Influence of long-term storage in cold hibernation on strain recovery and recovery stress of polyurethane shape memory polymer foam

In this paper, the effects of long-term storage in compressed cold hibernated elastic memory (CHEM) polyurethane foam, a kind of shape memory polymer, are investigated experimentally. The foams were pre-strained at a high temperature, which was above the glass transition temperature, to 80% and 93.4%, respectively, and then cooled back to room temperature. After various periods of cold hibernation (up to two months), they were heated up at fixed length or against different constant loads. It is found that: (1) the maximum stress that the foam could exert at fixed length depends heavily on the amount of pre-strain; (2) expansion rates of 380 and 1273% from the hibernated size against a 1 N load (pre-strained by 80 and 93.4%, respectively) are achievable. However, upon further increases in load, the expansion is reduced dramatically. It appears that the tested CHEM polyurethane foam retains its shape memory properties even after being stored in a compacted state for a long period. Complete strain recovery is attainable for a hibernation period of up to two months.