Abstract
Shape memory polymers are materials that are able to retain a deformed state until an external stimulus, most typically heat, triggers recovery to the original geometry. Whereas typically, shape memory polymers are required to recover fast (seconds to minutes), many applications, particularly in the medical field, would benefit from a slow recovery (days to weeks). In this work, we exploit the broad glass transition range of photo-cured poly(D,L-lactide) dimethacrylate networks to obtain recovery times of up to 2 weeks, at 11 °C below the peak glass transition temperature of 58 °C. Recovery times decreased considerably for higher recovery temperatures, down to ∼10 min at 55 °C. A large spread in glass transition values (53.3–61.0 °C) was observed between samples, indicating poor reproducibility in sample preparation and, hence, in predicting shape recovery kinetics for individual samples. Furthermore, a staged recovery was observed with different parts of the samples recovering at different times. The ability to prepare complex structures using digital light processing stereolithography 3D printing from these polymers was confirmed. To the best of our knowledge, this work provides the first experimental evidence of prolonged recovery of shape memory polymers.
Highlights
Shape memory polymers (SMPs) can be deformed and fixed into a temporary shape, to later be triggered into recovering to the original geometry by the application of a certain stimulus.1,2 The possible stimuli include heat,1 light irradiation,3–7 electric currents,8–10 magnetic fields,11,12 solvents,13–17 pH change,18,19 and redox reactions.20,21 Most SMPs are triggered by direct heating and, referred to as thermally actuated or thermoresponsive
Preparation and dynamic mechanical analysis of polylactide SMP networks The synthesis of poly(D,L-lactide)dimethacrylate (PDLLA-2MA) macromonomers with a molecular weight Mn of 2.85 6 0.18 kg/mol, a D,L-lactide (DLLA) monomer conversion of 97% 6 2.2%, and a degree of methacrylation of 94% 6 1.8% was confirmed through 1HNMR. These macromonomers were used in the formulation of a liquid resin, further containing a nonreactive diluent/solvent and a photo-initiator (TPO), from which rectangular strips were photo-polymerized on a low-cost digital light processing (DLP) stereolithography 3D printer
We demonstrated, for the first time, prolonged recovery of shape memory polymers up to 2 weeks
Summary
Shape memory polymers (SMPs) can be deformed and fixed into a temporary shape, to later be triggered into recovering to the original geometry by the application of a certain stimulus. The possible stimuli include heat, light irradiation, electric currents, magnetic fields, solvents, pH change, and redox reactions. Most SMPs are triggered by direct heating and, referred to as thermally actuated or thermoresponsive. Most SMPs are triggered by direct heating and, referred to as thermally actuated or thermoresponsive. Examples of such polymers are SMPs that typically hold their temporary shape upon cooling through crystallization below their melting temperature [e.g., poly(cyclooctene)22] or vitrification below their glass transition temperature [e.g., sodium montmorillonite poly(ethyl methacrylate) (PEMA) nanocomposites23]. The crosslinks act as netpoints that are not affected by the change in temperature applied to the polymer when triggering the shape recovery. They provide the overall geometrical stability required for the original, permanent, shape to be recovered.
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