Effects of chain topology on polymer dynamics: Configurational relaxation in polymer melts

Abstract

We report computer simulations of chain dynamics in monodisperse, linear polymer melts using a model which allows the dynamical effects of chain topology to be isolated from the individual effects of excluded volume and chain connectivity. The chain topology is controlled by either allowing or forbidding chain crossing events while maintaining the chain connectivity and excluded volume interactions between monomers. This chain crossing condition does not significantly alter the polymer structure or even the local, segment-level dynamics. For each topological condition, ‘‘crossing’’ and ‘‘noncrossing,’’ we decompose the polymer dynamics into a set of relaxation modes, calculate the autocorrelation functions for each mode, and obtain the complete distribution of internal relaxation times {τp}, where p labels the mode index. Several regimes of relaxation behavior are present, depending on the chain length N, the wavelength of the mode (N/p), and the chain topology. Noncrossing chains below a critical chain length, Nc, exhibit relaxation dynamics that are consistent with Rouse chains in the melt. Above Nc, noncrossing chains still exhibit Rouse-type dynamics in the short wavelength relaxation modes. For the modes of intermediate wavelength, however, the internal relaxation times scale with the mode wavelength according to τp∼(N/p)3 and the autocorrelation functions are strongly nonexponential. Relaxation behavior that is roughly consistent with the reptation model is finally seen for the first few relaxation modes of the noncrossing chains above Nc. In contrast to the noncrossing chains, the chains which are allowed to cross (in the presence of excluded volume and chain connectivity) exhibit Rouse-type relaxation dynamics for all long wavelength modes, even modes with characteristic wavelengths beyond the effective entanglement spacing of the noncrossing chains. Since we have controlled all factors other than chain topology which may in principle affect polymer dynamics, the results illustrate the role of chain topology in the transition from Rouse-type to reptationlike relaxation behavior in polymer melts.