Studies of the structure and shape memory effect of segmented polyurethane ionomer

Abstract

Shape memory polyurethane (SMPU) is composed of the reversible phase and the fixed phase. Thereof, the reversible phase having a melting or glass transition temperature of the soft segments as the transition temperature is used to hold the temporary deformation, whereas the fixed phase is referred to the hard segments covalently coupled to the soft segments and responsible for memorizing the permanent shape. Usually, the hard segment content, the soft segment length and the molecular structure are paramount on controlling the shape memory property in general. However, in some cases, increasing the hard segment content provides an option to alter their inter-connectivity at the expense of the crystallizable soft segment content, and thus lowering the shape fixity ratio. The chemical cross-linking hard segment structure was ever systematically incorporated into SMPU so as to improve its shape memory function. Nevertheless, the advantage of processing ability of this originally thermal plastic SMPU is lost due to the formation of cross-linkage points. Therefore, in this study, the Coulombic force produced by ionic groups within hard segments was considered as an adjustable internal force of the fixed phase, with which the processing ability of thermal plastic segmented PU can be kept. In addition, the introduction of novel functional groups can impart new functions to this kind of smart materials, while the shape memory function can be retained simultaneously. In this way, the application scope of shape memory polymer can be broadened immensely and more practical requirements for the novel SMP could be satisfied. Therefore, in this research programme, not only the effects of ionic groups within hard segments on shape memory function, but also the novel functions such as antibacterial activity with the choice of suitable ionic groups were studied. Moreover, ionic groups were introduced into shape memory polyurethane fibres to adjust the shape memory properties. For the above purposes, several series of SMPU anionomers and cationomers with various ionic group contents were synthesized with pre-polymerization. In each series of samples, the soft segment length and hard segment content were fixed so as to solely study the effect of ionic group contents on the morphology and shape memory effect. Considering the huge influence of soft segment crystallization on shape memory effect, the isothermal crystallization kinetic method was conducted with DSC to investigate the crystallization mechanism and process in SMPU ionomers. Microstructure and properties of SMPU ionomers were investigated by using DSC, DMA, FTIR, POM, Instron universal tensile tester. The molecular weight of SMPU ionomers was detected by GPC with DMF or THF as mobile phase. Result shows that ionic group contents have significant effect on the morphology of SMPU ionomers, accordingly affect the shape memory properties. The ionic groups within hard segments play the two-fold effect: (i) the disruption of the order of hard domain, and (ii) the enhancement of cohesion among hard segments. The former effect can be detected from the decrease of elastic modulus plateau in temperature higher than transition temperature, subsequently, the recovery ratio presents decreasing trend. The latter effect can be observed from the comparison of DMA results between SMPU ionomers and non-ionomers. Besides, although the ionic groups were located in hard segments, the morphology of the two-phase system in SMPU ionomers was affected significantly. Subsequently, the changes of morphology possibly can gives rise to the increasing crystallinity of soft segments in PCL based SMPU ionomers, which was observed to facilitate the fixity of temporary deformation under the specific cyclic SME testing condition. In application of SMPU ionomers, the substrate bonding antibacterial activity was introduced into shape memory matierals. At the same time, the rapid fixity ability to the temporary deformation can be achieved. In addition, the tiny ionic groups were introduced into the molecular backbone of SMPU fibres, which behave shape memory function with glass transition point as the switch temperature. The obvious effect from the introduction of ionic groups was found and it offers an effective mean to control shape memory function in SMPU fibres. The results in this study have offered a reference for the design and application of SMPU ionomer materials. In the part of future work, the theoretical research issues and potential application of these kinds of intelligent materials were suggested.