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

Mechanistic insights on nanosilica self-networking inducing ultra-toughness of rubber-modified polylactide-based materials

<p>Developing novel strategies to improve the impact strength of PLA-based materials is gaining a significant importance in order to enlarge the range of applications for this renewable polymer. Recently, the authors have designed ultra-tough polylactide (PLA)-based materials through co-addition of rubber-like poly(ϵ-caprolactone-<i>co</i>-d,l-lactide) (P[CL-<i>co</i>-LA]) impact modifier and silica nanoparticles (SiO₂) using extrusion techniques. The addition of silica nanoparticles into these immiscible PLA/P[CL-<i>co</i>-LA] blends altered their final morphology, changing it from rubbery spherical inclusions to almost oblong structures. A synergistic toughening effect of the combination of P[CL-<i>co</i>-LA] copolymer and silica nanoparticles on the resulting PLA-based materials therefore occurred. To explain this particular behavior, the present work hence aims at establishing the mechanistic features about the nanoparticle-induced impact enhancement in these immiscible PLA/impact modifier blends. Incorporation of silica nanoparticles of different surface treatments and sizes was thereby investigated by means of rheological, mechanical and morphological methods in order to highlight the key parameters responsible for the final impact performances of the as-produced PLA-based materials. Relying on video-controlled tensile testing experiments, a toughening mechanism was finally proposed to account for the impact behavior of resulting nanocomposites.</p