MOLECULAR STRESS AND STRAIN IN AN ORIENTED EXTENDED-CHAIN POLYMER OF FINITE MOLECULAR LENGTH

We have developed constitutive and molecular mechanics models to investigate the influence of chain-end defects on the macroscopic tensile properties of extended-chain polymers of finite molecular weight. Molecular mechanics simulations have been performed on the rigid-rod polymer PBZO, poly(p-phenylene benzobisoxazole), using the Dreiding II force field. The distance between chain ends (i.e., the chain length) can be varied systematically by increasing the size of the simulation unit cell in the chain direction. From this analysis it is possible to analyze the micromechanics of stress transfer between chains in detail. At chain ends, the applied tensile stress is transferred to the nearby chains through a shear lag region via secondary bonds. A constitutive model is developed for a geometry similar to the PBZO molecular simulations. The calculated strain distribution along individual chains describes well the strain distribution along the PBZO molecules. The model predicts a nonlinear response of the material and a transition in tensile failure mode from chain slip to chain scission, which depend on the interchain shear strength and the length of the polymer molecules. The influence of intermolecular shear modulus, shear strength, and molecular chain length on macroscopic properties such as tensile modulus, tensile strength, and elongation to break is examined. It is found that in the molecular engineering of strong, tough polymer fibers, an optimum combination of shear strength and chain length must be chosen.