Study On The Relaxation Behavior And Mechanism Of Polyether Polyurethane Elastomer Under High Moisture Exposure

Polyurethane elastomers have many characteristics of rubber,plastics,coatings and adhesives.Those charecters make it become a promising polymer material in aerospace,automotive interior trim,biological medical equipment and transportation.Polyurethane elastomers are viscoelastic,and the long-term performance is one of their major concerns.Stress relaxation is widely used to investigate the time-dependent viscoelastic properties of polymer.Moisture or water absorption are one of the most common environmental factors that Polyurethane elastomers are exposed to during service,which cause the deterioration of mechanical properties and limit the service life.Therefore,it is significant to study the internal mechanism of the deterioration of relaxation properties of polyurethane elastomer under high moisture exposure.The macroscopic relaxation behavior of polyurethane elastomers is strongly affected by its microphase separation structure,which in turn is significantly influenced by water molecules.So far,the research work on moisture absorption of polyurethane elastomers by domestic and foreign scholars has focused on the effect of water molecules on the mechanical properties of polyurethane elastomers,but pay insufficient attention to molecular dynamics and microphase separation structures.Therefore,comprehensively elaborate water absorption effects from microstructures to macroscopic properties are of great practical importance.In this work,the influence factors of water absorption kinetics of PTMG-PU were studied systematically.The influence of water absorption on molecular dynamics,microstructures and relaxation behaviors of polyurethane elastomers with polyether(poly(tetramethylene glycol),PTMG)soft segments has been studied.The main conclusions of this paper were as follows:(1)Hard segment content,immersion temperature and sample thickness are all factors affecting the water absorption kinetics of PTMG-PU.Kinetics of water absorption of polyurethane elastomer can be well fitted by Fick’s model.However,non-Fickian absorption behavior was observed in the late stage at 60 °C,which was presumed to be related to the disintegration of some hard domains.The diffusion coefficient and equilibrium water absorption rate decrease gradually with the increase of hard segment content,indicating that the hard domanins hinder the diffusion of water molecules.Furthermore,The higher the water immersion temperature,the faster the water absorption rate,the larger the water molecular diffusion coefficient,the larger the equilibrium water absorption rate,the equilibrium water absorption rate is not linear relationship with temperature.The thickness of the sample has an effect on the diffusion coefficient of water molecules,but has little effect on the equilibrium water absorption.(2)The results of two-dimensional correlation spectra show that water molecule interacts in the sequential order with free carbonyl,then with the bound C=O between hard and soft segments,and finally with the hydrogen-bonded C=O in hard domains.The effect of water molecules on the molecular dynamics of PTMG-PU was studied using BDRS.The analysis of α-relaxation and Maxwell-Wagner-Sillars interfacial polarization indicated that the water absorption has a great influence on the molecular mobility of soft segment and the interface regions between soft domains and hard domains.Microphase separation structures of PTMG-PU consisted of loose,partially regular and compact hard domains.Water molecules could interact with loose hard domains and lead to the disintegration.(3)The results of stress relaxation behavior indicated that water absorption leaded to remarkable increase in the stress relaxation ratio.The deterioration of relaxation properties caused by water absorption was not only related to the soft segments,but also caused by the destruction of the interface regions and the hard domains.Due to the water absorption properties of PTMG-PU,the tensile breaking strength,100% set tensile strength and tensile modulus are significantly reduced,both because of the plasticising effect of water on the soft segment domains of PTMG-PU and because of the action of water with the loose hard domains,which causes the disintegration of the loose hard domains.In this paper,the evaluation of the microphase separation structures of polyether polyurethane elastomers under high moisture exposure was methodically and thoroughly investigated,and the internal mechanism of the deterioration of relaxation properties of PTMG-PU caused by water absorption was clarified from the perspective of microphase separation structures.This provides conceptual direction for the development and manufacture of PTMG-PU with high reliability under high humidity exposure.