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Reversible Actuation of Thermoplastic Multiblock Copolymers with Overlapping Thermal Transitions of Crystalline and Glassy Domains
Polymeric
materials possessing specific features like programmability,
high deformability, and easy processability are highly desirable for
creating modern actuating systems. In this study, thermoplastic shape-memory
polymer actuators obtained by combining crystallizable poly(ε-caprolactone)
(PCL) and poly(3<i>S</i>-isobutylmorpholin-2,5-dione)
(PIBMD) segments in multiblock copolymers are described. We designed
these materials according to our hypothesis that the confinement of
glassy PIBMD domains present at the upper actuation temperature contribute
to the stability of the actuator skeleton, especially at large programming
strains. The copolymers have a phase-segregated morphology, indicated
by the well-separated melting and glass transition temperatures for
PIBMD and PCL, but possess a partially overlapping <i>T</i><sub>m</sub> of PCL and <i>T</i><sub>g</sub> of PIBMD in
the temperature interval from 40 to 60 °C. Crystalline PIBMD
hard domains act as strong physical netpoints in the PIBMD−PCL
bulk material enabling high deformability (up to 2000%) and good elastic
recoverability (up to 80% at 50 °C above <i>T</i><sub>m,PCL</sub>). In the programmed thermoplastic actuators a high content
of crystallizable PCL actuation domains ensures pronounced thermoreversible
shape changes upon repetitive cooling and heating. The programmed
actuator skeleton, composed of PCL crystals present at the upper actuation
temperature <i>T</i><sub>high</sub> and the remaining glassy
PIBMD domains, enabled oriented crystallization upon cooling. The
actuation performance of PIBMD-PCL could be tailored by balancing
the interplay between actuation and skeleton, but also by varying
the quantity of crystalline PIBMD hard domains via the copolymer composition,
the applied programming strain, and the choice of <i>T</i><sub>high</sub>. The actuator with 17 mol% PIBMD showed the highest
reversible elongation of 11.4% when programmed to a strain of 900%
at 50 °C. It is anticipated that the presented thermoplastic
actuator materials can be applied as modern compression textiles