Molecular modeling of polyethylene, polypropylene and ethylene-propylene block copolymer

Abstract

Structures of polyethylene (PE), isotactic polypropylene (iPP), and ethylene-propylene block copolymer (EPM) at 3 temperatures i.e. 0 K, glass transition temperature (T[subscript g]), and melting temperature (T[subscript m]) were investigated. Molecular Dynamics (MD) Simulations were carried out by the Discover module in the Cerius2 program for single chain polymers, double chain polymers as well as bundle polymers using PCFF molecular mechanics force field. Models with various number of repeating units of N=5, 10, 100, and 500 were applied. The starting geometries of these models were taken from the X-ray structures of PE and PP. It was suggested both of single chain and double chain polymers that the model with N=500 is the best representative of polymer at all temperatures. The conformation of all polymer models tends to be more entangle and turns into globule at high temperature, T[subscript m]. It was found that PE and EPM seem to orientate theirlong chains into ordered domain more easily than PP. These results correspond to the calculation data of surface area/polymer volume, in which lower value related to the formation of locally ordered structure. It was indicated that such value of PE was less than one of PP. For 10 chains with 50 repeating units per chain of PE and PP bundle models, structures at T[subscript g] and T[subscript m] are slightly deviated from the structure at 0 K, i.e., folding and entanglement are not observed. Probably, the chain length of 50 is too short. We then conclude that the double chain could be the best representative model of polymer at all temperatures. From the energies of chain-chain interaction, it can be observed that the shear energies of PE and PP are almost constant and PP has larger shear strength than PE, these results are agreed with the experimental data.