Solid phase rheology of a thermotropic nematic polymer.

摘要

We have performed a series of experiments at finite strain which indicate that a liquid crystalline copolyester, Vectra, has a region of viscoelastic behavior where all strain is recoverable and the stress response is approximately linear. We also find that at strains outside this region the polymer responds as a nonlinear viscoelastic material, and not all of the strain is recoverable. We have attempted to construct a simple model to describe this behavior.; At strains of 0.005 or greater the material clearly behaves as a nonlinear viscoelastic-viscoplastic material. The most readily visible aspect of this behavior is the development of non-recoverable strain and its dependence on the strain history. The plastic strain is a function of the applied strain. Moreover, short duration loading at a given strain produces less plastic strain than that resulting from a longer duration of loading at the same strain. This behavior is consistent with three types of displacement histories used to monitor the plastic strain response.; The stress behavior differs markedly from that expected for a linear viscoelastic material. We see that the stress developed is much lower than predicted for a viscoelastic material based on the infinitesimal strain frequency sweep, and the rate of relaxation of stress is more rapid than for that material. During quasi-static loading the stress begins to deviate significantly from linearity with respect to strain at approximately the value of strain where plastic strain is first observed during dynamic loading. This correlation between the deviation from linearity in the stress response and the evolution of plastic strain is not apparent at finite strain rates, where the stress deviates from linearity with respect to stress at a strain higher than that where plastic strain is first observed. Additionally, the stress response returns to linearity with respect to the difference between the strain and the plastic strain when the material is unloaded from a viscoelastic-plastic state.; When a small oscillatory strain is superimposed on a step strain the phase angle exhibits a nonlinear transient response. At long times both the plastic strain rate and the rate of decrease of the phase angle approach zero. Thus, at long time the material again appears to behave as a linear viscoelastic material.; We have developed a model to describe the viscoplastic response and the stress response, and the model has been used to predict the amount of plastic strain developed after several types of strain histories. Our model has been able to predict the qualitative behavior of the observed plastic strain response as a function of time and strain for applied strain experiments, and as a function of stress and time for the case of step stress. In predicting the stress response, the model was able to account for the qualitative trend of the phase angle with respect to time and frequency and the behavior of the phase angle as the strain is increased. In most cases the predicted values of the phase angle were far too low. Additionally, our attempt to predict the stress in other strain histories met with limited success. (Abstract shortened by UMI.)