Comparison of Swollen vs. Compounded Cross-linked High-Cis-1,4-Polybutadiene/n-Tetracosane Shape Memory Polymers

Blending elastomers with phase change materials offers a modular approach to fabricating smart materials, such as shape memory polymers. This study compares shape memory polymers prepared by peroxide cross-linking a blend of polybutadiene and n-tetracosane (compounded samples) versus swelling peroxide cross-linked polybutadiene in n-tetracosane (swollen samples). Characterization of the thermal, thermomechanical, and shape memory behavior of the two types of samples show very similar behavior as a function of polybutadiene content. The sample with ca. 30% polybutadiene displayed shape memory metrics of ~90% fixity and ~99% recovery at 25% applied strain. The main difference in the samples was the change in fixity and recovery with cycling where they were heated and cooled using water baths. The compounded samples exhibited a 1.4% decrease in fixity, 0.2% decrease in recovery, and 2.25% weight loss over 20 cycles. In contrast, the swollen samples have a 4.2% decrease in fixity, 0.4% decrease in recovery, and 5.63% weight loss over 20 cycles. This weight loss was attributed to the expulsion of the n-tetracosane that experienced a higher driving force in the swollen sample with stretched chains. The cyclic and long-term aging of these polymers is different depending on the preparation method to incorporate the n-tetracosane into the material.