A bridging model of a water-triggered shape-memory effect in an amorphous polymer undergoing multiple glass transitions

Hydrothermally-driven shape memory polymers (SMPs) have been extensively studied due to their advantage of having multiple response capabilities. In these SMPs,bound water reduces their glass transition temperatures (T (g)) by plasticizing the soft segments to achieve a water-triggered shape-memory effect (SME). However, the effect of bound water on hard segments, which has a synergistic effect on the T (g) and water-triggered SME of the soft ones, remains largely unexplored. In this study, we propose a new model to explore the working principles and hydrothermally-driven shape memory behaviors of amorphous SMPs. The bound water molecules are first divided into bridging and non-bridging bound water, and then a bridging effect is proposed to convert hard segments into soft ones, thus affecting the T (g) and water-triggered shape memory behavior in SMPs. An extended Gordon-Taylor model is formulated to identify the effects of bound water weight fraction and T (g). Furthermore, a constitutive relationship between strain and relaxation time has been developed to describe the effects of temperature and bound water weight fraction on the hydrothermally-driven shape memory behaviors. Finally, the effectiveness of the proposed models is verified using the experimental results of amorphous SMPs reported in the literature.