SIMULATIONS OF INDUCED ORIENTATION IN STRETCHED POLYMER MELTS

Molecular dynamics simulations of an entangled bead-spring polymer melt were used to study bond orientation induced by uniaxial stretching. Short free ''guest'' chains added to the melt showed an induced orientation 0.28 times that of the majority ''host'' chains. This orientation ratio epsilon is comparable to those recently observed in hydrocarbon polymers. For comparison, we oriented the host bonds using an explicit nematic field instead of stretching. The resulting orientation ratio was substantially reduced, to 0.17. This latter ratio was independently obtained using correlation functions of the isotropic, equilibrium Liquid. The discrepancy between these two orientation ratios suggests that the widely accepted theory of orientational coupling provides an incomplete explanation of induced orientation under stretching. Other potential mechanisms for induced orientation are discussed.