While a variety of adaptive and stimuli-responsive materials like hydrogels,liquid crystalline elastomers, and shape memory materialshave been developed, mimicking programmable behavior in a reversible way remains elusive. We have developed a general strategy for enabling reversible shifting of programmable shapes in conventional semi-crystalline elastomers with uniform chemical composition and without applying a persistent external force. This behavior stems from the correlated interplay between a crystalline scaffold and a network of chemical crosslinks, each capable of encoding a distinct shape. Thermodynamically driven relaxation of extended polymer chains on heating is inverted by kinetically preferred pathways of polymer crystallization on cooling.   Our studies of semi-crystalline elastomers have shown that Young's modulus exhibits three characteristic regimes with decreasing crystallinity upon melting.   Most significant shape shifting has been observed in regime II, which demonstrates characteristic scaling E~f3, tentatively ascribed to crystalline clusters behaving as hard inclusions within an amorphous matrix. A composite model has been applied to fit the data points, assuming a well-defined relation between volume fractions of clusters and crystallites fc~fa, where a depends on cluster organization. This strategy integrates three different shape memory (SM) behaviors: conventional one-way SM, two-way RSM, and one-way RSM. The corresponding strain contributions can be tuned by controlling the respective proportions of chemical crosslinks and crystallites.

Shapeshifting: General principles of reversible shape memory in semicrystalline elastomers

248th National Meeting (2014)

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