Mechanism of Behavior of Two-Way Shape Memory Polymer under Constant Strain Conditions

Abstract

In this paper, we report detailed investigation of structural changes occurring in two-way shape memory polymers (2W-SMP) upon thermal cycling at constant strain conditions. We demonstrate that cooling of stretched chemically cross-linked polycaprolactone results first in a slow decrease of stress, the stress drops rapidly upon further cooling, and further cooling results in sharp increase of stress. In this paper, we investigated structural changes occurring in each of these three phases using in situ force measurements and XRD analysis (SAXS and WAXS). In the first phase, the stress decreases due to decrease of entropic factor in the elastic state. In the second phase, polymer chains start to crystallize in the stretched state as a result of the number of active springs producing stress decreasing. This results in the decrease of the stretching degree of other polymer chains, and finally, unstretched polymer chains crystallize that result in an increase of stress─the polymer simply tends to shrink.