| The generation of heat during the deforming process of polymer materials are known as a result of molecular aggregation structure change, including dissipation heat by viscous friction, conformational entropic heat of molecular arrangement change, or the latent heat developed by crystallization. When the tests were performed on thicker samples with sufficiently fast extension rates, namely in approximate adiabatic conditions, heat generation can cause the temperature change of materials. Based on the feature of temperature change, the information of materials including molecular structure and molecular motion can be acquired. In this paper, infrared thermography (IRT) is used to online determine the thermal effects during the drawing process of polymer materials. By this technology, a method of thermodynamic analysis can be established, which can reveal the deformation mechanisms and molecular structure change of materials.In this paper, several thermal effects of polymer materials during plastic and elastic deformation are investigated, including:(1) Thermo-elastic effects and plastic thermal behavior upon uniform plastic deformationA uniform plastic deformation is observed during the uniaxial drawing process of ultra high molecular weight polyethylene (UHMWPE), which two characteristic phase of deformation can be noticed:the beginning of elastic deformation and the following large homogeneous plastic defonnation. The temperature change of UHMWPE during the uniaxial drawing process was determined by the infrared thermography. It was detected that at the beginning of the elastic deformation, temperature of specimen has a very small decrease, and then the temperature gradually increased in the following large deformation phase. It is also noticed that in the same stretch ratio, the temperature rise is less dependent of the drawing rate. Based on the thermal behavior of deformation, by the thermodynamics analysis, the initial temperature change can be deduced, i.e. dT=-α/ρCp·T0·dσ. which can make a conclusion that the initial temperature fall was correlative with the thermal expansion coefficient of specimens. So, a new method for values of linear thermal expansion coefficient can be estimated by equation α=ρ·Cp/T0·dT/dσ.result is agreed well with experimental data by dilatometer. In the large deformation phase, the temperature come to rise gradually, probably as a result of frictional dissipation, crystallization or conformational entropic changes. Because the temperature rise is less rate-dependent, it reveals large plastic process is essentially a forced high elastic deformation.(2) The thermal effects of the necking phenomenon upon nonuniform plastic deformationSignificant necking phenomenon can be observed when drawing polypropene (PP) fiber or polyester (PET) fiber. Infrared thermography is used to investigate the the thermal effects of the necking phenomenon. The change of temperature and diameter during the necking process has been online determined accurately by the infrared camera with special18μm close-up lens. The infrared thermal images showed that the self-heating of the fiber is only generated in the necking region and the maximum temperature part moves along the necking point. Depending on the drawing rate, this temperature of the necking region was found to exceed the ambient temperature by40-80℃. Although necking region is moving during the deformation process, the temperature gradient can be maintained constant in the specimen by given extension rate, so the deformation heat can be calculated by the thermal conduction formula, i.e. Q=K·A·dT/dx·dt. Internal energy increase for PP fiber calculated by the first law of thermaldynamics may be caused by melting of crystal, creation of new surfaces or flaws, as well as strored elastic strain energy. While the internal energy of PET fiber decreases with time, which is probablely caused by the induced crystallization. Comparing the temperature of the necking region, there is a higher temperature rise of PET than that of PP fiber. Analyzing by DSC, the change of crystallinity of PET is higher than that of PP fiber, which means that the temperature rise of PET fiber is not only caused by plastic friction dissipation, but the role of crystallization heat is not be neglected.(3) Thermo-elastic transition and elastic thermodynamics of rubber-like elastomersInfrared thermography is used to detect the thermal effects of rubber-like materials including nature rubber and nitrile butadiene rubber during the loading-unloading cycles. Sereral salient features of temperature change have been observed:(1) the temperature has a tiny decrease at first and then come to increase during stretching process, which is in agreement with thermo-elastic inversion effect.(2) temperature variation is partly reversible in the first cycle and rubber materials shows the viscoelastic properties.(3) temperature has a slight increase after the initial cyclic deformation and totally reversible in the following cycles. These phenomena are related to complicated deformation mechanism. It is clear that molecular slippage and reconstruction take place during the initial drawing process, which result in uniform distribution of the chain segments between the crosslink points.. Upon initial stretching sample, the thermal effects of the deformation is large rate-dependent and exhibite viscoelastic properties. While the rearrangement result in a perfect elastic structure, and the temperature effect is less rate-dependent. Samples have been stretched many times to the highest stretch ratio before analyzing of thermodynamics, and then the deformation of softened sample can be considered as a reversible process. For the two rubbers, it indicates that the internal energy contribution to the work was not negligible. At the same time, The relative contribution of the internal energy can also be determined by the thenno-elastic inversion. It is concluded that the relative contribution of the internal energy to the elasticity(△U/W)P.T was dependent of strain for both NR and NBR only at low stretch ratio. However,(△U/W)P.T approached a constant value at higher stretch ratio. Especially, at a very high extension (λ>4),(△U/W)P.T of NR had a sharply decreasing tendency, corresponding to a negative internal energy contribution to the force which can be ascribed to the incidence of strain-induced crystallization.(4) Thermal effects of thermoplastic elastomers upon loading-unloading cyclesPolyurethane (PU) is class of the thermoplastic elastomers. Infrared thermography is used to investigate the thermal effects of PU elastomers with dry-spinning, melt-spinning and filled with nano-particles during loading-unloading cycles, Several common thermal features are noticed that:1) the temperature increase during loading process and decrease during unloading, which is in agreement with entropic elasticity of elastomer.2) there is a inreversible temperature in the first cycle and totally reversible in the following loading-unloading cycles.(3) Temperature change in first loading process is higer than that of successive loading process. A hyperelastic-viscoplastic model was applied to explain the features of the thermal effects. The difference of thermal effects is noticed that the temperature difference value of dry-spinning polyurethane between the first two loading process is larger than that of melt-spinning. The phenomenon was derived from dry-spinning polyurethane fiber with larger hydrogen bond density, and therefore, the larger number of physical crosslinking pionts, which blocked the molecular motion to a certain extent. Parallel phenomenon is noticed that the temperature difference value of filled TPU is higer than that of unfilled TPU. The results indicated that with the addition of AT nanorods, small physical crosslinks were formed between AT and TPU matrix by hydrogen bond, which caused larger viscous friction between the molecular motions upon the deformation process and finally affected the fatigue resistence of the polyurethane elastomers. |
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