Effect of crosslinking and long-term storage on the shape-memory behavior of (meth)acrylate-based shape-memory polymers

Abstract

This work highlights the free- and fixed-strain shape-memory response of amorphous (meth)acrylate-based shape-memory polymers as the level of crosslinking is varied from uncrosslinked to highly crosslinked (corresponding to a decrease in failure strains and overall increase in rubbery moduli and failure stresses). The effect of long-term storage on the free-strain shape-memory response is also considered. Tensile deformation levels during the shape-memory cycle are 90% of failure strain values to determine the full extent of free- and fixed-strain recovery behavior. All materials demonstrate full shape-recovery under free-strain conditions (material is unconstrained during recovery); however, total recoverable strains increase with decreasing crosslinking level, with uncrosslinked and lightly crosslinked materials recovering strains on the order of 3–10× that of moderately and highly crosslinked materials. In contrast, under fixed-strain conditions (material is fully constrained in the fixed shape during recovery), the magnitude of recovery stress generation increases with increasing crosslinking level, with highly crosslinked materials demonstrating recovery stress levels 4–20× that of lightly crosslinked and uncrosslinked materials. The ability to produce recovery stresses on par with those reached during deformation also increases with crosslinking level. Stored-shape fixation and free-strain recovery levels remain stable after long-term storage in the deformed temporary state at 20 °C; however, recovery onset temperatures increase (by up to 9 °C) with storage time spanning ∼1 year, as do rates of free-strain recovery (by up to 9×), due to physical aging. Results indicate that aging could potentially be used as a method for shape-memory response optimization.