Hydrophobic association mediated physical hydrogels with high strength and healing ability
Graphical abstract
Introduction
The last three decades have witnessed the fast developments of hydrogels, which have similarity to soft biotissues, versatile responses to external stimuli, and broad applications in tissue engineering, drug delivery, soft actuators, etc [1], [2], [3]. The innovation of novel hydrogels with advanced properties or functions has greatly facilitated these developments. For example, Gong et al. have developed the double network principle to toughen hydrogels with excellent mechanical strength [4], [5], [6]; the resultant gels can be used as load-bearing artificial organs and actuators [6], [7], [8], [9]. Aida and coworkers have fabricated anisotropic hybrid hydrogels with cofacially aligned nanosheets, which show direction-dependent optical and mechanical properties, as well as extraordinary fast and reversible contraction without water uptake and release; these gels are ideal candidate to build optical devices and soft actuators [10], [11].
To meet special requirements, it is desired to develop hydrogels with multiple functions and high performances. For example, three-dimensional (3D) printed hydrogels are promising in artificial organs with complex structure and scaffolds for cell culture; it needs the gel material to be robust yet printable, which means that the precursor solution can be readily gelled or has a fast sol-to-gel transition [12], [13]. High strength physical hydrogels with tunable sol-to-gel transition should be an ideal candidate; however, most physical hydrogels such as jelly and collagen gels are weak. To develop robust physical gels with controllable sol-to-gel transition, supramolecular chemistry should be an effective approach. Supramolecular polymer gels are viscoelastic materials assembled by intermolecular noncovalent interactions [14], [15]. If the strength of multiple noncovalent bonds on a polymer chain is comparable to that of covalent bonds, the physical hydrogels will be of high strength. In recent years, tough physical hydrogels have been developed via the formation of polyion complex, multiple hydrogen bonding, and metal coordination, which possess high strength up to megapascal, self-healing, or shape memory properties [16], [17], [18], [19], [20].
Hydrophobic interaction is widely used in molecular self-assembly of block copolymers to form supramolecular gels, which are usually responsive yet mechanically weak [21], [22], [23]. To form tough supramolecular polymer hydrogels, the polymers should contain balanced hydrophilic groups to maintain water and hydrophobic groups to strongly associate as crosslinked junctions. Physical hydrogels mediated by hydrophobic association have been developed by compression moulding the copolymers and subsequently swelling the films in water [24], [25], [26], [27]. In addition, tough and self-healable hydrogels are developed by copolymerizing hydrophilic and hydrophobic monomers in a micellar solution of sodium dodecyl sulfate [28], [29], [30], [31], [32], [33], [34]. These physical gels show breaking stress and strain in tension of 0.2–0.5 MPa and 1000–2000%, respectively.
In this paper, we reinvent the classic system of acrylic acid (AA) and stearyl acrylate (SA) found by Osada and his coworkers. They prepared a chemically crosslinked hydrogel with shape memory properties by copolymerizing the two monomers with molar fraction of SA larger than 0.25, in the presence of N,N’-methylenebisacrylamide as the crosslinker. The alkyl chains of SA units form crystalline domains to freeze the temporary shape, which can recover to the original shape by melting the crystalline domains at high temperature [35], [36], [37], [38], [39]. At room temperature these hydrogels are whitish and fragile. In addition, gel-to-sol transition is absent due to the existence of chemically crosslinked network.
We report here robust physical hydrogels developed by casting the poly(SA-co-AA) ethanol solutions and swelling the resulting films in water. The fraction of SA is decreased to several percentages, so that transparent hydrogels with high tensile strength are obtained. The long alkyl chains of SA units form hydrophobic domains with different extent of crystallization, acting as the physical crosslinking junctions of the copolymers. The water content and mechanical property can be tuned by varying the composition of copolymers. These gels are responsive to temperature, pH, and ethanol solvent, which tune the stability of hydrophobic associations and lead to a gel-to-sol transition and healing ability. The microstructures and gel-to-sol transition mechanism of these physical gels are investigated by wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). These physical gels with high strength and healing ability are facile to fabricate and reconstructable, which should find applications as membranes, coatings, cell scaffolds, etc.
Section snippets
Materials
Acrylic acid (AA, Sinopharm Chemical Reagent Co., Ltd.) was purified by vacuum distillation at 90 °C before use. Stearyl acrylate (SA, Tokyo Chemical Industry Development Co., Ltd.) and 2,2′-azobis(isobutyronitrile) (AIBN, Sigma-Aldrich) were recrystallized from ethanol. Ethanol was distilled before use. Deuterated dimethyl sulfoxide (DMSO-d6, J&K Scientific Ltd.) for NMR measurements was used as received. Water was purified by ultra-pure water system (Heal Force).
Synthesis of poly(SA-co-AA)
Poly(SA-co-AA) with various
Gel synthesis and mechanical properties
The transparent solid copolymers of poly(SA-co-AA) are easily dissolved in ethanol to form homogeneous solutions. The weight-average molecular weight, Mw, of copolymers is around 1 × 106 kg/mol, as measured by laser light scattering. The molar fraction of SA unit in copolymers was determined by 1H NMR measurement from the intensity ratio of the peaks for the methyl proton of SA and the carboxyl proton of AA. As shown in Table 1, the fraction of SA unit in the copolymers is slightly larger than
Conclusions
We have developed robust physical hydrogels with hydrophobic associations serving as the crosslinking junctions. The gels are prepared by simple casting and subsequently swelling the film of copolymers synthesized by copolymerization of AA and a small fraction of SA in ethanol. The swelling ratio and mechanical properties of hydrogels can be tuned by varying the composition of copolymers. The tensile modulus E, breaking stress σb, and breaking strain εb of gels increase with the deformation
Acknowledgements
This research was supported by National Natural Science Foundation of China (51403184, 21422406), Natural Science Foundation of Zhejiang Province, China (Y14E030021, LR16E030003), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. WAXD and SAXS experiments were performed at BL16B beamline of SSRF, China.
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