Study on the shape memory mechanism of SMPUs and development of high-performance SMPUs

Abstract

The shape memory polyurethanes (SMPUs) drew much attention because of their extensive application potentials. They have either a melting transition temperature (Tm) or a glass transition temperature (Tg) as switch temperature (Ttrans) and thus can be abbreviated as Tm-SMPUs and Tg-SMPUs, respectively. Although a few investigations have been made, the relationships between structure and shape memory properties of SMPUs are less known. We synthesized a series of Tm-SMPUs and three sets of Tg-SMPUs and investigated their morphology with multiple techniques including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), wide angle x-ray diffraction (WAXD) and small angle x-ray scattering (SAXS). The shape fixity, shape recovery and recovery stress of the polymers were quantified via a series of thermomechanical tests. The relationships between chemical structure and composition, morphological structure, thermomechanical conditions and shape memory properties are attained and summarized as follows. With the increase of hard segment content (HSC), the Tm-SMPUs exhibit a series of morphological features from having no hard phase to isolated hard phase and interconnected hard phase. The shape fixity of the Tm-SMPUs decreases with the increase of HSC because of the increasing limitation from hard domains. The study reveals that the Tm-SMPUs with higher HSC should be deformed to large strain so as to develop significant strain-induced crystallization and to attain better shape fixity. Under the shape recovery temperature (Trec=Tm+20°C), the Tm-SMPUs having isolated hard domains give rise to better shape recovery (>90%). The shape recovery decreases dramatically when the hard domains change from isolated into interconnected state because of the prominent deformation of hard phase. When the deformed samples are heated up to a sufficiently high temperature above 100°C the shape recovery curves of all the Tm-SMPUs go up to over 95%, i.e. the irreversible deformation strains are less than 5%. This is because that the deformation of soft phase can be recovered at a low temperature above the Tm of soft phase while that of hard phase shall be recovered at a high temperature above 100°C. Stress relaxation can result in the decrease of shape recovery. Moreover the Tm-SMPUs with lower HSC show more decrease of shape recovery because of comparatively weak physical cross-links can not effectively resist stress relaxation. This study proved that the soft phase for triggering shape memory effect of Tg-SMPUs is indeed a mixed phase of soft and hard segments. The phase compositions of the Tg-SMPUs were estimated via the SAXS analysis. The three sets of Tg-SMPUs are proved to have no hard domains, isolated and interconnected hard domains with the variation of chemical composition. Tg can be readily located at a specific temperature via changing types of soft segments and chemical compositions, but the resultant Tg-SMPUs have very different morphological structure. The shape fixity of Tg-SMPUs is mainly dependent on their Tg and the shape fixing temperature. Roughly, the Tg-SMPUs should be at least cooled down below the starting point at the low temperature side of the glass transition region so as to attain better shape fixing. Under Trec=Tg+15°C, the shape recovery descends with the increase of HSC except two Tg-SMPUs having less content of or no hard domains. At higher HSC the shape recovery drops dramatically likely due to the discontiuousity and contiuousity transition of hard phase. The Tg-SMPUs with lower HSC exhibit narrow shape recovery region while those having higher HSC show rather broad shape recovery region. Silmilar to Tm-SMPUs, the distribution of deformation between soft and hard phases accounts for the results. The recovery stress is raised significantly with the increase of HSC because of the increasing physical cross-linking density. The Tg-SMPUs can exhibit pronounced shape recovery and shape fixity when deformation temperature Tdef is in a vicinity of Tg. Lower Tdef can lead to apparent decrease of shape recovery and shape fixity while higher Tdef is able to result in broad shape recovery region. In order to improve the shape memory properties of the Tm-SMPUs, a series of shape memory polymer polyurethane-ureas (Tm-SMPUUs) chain extended with aromatic diamines were prepared. In comparison with Tm-SMPUs with identical HSC, the Tm-SMPUUs exhibit lower crystallinity of soft phase but much higher rubbery modulus, deformation stress and shape recovery because of the stronger urethane-urea hard domains. It can be concluded that the shape recovery is proportional to the strength of hard domains and inversely proportional to the fraction of hard phase. The recovery stress of a polyurethane-urea is as twice high as that of the polyurethane with identical HSC. Hence it is possible to enhance the shape recovery and recovery stress without sacrificing much shape fixity by using rigid hard segments provided that the HSC is properly controlled. A series of non-aromatic Tm-SMPUUs, which can show excellent ultraviolet light resistance, were synthesized with isophorone diisocynates (IPDI). As would be expected, higher HSC favors the formation of hard domains and thus enhances the physical cross-linking density. Increasing the soft segment length can raise both shape fixity and shape recovery because on one hand the longer soft segments favor the strain-induced crystallization and on the other hand the increased hard segment length prompts the aggregation of hard domains. The polyurethane-urea chain extended with IPDA exhibits over 95% of shape fixity and shape recovery as well as excellent resistance to stress relaxation. The maximum recovery stress of the polyurethane-urea is ~4.2MPa which is comparable to those of the aromatic Tm-SMPUUs. A series of segmented polyurethanes having urethane chains as soft segments were prepared via a modified prepolymer copolymerization method. Employing urethane chains as soft segments can greatly enhance the degree of phase separation and thus reduce Tg of the Tg-SMPUs. Via this modified copolymerization some rigid chain extenders can be readily incorporated into Tg-SMPUs but does not greatly raise Tg. Under Trec=Tg+15°C, the Tg-SMPUs having urethane chains as soft segments yield more than 90% of shape recovery. All the Tg-SMPUs can recover most deformation strain (> 80%) in a narrow temperature region. The Tg-SMPUs have 61-72% of deformation stress converted into recovery stress. The maximum recovery stress can be elevated by increasing urethane chain length and HSC or by employing rigid chain extenders. In summary, Tg and shape memory properties of the Tg-SMPUs can be adjusted by employing the urethane chains as soft segments. It thus show a new way for developing high-performance Tg-SMPUs.