In two hydrolytic degradation studies the tensile (mechanical) and functional (thermo-mechanical) properties of a hydrolysis-stabilized shape memory poly(ester urethane) and its non-stabilized analog were investigated. Hydrolytic degradation was enforced by specimen immersion in de-ionized water at 80 °C. Significant differences in the fundamental shape memory parameters were monitored as function of aging time for the stabilized and non-stabilized polymer. This included the ability to recover strain (shape recoverability) and stress (stress recoverability) on heating after shape programming. Hydrolysis-related mechanical and functional changes were correlated with morphological ones, detected by differential scanning calorimetry (DSC). The shape memory poly(ester urethane), which was protected by a carbodiimide-based hydrolysis stabilizer, revealed significantly improved resistance towards hydrolysis with respect to various mechanical and shape memory parameters.
Hydrolytic degradation and functional stability of a segmented shape memory poly(ester urethane)
(2009)
In order to understand the effects of water and hydrolytic ageing on semi-crystalline poly(ester urethane) and its shape memory functionality, water immersion experiments at elevated temperature have been performed on a model substance and various parameters were monitored: change of the melting/crystallisation temperatures, substantial increase in crystallinity, temperature dependence of the water diffusion coefficient and solubility, hydrogen-bonding index and phase mixing by peak deconvolution of the FT-IR carbonyl region and day-to-day tensile and thermo-mechanical cyclic tensile tests. A rising fraction of freezable water agglomerates in the polymer was found for specimens cooled from the immersion temperature. The degradation process could be divided into three phases: an induction phase, a phase of continuous degradation and a phase of accelerated degradation. Shape recovery remains fairly constant during phase one and decreases slowly during phase two. The increase in crystallinity in phase two is accompanied by an increase in shape fixing ability.
Herein we demonstrate the realization of a new technological concept, which enables the use of shape memory polymers (SMPs) as switchable information carriers. At first, we applied a surface-specific dyeing process based on 'guest-diffusion' on two sophisticated polymeric host materials, including a thermoplastic poly(ester urethane) SMP and a thermoset epoxy-based SMP. Upon drying, self-assembly of the dye molecules inside the polymer surfaces occurred, resulting in homogeneous color penetration depths of about 100 µm. Subsequently, the colored surfaces were patterned with quick response (QR) codes. For this purpose, laser ablation was used. The resulting cavity depth was exceeding the color penetration depth. This assured sufficient surface contrast and rendered the QR codes machine-readable. In a progressive approach, two thermo-mechanical functionalization protocols were designed in accordance with the thermal properties of the polymers. As a result of programming, the tag prototypes were converted into stable, temporary shapes with non-decodable QR code information. When thermally triggering the shape memory effect on the functionalized tags, we verified the mostly complete recovery of the polymer surface and the associated restoration into the almost original shape. As such, the QR code could again precisely be read out. We anticipate that tagging products with these information carriers is helpful for the purpose of secure one-time identification.
A series of cyclic thermo-mechanical measurements was conducted on segregated poly(ester urethane) to study substantial changes in triple-shape properties as a result of hydrolytic aging (80 °C). Prior to the analysis of aging effects, a concept of triple-shape testing was elaborated, starting with the implementation of two distinct programming units. The first one included a deformation at 60 °C to εm1 = 100% (temporary shape B) and its fixing through soft segment crystallization by cooling to -20 °C under constant strain. The second one consisted of a deformation at -20 °C to εm2 = 200% (temporary shape A) and its stabilization through soft segment vitrification as achieved by cooling to -60 °C under fixed strain constraint. Then, gradual heating of the polymer from below to above its thermal transition temperatures gave two independent shape recovery responses in the reverse order of shape fixing: A → B through passing the glass transition by heating from -60 to 23 °C and B → C (back to the permanent shape), when heating the material from 23 to 60 °C and thus above its soft segment melting temperature. In a progressive approach, the storage of loading history through the sequential fixing of two temporary shapes was proven by the development of shape recovery stresses under constrained environment. With the implementation of the two testing methods several aging-related effects could be detected. Good shape fixing abilities ≥90% for both shapes were found and contrasted by significant changes in shape recoverabilities and stress storage capacities. Further insights derived from differential scanning calorimetry (DSC) measurements, indicating an aging-related growth in soft segment crystallinity, and dynamic mechanical analysis (DMA), suggesting a plasticizer effect of water onto the polymer matrix and that aging favoured an increase in cross-linking density.
The shape memory functionality of a segmented poly(ester urethane) and its hydrolytically aged specimens has been studied by cyclic thermo-mechanical measurements with an imposed strain of 100%. The shape memory effect was triggered by a melting transition in the soft segment phase. Aging was enforced by immersion in hot de-ionized water. In the course of the immersion the tensile properties (secant moduli, stress and strain at yield and break) were impaired by hydrolysis. Advanced specimen embrittlement finally led to rupture during the first thermo-mechanical cycle. This happened after 68 days of aging at 55 °C and correspondingly after 8 days at 80 °C. The residual strain after the first cycle, which was about 25%, increased significantly with aging time. Therefore, the total strain recoverability became ever smaller: aged specimens needed conditioning by at least two cycles for a full development of shape recoverability. Likewise the recovery force decreased continuously. Despite these degradation effects, it was observed that the shape fixity and the cycle-related shape recovery of appropriately conditioned specimens (number of cycles N > 2) remained on a constant high level (at round 100% and between 90% and 100%, respectively) throughout the whole aging period. These observations are discussed within the framework of a simplified model of the behavior of crystallizable shape memory polymers. The amorphous state of the polymer is described by the equation of the linear visco-elastic solid. As for the semi-crystalline state the material is assumed to react elastically with respect to deviations from the configuration, which was frozen up under constraint conditions. The curves of the dependence of the material behavior on aging time at 55 °C match perfectly those at 80 °C when the time axis is adjusted by a factor of 8.5, from which the apparent activation energy for hydrolytic aging in the amorphous state of 82 kJ mol-1 could be deduced.
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.
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.
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.