Folding and birefringence behavior of poly(vinyl alcohol) hydrogel film induced by freezing and thawing

Abstract

Band-like folds with high aspect ratio and birefringence behavior were observed on an in situ formed thin poly(vinyl alcohol) (PVA) hydrogel film via freezing–thawing treatment of PVA aqueous solution coated on glass. The crystallites generated during the freezing of the PVA solution cross-linked the PVA to form the hydrogel film. The volume expansion of the hydrogel film due to the absorption of condensed water in thawing induced the formation of folds. These folds show interesting birefringence behavior. The morphology, crystallization and birefringence behavior of the folds were characterized by polarized optical microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. A plausible principle for the fold formation is also discussed. It has been found that the moderate interaction between the hydrogel film and the substrate and the existence of condensed water on the frozen hydrogel film play important roles in the appearance of the folds.

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Introduction

Buckling is a popular phenomenon and is caused by mechanical instabilities triggered by thermal, mechanical or osmotic stress.^1,2 It plays a vital role in the formation of many natural structures such as the curling edges of leaves,³ the folds of brain tissue⁴ and friction ridges on fingers.⁵ Buckling instabilities have been utilized to prepare various patterns with potential application in flexible electronics, smart adhesives, microfluidic devices, microlens arrays and so on.^6–9 The most studied buckling usually relates to bilayer systems, where two layers have different moduli or stimuli-responsive behaviors.^10–14 There are two main types of bilayer systems: the extensively studied one involves coating a rigid skin on a pre-expanded soft elastomeric sheet,^15–17 and the relatively underexplored one of attaching a cross-linked polymer film on a rigid substrate.^18,19 In the latter system, volume increase of the cross-linked film induced by solvent swelling will lead to compressive stress, and in most cases the top film develops patters like creases or wrinkles.^20–22 However, these structures are usually shallow with low aspect ratio, which limits many proposed applications requiring high surface roughness or high local curvature. Recently, a folding structure with high aspect ratio was observed for a similar situation, where the compressive stress leads to the delamination of the film from the substrate, and the debonded free film folds upon itself to relieve the compressive stress. Tsukruk et al.^23,24 attached a 20–100 nm thick cross-linked poly(2-vinylpyridine) gel film on a silicon wafer, and uniform high-aspect folded structure lying on the flat surface was formed as a result of swelling of the gel film. Folds perpendicular to a flat film were also obtained by swelling a ten to several hundred micrometers thick, cross-linked PDMS film attached on glass.²⁵

In this work we demonstrate the formation of band-like folds with a high aspect ratio, parallel or perpendicular to a flat film from an in situ formed 1–2 μm thick poly(vinyl alcohol) (PVA) hydrogel film. Different from most previously reported buckling induced by dropping solvent on a cross-linked polymer, herein the folds appeared as a result of a freezing–thawing treatment of PVA aqueous solution coated on glass under humid conditions. Moreover, the fold shows optical birefringence behavior observed by a polarized optical microscope, which has been rarely reported for PVA film prepared by the aqueous solution casting method, as the crystallites do not have adequate birefringence to be observed by a polarized optical microscope except after undergoing special thermal treatment or melt crystallization.^26–30

Experimental section

Materials

PVA with an average molecular weight of 7.7 × 10⁴ and a degree of hydrolysis of 98% was purchased from Beijing Yili Chemical Inc. Deionized water was used as received.

Preparation of PVA hydrogel film with band-like folding structure

PVA aqueous solution (7 wt%) was prepared by dissolving the polymer in deionized water at 95 °C for 5 hours. After being slowly cooled to room temperature, the solution was spin-coated on a precleaned glass slide at 2000 rpm for 30 s. Water (0.2 ml) was sprayed in sealed vials to maintain a high humidity. The samples on glass were kept in the sealed vials and then frozen at −20 °C for 1 h and thawed at 25 °C for 0.5 h. PVA hydrogel films without folds were prepared under the same conditions but in a dry vial without spraying water. To prepare PVA bulk hydrogel, the solution was kept in a container with a depth of 2 mm and subjected to the same freezing and thawing process and then dried via lyophilization for characterization.

Characterization

The optical properties and the melting of the crystallites were investigated using a Nikon polarizing optical microscope (Eclipse E600W POL) with a Nikon (Coolplx4500) camera. A Linkam LTS350 hot stage was used to control the temperature. Morphology was characterized by scanning electron microscopy (SEM; Hitachi S-4300) operating at 15.0 kV. Fourier transform infrared (FTIR) spectra were recorded with a Bio-rad FTS6000/Raman III/UMA 500 system. X-ray diffraction (XRD) patterns were recorded in a transmission mode at room temperature with a Bruker D8 Discover diffractometer.

Results and discussion

Physically cross-linked PVA hydrogels prepared by the freezing–thawing method were first reported by Peppas.³¹ In the freezing process of the PVA solution, ice crystals formed and PVA was expelled from these crystals and concentrated in the interstitial sites. The high concentration of PVA was favorable for the formation of hydrogen bonds and microcrystalline sites. The as-formed crystallites with an average size of about 7 nm act as cross-linker.³² The freezing–thawing of bulk PVA solution has been studied extensively, whereas thin films of PVA solution coated on a substrate under freezing–thawing treatment have rarely been investigated. Herein, PVA aqueous solution was spin-coated on glass and then sealed in a humid vial for freezing and thawing treatment. SEM images of the resultant PVA film are shown in Fig. 1. The thickness of the dried PVA hydrogel film was about 1–2 micrometers. Interconnected band-like folds, several micrometers thick and 20 to 60 micrometers wide, can be observed on the film. The length of the folds can reach several millimeters. The folds do not always stand on the film. Part of the fold twists or lies on the surface. The “lying” region contacts the film (labeled A in Fig. 1d), while the “standing” part is perpendicular to the film (labeled B in Fig. 1d). There are also lots of nodes in the folds (Fig. 1b–d). For comparison, PVA film obtained by evaporating the PVA solution at ambient condition was flat without any folds.

Fig. 1 SEM images of PVA hydrogel film with (a) “standing” and (c) “lying” band-like folds prepared by freezing and thawing. (b and d) are the magnified images of (a and c), respectively.

The volume expansion induced by the absorption of condensed water from moisture in the thawing process plays a critical role in fold formation. In a control experiment, PVA solution coated on glass was sealed in a dry vial. Freezing–thawing treatment did not induce folding. These results demonstrate that it is absorption of condensed water that causes the self-folding.

A plausible formation mechanism of the band-like folds is described as follows. During freezing, PVA gelation takes place and a thin PVA hydrogel film forms on glass in the frozen state by using the PVA crystallites as the cross-linkers.¹⁰ Meanwhile, the moisture in the sealed vial condenses on the frozen hydrogel surface as ice particles. In the thawing process, water from these condensed ice particles is absorbed by the hydrogel film, inducing a volume expansion. The expansion of the film will be constrained by the adhesion between the film and the hydrophilic glass. Thus compressive stress is generated within the film. When reaching a critical point, the mechanical instability induces partial debonding of the film and sharp folds with high aspect ratio are formed to relieve the stress. Nodes will appear if different folds spread and meet.²³ These folding structures persist after the evaporation of the water. Different from ordinary swollen hydrogels, there was no crease formed on the PVA hydrogel film; the folds are formed directly from the flat state via buckle delamination.^24,25

The freezing–thawing treatment in humid conditions also induces the formation of folds for the as-formed PVA hydrogel film coated on glass. When the PVA solution underwent freezing–thawing treatment in a dry vial, a PVA hydrogel film was formed without folds. After spraying water in the sealed vial, similar folds appeared on the film by freezing–thawing again. Herein, the factors that affect the folding of the as-formed thin PVA hydrogel film were investigated. It is found that moderate adhesion between the PVA hydrogel film and the substrate is a crucial factor.²⁵ If the as-prepared PVA hydrogel film is dried in an ambient environment, the second freezing–thawing treatment under humid conditions cannot induce folding. This can be ascribed to the strong interaction between the substrate and the PVA hydrogel film after drying. The compressive stress resulting from the volume expansion cannot overcome the interaction between the film and the substrate. Thus delamination is not likely to occur. So keeping the hydrogel in a wet state is necessary for obtaining moderate adhesion during the freezing–thawing process.

To further study the mechanism of folding, the second freezing–thawing treatment was substituted by ordinary water swelling for the as-prepared wet PVA hydrogel, via dropping water on the hydrogel surface; however, similar folding did not occur. Though the wet PVA hydrogel film has a weaker interaction with the glass compared to the dry hydrogel, the interaction is still too strong for ordinary solvent swelling-induced compressive stress to concur. Thus the freezing–thawing treatment was required. It is proposed that, during the freezing–thawing treatment, the freezing decreases the interaction between the PVA chains and the glass substrate to a certain degree, via excluding the PVA chains from contacting the substrate by ice nucleation and growth on glass; thus the compressive stress induced by water swelling is sufficient for the partial debonding of the hydrogel film and finally self-folding occurs during the thaw process. The same process should occur for the spin-coated PVA solution, and thus the freezing–thawing treatment plays a key role in the formation of the folds for both spin-coated PVA solution and the as-formed PVA hydrogel films.

Previous research mainly focused on the formation of the folding. The difference in the crystallization and optical behavior between the folding and flat areas of the film was paid little attention. In this work, the optical properties of the folds were investigated and an interesting birefringence behavior was found. Normal and polarized optical micrographs of the PVA film are shown in Fig. 2. It can be seen that the “standing” folds, where the polarized light can pass, display an intensive birefringence behavior, while the “lying” folds and the flat area do not. The birefringence behavior usually is caused by the anisotropic structure of the crystals or the residual stress. Here the “lying” and “standing” folds underwent similar compression and stress release, and the birefringence disappeared when the “standing” fold was put to the “lying” state with a micromanipulator (Fig. S1†). So stress can be excluded as the main factor of the birefringence behavior. As PVA is a crystalline polymer, the birefringence behavior should be ascribed to the anisotropic structure of the PVA crystals.

Fig. 2 (a) Normal optical micrograph of freeze–thawed PVA film. (b) Corresponding polarized optical micrograph of (a). The “standing” band shows an optical anisotropic property.

It is difficult to compare the difference in crystallinity between the folds and the flat area of the film by XRD or differential scanning calorimetry because the folding occurs on the microscale. The difference in the crystallization behavior between the “lying” folds and the flat area was studied by FTIR in combination with a microscope. The incident light can be focused on an area of about 10 × 10 μm² with the assistance of the microscope, and thus the FTIR spectra of these two different areas can be collected separately. The FTIR spectra of the fold and flat areas are shown in Fig. 3. The absorption at 1144 cm⁻¹ was assigned to C–O stretching of doubly hydrogen bonded OH in crystalline domains of PVA.¹⁶ The presence of the peak at 1144 cm⁻¹ for the flat area indicates the existence of crystallinity. The FTIR spectrum of the fold area has a stronger peak at 1144 cm⁻¹, demonstrating a higher degree of crystallinity than that of the flat area.³³ These results imply that, during the freezing–thawing process, crystallization happens in the folds and also the flat area, but the folds have a higher degree of crystallinity. The reason may be due to the volume expansion-induced compression and the following stress release which allows the further ordered arrangement of the chains. For the folds, the “lying” and “standing” regions are subject to similar compression and stress release. Therefore, we presumed they have similar crystallinity.

Fig. 3 The FTIR spectra of the folding (a) and flat (b) areas of PVA film.

The birefringence behavior was also studied by polarized optical micrographs obtained at different temperatures. An optical anisotropic phenomenon can be clearly observed at room temperature (Fig. 4a). There is no change in the optical phenomenon when the sample was heated to 230 °C, around the melting temperature of PVA crystals.³¹ On further heating to 250 °C, the optical anisotropic phenomenon becomes weaker and almost disappears for some standing regions while the folding structure remains, indicating the fold becomes structurally isotropic (Fig. 4b).

Fig. 4 In situ observation of the melting process of PVA folds on glass under polarized light: (a) room temperature; (b) 250 °C.

From the above results, the birefringence could be ascribed to the crystallites in the “standing” folds. But why do the “lying” folds with crystallites not show birefringence behavior? As reported previously, crystalline PVA in a solution-cast film was found to lack adequate birefringence, so the birefringence behavior could not be observed from polarized optical microscopy of the flat area and the “lying” folds. But for the “standing” folds, their height is more than ten times the thickness of the flat area. In addition, the birefringence disappears when the “standing” band was altered to a “lying” one with the micromanipulator (Fig. S1†). We propose that the accumulation of the birefringence along the folds and also the sharp ridge structure make possible the observation of the enhanced birefringence behavior of the “standing” folds. This is consistent with the birefringence behavior also being observed for a thick hydrogel with ridge structure prepared via freezing–thawing treatment (Fig. S2†).

Fig. 5 shows XRD patterns of the PVA hydrogel film with folds and PVA bulk hydrogel. The XRD patterns show that the two samples have the same crystal structure: no new crystal structure was formed for the thin PVA solution film coated on glass under freezing–thawing treatment with high humidity.

Fig. 5 XRD patterns of the PVA hydrogel film with folds (a) and bulk PVA hydrogel (b).

Conclusions

PVA band-like folds were obtained by freezing–thawing treatment of PVA aqueous solution and PVA hydrogel film on glass sealed in a humid vial. The moderate adhesion between the PVA hydrogel film and the substrate, and the volume expansion due to water absorption played crucial roles in inducing the folding. The folding structure had a higher degree of crystallinity than the flat area. Birefringence of PVA standing folds was observed, which can be ascribed to the accumulation of the birefringence along the fold ridges.

Acknowledgements

We thank the financial support from 973 Project of China (2013CB933000) and NSFC (no. 21121001, 51073012, 51125010).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The optical micrographs of the PVA hydrogel film with folds before and after putting one “standing” fold to “lying”, the thick hydrogel with ridge structure, and their corresponding polarized micrographs. See DOI: 10.1039/c4ra06155a

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