FULL PAPER 1900309 (1 of 10) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mme-journal.de Mechanical Strengths of Hydrogels of Poly(N,N- Dimethylacrylamide)/Alginate with IPN and of Poly(N,N-Dimethylacrylamide)/Chitosan with Semi-IPN Microstructures Changzhuang Bai, Qiuhua Huang, Xiao Zhang, and Xiaopeng Xiong* C. Bai, Q. Huang, X. Zhang, Prof. X. Xiong Department of Materials Science and Engineering College of Materials Xiamen University Xiamen 361005, China E-mail: xpxiong@xmu.edu.cn The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/mame.201900309. DOI: 10.1002/mame.201900309 or randomly distributed crosslinking points. [4] Therefore, continuous efforts have been paid on improving the mechan- ical strength of hydrogels. [5–7] A variety of novel hydrogels with high mechanical strength have thus been developed based on their specific struc- tures, such as topological hydrogels, [8,9] nanocomposite hydrogels, [10,11] and inter- penetrating network (IPN) hydrogels. [12] Among them, the IPN hydrogel has attracted increasing attention, because the IPN structure can provide the mate- rial not only high mechanical strength but also functionalities. [13–15] The hydrogel with the IPN microstructure is usually prepared by a two-step sequential process, namely a first-network is formed from one polymer, and then the second network of another polymer is uniformly incorpo- rated into the former. [14,16] In other words, the two types of polymer networks inter- penetrate each other to form a homoge- neous hydrogel (IPN or full-IPN). Because of the relatively dense matrices and the synergistic interactions of the different components in the networks, the IPN hydrogel often displays very strong mechanical properties. [14] For example, Li et al. [17] successfully prepared gelatin–alginate IPN hydrogel with remarkably improved mechanical strengths, and the hydrogel exhibited excellent durability with steam sterilization to have promising potential in the field of tissue engineering. Lately, special attention has been paid on double network (DN) hydrogel, which also possesses the IPN struc- ture. [5,6,18] In those hydrogels, both the crack bridging of the loosely crosslinked and the rupture of the highly crosslinked networks would contribute to the energy dissipation to result in extreme strong performances. [19,20] In comparison, if the second polymer is not crosslinked but trapped in the first network, a semi-IPN microstructure can then be obtained for the resultant hydrogel. [14] The strength of the hydrogel can also be improved by the semi-IPN structure, where the disen- tanglement of trapped but un-crosslinked polymer chains in hydrogel network played important role on its strengthening and toughening. [21] However, more interest has been focused on the specific properties provided by the trapped polymer in Interpenetrating Networks Tough hydrogels receive continuous attention because of their prom- ising applications in many fields. Herein, tough hydrogels of poly (N,N-dimethylacrylamide) (PDMAA)/alginate (SA) are prepared, with inter- penetrating network (IPN) and of PDMAA/chitosan (CS) with semi-IPN microstructure, respectively. The toughening of the hydrogel by incorporating natural polymers is studied by compressing tests and dynamic mechanical analyses. Moreover, cyclic load–unload compressing of the two types of hydrogels are performed at low strains and under relatively high strains, in order to compare their strength and anti-fatigue properties. The results indi- cate that the mechanical strength can be markedly improved upon addition of the natural polymers, and the IPN hydrogel of PDMAA/SA reveals much higher mechanical performances but is less stable. However, the semi-IPN hydrogel of PDMAA/CS displays excellent anti-fatigue stability, but with rela- tively low strength. Swelling tests, scanning electron microscopy, and Fourier transform infrared spectroscopy are carried out to study the microstructures of the hydrogels, which are carefully analyzed to understand the difference in mechanical performances of those hydrogels. The results suggest that the presence of sacrificial unit and higher chain density in the IPN are helpful for toughening hydrogels, while the semi-IPN network is beneficial to improve the energy dissipation efficiency. 1. Introduction Polymer hydrogels are 3D networks of polymer chains cova- lently or physically crosslinked, and could be swollen but insoluble, so that those materials have a variety of applications in fields such as food, chemical industry, medicine, tissue engineering, flexible electronics, and smart devices. [1–3] Tradi- tional polymer hydrogels often suffer from the weak mechan- ical strength because of very few energy dissipation paths Macromol. Mater. Eng. 2019, 304, 1900309