Development of high performance shape memory polyurethane by cross-linking pp.727-754
Authors: (Yong-Chan Chung, Byoung Chul Chun, Department of Chemistry, The University of Suwon, Hwasung, Korea, and others)
Abstract: Effect of various cross-linking agents such as glycerol and pentaerythritol on the shape memory property of polyurethane block copolymer is investigated. The copolymer was composed of MDI, PTMG-2000, 1,4-butanediol (BD), and cross-linking agent. In addition to the conventional cross-linking method, shape memory polyurethane (SMPU) was laterally cross-linked by a polyethyleneglycol (PEG) spacer to remain flexible after cross-linking, where PEG connected the hydroxyl groups of two polyurethane chains via MDI as a connecting agent. Phase separation of hard and soft segment is dependent on cross-linking and hard segment content. Hydrogen bonding and dipole-dipole interaction between hard segments provides strong interaction between copolymer chains in addition to chemical cross-linking. As the hard segment content increases, the copolymer shows better tensile mechanical properties and higher melting temperature of soft segment (Tm). Effect of glycerol cross-linking on mechanical properties and Tm of soft segment is low compared to hard segment effect. Significant increase in shape recovery in the case of 30 wt% hard segment copolymer is observed after glycerol cross-linking. The four-way cross-linking by pentaerythritol also improved the mechanical properties and showed a very high shape recovery (94%), where the interaction between hard segments is mainly responsible for the very high shape recovery rate. In the case of flexible cross-linking, significant increase in maximum stress compared to linear polymer was attained without any sacrifice in strain. Especially, it is shown in the stress-strain curve that the flexibly cross-linked SMPU shows similar behavior and superior tensile mechanical properties to natural rubber. Excellent shape recovery (96.8%) was found and shape recovery time was 3 times faster than linear polymer. The drastic change of the properties of polyurethane block copolymer is discussed in the point of copolymer chain interaction.