Dynamic Mechanical Behavior of Photo-Cross-linked Shape-Memory Elastomers

Cross-linked elastomers containing reversibly binding side groups are capable of storing elastic energy on multiple time-scales, giving rise to shape-memory and self-healing properties. Photo-cross-linkable benzophenone side groups were incorporated into linear macromers containing reversible (2-ureido-4-pyrimidinone, UPy) side groups. This method enables melt-processing of shape-memory elastomers into complex permanent shapes, and samples can be prepared with much higher UPy-content. UV vis spectroscopy was applied to study the efficacy of the cross-linking process. Resulting elastomer networks with variable densities of covalent crosslinks and reversibly associating side groups were systematically prepared and studied. Dynamic mechanical analysis revealed the presence of two storage modulus plateaus: a high-temperature plateau attributed to covalent cross-links, and a lower temperature plateau attributed to both reversible and covalent cross-links. Results also show that dynamic cross-links behave nearly as effectively as permanent cross-links below the UPy hydrogen bond transition and that the presence of a covalent network supports cooperative binding of UPy side groups.