Shape memory polymers (SMPs) are responsive smart materials capable of physical transformation upon the application of an external stimuli, such as temperature. A large class of SMPs consist of a “permanent” cross-linked polymer matrix with a second reversible, shape fixing network. Two such crosslinked polymer matrices are chemically crosslinked elastomers, such as a sulfur cured natural rubber, and ABA triblock thermoplastic elastomers. It has recently been shown that these materials can be converted into high performing shape memory polymers by blending with a crystalline small molecule, such as a fatty acid, to form the reversible, shape fixing network.<br/><br/>This work investigates the relation between morphology and shape memory performance of polymer/crystalline small molecule blend SMP systems. In the case of natural rubber/fatty acid blends it will be shown that the deformation of the polymer matrix during shape programming can direct the crystallization of the small molecule additive leading to reinforcement along the strain direction, which enhances the shape fixity and provides the necessary resistance to the elastic forces in the deformed elastomer. It will be shown that this effect is dependent on both the concentration and chain length of the fatty acid and is more prevalent at high fatty acid loading and longer fatty acid tail length. In a second example, it will be shown that ABA triblock copolymer thermoplastic elastomers are able to act as SMPs without the addition of additional small molecule additive. It will be discussed how the shape memory properties depend on the molecular weight and programming of the SMP, which is in turn related to the chain dynamics and reorganization of the block copolymer morphology under load. Based on these findings a model is presented explaining the shape memory mechanism of neat block copolymer SMPs.

Morphology dependence of shape memory polymers

254th National Meeting (2017)

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