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- Shape memory polymer (8)
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Solvent-cast films from shape memory poly(ester urethane) (PEU) containing different weight contents of microencapsulated thermochromic pigments (T-PIGs) were prepared by drying in air. Spectrophotometric investigations unveiled that gradual loading with T-PIG black resulted in continuous darkening of the films up to filler contents of 20 wt%, accompanied by a steady enhancement of thermochromic properties. Taking this composition as standard, PEU films equipped with T-PIG black, blue and red were deposited atop PEU plaques to obtain laminate structures. Herein, the cover layer thickness (100 ± 5 µm) and the good dispersion of T-PIGs inside the polymer matrix were verified by scanning electron microscopy. Machine-readable information carriers were prepared by laser engraving quick response (QR) codes into the cover layer of the laminates and subsequently cutting cuboidal samples therefrom. Finally, thermo-mechanical programming of the QR code carriers was applied to randomly distort the code patterns, thus rendering them unreadable. Upon heating, surface decolorization and shape recovering occurred; during the ensuing cooling, the surface color and contrast reappeared whereupon the QR codes could be read out. Spectrophotometric, calorimetric and thermo-mechanical investigations gave evidence that the color switching temperature of the T-PIGs roughly coincided with the melting temperature of the ester-based switching segment and thus with the activation temperature of the shape memory effect. Apart from that unique functionality, manifold design concepts may render information carriers difficult-to-copy. Therefore, we anticipate tremendous potential as anti-counterfeiting technology.
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.
The selective compression of quick response (QR) and Data Matrix code carriers based on shape memory polymer (SMP) with a freely configurable steel ball type indenter and adjacent thermo-mechanical shape fixing gave notched, room temperature (23 °C) stable, temporary shapes with non-decipherable codes. The microscopic investigation of cryomicrotome sections unveiled indentation-related shape fixities of about 90 %. Independent of the selected two-dimensional code, the triggering of the SM effect resulted in sufficient shape recoveries to restore the code readability so that a maximum number of characters including 122 for a QR code (version 7) and 112 in case of a Data Matrix code (version 12) could be read with a scanning and decoding device. Due to the large number of difficult to copy shapes with on demand releasable information, SMPs may serve as viable information carriers for product and brand protection applications.
Ein neu entwickeltes thermoplastisches Polyurethan (TPU) ist alles andere als „vergesslich". Formteile aus dem Material lassen sich in einer temporären Form fixieren und wieder nahezu in ihre ursprüngliche Gestalt zurückverwandeln. Dieser Formgedächtniseffekt hat gute Anwendungschancen - vom Marken- und Produktschutz über den Maschinen- und Automobilbau, bis hin zur Freizeit- und Spielzeugindustrie.
Auch Kunststoffe können sich erinnern. Das zeigt das Beispiel eines neuen thermoplastischen Polyurethans. Das Formgedächtnispolymer kann vorübergehend in einem deformierten Zustand fixiert und dann wieder nahezu in seine ursprüngliche Gestalt zurückverwandelt werden. Einsatzchancen hat es unter anderem im Markenschutz und Maschinenbau sowie in der Textil-, Sport-, Freizeit- und Spielzeugindustrie.
Plastics can also remember things. This is made clear by a new thermoplastic polyurethane. The shape memory polymer can be temporarily fixed in a deformed state and then converted back almost entirely into its original shape. Potential applications exist in trademark protection and mechanical engineering as well as in the textile, sports, leisure and toy industries.
Here, we present an investigation of the triple-shape properties of star-shaped polyhedral oligomeric silsesquioxane-poly(ε-caprolactone) polyurethanes (SPOSS-PUs), which have three-dimensional network structures. In a typical 'triple-shape functionalization process', mostly consisting of two tensile deformations at different temperatures, chain immobilization of the polymer network component poly(ε-caprolactone) (PCL) was successfully realized first through crystallization and then through vitrification. Subsequently, large parts of the respective strains were released under stress-free recovery conditions. The two-fold fixed ('programmed') specimens responded to heating with two independent length contractions (switching steps); the first shape change was associated with PCL devitrification and the second one with the melting of hitherto crystalline PCL. It was revealed that the triple-shape properties of SPOSS-PU networks considerably depend on PCL network chain length. When applying exactly the same 'triple-shape creation procedure', larger strain releases were detected in the first transition for polymers with a higher PCL network chain length, whereas the second transition was more accentuated for SPOSS-PU networks with a shorter PCL chain length. In the course of thermo-mechanical cycling, the formation of a neck during the second tensile deformation was repeatedly detected for SPOSS-PUs with higher PCL network chain length; in the subsequent recovery process the specimens even exhibited the highest total strain recoverability. Finally, gradual strain release could be achieved at temperatures below the PCL melting transition through the selection of up to four temperature holding steps, at which every time stable shapes were formed.
Phase-segregated PEU exhibits two-way shape changes under various constant stresses. Most distinct actuation (expansion on cooling and contraction on heating) can be detected between +60 and -20 °C when the maximum stress applied during a thermo-mechanical pretreatment approached the point where deformation-induced crystallization of the PBA soft segment sets in. The driving forces for the strain-related two-way shape changes consist of PBA crystallization and PBA melting-induced entropy elasticity. At low constant stresses (<1.5 MPa), entropy elasticity also contributes to specimen expansion on cooling. The results of multiple- cycling experiments imply that PEU is an appropriate candidate for the fabrication of thermoresponsive actuators working under constant stress conditions.
Star-shaped polyhedral oligomeric silsesquioxane (SPOSS)-based multi-arm poly(ε-caprolactones) (PCLs) with various arm lengths were synthesized by ring opening polymerization of ε-caprolactone. Increasing the PCL arm lengths was observed to increase the melting temperature and also the melt crystallization temperature in SPOSS-PCLs. Star-shaped POSS-PCL polyurethanes (SPOSS-PUs) were then prepared by cross-linking SPOSS-PCLs with hexamethylene diisocyanate (HMDI) and their shape memory (SM) behaviour was determined. For this purpose two cyclic thermo-mechanical testing concepts with similar programming (deformation and shape fixing) but different recovery strategies (free strain versus fixed-strain constrained stress recovery) were used and the synthesized polymers were systematically investigated around the crystallization and melting temperatures of PCL. SPOSS-PU with the highest POSS content exhibited remarkable cycles-averaged (N = 2–5) shape fixities and strain recoverabilities of 98% and stress recoverabilities close to 100%. The main structural factors that were expected to reduce the SM performances with increasing PCL arm lengths were a lowering of cross-link density (content of octasubstituted POSS) and an increase in chain flexibility/molecular mobility.