On the short time dynamics of dense polymeric systems and the origin of the glass transition: A model system
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In order to model the short time (and distance) scale motions for dense polymeric systems, we have performed dynamic Monte Carlo simulations of chains on a diamond lattice at considerably greater densities than those done previously. Chain dynamics were simulated by a random sequence of three- and four-bond kink motions and end moves. For times shorter than the chain diffusion time, the single bead autocorrelation function g(t) exhibits three distinct regimes: a short time Rouse-like regime where g(t)~t[superscript ½]; a mid-region where g(t)~t β, followed by a longer time, Rouse-like regime where g(t)~t1/2. There is a smooth crossover from Rouse-like dynamics, β =1/2, at low density to smaller values of β at higher density, and β =0 at the glass transition density (Φ[subscript G] =0.92±0.01). It is shown that the major motion of the chains is transverse to the chain contour rather than along the chain. The observed motion is successfully analyzed in terms of the motion of defects (holes) through the sample. It is shown that the glass transition at Φ[subscript G] ±0.92 is caused by the shutting down of the orientation changing four-bond motions.