A MODEL OF RELAXATION IN SUPERCOOLED POLYMER MELTS

We present a dynamical mean-field model for molecular motions in a sup ercooled polymer melt. A macromolecule is represented by a harmonic ch ain undergoing Brownian motion whose bead mobilities fluctuate between zero and a finite value. These fluctuations mimic the dynamic obstacl es formed by the chain segments surrounding a given segment,;whose eff ects become more pronounced as T decreases. The rate of these mobility fluctuations is determined self-consistently by equating it to the as ymptotic long-time;relaxation rate of the shortest-wavelength Rouse mo de. The resulting fluctuating rate vanishes as c, the equilibrium frac tion of mobile beads, approaches a threshold value c. As c-->c*, rela xation times become;arbitrarily large, permitting the modeling of flui ds as T approaches T-g. Calculations of autocorrelation functions of R ouse mode coordinates and of segmental mean-squared displacements are presented: and compared to results from recent simulations of melts at low temperatures. The deviations from the Rouse model observed in the simulations are features of this theory. (C) 1998 American Institute of Physics.