Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer Toughness improvement and anisotropy in semicrystalline physical hydrogels Cigdem Bilici a , Damla Karaarslan b , Semra Ide b , Oguz Okay a,* a Department of Chemistry, Istanbul Technical University, 34469, Maslak, Istanbul, Turkey b Departments of Physics Engineering and Nanotechnology & Nanomedicine, Hacettepe University, 06800, Beytepe, Ankara, Turkey HIGHLIGHTS • High-strength physical hydrogels with anisotropic properties are prepared. • Directional toughness improvement is achieved in semicrystalline hydrogels. • Young's modulus of the hydrogel is 161 and 76 MPa along different directions. ARTICLE INFO Keywords: Mechanical anisotropy Physical hydrogels Semicrystalline hydrogels ABSTRACT A major challenge in the gel science is to create mechanically strong hydrogels with anisotropic properties as observed in many biological tissues. Here, we report a simple one-step method of producing high-strength physical hydrogels exhibiting microstructural and mechanical anisotropy. As the precursor material, we use semicrystalline shape-memory hydrogels consisting of poly(N, N-dimethylacrylamide) chains interconnected by n-octadecyl acrylate (C18A) segments forming crystalline domains and hydrophobic associations acting as switching segments and netpoints, respectively. To generate anisotropic microstructure, we impose a pre- stretching on the isotropic hydrogel sample above the melting temperature T m of its crystalline domains followed by cooling below T m under strain to fix the elongated shape of the gel sample. A significant microstructural and mechanical anisotropy was achieved that could be tuned by the magnitude of the prestretch ratio λ o . Directional brittle-to-ductile and ductile-to-brittle transitions could be induced by adjusting the prestretch ratio λ o . Small- and wide-angle X-ray scattering measurements and mechanical tests highlight a critical prestretch ratio λ o at which the hydrogel exhibits the highest microstructural and mechanical anisotropy due to the finite extensibility of the network chains. At λ o = 1.8, the hydrogel exhibits Young's moduli of 161 ± 14 and 76 ± 7 MPa, and toughness of 16 ± 1 and 1.3 ± 0.1 MJ m −3 along and perpendicular to the prestretching direction, respec- tively. 1. Introduction Owing to their similarities to biological tissues, hydrogels as soft and smart materials have important functions in a variety of biological and biomedical applications [1]. Although hydrogels are traditionally brittle and exhibit a low modulus of elasticity in the range of kPa, significant progress has been achieved in the past 15 years in the design of mechanically strong and tough hydrogels [2]. Several techniques developed so far enable preparation of hydrogels with mechanical performances approaching to those of biological systems. Another challenge to be addressed in the gel science is to create mechanically strong hydrogels with anisotropic properties, as observed in many biological tissues such as skin, muscle, and articular cartilage possessing anisotropically oriented hierarchical structures [3]. To achieve this goal, nanofillers such as nanofibers [4,5], graphene oxide [6], nanosheets [7], nanotubes [8], or nanodisks [9,10] in a precursor dispersion were first oriented and then the oriented microstructure was fixed by gelation. Anisotropic hydrogels were also produced by direc- tional freezing [11–13], or by orienting the network chains of isotropic hydrogels under an external force followed by fixing the anisotropic structure via in situ polymerization [14–18]. Kajiyama et al. reported stress-induced orientation of lamellar crystals in covalently cross-linked semicrystalline hydrogels [19]. Although not reported, these hydrogels should exhibit anisotropic mechanical properties. Such hydrogels were https://doi.org/10.1016/j.polymer.2018.07.077 Received 4 May 2018; Received in revised form 7 July 2018; Accepted 28 July 2018 * Corresponding author. E-mail address: okayo@itu.edu.tr (O. Okay). Polymer 151 (2018) 208–217 Available online 30 July 2018 0032-3861/ © 2018 Elsevier Ltd. All rights reserved. T