Construction of Dual Orientation Crystalline Structure in Poly(vinyl

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 10908−10921

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Construction of Dual Orientation Crystalline Structure in Poly(vinyl alcohol)/Graphene Oxide Nano-Composite Hydrogels and Reinforcing Mechanism Yeqiao Meng, Xiaowen Zhao, and Lin Ye* State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China

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ABSTRACT: Oriented poly(vinyl alcohol) (PVA)/graphene oxide (GO) nanocomposite hydrogels with different draw ratios were prepared by stretching during a freezing/thawing process. The PVA/GO samples with a draw ratio of 200% had significantly enhanced mechanical properties by formation of a microfibrillar structure, while the tensile strength and compressive modulus respectively reached as high as 11.60 and 4.05 MPa, outperforming the unoriented samples with an increase of 167% and 540%, respectively. During the freezing/ thawing process under stress, the orientation factor, crystallinity, and cross-linking density (υe) of PVA/GO samples increased steadily, while the regularly arranged fibrillar bundle structure increasingly formed, and the size of the porous structure decreased. Besides that, additional PVA crystals formed with decreasing long period (L), crystalline region (Lc), and lamellae size (Llateral). Due to the synergic orientation of GO sheets and nucleation effect, the oriented PVA/GO samples showed an orientation factor, crystallinity, and υe that were higher with lower crystal size in comparison with PVA samples. Thus, the dense dual orientation crystalline network structure formed, and the highly reinforcing PVA/GO hydrogels were achieved. properties.14−16 Kazuhiro et al. prepared a robust polyacrylamide hydrogel reinforced by oriented imogolite, and the tensile strength of which was improved from about 80 kPa to more than 250 kPa.17 Morales et al. reported the self-shaping poly(N-isopropylacrylamide) hydrogel sheets with oriented polystyrene particles by using an electric-field-induced strategy.18 By linearly remodelling under drawing and fixing it with ionic cross-linking of chains of alginate, Choi et al. prepared anisotropic Ca-alginate/polyacrylamide hydrogel with improved tensile strength (almost 700 kPa) and elastic modulus (1100 kPa).19 However, the mechanical strengths of the reported oriented hydrogels were far less than expected, and few literature reports on the PVA hydrogels with an oriented structure are available. In our previous study, PVA/graphene oxide (GO) nanocomposite hydrogel was prepared via a freezing/thawing process.13 The distinctive properties of GO, such as its extremely high mechanical strength, excellent lubricating property, and high load-bearing capability, were achieved by exfoliation and uniform distribution of GO in PVA matrix due to an efficient graft of PVA molecules into GO layers by

1. INTRODUCTION Cartilage is a viscoelastic connective tissue covering the bone end in diarthrodial joints, providing efficient aqueous lubrication systems with both high load-bearing and lowfriction properties.1−5 However, due to its avascular nature and very poor self-repair ability, the cartilage is hard to heal once a lesion occurs, and the development of cartilage replacement is quite urgent in clinics at present.6,7 Poly(vinyl alcohol) (PVA) hydrogel, with an excellent biocompatibility, high mechanical property, and low friction coefficient, is regarded as a promising candidate for cartilage replacement which is required to sustain high loading and comparable biocompatibility and lubricating properties.8−11 However, PVA hydrogels normally possess inadequate mechanical and wear-resistant properties for supporting the loads present in joints, thereby limiting their wide applications. Although recent efforts have been made on enhancing the mechanical properties of PVA hydrogel by incorporation of PVA with rigid nanomaterials, such as hydroxyapatite (HA), carbon nanotubes (CNT), and montmorillonite (MMT), it is still a big challenge for them to meet the demanding properties in cartilage applications.12,13 Inspired by super tough tissues with ordered structures in cartilage and skeletal muscle in human bodies, presenting highly anisotropic mechanical strength and functionality, researchers have increasingly studied ordered hydrogels as biological soft materials with highly enhanced mechanical © 2019 American Chemical Society

Received: Revised: Accepted: Published: 10908

April 3, 2019 May 15, 2019 June 3, 2019 June 3, 2019 DOI: 10.1021/acs.iecr.9b01819 Ind. Eng. Chem. Res. 2019, 58, 10908−10921

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic illustration of the preparation process of oriented PVA and PVA/GO hydrogels via a freezing/thawing process.

pared as follows. First, 0.288 g of GO was sonicated in 100 mL of deionized water. Next, 0.230 g of PEG was then added, and the solution was sonicated at 70 °C for 1 h to obtain GO-PEG hybrid. Afterward 19.04 g of PVA was added, and the mixture was stirred at 95 °C until the PVA dissolved to obtain the PVA/GO solution. The PVA/GO solution was then poured slowly into a mold, and PVA/GO hydrogel sheets with a 2 mm thickness and cylinders with a 15 mm diameter were formed after one freezing/thawing process (12 h of freezing at −20 °C and 4 h of thawing at room temperature), denoted as PVA/GO-0-1. Afterward, PVA/GO-0-1 hydrogels were mechanically stretched to different draw ratios and fixed onto a customized holder to maintain the deformation of the hydrogels, following another 4 times of the freezing/thawing processing to form the oriented PVA/GO nanocomposite hydrogels with different draw ratios. The oriented sample was denoted as PVA/GO-x-y, respectively (x represented the draw ratio, while y represented the number of freezing/thawing times). For comparison, PVA hydrogels were prepared in the absence of GO hybrid, which was denoted as PVA-x-y via the same method. The draw ratio of the samples was calculated with following equation:

introducing polyethylene glycol (PEG), thereby obtaining a new material for cartilage replacement with combination of high mechanical property and excellent lubrication.20−22 In this study, the oriented PVA/GO hydrogels with different draw ratios were produced by stretching during a freezing/thawing process, and GO sheets were expected to form a joint orientation with the PVA matrix for further reinforcing PVA hydrogels. The influence of the draw ratio on the mechanical properties of oriented PVA and PVA/GO hydrogels was studied in comparison with unoriented samples. Meanwhile, the formation of dual orientation structure and the crystalline network structure in oriented PVA/GO hydrogels during the freezing/thawing process was studied, and the structure evolution and reinforcing mechanism were explored.

2. EXPERIMENTAL SECTION 2.1. Materials. PVA with an Mn value of 74800 g/mol was provided by Sichuan Vinylon Co. (China). GO with a micrometer grade size was provided by the Sixth Element Materials Technology Co., Ltd. (Changzhou, China). PEG with a molecular weight of 20000 g/mol was purchased from Kelong Chemical Reagents Co., Ltd. (Chengdu, China). 2.2. Preparation of Oriented PVA and PVA/GO Nanocomposite Hydrogels. PVA/GO solution was pre10909

DOI: 10.1021/acs.iecr.9b01819 Ind. Eng. Chem. Res. 2019, 58, 10908−10921

Article


Figure 2. Stress−strain curves (a, b) and tensile mechanical properties (c) of oriented PVA and PVA/GO hydrogels with different draw ratios.

draw ratio (%) =

L − L0 × 100% L0

were measured with a cylinder shape of 8 mm (height) × 15 mm (diameter) at 1 mm/min. 2.3.2. Two-Dimensional Wide-Angle X-Ray Diffraction Analysis (2D-WAXD). Two-dimensional wide-angle X-ray diffraction (2D-WAXD) analysis of PVA-based hydrogel samples was conducted with a D8 Discover two-dimensional wide-angle X-ray diffractometer (2D-WAXD) (Bruker AXS Co., Germany). The sampling time was 180 s, and Cu (Kα) radiation was applied. 2.3.3. Two-Dimensional Small-Angle X-Ray Scattering Analysis (2D-SAXS). Two-dimensional small-angle X-ray scattering analysis of PVA-based hydrogel samples was performed with a Xeuss 2.0 system from Xenocs, France,

(1)

where L0 is the length of the samples prior to drawing and L is the final length of the samples after drawing. The water content of the PVA-based hydrogels were in the range of 79∼81 wt %. 2.3. Measurements. 2.3.1. Mechanical Properties. The tensile properties of PVA-based hydrogel samples were measured with a 4302 material testing machine from Instron Co. (USA) according to ISO527/1-1993 (E). The samples with a dumbbell shape and a size of 75 × 4 × 2 mm3 were stretched at 50 mm/min (23 °C). The compressive properties 10910


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Figure 3. Unconfined compressive properties of oriented PVA and PVA/GO hydrogels with different draw ratios.

and Cu (Kα) radiation was applied. Data processing was performed with Fit2D 2012 software. 2.3.4. Rheological Property. The viscoelasticity property of PVA-based hydrogel samples was analyzed with a Gemini 200 Rheometer System from Malvern Instrument Co. (U.K.) at 30 °C in the linear viscoelastic regime, with frequencies of 0.1− 100 Hz, at a maximum strain, γ, of 0.1%. 2.3.5. Scanning Electron Microscope Analysis (SEM). The fractured surface morphology of PVA-based hydrogel samples was observed with a JEOL JSM-5900LV scanning electron microscope (SEM, JEOL Co., Japan) with an acceleration voltage of 20 kV. The samples were freeze-dried, cryogenically fractured in liquid nitrogen, and finally sputter-coated with gold. 2.3.6. Transmission Electron Microscopy (TEM) Analysis. The TEM image of PVA-based hydrogel samples was observed on a JEOL JEM 100CX II TEM instrument (Japan) at an acceleration voltage of 200 kV. The samples were obtained by cryogenic thin sections.

mechanical properties are shown in Figure 2(c). All samples exhibited untypical stress yield behavior, and no yield point was observed. Notably, with the increase of tensile strain, a strain-hardening behavior can also be observed due to orientation of polymer chains along drawing direction. As shown in Figure 2(c), the PVA-0%-5 sample showed a very low tensile strength (1.78 MPa), a very low tensile modulus (1.96 MPa), and a relatively low elongation at break (171%), while the PVA/GO-0%-5 sample showed an improved elongation at break (248%) but still a low tensile strength (4.33 MPa) and tensile modulus (2.17 MPa). Only a flat section could be observed in SEM micrographs of cryogenically fractured surfaces of both PVA-0%-5 and PVA/GO-0%-5 samples. Apparently, the orientation brought PVA and PVA/GO samples with significant improvement in mechanical properties, showing a tensile strength and tensile modulus up to 6.13 and 5.40 MPa for PVA and 11.60 and 8.18 MPa for PVA/GO, respectively. Obviously, microfibers arranging along the drawing direction were observed in SEM micrographs of both PVA-150%-5 and PVA/GO-200%-5 samples, contributing to the significantly enhanced mechanical properties of the oriented samples.23 Notably, for the PVA and PVA/GO samples prepared with relatively low draw ratios (50% and 100%), the elongation at break showed surprising promotion, indicating successful achievement of simultaneously strengthening and toughening properties. With further increasing the draw ratio, the elongation at break of both PVA and PVA/GO samples decreased, while for PVA samples, it still remained relatively higher than the unoriented samples. Besides, the tensile strength and tensile modulus of PVA/GO samples were superior to that of PVA samples with the same draw ratio, owing to the coexistence of orientated GO.

3. RESULTS AND DISCUSSION 3.1. Preparation and Mechanical Properties of Oriented PVA/GO Hydrogels. In order to obtain PVA and PVA/GO samples with different oriented structures, stress was applied to the hydrogels after one time of the freezing/thawing process, which led to the regular arrangement of PVA molecular chains along the drawing direction, and the oriented PVA and PVA/GO samples with different draw ratios were achieved (as shown in Figure 1). For PVA samples, the maximun draw ratio only reached 150%, while for PVA/GO samples, the maximun draw ratio of 200% could be achieved. The tensile stress−strain curves of the oriented PVA and PVA/GO composite hydrogels with different draw ratios are shown in Figure 2(a) and (b), and the obtained tensile 10911


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Industrial & Engineering Chemistry Research Unconfined compressive properties of the oriented PVA and PVA/GO hydrogels with different draw ratios were illustrated in Figure 3(a−c). Compressive stress−strain curves of all samples increased exponentially with the rise of compressive strain, displaying a nonlinear stress−strain response.24 As shown in Figure 3(c), with the increase of draw ratio, the compressive strength and modulus of PVA and PVA/GO hydrogels increased dramatically, reaching up to 2.99 and 1.83 MPa for PVA-150%-5 and 5.40 and 4.08 MPa for PVA/GO200%-5, respectively, outperforming the unoriented samples with an increase of 393% and 361% for PVA-150%-5 and 377% and 540% for PVA/GO-200%-5, respectively. 3.2. Formation of Dual Orientation Structure in PVA/ GO Hydrogels Oriented during the Freezing/Thawing Process. For investigating the formation mechanism of the orientation structure in PVA and PVA/GO hydrogels, the structure evolution of oriented PVA and PVA/GO samples with the draw ratio of 150% during the process of 2∼5 times of freezing/thawing was studied. For comparison, the unoriented PVA and PVA/GO samples were also studied. The 2D-WAXD images of PVA and PVA/GO hydrogels oriented during the freezing/thawing process are shown in Figure 4(a). Obviously, the isotropic PVA and PVA/GO samples with only one time of freezing/thawing did not show any Debye−Scherrer diffraction rings due to low crystallinity and random arrangement of PVA crystal lamellas and GO sheets. For the oriented samples, the diffraction planes (101) of crystalline PVA appeared as diffraction arcs on the meridian.25 With increasing freezing/thawing times, the diffraction arcs became increasingly prominent, indicating PVA crystal axis was oriented perpendicularly to the drawing direction. The orientation factor (F) was calculated with Hermans’s functions: 3 < cos2ϕ > −1 2

F=

2

⟨cos ϕ⟩ =

∫0

π /2

∫0

(2)

I(ϕ) sin ϕ cos2ϕ dϕ π /2

I(ϕ) sin ϕ dϕ

Figure 4. 2D-WAXD patterns (a) and orientation factor (b) of PVA and PVA/GO hydrogels oriented during a freezing/thawing process.

(3)

where I(ϕ) is the scattering intensity along angle ϕ. When the crystals are oriented with the direction parallel to the reference axis, F = 1; when the crystals are oriented with the direction perpendicular to the reference axis, F = −0.5, and for a random orientation, F = 0. Orientation factors of PVA and PVA/GO hydrogels oriented during the freezing/thawing process were calculated with eqs 2 and 3, as shown in Figure 4(b). Before orientation, PVA and PVA/GO samples prepared with only one time of freezing/ thawing showed an extremely low F value (0.013 and 0.019, respectively), owed to the randomly arranged PVA crystals and GO sheets. For the oriented samples, with the increase of freezing/thawing times, the F value of PVA and PVA/GO samples exhibited a significant increase from 0.013 and 0.019 to 0.31 and 0.38 for the sample prepared by five times of freezing/thawing, respectively, indicating steady improvement of the orientation degree in the further process of freezing/ thawing. Moreover, in comparison to the oriented PVA samples with the same freezing/thawing times, the oriented PVA/GO samples showed a higher orientation factor.

The TEM micrographs of PVA/GO-0%-5 and PVA/GO150%-5 hydrogels are shown in Figure 5. For PVA/GO-0%-5, GO sheets were randomly dispersed PVA matrix. As expected, for PVA/GO-150%-5, the permanent deformation of the PVA matrix caused by tensile stress induced GO sheets to array orderly along the drawing direction simultaneously, resulting in a higher orientation factor of PVA/GO samples in Figure 4(b). 3.3. Formation of the Crystalline Network Structure in PVA/GO Hydrogels Oriented during the Freezing/ Thawing Process. The XRD diffraction analysis of PVA and PVA/GO hydrogel samples oriented during the freezing/ thawing process are shown in Figure 6(a−d). The diffraction profiles of all samples exhibited a halo centered at 2θ ≈ 28°, forming from water in the samples, while a weak peak in the 2θ range of 18−21° corresponded to the diffraction planes (101) of crystalline PVA.26 For the oriented PVA and PVA/GO samples (Figure 6(c, d)), with increasing freezing/thawing times, the peak intensity of the PVA diffraction planes (101) increased obviously, while unoriented PVA and PVA/GO samples only showed an inconspicuous increase in the peak 10912


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Figure 5. TEM micrographs of PVA/GO-0%-5 and PVA/GO-150%-5 hydrogels.

Figure 6. 1D-WAXD curves of PVA (a, c) and PVA/GO (b, d) hydogels unoriented and oriented during the freezing/thawing process, and the Xc (e, f) results of PVA and PVA/GO hydrogels unoriented and oriented during the freezing/thawing process.

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Figure 7. 2D-SAXS patterns of PVA and PVA/GO hydrogels unoriented and oriented during the freezing/thawing process.

intensity of PVA diffraction planes (101) (Figure 6(a, b)). Moreover, especially for PVA/GO-150%-4 and PVA/GO150%-5 samples, diffraction peaks of GO in the 2θ range of 10−13° appeared, attributed to the orderly arrangement of GO sheets simultaneously. The crystallinity (Xc) of PVA and PVA/GO hydrogels unoriented and oriented during the freezing/thawing process were evaluated from Figure 6(a−d) as the ratio of the diffraction planes (101) to the whole diffraction of the sample.30 As shown in Figure 6(e, f), with increasing freezing/ thawing times, the Xc of unoriented PVA and PVA/GO samples both increased from 0.11% and 0.51% for PVA-0%-1 and PVA/GO-0%-1 to 2.72% and 4.01% for PVA-0%-5 and PVA/GO-0%-5, respectively. Similarly, for the oriented PVA and PVA/GO samples, with increasing freezing/thawing times, Xc increased dramatically to 7.72% for PVA-150%-5 and 10.1% for PVA/GO-150%-5. Moreover, in comparison to that of unoriented PVA and PVA/GO samples prepared with the same freezing/thawing times, oriented PVA and PVA/GO samples showed a much higher value of Xc, indicating the contribution of the orientation effect to crystallization of PVA during the freezing/thawing process. Additionally, GO sheets serving as a crystal nucleus promoted the crystal growth of PVA molecules, resulting in the higher crystallinity of PVA/ GO in comparison to PVA with the same draw ratio and freezing/thawing times. In order to reveal the growth of PVA crystallization during the freezing/thawing process under stretching stress, the 2DSAXS analysis of PVA and PVA/GO hydrogels were

performed, as shown in Figure 7. As for both unoriented PVA and PVA/GO samples, only isotropic scattering rings could be observed, revealing the nearly amorphous state, and with increasing freezing/thawing times, hardly any changes could be observed. For the oriented PVA and PVA/GO samples, with increasing freezing/thawing times, initial isotropic SAXS pattern gradually became two-bar-like on the meridian and ultimately shaped the spindly patterns, which could be indicative of increasing lamellar stacks generating perpendicularly to the drawing direction.27 The structural parameters of lamellae were calculated from the 2D-SAXS analysis and discussed. The scattering intensity of the samples (I(θ)) was modified by the Lorentz correction, as shown in Figure 8(a−d). Generally, the peaks were suggestive of the ordered structures in the samples. The long period (L) of the oriented samples could be calculated with the Bragg equation: 2L sin θ = nλ

(4)

where θ is the diffraction angle; n is the order of diffraction, and λ is the incident wavelength. As shown in Figure 8 (e) and (f), after one time of freezing/ thawing, the value of L of PVA-0%-1 and PVA/GO-0%-1 was 10.98 and 10.15 nm, respectively. With increasing freezing/ thawing times, the value of L for the unoriented PVA and PVA/GO samples decreased slightly, owing to the increasingly compact crystalline lamellar stacks. In comparison, PVA-150%2 and PVA/GO-150%-2 showed an extremely high L value, owing to the deformation of PVA crystals caused by stretching. Likewise after orientation, the increase of freezing/thawing 10914


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Figure 8. 1D-SAXS curves (a−d) and L values (e, f) of PVA and PVA/GO hydrogels unoriented and oriented during the freezing/thawing process.

times contributed to reduction in the value of L for both oriented PVA and PVA/GO samples. Additionally, compared with oriented PVA samples, the oriented PVA/GO samples had a much smaller L value, owing to the denser crystallization structure of PVA/GO samples promoted by GO sheets. The electron density correlation function K(z) is derived from the inverse Fourier transformation of the experimental intensity distribution I(q) as follows:28,29

the crystalline (Lc) and amorphous region (La) could be derived from following equation, as shown in Figure 9(e) and (f): La = Lac − Lc

where Lc equaled the value of z of at the intersection and Lac was the long period calculated from the K(z) curves, equaled to the value of z at the peak following the minimum point (pointed to by arrows in Figure 9(a−d)). As shown in Figure 9(e) and (f), for PVA-0%-1 and PVA/ GO-0%-1, Lc/La were 2.59 nm/8.59 nm and 2.44 nm/8.08 nm, respectively. With increasing freezing/thawing times, the value of Lc/La for the unoriented PVA and PVA/GO samples showed a slight decrease, only sliding to 2.30 nm/4.60 nm for PVA-0%-5 and to 2.22 nm/4.48 nm for PVA/GO-0%-5. Similarly, after stretching, the Lc/La of both PVA-150%-2 and PVA/GO-150%-2 increased conspicuously. As the freezing/ thawing process increased from 2 to 5 times, the Lc/La of both oriented PVA and PVA/GO samples decreased steadily, ending at 7.02 nm/10.13 nm for PVA-150%-5 and at 6.38

∞

K (z ) =

∫0 I(q) q2 cos(qz) dq ∞

∫0 I(q) q2 dq

(6)

(5)

where z is perpendicular to the loading direction and I(q) is obtained by integrating along the equator. The resultant K(z) curves of unoriented and oriented PVA and PVA/GO samples are shown in Figure 9(a−d). After heading down straightly at the beginning, K(z) curves reached the minimum and then climbed slowly to attain the following peak. The straight-line fit of the declining region of K(z) had an intersection with the horizontal baseline of the minimum point, and the thickness of 10915


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Figure 9. K(z) curves (a−d) and parameters of the inner crystalline structure (e, f) in PVA and PVA/GO hydrogels unoriented and oriented during the freezing/thawing process.

The scattering vectors along and perpendicular to the stretching direction were defined as qx and qy, as shown in the upper left corner of Figure 10(a−d). The size of the lamellae (Llateral) can be derived from the scattering intensity distribution (I(qy)) along the qy direction. The width of the resulting Lorentz function Δqy (the width of the peak at half height of the I(q)−q curves) was related to the Llateral value

nm/5.85 nm for PVA/GO-150%-5, indicating formation of a dense stack of crystalline lamellas. Meanwhile, compared with unoriented PVA and PVA/GO samples, the oriented samples had a much higher Lc and La, owing to the deformation of PVA matrix caused by stretching. Additionally, compared with PVA samples, the PVA/GO samples had a relatively smaller Lc and La, resulting from the denser crystallization structure of PVA/ GO samples promoted by GO. 10916


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Figure 10. Azimuthal scan of the lamellar peaks along qy (a−d) and Llateral (e, f) of PVA and PVA/GO hydrogels unoriented and oriented during the freezing/thawing process.

according to Llateral = 2π/Δqy,23 and the obtained results are shown in Figure 10(e) and (f). Llateral values of PVA and PVA/GO samples oriented during the freezing/thawing process are shown in Figure 10(e) and (f). With increasing freezing/thawing times, the Llateral value of unoriented PVA and PVA/GO samples decreased steadily. Unexpectedly, the Llateral of the PVA-150%-2 and PVA/GO150%-2 increased rather than exhibited a decrease in comparison to that of PVA-0%-1 and PVA/GO-0%-1, which indicated that the crystalline lamellas did not tear up but rearranged orderly from disordered arrangement after stretching. After that, with increasing freezing/thawing times (2∼5 times), Llateral of oriented samples decreased; although, after 5 times of freezing/thawing, the oriented PVA and PVA/GO samples still showed a higher value of Llateral in comparison to unoriented samples, attributed to the deformation of PVA matrix after stretching. Besides, with the same freezing/ thawing times and draw ratio, PVA/GO samples showed a

slightly lower value of Llateral than that of PVA samples, owing to the high crystallinity improved by GO. Above all, the obtained crystalline parameters of PVA and PVA/GO samples indicated that the higher crystallinity of the oriented samples promoted by orientation could help the gradual formation of the crystalline structure with decreasing small size and increasing dense stack during the freezing/ thawing process. As shown in Figure S1, for all samples, G′ was higher than G′′ and independent of frequency in the range of 0.1−100 Hz, indicating the formation of a network structure.21 With increasing freezing/thawing times, all of the samples showed a steadily increased G′. The value of υe and Mc of the network for PVA and PVA/ GO hydrogels unoriented and oriented during the freezing/ thawing process are shown in Figure S2. Obviously, for unoriented PVA and PVA/GO samples, with increasing freezing/thawing times, υe increased dramatically, while Mc declined sharply, owing to the increasing crystallinity during 10917


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Figure 11. SEM micrographs of cryogenically fractured surfaces for PVA and PVA/GO hydrogels oriented during the freezing/thawing process ((A−J) at magnification of 500×; (a−j) at magnification of 10000× )

only a flat section could be observed. Contrarily, after orientation, rather than flat sections, the arranged fibrillar structure appeared in the oriented samples regardless of freezing/thawing (Figure 11(B−E) and (G−J)), which was mainly composed of highly oriented lamellar crystals of PVA and amorphous parts oriented along the drawing direction. With increasing freezing/thawing times, the arranged fibrillar structure tended to be increasingly distinct. It was worth noting that the fibrillar bundle structure strongly depended on the freezing/thawing times, and it became more distinct as the freezing/thawing times increased. The fibrillar structure showed apparent fractures in samples of PVA-150%-5, owing to the brittle matrix of PVA, whereas no obvious fractures appeared in the GO-reinforced PVA/GO-150%-5 samples. The fibrillar structure had been studied using a magnification of 10000× , as shown in Figure 11(a−j). PVA-0%-1 and PVA/GO-0%-1 both exhibited a porous structure with a large size of network pores. As expected, after orientation, for PVA-

the freezing/thawing process. Likewise, with increasing the freezing/thawing process, the same trend was found in υe and Mc of oriented PVA and PVA/GO samples. Compared with that of unoriented samples, due to the higher crystallinity of the oriented samples promoted by orientation, the oriented PVA and PVA/GO samples showed a relatively higher value of υe and, certainly, a lower value of Mc. Besides, with the same freezing/thawing times and draw ratio, the higher value of υe in PVA/GO samples in comparison to that of PVA samples was attributed to the higher crystallinity of PVA/GO samples which could promote the formation of cross-linking points for hydrogels. Above all, both the process of freezing/thawing and dual orientation could lead to the formation of increasing denser network structure. The SEM micrographs (magnification of 500× ) of cryogenically fractured surfaces of oriented PVA and PVA/ GO samples oriented during the freezing/thawing process are shown in Figure 11(A−J). For PVA-0%-1 and PVA/GO-0%-1, 10918


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Figure 12. Schematic illustration of structure evolution of PVA and PVA/GO hydrogels oriented during the freezing/thawing process.

bundle structure formed, while the size of porous structure decreased apparently. Besides, more and more PVA crystals formed, and the increase of the already formed crystalline region led to the decreasing residual region to form new PVA crystals, resulting in smaller PVA crystals with lower L, Lc, La, and Llateral values, indicating the formation of an increasingly dense network structure with many oriented microfibers, a dense stack of crystalline lamellas, and small-sized pores. Compared with the unoriented samples, the oriented samples had a much higher orientation degree, crystallinity, and υe. Additionally, compared with the oriented PVA sample, for the oriented PVA/GO samples, the synergic orientation of GO sheets and nucleation effect resulted in a higher orientation factor, crystallinity, and υe and lower L, Lc, La, and Llateral values. The dense dual orientation crystalline network structure formed, thereby achieving the significantly enhanced mechanical properties for oriented PVA/GO hydrogels.

150%-2 and PVA/GO-150%-2, microfibers appeared and arranged regularly along the drawing direction. With increasing freezing/thawing times, both for the oriented PVA and PVA/ GO samples, the microfibers arranged increasingly denser, while the size of the porous structure decreased apparently, which further indicated the increasing of the cross-linking density during the freezing/thawing process. In comparison, the oriented PVA/GO samples showed a much denser porous structure and regularly arranged structure. When 5 times of freezing/thawing were employed, obvious microfibers aligning along the drawing direction were observed in oriented PVA and PVA/GO samples, contributing to the significantly high mechanical properties of the samples. 3.4. Structure Evolution and Reinforcing Mechanism of Oriented PVA/GO Hydrogels during the Freezing/ Thawing Process. The schematic illustration of structure evolution of PVA and PVA/GO hydrogels oriented during the freezing/thawing process is shown in Figure 12. Before freezing/thawing, no crystalline structure existed in PVA and PVA/GO samples and the PVA molecules could move independently. After one time of freezing/thawing, very few PVA crystals formed and arranged disorderly, and PVA samples showed very low crystallinity, resulting in a loose network structure with a large size of network pores. After two times of freezing/thawing, by applying stress to the hydrogels, the random PVA molecular chains were stretched and arrayed orderly along the drawing direction, resulting in a much higher orientation degree and crystallinity. Meanwhile, some oriented molecules formed a crystalline phase, and some of them formed a mesomorphic phase with an extended-chain conformation, resulting in an increase of La, Lc, and Lac values (L). With further increasing freezing/thawing times, the orientation factor, crystallinity, and υe of PVA and PVA/GO samples increased steadily. The regularly arranged fibrillar

4. CONCLUSIONS In the present work, PVA and PVA/GO hydrogels with different draw ratios were prepared by stretching during a freezing/thawing process. The orientation brought PVA and PVA/GO samples with a significant improvement in mechanical properties by formation of a microfibrillar structure. During the freezing/thawing process under stress, the samples showed a steadily increased orientation factor, crystallinity, and υe. The regularly arranged fibrillar bundle structure formed and microfibers arranged increasingly denser, while the size of the porous structure decreased. Besides, with the increase of freezing/thawing times, more and more PVA crystals formed, and the increase of the already formed crystalline region led to the decreasing residual region to form new PVA crystals, resulting in the smaller size of PVA crystals 10919


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with lower L, Lc, La, and Llateral values, indicating the formation of an increasingly dense network structure with very oriented microfibers, a dense stack of crystalline lamellas, and a small size of pores. Compared with the PVA sample, for the oriented PVA/GO sample, the synergic orientation of GO sheets and the nucleation effect resulted in a higher orientation factor, crystallinity, and υe with a lower crystal size. In addition, the dense dual orientation crystalline network structure formed, thereby achieving the significantly enhanced mechanical properties for oriented PVA/GO hydrogels.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01819. (Figure 1a−d and 1a′−d′) The frequency dependence of the shear storage modulus (G′) and loss modulus (G′′) for PVA and PVA/GO hydrogels unoriented and oriented during the freezing/thawing process, respectively, and (Figure S2) υe and Mc of PVA and PVA/GO hydrogels unoriented and oriented during the freezing/ thawing process (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-28-85408802. Fax: 8628-85402465. ORCID

Lin Ye: 0000-0003-4164-6038 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the State Key Laboratory of Polymer Materials Engineering of China (sklpme2016-2-07).

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REFERENCES

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Construction of Dual Orientation Crystalline Structure in Poly(vinyl

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