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Observation of Water-Stimulated Supercontraction of Uniaxially Oriented Poly(vinyl alcohol) and the Related Hierarchical Structure Change Revealed by the Time-Resolved WAXD/SAXS Measurements Taiyo Yoshioka,*,† Kohji Tashiro,*,† and Noboru Ohta‡ †

Department of Future Industry-oriented Basic Science and Materials, Graduate School of Engineering, Toyota Technological Institute, Tempaku, Nagoya 468-8511, Japan ‡ Japan Synchrotron Radiation Research Institute, 1-1 Koto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan S Supporting Information *

ABSTRACT: A uniaxially oriented poly(vinyl alcohol) (PVA) film was found for the first time to respond to the humidity change in the two different modes under the restrained condition, which are essentially the same as the modes of the supercontraction and cyclic contraction observed in spider dragline silk. Once the atmospheric humidity started to increase, the PVA film showed at first the irreversible stress generation (supercontraction stress), followed by the reversible stress generation synchronizing with the cyclic change of humidity. These irreversible and reversible stress changes have been connected to the changes of higher-order structure caused by the cyclic change of wet/dry atmosphere as revealed by the detailed in situ measurements of the 2-dimensional wide-angle and small-angle X-ray scatterings during these processes. On the basis of a simple mechanical model and the thus-collected information on the higher-order structural change, it was concluded that the irreversible and reversible stress generations are governed mainly by the irreversible hydration-induced stress relaxation in the highly tensioned amorphous region and the reversible swelling in the normal amorphous region, respectively. combining with carbon nanotube.15 Recently, Yoshioka et al. have found that such a reversible stress generation driven by wet/dry change is observed not only for spider dragline silk specifically but also for the uniaxially oriented products of regenerated silk fibroin from Bombyx mori silkworm silk.16 As will be reported here, we have detected a similar phenomenon also for the uniaxially oriented poly(vinyl alcohol) (PVA).17 Compared with the spider dragline silk, both regenerated silk fibroin and PVA are considered to be more suitable candidates for the actuator materials because of their superior processability and mass producibility. Additionally, because of their excellent balance between high biocompatibility and mechanical properties, the function of reversible stress generation is expected to be applicable to the biomedical uses. A common chemical characteristic among these materials including spider silk is a formation of intra- and interchain hydrogen bonds. The supercontraction of spider dragline silk was interpreted as the result of hydration-driven breakage of interchain hydrogen bonds and subsequent entropy-driven recoiling of oriented amorphous chains.18−22 The cases of

1. INTRODUCTION Some polymers are sensitive to the external stimulus, and they are called the stimulus-responsive polymers. As well-known, these stimulus-responsive polymers change their internal structure reversibly, leading to a reversible change of their shape or a reversible generation of mechanical force in response to various environmental changes (e.g., electric field, light source, pH, ionic strength, temperature, and water or solvent). These polymers have attracted strong interests because of their various potentialities for the applications to sensors and actuators and also for the medical applications.1−4 In particular, polymeric actuator of water-responsive type may be regarded as one of the promising energy generation systems.5−9 Among them, spider dragline silk is one of the hot materials showing a reversible response to humidity change. Their remarkable shrinkage, exceeding 50% of its original length by wetting under unrestrained condition and being called “supercontraction”,10 generates a significant contraction stress over 50 MPa under restrained conditions.11,12 Blackledge et al. found that the stress generation is repeated cyclically if the alternative wet/dry change is applied repeatedly.13 After that, the basic researches were reported on the cyclic response of spider silk to the humidity change at the aim of the various applications to the actuator, such as artificial muscle14 and electrical sensor by © XXXX American Chemical Society

Received: November 24, 2016 Revised: March 17, 2017

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The sample with a dimension of 20 mm (length) × 5 mm (width) × several micrometers (thickness) was fastened to the clamps and was lightly tensioned by applying a negligibly small tensile force (ca. 1 MPa order) along the stretching direction so that the sample did not have any flexure. The RH of the sample environment was controlled cyclically, during which the changes of generated stress was detected by a load cell. The structural change was investigated for the samples of about several hundred micrometers thickness by measuring the wide-angle X-ray diffraction (WAXD) using an R-Axis VII X-ray diffractometer (Mo Kα) (Rigaku Co., Japan) with an imaging plate detector and the small-angle X-ray scattering (SAXS) using a Nanoviewer X-ray diffractometer (Cu Kα) (Rigaku Co., Japan) equipped with a highly sensitive single-photon-counting pixel detector (Pilatus 300 K; Dectris Ltd., Switzerland). A fully wetted sample was used for these measurements, which was prepared by immersing the uniaxially oriented film in pure water for 1 day at room temperature with the both ends fixed by a handmade metal holder. To avoid the drying of the sample during the X-ray scattering measurements, the sample was covered with a thin Mylar film (2 μm thickness) and set in the humidcontrol cell with a pair of Kapton windows (12.5 μm thickness) at the RH higher than 95%. After the X-ray measurement in wet condition was finished, the Mylar film was removed away, and the sample was dried overnight under dry-air flow, which was used as a dry sample for the X-ray measurement. Time-resolved WAXD and SAXS simultaneous measurements were also performed during the wetting process under humid-condition (RH > 95%) by using the Nanoviewer. In this case, the two single-photon-counting detectors, Pilatus 100 and 300 K, were used for WAXD and SAXS date collection, respectively. The geometry of the experimental setting is depicted in the Supporting Information (Figure S2). A similar measurement was made also by utilizing the synchrotron radiation X-ray beam (wavelength (λ): 0.0709 nm) at SPring-8 beamline 40B2 (Hyogo, Japan). A flat panel detector (Hamamatsu Photonics K. K., Japan) and a CCD camera detector (Hamamatsu Photonics K. K., Japan) were utilized for WAXD and SAXS measurements, respectively. The thermal energy change and water evaporation were investigated by means of a differential scanning calorimetry (DSC) using a DSCQ1000T (TA Instruments, USA) and by thermogravimetric analysis (TGA) using a TGA-5000IRS (TA Instruments, USA), respectively. For the DSC measurements, a sample of about 10 mg was sealed in liquid-type aluminum pan, which was pretreated in boiling water for 2 h to avoid the oxidization reaction with water during the measurements. The DSC thermograms were obtained in the temperature range from −90 to 50 °C at a scanning rate of 2.5 °C/min under nitrogen gas flow of 50 mL/min. For the TGA measurements, a sample of 5−10 mg with a thickness of about 150 μm was put into a 100 μL platinum pan, and the TGA curve was measured in the temperature range from 50 to 300 °C at a heating rate of 5 °C/min under nitrogen gas flow of 25 mL/min.

regenerated Bombyx mori silk and PVA might be also interpretable in a similar way. But these speculations must be checked experimentally by investigating the detailed structural changes during the contraction/elongation processes. It is also an important subject to clarify how the structural change is related with the stress generation mechanism under restrained conditions. In the present paper, we report the first finding of a waterstimulated reversible stress generation/relaxation behavior of the uniaxially oriented PVA films. The phenomenon itself is similar to the spider dragline silk,13 but the PVA sample case may give us clearer information on the structural origin of this phenomenon compared with the case of silk because of the well-analyzable stacking structure of crystalline lamellae in the oriented PVA sample. In fact, we have successfully observed the hydration-induced changes in the higher-order structure of PVA sample by performing the simultaneous time-resolved wide-angle and small-angle X-ray scattering measurements, allowing us to extract several common features useful for the explanation of the reversible stress generation mechanism in the water-absorbable polymer materials.

2. EXPERIMENTAL SECTION PVA pellet was kindly supplied by Kuraray Co., Ltd., Japan, whose degree of saponification was 99.7 mol %, and its average degree of polymerization was 4000. Uniaxially oriented PVA films of several to several tens micrometers thickness were prepared by hot-stretching the aqueous-solution-cast films by 500% the original length in the oil bath at 150 °C. The thus-prepared highly oriented and annealed sample has the high degree of crystallinity and shows a high water resistivity; that is to say, this sample is insoluble in water even when soaked for a long time. This point becomes important in the later discussion made in section 3.8. The contraction stress, which was generated during the humidity change under the restrained condition, was measured at room temperature using a mechanical tensile stage (Linkam Scientific Instruments Ltd., UK). The schematic drawing of the tensile stage is shown in Figure 1. The sample was set in a handmade humiditycontrol cell, and the relative humidity (RH) was controlled by switching the wet air of RH higher than 95%, which is a measurable upper limit in the humidity sensor used, and the dry air of RH lower than 15%. An example of cyclic humidity change achieved by this control system is shown in the Supporting Information (Figure S1).

3. RESULTS AND DISCUSSION 3.1. Wetting Behavior of Uniaxially Oriented PVA under Unrestrained Condition. Before showing the results of wet/dry-induced stress generation behavior of uniaxially oriented PVA sample under restrained condition, the dimensional and structural changes under unrestrained condition are described briefly. When the sample was immersed in water for 24 h, the sample shrunk by about 50% along the drawn direction and expanded by about 50% in both the width and thickness directions. This dimensional change is in quite similar level as that observed for the supercontraction of spider dragline silk,10 although the response to the humidity change is much slower in the PVA case. As a result, the crystalline orientation became significantly worse as revealed from the comparison of 2-dimensional (2d)-WAXD patterns measured before and after wetting (see Figure 2). Once becoming worse,

Figure 1. Schematic drawing of the sample joint to the load cell. The load cell is depicted as a spring with a spring constant Kcell (N/mm), and the positive and negative values of the generated stress are defined as shown in the drawing. For example, when the sample is shrunk, the spring is tensioned with the positive stress. B

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Figure 2. Changes of 2d-WAXD pattern measured for the uniaxially oriented PVA sample: (a) the intact sample, (b) the wet sample after immersed in water for 24 h under unrestrained condition, and (c) the sample dried up from (b). The observed dimensional change before and after wetting was −56% (length), +53% (width), and +50% (thickness).

hand, the thus-generated stress was relaxed partially and kept at a constant value in the second wetting process. After the end of the second cycle (∼150 min), the stress relaxation and stress generation in the wetting and drying processes, respectively, were observed reversibly in the stress range 16−41 MPa. In this way, the behavior can be divided into two different types of stress generation mode. One is an irreversible mode observed only once in the first wetting process. Another is the reversible mode observed in every cyclic drying/wetting process after the first irreversible stage. This stress generation/relaxation behavior of PVA film is essentially identical to the observation made for spider dragline silk.13 The irreversible and reversible stresses are called here “supercontraction stress” and “cycliccontraction stress”, respectively, following the case of spider silk. In contrast to the stress generation/relaxation behavior shown in Figure 3a, the thicker PVA films of 85 μm thickness showed a little different behavior as shown in Figure 3b. The first wetting generated a remarkable contraction stress of 46 MPa. By starting the drying of the sample, a small quick stress relaxation was detected, followed by a slow stress increase during the drying process. Such a stress relaxation was detected also for the thinner sample, as shown in Figure 3a, although the data were noisier. At the present stage, unfortunately, the origin of this quick stress relaxation detected in the beginning of drying process is not clear. The stress generated in the drying process was saturated at the level exceeding the supercontraction stress of the first wetting. The observed stress change was almost common to both the thin and thick samples, but the clearer observation was made for the thicker sample because of the higher stress generation. It is noted here that the generation of the contraction stress looks remarkably high as known from Figure 3, but the magnitude itself is about 50 MPa, which is enough low to keep the sample from the breakage due to the generated stress. This can be checked by measuring the stress−strain curves of the PVA sample under the dry and wet conditions, which are reproduced in the Supporting Information (Figure S3). In order to investigate the structural changes occurring in these stress changes, the 2d-WAXD patterns were measured for the intact, wet, and dry PVA samples as shown in Figure 4a. The equatorial and meridional diffraction profiles obtained from the 2d-WAXD patterns are shown in Figures 4b and 4c, respectively. In contrast to the case of unrestrained wetting (see Figure 2), the crystalline orientation level was kept almost constant in the restrained wetting. When the meridional diffraction profiles are compared among the samples subjected

the crystalline orientation was not recovered even after the sample was dried again (Figure 2c). 3.2. Two Types of Contraction Stress Generated by Wet/Dry Process under Restrained Conditions. Under restrained conditions or under the constant length conditions a contraction stress was generated instead of the dimensional change induced under unrestrained conditions. A typical cyclic change of contraction stress is shown in Figure 3a which was

Figure 3. Time dependence of the contraction stress characteristics of the restrained PVA films induced by the cyclic change of wet/dry condition. (a) and (b) were obtained for the samples with the different film thickness of 6 and 85 μm, respectively.

detected for a restrained PVA film of 6 μm thickness under the cyclic wet/dry atmosphere change. As indicated in Figure 1, the stress of the sample, which pulled the load cell, was defined to be positive. Thus, as seen in Figure 3, the sample was a little tensioned at the starting point, as known from the negative stress of ca. −13 MPa. Once the wetting process was started, the film generated the strong contraction stress of ca. 29 MPa (from −13 to +16 MPa). In the subsequent drying process, the stress was increased further up to ca. 34 MPa. On the other C

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overwhelmingly high compared with the Ea of about several hundred MPa. Therefore, we can put the relation Ec ≫ Ea in the equation. The degree of crystallinity is Xc. The stress and strain relation is given as σsample = Esampleεsample

1 Esample

=

(1)

Xc 1 − Xc 1 − Xc + ∝ Ec Ea Ea

( ∵ Ec ≫ Ea)

(2)

σcell = Ecellεcell

(3)

σtotal = Etotalεtotal

(4)

The total length ltotal is ltotal = lsample + lcell

(5)

Therefore, the strain of the whole system is given as εtotal =

lsample Δlsample Δltotal l Δl = + cell cell ltotal ltotal lsample ltotal lcell

= XLεsample + (1 − XL)εcell

(6)

where XL = lsample/ltotal. In the restrained condition, εtotol = 0 and so eq 6 gives εsample as follows. ⎛ 1 − XL ⎞ εsample = −⎜ ⎟εcell ⎝ XL ⎠

(7)

It must be noticed that the restraining condition is applied to the total system consisting of the sample and the load cell, and so εtotol = 0, not εsample = 0. If εsample = 0, no stress is generated actually (σsample = Esampleεsample = 0). At the same time, we can get the following relation from eqs 2 and 7: ⎛ XL ⎞ σsample = Esampleεsample = −Esample⎜ ⎟εcell ⎝ 1 − XL ⎠

Figure 4. 2D-WAXD patterns (a) and the meridional (b) and equatorial (c) profiles of restrained PVA oriented film measured in intact, wet, and dry conditions. Insets in (b) and (c) show the enlarged image of the 101 and 110 reflections, respectively.

=−

to the various conditions, the diffraction peak coming from the crystalline phase did not shift at all in the wet and dry conditions within the experimental error. But, the crystalline peaks along the equatorial line, in particular the 110 reflection, were found to shift slightly and reversibly by ∼0.6% the original d-spacing: the d-spacing was expanded by wetting as a result of water absorption. However, this slight dimensional change of the crystal phase in the lateral direction may be negligible to interpret the stress data shown in Figure 3 as long as the mechanical deformation along the draw direction of the oriented sample is discussed. The 101 reflection did not shift, indicating the crystal lattice is not deformed along the chain axis within the experimental error. Rather, the role of the amorphous phase is needed to consider more preferentially. 3.3. Importance of Amorphous Phase for the WaterInduced Mechanical Stress Generation. Let us consider the mechanical model in which the sample is connected to a load cell in series. The Young’s modulus of the load cell is Ecell. The lengths of the sample and load cell are lsample and lcell, respectively. The total length of this system is ltotal. The sample is assumed to consist of the crystalline and amorphous phases in a series mode, the Young’s modulus of which is Ec and Ea, respectively. The Ec of PVA crystal is about 290 GPa,23 which is

Esample ⎛ XL ⎞ E XL 1 σcell ⎜ ⎟σcell ≅ − a Ecell ⎝ 1 − XL ⎠ Ecell 1 − Xc 1 − XL (8)

If Ec ≫ Ea ,

εsample = Xcεc + (1 − Xc)εa ≅ (1 − Xc)εa (9)

where εc =

σc ≈0 Ec

and

εa =

σsample σa = Ea Ea

(10)

Equation 8 says that the stress of the sample σsample is detected by measuring the stress of the load cell (σcell). Figures 9 and 10 show clearly that the σsample is governed mainly by the mechanical response of the amorphous phase. 3.4. Experimental Evaluation of the Contribution of the Amorphous Phase. In this way, the contribution of the amorphous phase should be large for the reasonable interpretation of the mechanical behavior observed in Figure 3. The X-ray diffraction data support this speculation. In the meridional 2θ profile obtained for the intact sample (Figure 4b), a broad peak was detected at 2θ of around 9.2°. This broad peak shifted to higher angle side by ∼12% from the original position during the wetting process and shifted back almost D

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the penetration of water molecules into the dried sample stepwise. In the early stage of this type of experiment, the water content penetrating into the sample is not very large to disturb the detection of the amorphous halo peak shift. The concrete discussion is made in section 3.6 (Figure 9). Another possible way to separate these two effects of the amorphous phase and absorbed water is to use the wide-angle neutron diffraction. The neutron scattering from H2O may become an incoherent background if the deuterated PVA sample is used as an oriented sample, which may give the strong and clear amorphous halo due to the relatively strong coherent scattering of the deuterated species. Clearer experimental evidence of the structural change in the amorphous phase was obtained on the basis of the SAXS measurement. On the basis of this information on the amorphous region (namely, the change of εa), we can deduce the change of the higher-order structure since the amorphous region plays an important role in the change of the whole system (eq 9). In order to investigate the changes of the higherorder structure for the intact, wet, and dry samples, the SAXS patterns were measured as shown in Figure 6a. All the SAXS patterns are basically of the four-point scattering pattern, which changed drastically and reversibly by wetting and drying. The SAXS patterns were analyzed on the basis of a stacked-tiltinglamellar structure model shown in Figure 6b. In the wetting process of the intact sample, the lamella tilt angle (ϕ) changed drastically from 48° to 64° accompanying the increase of the long period along the stretched direction (Lp). The Lp expanded by 30% of the original value. Because no significant change of the crystallinity was observed between the intact, wet, and dry samples as known from the WAXD profiles, the dimensional change of Lp is considered to be attributed mainly to the dimensional change of the amorphous phase existing between the stacked crystalline lamellae. In other words, the reversible increase/decrease of Lp induced by wet/dry change may be directly related to the contraction/relaxation of the amorphous phase along the stretched direction. The phenomenological eqs 9 and 10 describe this situation clearly. Another point to be noticed is the change in the tilting angle of the lamellae as seen in Figure 6. Because the WAXD measurements demonstrated no occurrence of significant change in the crystallite orientation between the dry and wet conditions, the drastic change of ϕ is not caused by the simple rotation of lamellar crystals. A possible mechanism is the chain sliding along the chain axis in the lamellar crystals, so that the normal of the lamella changes by keeping the chain axis direction with the chain packing structure in the lamella kept unchanged. In addition to the change of the four-point SAXS pattern, the streak line was detected on the equatorial line in the wetting state with a peak maximum at around q = 0.77 nm−1 (8.2 nm). This suggests that water molecules preferentially penetrate into the amorphous regions between the crystallites arrayed in the lateral directions with the averaged period of ca. 8.2 nm and increase the electron density contrast. A concrete image of the higher-order structure change will be shown in a later section. 3.5. Higher-Order Structure Change in Cyclic Wetting Process. To investigate the details of structural change during the wetting process in the cyclic-contraction period, the timeresolved simultaneous WAXD and SAXS measurements were performed using Rigaku Nanoviewer X-ray diffractometer. The changes of 2d-SAXS and WAXD patterns during the second wetting process are shown in Figures 7a and 7b, respectively.

perfectly to the original 2θ position by drying. It suggests that the peak shift of this halo corresponds to the structural change of the amorphous phase. However, here we need to notice the contribution of water component in the observed halo profile. The difference profile between the wet and dry profiles (wet− dry in Figure 4b) has revealed that the bulk water absorbed into the sample gives a peak maximum at 2θ around 12.3° (Figure 4b). This means that the apparent peak shift observed for the WAXD halo might be affected more or less by the overlap of the two peaks originating from the amorphous peak of the PVA sample and the scattering by bulk water absorbed into the PVA sample. The presence of the bulk water in the wet sample was confirmed by the DSC thermograms. The DSC heating traces of the intact, wet, and dry samples are shown in Figure 5. While

Figure 5. DSC thermograms measured for intact, wet, and dry PVA samples in the heating process.

no sharp peak corresponding to the melting of free water ice (namely, the melting of bulk water ice) was observed for both the intact and dry samples, the wet sample clearly shows a narrow DSC peak at around 0 °C, which corresponds to the melting peak of free water ice. The broad peak overlapped in a wide temperature region of −23 to 5 °C, which may be attributed to the freezable bound water.24 (The detailed study of the aggregation state of water molecules absorbed in the PVA sample may be made through such a spectroscopic measurement as IR and/or Raman spectra,25 which will be a future work.) By referring to these DSC data, the broad X-ray scattering peak observed in the meridional 2θ profile in the wet state (Figure 4b) is considered to come partially from the free water and/or freezable bound water and also from a contribution of the amorphous halo of PVA sample. We tried to subtract the contribution of the profile coming from the bulk water from the observed profile so that the amorphous halo peak component can be extracted nicely. But the result was not very satisfactory. Therefore, because of the difficulty of quantitative peak separation into the scattering components of the amorphous and water phases, the clear experimental evidence showing the structural change of amorphous phase (namely, the change of εa) during the wet/dry change might be difficult to deduce from only the WAXD measurement. The exception might be there in the early stage of the wetting process. Since the total amount of the absorbed water is smaller for the dried and intact samples (as seen from the TGA data shown in a later section), the WAXD data in the early wetting stage may be used for the analysis of the amorphous halo peak shift to occur at this stage. This idea was applied actually for the X-ray diffraction data collected using synchrotron radiation. Different from the case of laboratory-level experiment (Figures 4 and 7), the synchrotron-sourced X-ray diffraction measurement can be made in a short time interval, allowing us to trace E

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until the saturation occurred, indicating that the higher-order structural change occurred cooperatively. This structural change was saturated at around 10 h. On the other hand, the shift of halo peak position, which is considered to be an index of the progressing of wetness as mentioned above though the contributions from water and amorphous region are included, achieved almost a saturated level at the wetting time of 4 h. In this way, there might be a time lag in the saturation between the water absorbance (WAXD) and the higher-order structural change (SAXS). All of the thus-experimentally obtained information is summarized briefly in Figure 8. By wetting the intact sample, the stress was generated, just when the long period of the stacked lamellae (Lp) was increased and the originally tilted lamellae increased the tilting angle furthermore. At the first wetting stage, the stress increased drastically and saturated (Figure 3). More detailed observation in this first wetting process (or supercontraction) will be described in the next section. The drying this sample increased the stress further. After the phenomenon was saturated, the second wetting caused the steep decrease of the stress down to the saturated level in the first wetting stage. After that, the alternative change between the dry and wet conditions caused the reversible change of stress. The stress increments observed for the intact sample in the two stages of the first wetting (A) and the following drying process (B) must be related to the significant change of the higher-order structure, in particular, in the amorphous region. The cyclic change of stress caused by the repeated wet/dry process (B and C) is intimately related to the reversible change of Lp and ϕ as known from the SAXS data. 3.6. First Irreversible Supercontraction. Next, we consider the structural change and the generation mechanism of supercontraction stress induced in the first wetting process. By wetting the intact sample, the contraction stress was generated remarkably but irreversibly as seen in Figure 3. Since the supercontraction occurs quickly, we investigated the structural change during the first wetting process by measuring the time dependence of WAXD/SAXS data utilizing the synchrotron radiation X-ray beam. The changes of contraction stress, the structural parameters of Lp and ϕ estimated from SAXS data, and the WAXD halo position are plotted against the wetting time (Figure 9). An induction time of about 20 min was needed before the generation of the contraction stress. This long induction time was due to the relatively slow diffusion of water into rather thick sample of about 200 μm. In this induction time region the peak position of WAXD halo started to shift to the higher angle side gradually after wetting. The lamellar tilting angle ϕ and long period Lp were also detected to change in this induction time. The stress generation started to occur at around 20 min. The saturation behavior of the stress curve is quite similar to that of the 2θ halo peak shift. But, even in the induction time region where the generated stress was zero, the 2θ halo peak was detected to shift to the higher angle side, suggesting some shrinkage in the amorphous region. (As pointed out in section 3.4, in the early stage of the water penetration process the contribution of the X-ray scattering peak of the absorbed bulk water may be assumed to be small, allowing to trace the change in the amorphous phase by reading the shift of the halo peak detected in the time region of 0−20 min.) Correspondingly, the ϕ and Lp change also in this time region. Once when these structural changes reach some critical point at around 20 min, the shift of the 2θ halo peak became remarkable, and the supercontraction stress started to generate

Figure 6. Changes of (a) SAXS pattern induced by wet/dry change under restrained condition and of (c) the structural parameters Lp and ϕ, evaluated on the basis of a tilted lamella stacking model (b). The 1d-SAXS profile of the wet sample is shown in (d), which was obtained by scanning the 2d-SAXS pattern along the equatorial streak line (surrounded by yellow colored broken line in (a)).

From the SAXS patterns, the structural parameters Lp and ϕ are evaluated, which are plotted against the wetting time (Figure 7c). The changes of peak position (2θ) of the halo peak in the meridional 2θ profile of WAXD pattern are also plotted against the wetting time in Figure 7c. Because the structural changes induced by wet air conditions were almost ceased after 12 h, the wet sample after 24 h was dipped directly into pure water for another 20 h to accelerate the further wetness. The SAXS and WAXD patterns indicated as “dipwet20h” in Figure 7 were obtained for this sample. The structural parameters derived from the data (a) and (b) are plotted in Figure 7c. It is clearly shown that both of the parameters Lp and ϕ changed continuously at a similar timing during the wetting process F

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Figure 7. Time-resolved (a) SAXS and (b) WAXD patterns measured simultaneously during the second wetting process of restrained PVA sample. The stretched direction of the sample corresponds to the vertical direction in both of the SAXS and WAXD patterns. After 24 h of wetting using the wet-air flow, the structural change became quite slower. Then, the sample was dipped into water for another 20 h to accelerate further wetness (dipwet20h). Changes of the structural parameters Lp and ϕ, evaluated from SAXS data, and of the peak position of WAXD halo are plotted against the wetting time in (c).

Figure 8. Summary of the experimental data of stress, long period Lp, and the lamellar tilting angle ϕ obtained for the dry/wet PVA sample. The details of the structural change in the beginning of first wetting stage (namely, during the induction time) are described in Figure 9.

and increased with time. The lamellar stacking structure also changes in parallel: the ϕ started to decrease and Lp increased slightly. In this way, Figure 9 indicates the strong correlation between the higher-order structural change represented by the shift of the 2θ halo peak, ϕ, and Lp and the stress generation process. A possible stress generation mechanism of supercontraction is discussed in the next section. 3.7. Interpretation of Stress Generation and the Related Hierarchical Structure Change. The irreversible and reversible structural changes occurring during the first stage of wetting and the second stage or the cyclic dry/wet procedures may be interpreted qualitatively at least by focusing on the change of the molecular chain segments included in the amorphous region. The key points necessarily for the interpretation of the stress generation are the following three. (i) The mechanically tensioned structure: Some parts of the amorphous chain segments in the intact PVA sample may be highly tensioned because of the residual strain in the uniaxial stretching of PVA film. But, the relative content of such highly tensioned amorphous part may be low, and most of the amorphous chains are in a more relaxed state. In order to

Figure 9. (top) Time dependence of supercontraction stress time characteristic of the restrained PVA film during the first wetting process. (bottom) Changes of the structural parameters ϕ and Lp, evaluated from the SAXS data, and of the peak position of the WAXD halo. The SAXS and WAXD data were obtained by utilizing the synchrotron radiation X-ray beam.

distinguish these two kinds of amorphous states, i.e., the highly tensioned and more relaxed states, we call them the tautamorphous and normal-amorphous, respectively, in the following discussion. (The concept of taut tie chain is well-known in the interpretation of the mechanical property of crystalline polymer, the content of which is quite small, but it plays an important role in the analysis of the heterogeneous stress distribution in the sample subjected to a tensile force.26−31 This concept is hypothetical but supported by the various kinds of experimental evidence: the interpretation of the mechanical deformation of the crystal lattice,26−30 the structural evolution in the isothermal crystallization process from the melt,31 and so G

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“contraction” stresses are generated correspondingly under the constrained condition of the sample. In this way, the contribution of the structure change in the amorphous region is quite important for the interpretation of the irreversible supercontraction in the first stage and the cyclic generation of the stress in the second stage. Let us now see the individual stages in more concrete way. Second Stage. As already mentioned, the hierarchical structure changes in these processes. The contraction of the highly tensioned taut-amorphous chain segments may induce the remarkable decrement of the long period (Lp) of the stacked lamellae. Contrarily, the swelling of the higher-order structure may give the increase of the long period. The evaluation of the long period in the second stage or in the cyclic dry/wet process shows this situation clearly: the decrease of Lp in the dry state and the increase of Lp in the wet state (see Figure 8). Correspondingly the generation of the contraction stress and the expansion stress is observed well. These processes occur reversibly. First Stage. On the other hand, the first process or the wetting of the intact sample causes the contraction of the bulk sample length (under the free end condition) or the generation of contraction stress (under the constraining condition). At the same time, the SAXS data revealed the increase of Lp. These apparently contradicted phenomena may be interpreted rationally in the following way by assuming the heterogeneous geometrical change in the amorphous region. Here we assume some parts of the amorphous region in the intact PVA sample are abnormally largely tensioned while the other parts are in the normal relaxed state as illustrated in Figure 10a. That is to say, the original sample is assumed to contain the heterogeneous structure distribution of the amorphous region. The sample length is expressed as below. The bulk sample length

on. In the present case, also, the situation is similar. The introduction of such taut chains in the amorphous region helps us to interpret the phenomenon observed here rationally. The experimental extraction of such hypothetical taut chains is quite difficult at present.) (ii) The water-swollen structure: Once the water molecules penetrate into the amorphous region and break some content of intermolecular hydrogen bonds between the networked amorphous chains. The amorphous chain segments in the normal state are expanded due to the swelling by absorbing water. (iii) The dried structure: the amorphous chain segments are contracted by desorbing water. When the uniaxially oriented PVA sample is subjected to the water vapor atmosphere, the originally highly tensioned tautamorphous chain segments are relaxed and transform to the normal swollen structure (ii). That is the mechanically tensioned taut amorphous chain segments are forced to contract drastically. However, because both ends of the sample is constrained strongly, the stress (contraction stress) is generated as a result. This phenomenon was named a supercontraction in the above sections. Figures 10a and 10b

L = Lc + La(normal) + La(taut)

(11)

This equation expressed the heterogeneous contribution of the individual components, but is not necessarily the mechanical series model. The corresponding strain is given by ε = ΔL /L = (ΔLc + ΔLa(normal) + ΔLa(taut))/L

Figure 10. A structural change model of the stacked-lamellar structure induced by the first wetting ((a) to (b)) and the subsequent cyclic wet/dry processes ((b)/(c) to (c)/(b)). The irreversible structural change from (a) to (b) and the reversible one from (b)/(c) to (c)/(b) correspond each to supercontraction and cyclic-contraction phenomena, respectively. The intact stacked-lamellar structure (a) is composed of two kinds of oriented amorphous that is of normal oriented one and of taut one. In the first wetting process, while the normal-oriented amorphous expands by swelling (La(normal) ⇒ La(swollen)), the taut amorphous shrinks (La(taut) ⇒ La(swollen)), resulting in the generation of the supercontraction stress. In the subsequent cyclic wet/dry processes, the dimension of amorphous phase changes reversibly (La(swollen) ⇔ La(dried)), resulting in the generation of cyclic contraction stress.

= (ΔLc /Lc)(Lc /L) + (ΔLa(normal)/La(normal))(La(normal)/L) + (ΔLa(taut)/La(taut))(La(taut)/L)

= εcXc + εa(normal)Xa(normal) + εa(taut)Xa(taut)

(12)

In the present experiment, the sample length is fixed and so the corresponding stress is generated as σ = E bε = E b[εcXc + εa(normal)Xa(normal) + εa(taut)Xa(taut)]

= σc + σa(normal) + σa(taut) (normal)

(13)

(taut)

The Xc, Xa , and Xa are the fractions of the crystalline, normal amorphous, and taut-amorphous regions, respectively, and Xc + Xa(normal) + Xa(taut) = 1. The absorption of water causes the expansion of the normal amorphous region (εa(normal) > 0) and the contraction of the taut-amorphous region (εa(taut) < 0). The crystalline region is almost constant irrespective of the wetting and drying states (εc ∼ 0), as supported by the abovementioned X-ray scattering data. Then, we have the following equations.

show this situation concretely. When this swollen sample is dried, the water molecules are taken out of the amorphous region, and then the molecular chain segments are shrunk to the dried structure (iii) (see Figure 10c). This structure returns to the wet and swollen structure (ii) by absorbing water again, and the stress is relaxed. The transformation between these two structures (ii) and (iii) occurs reversibly during the cyclic wet/ dry conditional changes, and the “relaxed (or expansion)” and H

DOI: 10.1021/acs.macromol.6b02523 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Under the end free condition ε ≈ εa(normal)Xa(normal) + εa(taut)Xa(taut)

The water swelling of the normal amorphous region causes the expansion of the whole sample just like an aff ine deformation, and the reversible change of the stress and long period may occur as actually observed in the second stage (see Figure 8). Not only the long period but also the lamellar tilting angle is changed correspondingly. The thus-predicted changes of the long period and lamellar tilting angle are shown in Figure 10, which are in good correspondence to the actually observed data. (It might be interesting to point out the difference in the stress change rate between the drying and wetting procedures, as shown in Figure 3. The above-mentioned structural change occurs in parallel to the diffusion and evaporation of water molecules in the PVA film. As a future work, we need to clarify the relationship between the rate of the structural change proposed in Figure 10 and the stress change rate in both the drying and wetting processes in association with the migration behavior of water molecules.) 3.8. Hydration State. The remarkable difference in the stress generation between the intact and dried states or between the supercontraction and cyclic contraction is considered to come from the difference in the aggregation state of the stacked lamellae and the amorphous phase sandwiched between them. Especially the (assumed) presence of the taut-amorphous region is important in the first wetting process, which is created in the sample preparation process by the uniaxial drawing of the cast film. Such a difference of the higher-order structure in the first and second stages may reflect also on the hydration state in the first and cyclic wetting processes. This can be investigated by the TGA data for the samples picked up from the intact, wetted, and dried stages. The TGA thermograms for the intact, wet, and dried oriented samples are shown in Figure 11. Similar curves are observed for the wet

(14)

Under the fixed end condition σ ≈ E b[εa(normal)Xa(normal) + εa(taut)Xa(taut)] = σa(normal) + σa(taut)

(15)

σa(normal) = E bεa(normal)Xa(normal)

(16)

σa(taut) = E bεa(taut)Xa(taut)

(17)

The SAXS data gave us an information on the long period which is the average between the long period coming from the normally stacked lamellar structure (or the repetition of the crystalline and normal amorphous regions) and that from the highly tensioned amorphous region (see Figure 10). The averaged long period Lp is given as Lp = Xa(normal)Lp(crystal‐normal amorphous) + Xa(taut)Lp(crystal‐taut amorphous)

(18)

where the notation Lp (crystal-normal amorphous) means the long period corresponding to the stacked lamellar structure with the normal amorphous region in between. The fraction Xa(taut) of the tensioned taut-amorphous region in the uniaxially oriented sample is speculated to be negligibly small, and then eq 18 becomes approximately Lp ≈ Xa(normal)Lp(crystal‐normal amorphous)

(19)

Since the normal amorphous region is swollen by wetting, the Lp (crystal-normal amorphous) is increased, and as a result the averaged long period increases in spite of the generation of the large contraction stress predicted for the highly tensioned tautamorphous region. In the derivation of the stress expressed in eqs 14 and 15, the contribution of the contraction strain εa(taut) must be taken into account although the content Xa (taut) is quite small as pointed out in eq 19. Since the contraction stress σa(taut) coming from the contraction of the highly strained and highly extended taut-amorphous chain segments is overwhelmingly large, the product between Ebεa(taut) and Xa(taut) cannot be neglected, resulting in the generation of the large contraction stress as a whole. Of course, the expansion stress originated from the swollen normal amorphous region may contribute also to some extent. σ ≈ σa(normal) + σa(taut) ≈ σa(taut)

Figure 11. TGA thermograms of intact, wet, and dried oriented PVA films. For the wet and dried samples, two different results are shown together for the different treatment time of 24 and 72 h of wetting and of 24 and 48 h of drying.

(if σa(normal) ≪ σa(taut)) (20)

Second Cyclic Stage. Once the PVA sample is gotten wet, the highly tensioned taut-amorphous region is mostly relaxed, and the normal expansion and contraction of the whole amorphous region are expected to occur by following the equations ε ≈ εa(normal)Xa(normal)

samples immersed in water for 24 and 72 h (blue curves), indicating that the wetting time of 24 h used in this study is enough long to achieve the fully hydrated state in the thick sample. (It must be noticed here that the PVA samples used in the present experiment were highly annealed during stretching at 150 °C, and they were quite tough against water even being immersed for 1−3 days in water.) The saturation of the dry state was also detected in a similar manner for the samples dried for 24 and 48 h (red curves). The details are read out of these curves. The wet sample, picked up from the cyclic wetting period, lost the water by about 15% in the temperature region up to 150 °C, which corresponds to the endothermic peak detected in the DSC

(under tension‐free condition) (21)

σ ≈ E bεa(normal)Xa(normal) = σa(normal)

(under fixed end condition) (22)

Lp ≈ Xa(normal)Lp(crystal‐normal amorphous)

(23) I

DOI: 10.1021/acs.macromol.6b02523 Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

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thermogram. On the other hand, the intact sample and the dried sample, extracted from the cyclic drying process, lost the water by 4.4% and 6.8%, respectively, in a similar temperature region. The existing state of these absorbed water can be speculated from the DSC thermograms: the intact and cyclically dried samples did not show any detectable thermal energy change in the temperature region of −30 to 10 °C, different from the case of the wet sample (Figure 5). This indicates the absorbed water may exist as a nonfreezing strongly bound water. Another important point is the difference in the water content between the intact (dry) and cyclically dried samples. The difference in the higher-order structure may cause the difference in the aggregation state of these strongly bounded water in the intact and cyclically dried samples. The details must be clarified by measuring the vibrational spectra or NMR spectra for these samples, which is a future work.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02523. S1: an example of cyclically changed relative humidity in the stress measurement; S2: a schematic drawing of the simultaneous SAXS/WAXD measurements of a PVA sample subjected under a humid atmospheric condition; and S3: comparison of the stress−strain curves of the oriented PVA sample under dry and wet conditions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.T.). *E-mail: yoshiokat@affrc.go.jp (T.Y.).

4. CONCLUSION

ORCID

We have found that the uniaxially oriented PVA films show two different stages of response to the wet/dry change under restrained conditions. While PVA films show the irreversible stress generation in the first wetting (supercontraction stress), in the second stage they show the reversible stress generation (cyclic-contraction stress) synchronizing with the cyclic humidity change. In addition, it has been successfully revealed that the higher-order structure changes significantly and reversibly during the cyclic stress generation. From a simple mechanical model and the time-resolved analyses of WAXD and SAXS data, we concluded that the irreversible and reversible stress generations are governed mainly by the structural changes of the highly tensioned taut-amorphous phase and the normal amorphous phase, respectively. In the former case, the hydration-induced structural change is irreversible, which is caused by the relaxation of the highly tensioned taut amorphous part. In the latter case the swelling and deswelling occur reversibly corresponding to the cyclic dry/wet condition changes. In addition, the TGA analysis revealed non-negligible increase of the strongly bound-water content in the first wetting process. This permanent change of the water content is considered to be related with the irreversible supercontractionstress generation. In section 3.7, the stress generation mechanism was described using the several equations based on the simple mechanical models for both of supercontraction and cycliccontraction stresses. These equations can emphasize the significant contribution of the amorphous region in the hydration-induced change of the mechanical property. It might be allowable to emphasize here that the detection of the higher-order structural changes in the supercontraction and cyclic-contraction processes of drawn samples was not reported for the silk fibers of spider and Bombyx mori silk warm. The knowledge obtained in the present study may give us a useful hint to discuss the mechanism of supercontraction and cyclic contraction of spider dragline silk and regenerated silk fibroin as well. The cyclical stress generation is expected to be applicable for the various kinds of application fields, such as actuator, sensor, and biomedical applications of such conventional polymers as PVA.

Kohji Tashiro: 0000-0002-7543-2778 Present Address

T.Y.: National Agriculture and Food Research Organization (NARO), 1-2 Ohwashi, Tsukuba, Ibaraki 305-8634, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synchrotron radiation experiments were performed at the BL40B2 of SPring-8 with the approval of Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2013A1285 and 2013B1475). This study was supported financially by MEXT “Strategic Project to Support the Formation of Research Bases at Private Universities (2010− 2014 and 2015−2019)”.

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K

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