Designing Multiple-Shape Memory Polymers with Miscible Polymer Blends: Evidence and Origins of a Triple-Shape Memory Effect for Miscible PLLA/PMMA Blends

Shape memory properties of polymers represent one of the most expanding fields in polymer science related to numerous smart applications. Recently, multiple-shape memory polymers (multiple-SMPs) have attracted significant attention and can be achieved with complex polymer architectures. Here, miscible PLLA/PMMA blends with broad glass transitions are investigated as an alternative platform to design multiple-SMPs. Dual-shape memory experiments were first conducted at different stretching temperatures to identify the so-called temperature memory effect. The switch temperature of the symmetric 50% PLLA/50% PMMA blend smoothly shifted from 70 to 90 C for stretching temperatures increasing from 65 to 94 C, attesting for a significant temperature memory effect. Asymmetric formulations with 30% and 80% PMMA also present a temperature memory effect, but the symmetric blend clearly appeared as the most efficient formulation for multiple-shape memory applications. A programming step designed with two successive stretchings within the broad glass transition consequently afforded high triple-shape memory performances with tunable intermediate shapes, demonstrating that the symmetric blend could represent an interesting candidate for future developments. Advanced shape recovery processes are consistent with a selective activation of specific soft domains or nanodomains arising from the broad glass transition and the large distribution of relaxation time observed by DSC and dielectric spectroscopy. Polarized IR measurements pointed out that the composition of activated/oriented soft domains could vary with stretching temperature, giving rise to the temperature-memory effect. Consequently, from a polymer physics standpoint, nanoscale compositional heterogeneities within the symmetric blend could be suspected and discussed on the basis of available models for miscible blends and for multiple-SMPs.