Free Volume Study on Three Types of Shape Memory Polymers by Positron Annihilation

Temperature dependence of nanoscopic environment of shape memory polymers such as polynorbornene (T$\text{}_{g}$≈313 K), polyurethane (T$\text{}_{g}$≈321 K=dynamic viscoelasticity, destruction of hydrogen bond ≈353 K) and styrene-butadiene copolymer (T$\text{}_{m}$=333 K) is observed in terms of free volume parameters - average size, numerical concentration and size distribution - estimated from positronium lifetime measurement, and mechanisms of their shape recoveries are discussed at a molecular level.