Influence of protocol history on shape memory effects in polymers

Shape memory materials are smart materials that can reversibly switch between an equilibrium and a mechanically programmed state via an external trigger. While shape memory alloys (SMAs) were discovered first and are used widely, cross-linked polymers offer significant advantages. Today, the field of shape memory polymers (SMPs) includes systems that deform reversibly with strains far beyond 100% in response to heat, light, electrical, and chemical stimuli, offering a wide range of applications. Conventional study and usage of SMPs involves applying a constant stress or strain, then cooling to “fix” the temporary shape via crystallization or vitrification. Here we explore how the temporary state can represent a wider variety of protocols. We use a semi-crystalline polymer, crosslinked polycyclooctene (PCO), whose shape memory arises from network chain crystallization. Our results illustrate that different protocols during cooling may result in the same “fixed” shape, but with different mechanical properties and crystalline microstructure.