Shape-memory properties of hydrogels having a poly(Epsilon-caprolactone) crosslinker and switching segment in an aqueous environment

Abstract

Multiphase copolymer networks are of great relevance as their thermal and mechanical as well as shape-memory properties can be adjusted by the variation of the composition and crosslink density. In this context recently grafted copolymer networks (named CLEG), prepared by thermal free-radical polymerization with different ratios of the hydrophobic telechelic crosslinker poly(ε-caprolactone) diisocyanoethyl methacrylate (PCLDIMA, Tm,PCL = 55 °C) and hydrophilic poly(ethylene glycol) monomethyl ether monomethacrylate (PEGMA, Tm,PEG = 38 °C) as co-monomer, were introduced, which additionally allow the alteration of the overall elastic properties via controlled water uptake. Here we study the thermomechanical properties as well as the shape-memory behavior of a series of CLEG copolymer networks in an aqueous environment. The mechanical properties of the networks at 25 °C in aqueous environment were found to increase from 4 MPa to 77 MPa with increasing crosslink density. The shape-memory properties of the copolymer networks were examined in an aqueous environment by both bending as well as uniaxial elongation experiments. Excellent dual-shape properties with high shape fixity ratios around Rf = 79–100% and shape recovery ratios in the range of Rr = 59% to Rr = 100% were obtained for copolymers with a PCLDIMA weight fraction ⩾50 wt% in the starting composition. The swelling of CLEG in H2O resulted in a reduction of the switching temperature. Furthermore the structural changes during programming of CLEG were assessed in situ by small and wide angle X-ray scattering (SAXS, WAXS) experiments, which confirmed that the overall degree of crystallinity as well as the orientation of the crystalline domains controlled the dual-shape performance. We have found that an appropriate switching segment should result in a degree of crystallinity higher than 10–20% to enable high strain fixity ratios. Furthermore, hydrogels having dual shape-memory capability could be created by using semi-crystalline crosslinker simultaneously acting as the switching segment.