Relationship between recovered enthalpy and the shape‐memory effect in shape memory polymers

• Published in: Journal of applied polymer science 2023-12, Vol.140 (48)
• Main Authors: Siwakoti, Midhan, Mailen, Russell W.
• Format: Article
• Language: English
• Subjects: Aging; Chains (polymeric); Dynamic mechanical analysis; Enthalpy; Mechanical properties; Polymers; Recovery time; Shape effects; Shape memory; Strain rate; Thermodynamic equilibrium
• ISSN: 0021-8995; 1097-4628
• DOI: 10.1002/app.54727

Shape memory polymers (SMPs) maintain a temporary shape after pre‐straining, wherein the polymer chains are constrained in a non‐equilibrium thermodynamic state. Physical aging lowers the chain conformational energy, which affects the mechanical properties. Herein, we investigate the relationship between physical aging and the shape recovery of SMP sheets, whereas both processes involve motion of polymer chains. We induce conformational changes to polymer chains either by physical aging or via a thermomechanical pre‐straining process. We then quantify structural relaxation via recovered enthalpy measurements using modulated differential scanning calorimetry (MDSC), and the shape recovery performance using dynamic mechanical analysis (DMA). We vary pre‐straining holding time, amount, and rate and observe the relationship between physical aging, recovered enthalpy, and the shape recovery performance. The results indicate that an increase in recovered enthalpy correlates with an increase in characteristic shape recovery time. Further, a maximum decrease in recovery time of 65% is observed at the highest strain rate, and only small amounts of recovered enthalpy occur for aging times longer than 16 h. The results provide insight into the relationship between physical aging and its effects on shape memory, which is important for applications requiring storage for long durations.