE-2BA” solution (Figure 3a). Moreover, upon application of one and five FT cycles, the solutions exhibited characteristics similar to those previously observed (Figure 3b). It is important to notice that after

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RESULTS AND DISCUSSION TPE-2BA has distinct fluorescence properties in aggregate and fibril structural organization. TPE-2BA (chemical structure shown in Figure 1a), with the excitation wavelength (λex) at 365 nm, is highly miscible in dimethyl sulfoxide (DMSO) and immiscible in water. To evaluate the AIE characteristics of TPE-2BA, its FL properties were measured (Figure 1b) using a water−DMSO mixture at different volume fractions of water (WF). TPE-2BA has an emission maximum (λem) at 460 nm in high WF of 90%. It was found that decreasing the WF to 60% resulted in an obvious shift in the FL spectrum to 420 nm (Figure 1b) and a significant decrease in FL intensity (Figure 1c). To investigate the reason for the observed shift in FL spectra, fluorescent microscopy (Figures 2a−d) and SEM (Figures 2e−h) imaging were performed for samples with different WF. It was revealed that the observation of shift in FL spectra from 420 to 460 nm could be attributed to the increase in WF leading to aggregation and organizational alteration of TPE-2BA aggregates from particle to fibril structure. It was also found that the FL intensity increased followed by the increase in WF because the capture of high-resolution (brighter) fluorescent images (Figures 2c,d) required higher intensity of the light source (100 times more). The decrease in WF led to the creation of larger aggregations of TPE-2BA molecules (Figures 2d,h compared to Figures 2a,e).34 The weaker FL emission was consistent with the fact that the pack of TPE2BA molecules in the larger aggregated state weakened the RIM and resulted in less emission.35 In contrast, when TPE2BA molecules were packed in smaller areas (Figures 2a,e), higher emission was observed. On the basis of the SEM images, it was found that when WF was >60%, TPE-2BA transformed into dendrite microstructures in aggregated state with high FL intensity. In contrast, weak emission was observed for WF < 60%, where fibril structures with larger sizes of TPE-2BA were seen. The SEM images also identified smaller dendrites in the aggregated state (Figure 2e) than those in the samples with WF = 30 or 10% (Figures 2g,h). The smaller size aggregates resulted in closer packing of TPE-2BA molecules that led to the higher FL intensity. Moreover, appropriate photostability of TPE-2BA in WF = 90% was observed under ultraviolet light (365 nm) illumination for 60

Figure 3. Schematic drawing and FL spectra for PVA 6% solution containing TPE-2BA (a) as prepared (0 FT cycle) with 60 and 70% of WF and (b) after application of one and five FT cycles. The schematic drawing and fluorescent microscope images represent the effects (a) WF comparing water- and DMSO-rich systems and (b) application of FT cycles on organization of TPA-2BA molecules in PVA solution (polymer, TPE-2BA, and water molecules denoted by black, red, and blue markings, respectively). D

DOI: 10.1021/acsapm.9b00173 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials completion of five FT cycles two peaks were found in the FL spectra at 420 and 460 nm, indicating the presence of two different structural organizations of TPE-2BA molecules, including aggregated microparticles and fibrils within PVA hydrogels (Figure 3b, inset). This observation was confirmed by the images captured using a fluorescent microscope at 360 nm excitation wavelength (Figure 3b, inset). This could be explained by the occurrence of phase separation followed by the formation of fibril-like crystals in the PVA solution. For the first FT cycle, PVA molecules are not well-ordered. The aqueous phase consisting of water + DMSO + TPE-2BA are more likely to be randomly distributed across the PVA network. This results in formation of aggregated TPE-2BA microparticles with in the aqueous phase (Figure 3b, inset; bottom). Increasing the number of applied FT cycles to five results in densification of PVA molecules with more ordered structure within the crystal domain. As a result, accumulation of water occurs in the amorphous regions.14 Consistent with the images that were captured by a fluorescent microscope, the aggregated TPE-2BA particles in the amorphous region become smaller and also TPE-2BA fibrils form and are likely to be packed between PVA molecules in the crystalline districts (Figure 3b, inset; top). Within the crystalline districts, it is more likely that RIM occurs by the formation of hydrogen bonds between OH groups of TPE-2BA and PVA molecules. The FL characteristics of PVA film containing TPE-2BA are enhanced by application of FT cycles, reflecting the crystallization process. A significant increase in FL intensity was observed for “PVA film (6% w/w) containing TPE-2BA” by increasing the number of FT cycles (Figure 4). Consistent with the previous observation, an emission maximum was seen at 420 nm for all 1, 3, and 5 FT cycles. Opinions in the literature agree on the formation and growth of crystalline regions by the application of FT cycles, and our findings clearly showed that the observed increase in FL intensity reflected changes in the crystallinity of PVA films. To obtain deeper insight into the relation between FL intensity and the crystallization process, differential scanning calorimetry (DSC) was performed on both pure PVA films and “PVA films containing TPE-2BA” (Table 1), and the percentage of crystallinity and latent heat was measured. The percentage of crystallinity was measured using the formulation in eq 1, considering ΔHc = 138.6 J/g.7 crystallinity (%) =

ΔH × 100 ΔHc

Figure 4. (a) FL spectra and (b) FL relative intensity and fluorescence microscope images (insets) for “PVA film (6% w/w) containing TPE-2BA” exposed to different FT cycles.

Table 1. DSC Results for PVA Film (6% w/w, with and without TPE-2BA) for Different Numbers of FT Cycles sample PVA film

PVA film containing TPE2BA

(1)

The latent heat (reflected by the crystalline region area) could be understood as heat energy in a hidden form that was supplied or extracted to change the state of a substance without changing its temperature. The measured melting temperature was in the range 225− 227 °C, i.e., 226.4 ± 0.59 °C here. It was revealed that all samples reached more than 35% crystallization after the first FT cycle. Exposure to FT cycles resulted in an increase of ∼5% in the degree of crystallization after application of one extra FT cycle. However, >90% enhancement of latent heat was observed for the fifth FT cycle compared to that of the first cycle. These two findings indicate that the stability of crystalline regions was enhanced when PVA film was exposed to an increased number of FT cycles. In fact, it was apparent that repeated FT cycles reinforced crystals. Therefore, an increase in the number of FT cycles did not necessarily

FT cycle

ΔH (J/g)

% crystallinity

latent heat (mJ)

1 2 3 4 5 1

49.8 52.6 57.8 62.1 69.4 55.2

35.9 37.9 41.7 44.8 49.7 39.5

75.1 87 115 123.9 132 78.2

2 3 4 5

59.3 61.1 69.5 71.5

42.8 44.1 50.1 51.6

91.3 105.8 131.2 146.5

increase the overall initial degree of crystallinity.7 These observations were further evidenced by the captured fluorescent microscope images (Figure 4b, inset) and Fourier transform infrared spectroscopy (FTIR) (Figure S1), in which the increase in the number of FT cycles was shown to lead to larger and brighter regions as well as changes in the intensity of the absorbance bands at 1142, 1095, and 3000−3600 cm−1, respectively.36 It is well-known that the intensity of the absorption bands at 1142 and 1095 cm−1 is strongly correlated to the crystallinity of the PVA. The bands are assigned to C−O and C−C stretching vibrations in the crystalline phase, E

DOI: 10.1021/acsapm.9b00173 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials respectively. In fact, a number of intra- and interchain hydrogen bonds are formed followed by the application of FT cycles, resulting in the increase of crystallinity. By the noncovalent interaction, the amount of free hydroxyl groups decreases, leading to a less intense peak at 3000−3600 cm−1. Similar to the results obtained from DSC, a significant drop in intensity is seen for free hydroxyl groups (3000−3600 cm−1) and C−O (1142 cm−1) and C−C (1095 cm−1) stretching vibrations after the first FT cycle. However, the drop in intensity ratio for those peaks remains approximately unchanged during the application of extra FT cycles. It was concluded that the increase in FL intensity with the first FT cycle reflected the initial increase in percentage of crystallinity, whereas the amplified FL intensities with the second to fifth FT cycles were more likely evidence of the growth and enhancement of regions varying in crystalline perfection. It could also be seen that an increase in the number of FT cycles beyond five increased neither the percentage of crystallinity nor the structural enhancement (Figure 4b, inset). This finding was consistent with our DSC results and with previously reported studies9,37 (Figure S2 and Table S1). The development of stable crystalline districts was more likely to occur with increased concentration due to an increase in the overlap of PVA chains and the promotion of polymer chain folding. However, with high or low concentrations of PVA, crystallinity was reported to level off or start decreasing, respectively.8,38 In high concentrations of PVA, entanglements among the polymer chains could inhibit the folding of polymer chains and thus the formation of crystals. In contrast, in low PVA concentrations, properly solvated PVA chains were unlikely to overlap and promote crystallinity. Our further investigations into the effects of concentration revealed more consistent results than those in conventional previous studies (Figure S3). More in-depth insight was sought by investigating whether the FL spectra could represent crystalline nonamorphous regions via examining the stability of “PVA films (6% w/w) containing TPE-2BA” upon swelling in water. Because crystalline regions in PVA films are served as physical crosslinking, their stability that is increased during application of FT cycles affects the swelling properties. It is hypothesized that release of TPE-2BA from PVA films with more stable crystalline regions (exposed to five FT cycles) should be lower compared to that of the PVA films with lower degree of crystallinity (i.e., exposed to one FT cycle). To examine this hypothesis, the swelling properties of PVA films and release of TPE-2BA in swelling environment were measured. To do this, PVA films exposed to 0, 1, 3, and 5 FT cycles were placed in vials containing distilled water and both the FL intensities of the TPE-2BA expelled into water (Figure 5a), and the swelling ratios of the PVA films (Figure 5b) were measured at different time points. PVA solution (6% w/w) containing TPE-2BA served as the control. The amount of TPE-2BA released from the PVA film that was exposed to 0 FT cycle became approximately similar to that of the control (Figure 5a). This observation was consistent with the fact that in the absence of FT the crystallization of PVA film was negligible. After 24 h of swelling, the release of TPE-2BA from PVA films that had been exposed to 1, 3, and 5 FT cycles was stopped. The magnitude of FL intensity differed across all samples. It was seen that the release of TPE-2BA into the water increased when the number of FT cycles decreased. No change in the FL intensity in any of the films was found after 24 h of swelling. The observation of

Figure 5. (a) Change in FL intensity and relative intensity (inset) of “PVA film (6% w/w) containing TPE-2BA” exposed to different numbers of FT cycles during 96 h of swelling and (b) for the swelling ratio of “PVA (6% w/w) solution containing TPE-2BA” exposed to different FT cycles versus time.

11−18% decrease in FL intensity at 24 h of swelling for “PVA film (6% w/w) containing TPE-2BA” compared to t = 0 was consistent with our DSC results that identified a 15% decrease in crystallinity (ΔH for PVA film (6% w/w) exposed to five FT cycles was 69.497 and 59.651 J/g before and after swelling, respectively, Figure S4). These findings were consistent with those of the previous study that identified a 15% decrease in the initial degree of crystallinity of PVA samples exposed to seven FT cycles after 1 day swelling.7 Further investigations of the change in FL intensity of “PVA film (6% w/w) containing TPE-2BA” exposed to 1 and 5 FT cycles after 24 and 96 h of swelling resulted in similar observations (Figure S5). Moreover, comparisons of the images captured by polarized and fluorescent microscopes revealed that the bright regions identified by the fluorescent microscope were crystalline regions (Figure S6). While different studies have reported that boric acid compounds can contribute to the structural integrity of PVA hydrogels, our results identified that TPE-2BA has fluorescent properties when presented in crystalline regions. This belief is based on observations including negligible FL properties of TPE-2BA for the PVA solutions exposed to FT = 0 cycle, shift in the FL intensity peak from 460 to 420 nm after crystallization, the presence of TPE-2BA in the crystalline regions compared to the amorphous regions using EDAX, polarized and fluorescent microscopes, and the stability of crystalline regions and associated FL properties upon swelling for 96 h. F

DOI: 10.1021/acsapm.9b00173 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

ACS Applied Polymer Materials

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i Dt yz m60% − m0 z = 4jjjj z 2 z meq − m0 k πδ {

The FL characteristics of PVA film containing TPE-2BA could be used for prediction of PVA hydrogel properties (mechanical, swelling, and diffusion). When freeze−thawed gels are considered for various medical and pharmaceutical applications, it is crucial to characterize their physicochemical (swelling, diffusion, permeability, etc.) and mechanical properties over short and long time periods. Dissolution of PVA chains, changes in crystallinity in the long term, and crystallites melting out are some complications associated with PVA hydrogels that are physically cross-linked by the FT method. These problems need to be addressed because they can significantly alter the behavior of hydrogels over time. As stated in a significant number of studies, the equilibrium swelling ratio of PVA film (6% w/w) decreases when the number of FT cycles increases39−41 (Figures 6a,b). As illustrated previously,

(2)

It is clear that the coefficient of diffusion decreases as the number of FT cycles increases. Interestingly, it could be identified that the FL intensity at 420 nm, upon excitation at 365 nm, reflected the change in crystallinity of PVA hydrogel (Figure 4b) and therefore represented the reduction in the equilibrium swelling ratio and the diffusion coefficient (Figure 6a) and the increases in the mechanical properties (Figure 6b). Moreover, and consistent with previous studies, both the ultimate strength at failure and the Young’s modulus of PVA hydrogels increased significantly when more FT cycles were applied to the PVA film, resulting in enhancement of their mechanical properties (Figures 6b). On the basis of the FL spectra of “PVA film (6% w/w) containing TPE-2BA”, it was found that the increase in emission maxima at 420 nm reflected the increases in mechanical properties and the decreases in swelling and diffusion properties (Figure 6b).

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CONCLUSION In conclusion, a new and simple method was developed for in situ visualization of the kinetics of the formation of crystalline regions in physically cross-linked PVA-based hydrogels and for directly linking the FT process to the hydrogels’ final properties. This approach can be employed to effectively demonstrate the crystallization process using significant color and brightness differences in crystalline regions compared to amorphous areas. This capability allows researchers to visualize the crystalline regions clearly and to study their kinetics, formation, and transition in the solid state. Moreover, based on the findings, the FL spectroscopy information collected during FT cycles can be used to reflect the changes in crystallinityinduced properties of hydrogels. Results from this study have implications for the broad class of gels derived through polymer crystallization from solution.

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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/acsapm.9b00173.

Figure 6. Relationship between changes in the relative FL intensity and (a) swelling properties (equilibrium swelling ratio and diffusion coefficient) and (b) mechanical properties (ultimate tensile strength and modulus) of PVA films exposed to different FT cycles.

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Additional figures on FTIR spectra for PVA film containing TPE-2BA, freeze−thaw cycles on fluorescent property, DSC results, the FL spectra and relative intensity for “PVA films containing TPE-2BA” with different PVA concentrations, DSC curve for PVA film before and after swelling, effect of swelling on the FL spectra and relative intensity of “PVA film (6% w/w) containing TPE-2BA” exposed to different freeze−thaw cycles, and images of crystalline regions in PVA films captured using polarized and fluorescent microscopes at different freeze−thaw cycles (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.Z.T.). *E-mail: yohong.tang@flinders.edu.au (Y.T.).

the transport of water into a PVA hydrogel network follows Fick’s laws of diffusion in nature because the first 60% of water uptake can be fitted to a square root of time, and the coefficient of diffusion can be estimated by eq 2:38

ORCID

Ben Zhong Tang: 0000-0002-0293-964X Youhong Tang: 0000-0003-2718-544X G

DOI: 10.1021/acsapm.9b00173 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.T. and Y.T. are grateful to the International Research Grant (International Laboratory for Health Technologies) of South Australia for support. The authors acknowledge the expertise, equipment and support provided by Microscopy Australia and the Australian National Fabrication Facility (ANFF) at the South Australian nodes under the National Collaborative Research Infrastructure Strategy.

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ABBREVIATIONS AIEgen, aggregation-induced emission fluorgen; PVA, poly(vinyl alcohol); FT, freeze−thaw; XRD, X-ray diffraction; DSC, differential scanning calorimetry; NMR, solid state nuclear magnetic resonance; SAXS, small-angle X-ray scattering; TPE-2BA, tetraphenylethene-cored diboronic acid; FL, fluorescence properties; RIM, restriction of intramolecular motion; DMSO, dimethyl sulfoxide; WF, volume fractions of water; FTIR, Fourier transform infrared spectroscopy.

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DOI: 10.1021/acsapm.9b00173 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX