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We analyzed the effects of crosslinking fraction and number of functional sites per hardener molecule on the stress recovery and topology of thermoset shape memory polymers (TSMPs) via coarse-grained molecular dynamics simulations. After systematically varying the quality of the crosslinked network by manipulating the number of unique epoxies reacted with each hardener, we found that two fingerprints correlate well with stress recovery of TSMPs. These fingerprints are the fraction of epoxy molecules connected to two distinct hardener molecules, and the fraction of molecules that are part of the largest or main network in the system. Their product can be used as a topological score ($S_{\text{topo}}$) to quantify the topological feature of the network. When analyzing stress recovery as a function of$S_{\text{topo}}$, we found a strong correlation between$S_{\text{topo}}$and recovery stress. Moreover, we observed that while a higher crosslinking fraction did frequently lead to a higher stress recovery, many exceptions existed. High functionality hardeners tend to exhibit higher stress recovery at similar$S_{\text{topo}}$, especially at high (>0.65)$S_{\text{topo}}$. These results suggest that increasing the number of functional sites per hardener molecule combined with improving the topology of the network with a method such as semi batch monomer addition can lead to an improvement in the stress recovery of TSMPs.

 

Abstract Defects in crosslinked networks have a negative effect on mechanical and functional properties. In this study, an epoxy resin diglycidyl ether of bisphenol A crosslinked by a hardener 4,4-diaminodiphenyl methane with various cyclic topologies was simulated to find correlations between the mechanical/shape memory properties (i.e. glassy/rubbery elastic modulus, shape recovery ratio, and recovery stress) and cyclic topologies (i.e. number of total loops, number of defective loops (DLs), etc). The effect of cyclic topology on shape memory properties was more significant than its effect on mechanical properties, altering recovery stress by more than 25% on average. After analyzing several topological fingerprints such as total number of loops, number of DLs, and number of higher order loops, we found that the effect of cyclic topology on the mechanical/shape memory properties of the systems can be best understood by the fraction of hardeners reacted with four distinct epoxy molecules (tetra-distinctly-reacted (TDR) hardeners). By increasing the number of TDR hardeners, the network is closer to ideal, resulting in an increase in the number of higher order loops and a reduction in the number of DLs, which in turn leads to an increase in rubbery elastic modulus and shape recovery ratio to a lesser degree, but a substantial increase in recovery stress. These results suggest that utilization of experimental techniques such as semibatch monomer addition, which leads to a more expanded and defect-free network, can result in a simultaneous increase in both shape recovery ratio and recovery stress in thermoset shape memory polymers (TSMPs). Moreover, topology alteration can be used to synthesize TSMPs with improved recovery stress without significantly increasing their stiffness.