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