Polyurethane Viscoelastic Behavior Of Composites

As an important polymeric material, polyurethane (PU) has been widely used in both the common and engineering areas because its performance is designable by controlling its short-range and long-range structures. Recently, interpenetrating network (IPN) and nanocomposite technologies have attracted much attention to develop new type of PU materials. The research work focuses mainly on the molecular structure design, nano-morphology control, and property improvement such as mechanical strength, thermal stability and biocompatibility, etc. It is well known that dimensional stability is very important to the PU materials, which is closely related to the viscoelasticity. Because the dimensional instability caused by viscoelastic alteration highly restricts their further applications, it is hence very important and also meaningful to explore the viscoelastic behaviors such as creep, creep-recovery and stress relaxation of PU and its composites.In this work, therefore, polyurethane/epoxy interpenetrating network systems (PU/EP IPNs), polyurethane/multi-walled carbon nanotube composites (PU/MWCNTs) and polyurethane/epoxy system filled with multi-walled carbon nanotubes (PU/EP/MWCNTs) were prepared by interpenetrating network technology and in-situ polymerization, respectively for the structure and property studies in terms of nano-scaled morphology, mechanical strength, thermal stability and viscoelasticity to describe the structure-property relations of PU composite systems preliminarily. Then, the effects of EP and MWCNTs on the creep, creep-recovery and stress relaxation behavior were investigated in detail. Some commonly used constitutive equations and viscoelastic models were further utilized to establish the relations between the hierarchical structure of MWCNTs and the viscoelastic behaviors of the PU composite systems, aiming at understanding the structure design and viscoelasticity control of the PU based composites, and finally, providing the comprehensive experimental and theoretical evaluation on the fabrication and application of PU composites with high performance. The preliminary conclusions are as follows.(1) PU/EP IPN systemsThe curing rate of PU increased with addition of EP component, accompanied by the reduction of the crosslinking degree of the IPN composites. At the lower EP content (10wt%), the tensile strength, Young’s modulus, storage modulus and thermal decomposition temperature of the IPN systems were lower than those of the neat PU. With increasing EP content, those properties were remarkably improved, and the IPN systems showed performance much higher than the neat PU. Moreover, the creep and stress relaxation behaviors of the IPN systems were strongly dependent on the experimental temperature and applied loads as well as the EP content. The creep and stress relaxation rate were increased with the temperature and the increased of the applied load for both the neat PU and the IPN systems. The latter, however, showed significantly stronger creep and stress relaxation resist ability than the neat PU, especially at the higher content of EP.(2) PU/MWCNT compositesThe MWCNT5are good reinforcement to the PU. The tensile strength, storage modulus and a transition temperature of the composite containing1wt%MWCNTs, increased by about16%,56%and22℃relative to the neat PU and, the creep resistance ability could also be improved strongly in the presence of MWCNTs. The composite with1wt%MWCNTs showed the lowest creep strain level, indicating the best creep resistance ability at that MWCNT loadings. Moreover, the creep behavior could be well evaluated by Findley model and Burgers model, and creep-recovery process could be described by Weibull model for both the neat PU and the composites. In addition, the long time scale creep behavior of the composite systems could be obtained by time-temperature superposition and the result showed that, the creep platform time were about109s and1011s (2.5MPa and40℃) for the neat PU and the composite containing0.5wt%MWCNTs. This again indicated that the creep of the PU chain was restrained by the presence MWCNTs.(3) PU/EP/MWCNT compositesThe mechanical properties and thermal stability of the PU/EP IPN composite systems could be further improved with addition of MWCNTs. Compared with those of the neat IPN (PU70/EP30), the Young’s modulus and thermal decomposition temperature of the composite with2wt%MWCNTs increased by about28%and10%, respectively. Moreover, the relaxation stress of the composites increased monotonically with the increasing loadings of MWCNTs. The stress relaxation behavior could be well evaluated by Power-law model and Stretched-exponential model. The long time scale stress relaxation obtained through time-temperature superposition further elucidated that the stress relaxation of the PU chain were highly suppressed by the presence of MWCNTs.