Short CommunicationSynthesis mechanical properties and self-healing behavior of aliphatic polycarbonate hydrogels based on cooperation hydrogen bonds
Graphical abstract
Introduction
Polymer hydrogel is a kind of material with broad prospects because of its unique structure. It has extremely high water content and a water-soluble three-dimensional network [1]. Advantages such as good stability [2] and strong water absorption [3] make it an ideal material in many pharmaceutical fields [[4], [5], [6]], industrial products [[7], [8], [9]], and scientific researches [10,11]. However, most conventional polymer hydrogels are weak in mechanical strength [12], lack of toughness [[13], [14], [15]] with low self-healing efficiency [16,17], which block its deepen development. To make up for these defects, many ways have been explored. Constructing double-networks [[18], [19], [20]], using inorganic nanoparticles [21,22] as the reinforcing phase or connecting molecules with non-covalent bonds [[23], [24], [25], [26]] generate supermolecular hydrogels to provide sufficient mechanical strength of polymer hydrogels. As for improving its recoverability, hydrogels are mainly divided into two categories according to the modes of reparations, intrinsic self-healing hydrogels, and extrinsic self-healing hydrogels [27]. Intrinsic self-healing hydrogels possess this property through doping with materials with self-healing properties [[28], [29], [30]]. Extrinsic self-healing hydrogels generally depend on dynamic bonds [31,32], such as D-A bonds and hydrogen bonds. The thermal stability of hydrogels has also made a breakthrough by the collective effect between hydrogen bonding and hydrophobic interactions [33]. And now further research focuses on the preparation of composite functional hydrogels [[34], [35], [36]].
The general strategy for the preparation of hydrogels with improving mechanical properties is incorporating reversible weak physical cross-linking into hydrogels structure [37,38], typically hydrogen bonds [[39], [40], [41]]. For example, PAMPS/DMAA hydrogels, which are composed by physical Poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) network decorated with N,N-dimethylacrylamide (DMAA) segments, shows a higher modulus [42] than purely PAMPS hydrogels [43]. The hydrogels are prepared by heating aqueous acrylamide (AAm) solution in the presence of poly(N-vinylpyrrolidone) (PVP). The obtained samples exhibit higher tensile strength, higher tensile extensibility, and higher compressive strength than untreated ones [44]. The same result is reflected in silk fibroin hydrogels induced by a small peptide gelator [45], poly(4-vinylpyridine) hydrogels are interacted by glutaric [46] and acrylic acid with 2 mol% stearyl methacrylate in a solution of sodium dodecyl sulfate micelles [47]. The above phenomena have a common feature that the modified hydrogel has more types of hydrogen bonds, and the cooperative H-bonds in hydrogels cause the improved mechanical properties of hydrogel [[48], [49], [50]]. The existence of stable cooperative hydrogen bonds makes the shape of the hydrogel network much firmer [51,52] and experimental proofs that ionic cross-linking are promoted by cooperative hydrogen bonds [53,54,59], thus the tensile strength and elastic modulus of hydrogels are evidently increased [55]. Besides, they enable the formation of hydrogels with anisotropic structures [56,57], which make the synthetic one has different mechanical properties in different directions, liking the natural one. Considering the superiority that cooperative hydrogen bonds enhance mechanical properties in many ways, it is a significant issue that how to improve the mechanical properties of hydrogels to a greater extent and maintain their original excellent performance even endow their new characteristics, including self-healing properties or shape memory abilities.
As far as we know, the polymer obtained from the monomer MBC is an aliphatic polycarbonate material, a non-toxic biological material approved by the American Food and Drug Association (FDA) because of its excellent biocompatibility, lower than the human body temperature of the glass transition temperature (Tg) and without acidic degradation products during the degradation process [58,65,67,68].
Herein, the purpose of this work is to further enhance the mechanical properties and self-healing abilities of hydrogels based on the bulk polymerization approach. The ring-opening polymerization of 2-methyl-2-benzyloxy carbonyl propylene carbonate (MBC) is initiated with methoxy poly(ethylene glycol) (mPEG113, Mn = 5000) to synthesis and characterize hydrogels with dramatically enhanced mechanical properties and excellent self-healing ability. Then the properties of the hydrogel are explored by the different ratio of catalyst. In addition, the MBC is selected as a polymerization monomer because the MBC is a rigid segment containing ester carbonyl and benzene ring groups, and has a relatively high dynamic because of the steric effect of the benzene ring [64,69]. Meanwhile, the raw materials of MBC are environmentally friendly [63]. The mPEG113 is soft chains and has the advantages of good biodegradability, excellent biocompatibility, and low toxicity [60]. And now its application in biology is relatively mature [61,62]. During the polymerization, the number of ester carbonyl and benzene ring groups of mPEG113–b-PMBCn are continuously increased, then, the sp2 hybrid oxygen element in ester carbonyl has a strong electronegativity to attract lone electron pairs, which will facilitate the carbonyl group combines with the donor to form a hydrogen bond. Thus, this kind of hydrogel will simultaneously have good mechanical properties and self-healing properties (Scheme 1).
Section snippets
Materials
Benzyl chloride (C7H7Cl, 99%, Shanghai Chemical Reagent plant), 2, 2-bis (hydroxymethyl) propionic acid (C5H10O4, 99%, Tianjin Bodi Chemical Co., Ltd.), triethylamine (C6H15N, 99%, Tianjin Damao Chemical Reagent Factory), methoxypolyethylene glycols (mPEG113, 5.0 kDa, Alpha chemical co., Ltd.), stannous octoate (Sn(Oct)2, 99%, Sigma-Aldrich), Ethyl chloroformate (C3H5ClO2, 99%, Xinyi Huili Fine Chemical Co., Ltd.), toluene (C7H8, 99%) were analytical grade. Toluene was dried by treatment with
Synthesis and characterization
As illustrated in Fig. 1, the ring-opening of the monomer MBC is initiated by the hydroxyl group of the initiator mPEG113, using stannous octoate as a catalyst forming a macromolecular chain to promote the formation of the hydrogen bond between the carbonyl and hydroxyl groups, which facilitate further the formation of a network of mPEG113-b-P(MBC)n hydrogels. The schemes and pictures of the mPEG113-b-P(MBC)n gels are exhibited in Fig. 2(a) and (b), respectively. The monomer and initiator are
Conclusion
In this study, the hydrogels (mPEG113-b-P(MBC)n) with strong mechanical properties and highly efficient self-healing have been successfully synthesized through the ring-opening bulk polymerization of MBC, using mPEG113 as the initiator. The cooperative hydrogen bond between the carbonyl and hydroxyl groups is the roots of the self-healing properties of the obtained hydrogels. The swelling experiments show that as the chain grows, the gel fraction of the materials and the swelling ratio in
CRediT authorship contribution statement
Chaoxian Chen: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Resources, Software, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing. Nan Duan: Writing - original draft. Siwen Chen: Conceptualization. Zhihao Guo: Data curation. Jianshe Hu: Funding acquisition, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing. Jing Guo: Resources.
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgements
This work was supported by the Liaoning Revitalization Talents Program (XLYC1807142), Fundamental Research Funds for the Central Universities (N180705004), Science and Technology Department of Liaoning Province (2018225079, 20180037), the Educational Department of Liaoning Province (ZF2019040), and the Shenyang Science and Technology Bureau (RC190426).
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