Structure and Mobility of Poly(ethylene terephthalate): A Molecular Dynamics Simulation Study

An accurate force-field has been parametrized for PET and is employed to simulate a long chain, consisting of 120 repeat units, over a wide temperature and pressure range, for long time scales. The force field has been validated against the experimental results for the pressure−volume−temperature properties, the characteristic ratio, the dipolar ratio, and the population of ethylene glycol bond conformer. In all cases the agreement with experiment is quite good. The glass transition temperature is calculated very accurately as a function of pressure. The local dynamics of the chain has been investigated by calculating phenylene reorientation, collective dipole moment, and individual dipole moment correlation functions. The calculated relaxation times for collective dipole moment and phenylene reorientation correlation functions show activation energies close to the corresponding experimental β-relaxation value. Local translational mobilities of phenylene groups in the backbone and end phenylene groups have also been investigated. It is concluded that the end groups have much higher mobility than the polymer backbone, and increasing the distance from the chain ends decreases the mobility dramatically. The long-time asymptotic slope of chain displacement at temperatures higher than the glass transition temperature is around 0.65 in close agreement with Rouse model. On the other hand at the glassy state, the polymer segments perform very restricted movements in the cages formed by their surrounding segments.