Optimized Association of Short Alkyl Side Chains Enables Stiff, Self

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Optimized Association of Short Alkyl-Side Chains Enables Stiff, Self-Recoverable and Durable Shape-Memory Hydrogel Shuting Wang, Mengjuan Liu, Liang Gao, Guoqiang Guo, and Yanping Huo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06716 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Materials & Interfaces

Optimized Association of Short Alkyl-Side Chains Enables Stiff, Self-Recoverable and Durable Shape-Memory Hydrogel Shuting Wang, # Mengjuan Liu, # Liang Gao,* Guoqiang Guo, Yanping Huo* #These

authors contribute equally to this work

M. Liu, S. Wang, Dr. L. Gao, G. Guo, Dr. Y. Huo School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, 510006, China E-mail: ([email protected]; [email protected] ) Abstract This work reports a self-healing and shape-memory hydrogel integrating multiple mechanical properties. The network configuration is featured as entangled networks crosslinked by distributed association of very short alkyl chains (hexyl, six carbons). These crosslinking knots are interconnected by the long hydrophilic polyvinyl alcohol backbone. The optimal aggregation of hexyl side chains leads to the broadened distribution in bonding strength as verified by static and dynamic mechanical characterization. These structural features contribute to high strength, toughness, stiffness yet fast recoverability. Furthermore, the hydrophobic and supramolecular nature of aggregated alkyl chains offers high durability and solvent-assistant healing function. Finally, distributed association of hexyl-side chains confers a broadened temperature-dependent modulus, allowing for encoding step-wise shape recovery from a temporary shape at different temperatures and/or times.

Keywords: (Hydrogel, Polyvinyl alcohol, Shape memory, Toughness, Hydrophobic association)

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1. Introduction Wet and soft hydrogels uniquely bear some similarities to the soft tissues in biological systems.1-3 Tough hydrogels with a fracture energy over 1,000 J/m2 have been convincingly demonstrated.4-15 Many scientific and technological applications, however, require a systematic integration of mechanical and physiochemical properties including strength, stretchability, stiffness, self-healing, fatigue resistance, durability and encodable shape-shifting, along with toughness. However, most tough hydrogels are still soft (i.e., with an elastic modulus of 11) to achieve crystal formation of alkyl chains.24, 31-35 Otherwise, the formed hydrogels are significantly weak, with a tensile stress in the range of kPa.24,

31, 36-38

One notable issue is that using long alkyl chains often

compromise the extensibility and toughness.30-31 For example, ultra-stiff semicrystalline hydrogels with an elastic modulus of 300 MPa are obtained via the crystallized association of n-octadecyl (C18) chains, but they are highly brittle, with a fracture strain of only 20%.30-31 The absence of yielding at the typical strain rates suggests the low toughness. The unfavourable mechanical properties are related to the lamellar clusters of long alkyl chains, which probably have extra-long relaxation time due to the strong steric hinderance and bonding strength.30-31 Shortening the average length of side-chains could introduce ‘active ties’ that can be extended during deformation.31 The significantly improved extensibility and toughness is achieved by 3



replacing the C18 chains with a very small fraction of a lauryl (C12).31 However, when the fraction of short chains increases over a critical value (e.g., 20 mol% C12 of all monomer units),31, 38 the mechanical strength drastically decays to kPa levels because C12 segments do not contribute to the formation of alkyl crystals. In addition, the hydrophobes of alkyl crystals present a monotonous bonding strength as evidenced by abrupt thermal-softening behaviours.31-34, 38 Thus, they do not have a multi-stage shapememory function, which is desirable for some specific applications.14, 39-40 Although association of short side alkyl chains (average number of carbons typically20 MPa). The folded long backbones can be further extended until the rupture of strong bonds and/or disentanglements, offering reasonable fracture strain (>300%) and fracture stress (≈4 MPa). The existence of strong bondings and entanglement effectively prohibits the slippage of backbone, and the re-association of dynamic short alkyl chains offers rapid recovery (90 min at a strain of 150%) after mechanical unloading. In addition, the structural features of our hydrogel benefit the high chemical stability, solvent-assistant healing and multi-stage shape-memory properties. 2. Results and Discussion 2.1. Design and Synthesis of PVA-C6-gel Our target network can be constructed by combining two components: (1) the hydrophilic backbone and (2) short alkyl side chains. A commercialized polyvinyl alcohol (PVA) with a high molecular weight (e.g., Mw=146,000-186,000) is selected as a long hydrophilic backbone due to its hydrophilicity and biocompatibility.41 To establish distributed and dynamic hydrophobic interaction with short alkyl side chains, we adopt randomly appendant hexyl segments with the following rationale: the dissociation energy of hydrophobic associations is approximately 0.98kBT per methylene group of alkyl chains,42 while the typical covalent bonding is about 140kBT at room temperature. Inspired by the design principle of polyampholytes,22 the weak dynamic bonding should be composed of several to tens of methylene (-CH2-) units, while the strong bonding should be composed of tens to a hundred of -CH2- units. We presume that the distributed crosslinking can be produced by simply varying the number of short alkyl chains in crosslinking knots. The minimum unit of hydrophobic 5



association may be constructed using two alkyl chains, an approach which is required to form a weak dissociation energy of ≈10kBT in order to sustain initial tensile loadings. In addition, most fatty acids in organisms have an even number of carbon atoms. We thus choose the hexyl alkyl segment as the side chain.

Figure 1. Synthesis of PVA-C6-gel with superior mechanics. (A) (I) Synthesis of PVA-C6 is based on a 1-step chemical modification of a PVA polymer; (see Synthesis of PVA-C6 in ESI); (II) Gelation of PVA-C6 is based on three steps: (i) dissolving PVA-C6 in DMF and casting the PVA-C6/DMF solution in a Teflon mould; (ii) DMF (solvent)-water (nonsolvent) vapor exchange inducing the conversion of PVA-C6/DMF solution into PVA-C6-as gel; and (iii) hydrothermal treatment by immersing the asprepared PVA-C6-as in 90 oC water for 12 h to produce PVA-C6-gel; When the hydrogels are dried, they can be re-dissolved in DMF. (B) Experimental images to illustrate that notched PVA-C6-gel can bear a load and the large deformation can be spontaneously recovered under ambient conditions (24 oC) (see Video S1). To synthesize this target polymer, we first partially replace the -OH groups on polyvinyl alcohol (PVA) with a hexyl-carbamate side chain according to the strategy reported in our previous papers (see details in ESI).43-44 These modified PVAs, with explicit 6


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structure (Figure S1), are denoted as PVA-C6, where C6 stands for the six carbons of the terminal alkyl chain on carbamate segment (Figure 1A). The mole ratio of the carbamate segment to that of -OH groups of the initial PVA, which is formally the degree of substitution (DS), is controlled to be approximately 50% by adjusting the reaction course.43 PVA-C6 is biocompatible, as verified by cytotoxicity testing on RAW 264.7 cells. (Figure S2 in ESI). PVA-C6 can be readily dissolved in several water-miscible organic solvents (e.g., dimethyl formamide (DMF) and dimethylsulfoxide (DMSO)), but never in water. This solubility feature allows gelation via the solvent-nonsolvent exchange strategy.45 As illustrated in Figure 1A, three simple steps, indicated as (i), (ii) and (iii), convert the PVA-C6 powder into stiff and tough hydrogel. In step (i), the PVA-C6 powder is fully dissolved in DMF at a concentration of 100 mg/mL. In step (ii), the PVA-C6/DMF solution undergoes gelation upon exposure in water vapor at 24 oC. During the vaporphase solvent exchange process, the hydrophobic hexyl chains gather to form crosslinking knots due to the increase of the solvent interaction parameter-χ of the hexyl chains.45 The as-prepared hydrogels (denoted as PVA-C6-as, where the as stands for as-prepared) are weak. In step (iii), the weak PVA-C6-as is immersed in hot water

at

90 oC for 12 h and then cooled to 24 oC to produce a tough and opaque hydrogel, which is donated as PVA-C6-gel. For the sake of simplicity, such treatment method of immersing the as-prepared PVA-C6-as in hot water is refered to as hydrothermal treatment in this paper. The optimial temeprature of hydrothermal treatment is selected as 90 oC as this treatment condition can offer superior mechanical properties while 7



simultaneously avoiding water being boiled off (Figure S3).The detailed gelation process can be found in ESI. The PVA-C6-based hydrogels can be completely redissolved in solvents (e.g., DMF and DMSO). Thus, they are truly physical hydrogels, benefiting recyclability and solution- processability (Figure 1A). In Figure 1B and Video S1, we first vividly present the superior mechanical properties of PVA-C6-gel: A thin PVA-C6-gel (thickness~1.2 mm) with a large hole (diameter=5 mm) can handle up to a weight of 1 kg, suggesting its high strength. Such intensive mechanical loading results in significant deformations. After several minutes of selfrecovery, the deformed sample with a macroscopic crack is almost fully restored to its original shape without external stimulations. 2.2 Optimization on the Association of Short Alkyl Chains We find that hydrothermal treatment effectively improves the mechanical properties of PVA-C6-as through optimizing the association of hexyl chains. As shown in Figure 1B, treating PVA-C6-as in 90 oC water causes volume shrinkage by approximately 35%. Accordingly, the water content also decreases from ~78 wt.% to ~50 wt.% (Figure S3). The compactness of hydrogel is enhanced at a microscopic level, as evidenced by the cross-section image taken using scanning electron microscopy (SEM) (Figure 2A).

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Figure 2. Structural differences between PVA-C6-as and PVA-C6-gel. (A) SEM images for the cross section of freeze-dried PVA-C6-as and PVA-C6-gel (scale bar=10 µm); (B) XRD patterns for wet PVA-C6-as, PVA-C6-gel and PVA cryogel (recorded at 24 oC, scanning rate: 1 o/min; scanning region 1o-50o); (C) Schematic on the optimization of hydrophobic association of hexyl chains and the proposed network configurations of PVA-C6-as and PVA-C6-gel. The characteristic length of the peaks in the X-ray diffraction (XRD) patterns can characterize the backbone-to-backbone distance (d1) and side-chain spacing (d2) of side alkyl chains for the polymer systems with alkyl side chains.46 We therefore further perform XRD to probe the molecular arrangement of hexyl side chains in PVA-C69



based hydrogel. To understand the contribution of PVA main chains, a PVA cryogel are also characterized by XRD under the same conditions. PVA cryogel is prepared by conventional freezing/thawing cycles (ESI).49 In Figure 2B, both wet PVA-C6-as and PVA-C6-gel exhibit an XRD peak centred at 2θ=4.1o, while no such peak is present for the PVA cryogel. This peak of PVA-C6-based hydrogels corresponds to the distance (d1) between backbones of adjacent PVA-C6 chains as schemed in Figure 2C. The average backbone distance is estimated to be d1=2.12 nm based on the Bragg equation (2d1sinθ=0.154 nm, where 0.154 nm is the wavelength of X-ray used for XRD collection). The average length of the appendent hexyl side chain (including the carbamate segment) is calculated to be 0.956 nm (Figure S1). The average backbone distance (d1=2.12 nm) is slightly larger than twice of the hexyl side chain (0.956×2=1.912 nm). These results may signify tail-to-tail alignment of the hexyl side chains without overlapping as schematically illustrated in Figure 2C. There is a highly broadened peak for both wet PVA-C6-as and PVA-C6-gel between 2θ=15o and 2θ=45o. This broadened peak is partially overlapped with that of PVA cryogel. It is known that this broadened XRD peak of PVA cryogel is from the swollen amorphous PVA and free water.49 Therefore, both PVA-C6-as and PVA-C6-gel should contain similar hydrophilic domains to that of PVA cryogel, contributing to a reasonable hydration level. With comparison to that of PVA-C6-as, PVA-C6-gel exhibits much more distinct peak centred at 2θ=20.1o. The spacing distance between side chains (d2) is thus estimated to be 0.44 nm as schemed in Figure 2C. The relative intensity of the XRD peak at 2θ=4.1o of the PVA-C6-gel is markedly higher than that 10


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of PVA-C6-as. The intensified XRD peak may suggest the increased average number of alkyl chains that are involved in crosslinking knots, in line with the decreased hydration level. A remaining open question is how the hydrothermal treatment induces such significant impacts on the association of hexyl side chains. One of the reasonable mechanism is envisioned as follows: The hydrothermal treatment first effectively disturbs the hexyl side chains in PVA-C6-as, resulting in relaxation of conformation. Subsequently, the disturbed chains with high freedom undergo spontaneous and irreversible rearrangement, as verified above. The molecular arrangement in PVA-C6-gel can be more thermodynamically favourable (e.g., with lower Gibbs free energy) than that of PVA-C6-as, resulting in optimized association of hexyl side chains. This optimization process requires that hexyl chains are fully activated. Therefore, the treated temperature should be sufficiently high in order to overcome the activation energy and relax the hexyl chains. Indeed, when the temperature of hydrothermal treatment decreases from 90 oC to 40 oC, there is almost no volume contraction and dehydration for PVA-C6-as (Figure S3), implying no impacts on structure. Further characterization or/and molecular simulation is required in the future work in order to fully understand the hydrothermal effect, but the general goal of the current study is not compromised due to the lacking of such analyses. It is important to note that PVA-C6-gel does not exhibit well-defined melting behaviour (Figure S4A). This observation is consistent with the thermal behavior of ultra-high molecular weight poly(1-hexene) melt.48 The lack of melting behaviour of PVA-C611



gel is distinct from conventional semi-crystalline hydrogels, which typically exhibits a sharp melting peak in DSC due to the melting of lamellar packing composed of long alkyl chains.28 In contrast, PVA-C6-gel exhibits a highly broadened softening temperature (Figure S5). Considering that the hexyl side chains are randomly attached on PVA backbone in PVA-C6-gel, the amount of alkyl chains in these dynamic clusters can be widely distributed as per the schemed illustration in Figure 2C. The hexyl side chains may be too short to contribute to the formation of long-range orders.31, 47 The observed XRD peaks may be related to distributed domains of similar size (e.g., glassy sphere) or amorphous halo rather than the long-range orders of lamellar structures.24, 46, 50 Besides,

a very weak birefringence is observed under a microscope with both parallel

and crossed polarizers (Figure S6). The liquid crystal-like structures (e.g., smectic phase) are not likely to exist in PVA-C6-gel. In addtion, we should particularly point out that there exist no crystalline phase of PVA backbone in PVA-C6-gel, as confirmed by a series of experimental results (Figure S4). First, DSC trace of PVA-C6-gel does not exhibit any melting peak corresponding to the crystalline domain of PVA backbone; Secondly, XRD patterns of PVA-C6-gel do not exhibit crystallization peak of PVA; Finally, the PVA-C6-gel has undergone prolonged treatment in water at 90 oC, destructing any crystallized PVA phase that may exist (see more details in Figure S4).

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Figure 3. Tensile behaviours of PVA-C6-as and PVA-C6-gel. (A) Stress-strain curves (24 oC); (B) Elastic modulus and (C) The corresponding Mooney–Rivlin curves of PVA-C6-as and PVA-C6-gel (strain0) or hardening (C20 at a small strain, suggesting large amount of weak bonding (cluster with small amount of hexyl side chains) is broken in the initial tensile deformation. This feature 14


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contributes to a large elastic modulus. Further elongation is limited by the strong bonding ( cluster with large amount of hexyl side chains) as indicated by the much less pronounced strain softening behaviour at a large strain. We should emphasize that the possible hydrogen-bonding (H-bonding) between the residual -OH groups is highly unlikely to contribute to the mechanical strength of the PVA-C6-gel, because its tensile behaviour barely changes after being immersed in Hbonding-breaking reagent (NaSCN) for 24 h (see below in Figure 7A-7B). The main factors influencing the mechanics are thus identified as (i) the association/amount of hexyl chains involved in junction points and (ii) their interconnection. When the mole ratio of the carbamate segment to that of the -OH groups of the initial PVA decreases from 50% to 30% in PVA-C6, the mechanical strength of the resultant hydrogels clearly decays (Figure S7). In addition, the yielding behaviour also almost disappears. This result perhaps suggests that a sufficient amount of pendant hexyl chains is necessary for the formation of clusters with distributed bonding strength. Otherwise, the hydrogel cannot be tough. On the other hand, the hydrophobic association of alkyl chains must be sufficiently interconnected by the entanglement of the PVA backbone. This argument is supported by the fact that PVA-C6-gel prepared by using low-molecular weight PVA (e.g., Mw=61,000) is very weak (Figure S7). Generally, low molecular weight results in inefficient entanglements.53 We also find that when the molecular weight of the initial PVA further decreases to Mw=27,000, there is even no gelation occurring in the step of DMF-water vapor exchange. Overall, the cooperation of the

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entanglement of a long PVA backbone and optimized hydrophobic association of hexyl side chains contribute to superior tensile behaviours. 2.3 Dynamics of PVA-C6-gel

Figure 4. Linear dynamic behaviour of PVA-C6-gel. (A) Constructed master curves of storage modulus G′ (filled square) and loss modulus G″ (cross); (B) Arrhenius plot for the shift factors aT. Average activation energy Ea is calculated from the slope of the fitted curves. Time-temperature superposition (TTS) is a powerful strategy to understand the dynamics of polymeric materials. TTS is demonstrated to be valid for high-molecular weight poly(1-hexyl).48 Thus, the entanglement and hydrophobic association of PVAC6-gel can be characterized by TTS. We first acquire the frequency-dependent storage modulus G′ and loss modulus G″ at different temperatures for a small range of frequencies (0.01~1 Hz) from a rheology test. According to the TTS principle, we superimpose these G′and G″ by using a time scale multiplicative horizontal shift factor aT and a modulus scale multiplicative vertical shift factor bT:54

G '( , T )  bT G '(aT  , T0 ) and G ''( , T )  bT G ''(aT  , T0 )

Equation 2

The aT and bT are adjustable parameters. T0 is a reference temperature. During the shifting, we maintain bT≈1, while aT is varied in a large range from 625 (10 oC) to 16


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2.5×10-7 (90 oC) at a reference temperature of 30 oC, which is almost the softening temperature for PVA-C6-gel (Figure S5). As shown in Figure 4A, the G′ and G″ can be roughly superimposed into a single master curve over a wide frequency range of 10-10104 rad/s. Interestingly, the master curve of G′ and G″ exhibit three critical frequencies where G′=G″. This linear viscoelastic response mode is usually observed in both branched and linear polymer melts.48, 55 But, to the best of our knowledge, this is the first demonstration in a hydrogel system, suggesting the rare dynamics of PVA-C6-gel. The reciprocal of these three frequencies can be assigned as reptation time (τrep), Rouse time (τe) and relaxation time of the Kuhn monomer (τ0), respectively.56 An elastic modulus plateau can be generally identified between τrep and τe, suggesting an entangled configuration. The reptation time τrep reflects the time for the chain to break away from the topological restriction of molecular motion by other chains, namely the effect of polymer chain entanglements on chain relaxation time. The τrep is calculated to be 3.6×108 s, suggesting that the chains in PVA-C6-gel are permanently entangled on the observation time scale. The Rouse time (τe) with a calculated value of 23 s is an indicator of time-dependent viscoelasticity. Generally, when the deformation rate of applied force is faster than the 1/23≈0.043 s-1, the hydrogel behaves viscoelasticity. While the deformation rate is slower than this value 0.043 s-1, the hydrogel behaves as an elastic network.56 The validity of this Rouse time in our system is confirmed by strain-rate dependent tensile testing at a large strain (see Figure 5A as below). In Figure 4B, an inversely linear relationship between ln(aT) and absolute temperature (T) can be established as follows: 17



ln(aT ) 

Ea 1 ( ) R T

Equation 3

where R=8.314 J/(mol•K) is the gas constant, Ea is the average activation energy, which is related to bond disassociation. The Ea is estimated to be 230 kJ/mol (93kBT). This value is lower than the disassociation energy of the covalent bond (350 kJ/mol, 140kBT), but much higher than the individual hydrophobic association composed of two hexyl alkyl chains (10kBT). Thus, dynamic clusters of alkyl chains must be formed during the synthesis of PVA-C6-gel. Further, we observe several relaxation regimes for over 15 decades in frequency in constructed master curves. Such broadened dynamics should be the collective contribution of the distributed association of hexyl chains with fast as well as slow dynamics. 2.4 Viscoelasticity and crack-propagation resistance of PVA-C6-gel

Figure 5. Viscoelasticity and crack-propagation resistance of PVA-C6-gel: (A) Tensile behaviours under different strain rates ranging from 0.016 s-1 to 1.02 s-1; (B) Stress-strain curve of notched PVA-C6-gel; (C) Experimental images of the crack 18


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propagation process and (D) Fitting curve of yield stress as a function of logarithm natural form of strain rate. The viscoelasticity of PVA-C6-gel is also verified by the strain-rate-dependent tensile behaviour at large strains (Figure 5A). It is notable that when the strain rate is 0.016 s1,

slower than 1/τe≈0.043 s-1, the stress-strain curve almost behaves as a pure elastic

network. This time scale is consistent with the τe based on TTS (Figure 4A). The viscoelasticity is conducive to highly efficient energy dissipation (Figure S8), which can prohibit the crack propagation.14 A notched PVA-C6-gel can still be stretched by 250% (Figure 5B-5C, and Video S2), which is close to the fracture strain of 330% of an intact PVA-C6-gel. No propagation is observed until a strain of

Optimized Association of Short Alkyl Side Chains Enables Stiff, Self

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