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.