Temperature-memory polymers are able to generate a substantial mechanical response when heated above the temperature, at which a preceding deformation was carried out. Here we show how to design the temperature-memory effect (TME) by thermomechanical treatment. As a model polymer, phase segregated poly(ester urethane) (PEU) containing crystallizable segments of poly(1,4-butylene adipate) (PBA) was used. For programming, strain elongation was applied at temperatures within the PBA melting transition area, before temperature holding, unloading and cooling were carried out. Upon heating under stress-free or constant strain recovery conditions, precisely set temperature-memory onsets could be witnessed. Most importantly, strain fixities and recoverabilities the same as maximum recovery stresses turned out to be controllable by strain rate and temperature holding time after deformation, while transition temperatures remained largely unaffected. The tailoring of thermoresponsiveness was structurally enabled by different PBA crystallinities in the programmed state as verified by wide-angle X-ray scattering (WAXS). The reported studies intend to design TMEs in semicrystalline polyurethanes according to user-defined needs to make this technology broadly applicable.
We demonstrate that phase-segregated poly(ester urethane) (PEU) with crystallizable switching segments of poly(1,4-butylene adipate) (PBA) excels as high-performance temperature-memory polymer. Temperature-memory effects (TMEs) with regard to strain and stress recovering could be programmed by polymer elongation at temperatures below or within the PBA melting transition, followed by cooling under constant stress below the PBA crystallization transition and unloading. Beyond that conventional approach, a novel TME programming route was designed, mostly consisting in specimen elongation and unloading at the same temperature. As a result, an enhanced control over the onsets of strain and stress recovering could be achieved. With these findings, the TME could be exploited to switch quick response (QR) codes in recently developed information carriers from unreadable to readable. We conjecture that such behavior can be programmed into virtually all semicrystalline elastomers and anticipate applicability as label technology to monitor temperature abuse of food and pharmaceuticals.
Here we present a suitable tag prototype with phasesegregated
poly(ester urethane) (PEU) as base material for
effectively switching a quick response (QR) code in its surface
from non-readable to readable. In comparison with recently
introduced tags (different geometry) we minimized the
thickness from plaque (2 mm) to foil size (0.5 mm) and reduced
the lateral QR code length from 15 to 5 mm. Subsequent to
surface-dyeing by means of guest diffusion, the QR code was
laser-engraved. The implementation of thermo-mechanical
functionalization via tensile deformation and cooling resulted in
the formation of stable shapes, which exceeded the barrier of
QR code readability at an elongation of 20%. Once
functionalized, tags were switched on demand by heating. As
such the recovery of the PEU was accomplished and the QR
code could again be read out. QR code carriers based on shape
memory polymer can be used in product and brand protection
applications.