Creep and Stress Relaxation in a Film of Shape Memory Polymers of Polyurethane Series.

Abstract

Basic properties of creep and stress relaxation in a film of shape memory polymers of polyurethane series above and below the glass transition temperature Tg were investigated experimentally. The results are summarized as follows. Modulus of elasticity, yield stress and 100% modulus are large below Tg but small above Tg. The relationships between normalized stress at each temperature and creep strain almost coincide without depending on temperature. Creep strain above Tg recovers after unloading but creep strain below Tg does not recover. Irrecoverable creep strain which remains below Tg recovers at temperatures in the vicinity of Tg during the heating process. A half of initial stress decreases under constant strain after a certain duration of time for each temperature and strain. The ratio of relaxed stress to initial stress increases with an increase in strain below Tg. The deformation properties of the materials with different Tg are almost same except for difference in Tg.