Dielectric relaxation of dipole-inverted polar polymers as studied by computer simulations

We use molecular dynamics simulations to examine the dielectric relaxation of polar macromolecules with dipoles parallel to the chain backbone. Analysis of the simulation trajectories closely follows the treatment of Watanabe and co-workers for experimental results of dipole-inverted cis-polyisoprene solutions (Macromolecules 1995, 28, 6443). The important observable quantity in experiments and simulations is the dielectric loss spectrum epsilon "(omega), whose shape reflects the distribution of relaxation processes for the global chain motion. The observed broadening of the spectra with increasing polymer concentration, classically attributed to overlapping of the chains, is analyzed quantitatively using a local correlation function C(n,t;m) = 1/a(2) [u(n,t).u(m,0)], where u(n,t) is the bond vector of the nth segment of the chain at time t, and a(2) = [u(2)]. At long times, C(n,t;m) can be expanded as a sum of its eigenmodes: C(n,t;m) = 2/N(n)Sigma(p=1)f(p)(m)f(p)(m) exp(-t/tau(p)), where N is the size of the chain and tau(p) and f(p) are the relaxation time and the eigenfunction of the pth mode. From simulations we calculate the time correlation functions and dielectric loss spectra of multi-inverted and asymmetrically inverted polar polymers. We compute the relaxation times tau(p) and the eigenfunctions of C(t,n;m). The relaxation times follow a power-law dependence tau p proportional to p(-gamma), with gamma similar or equal to 2.08, and the ratios tau(p)/tau(1) remain independent of the concentration, a behavior predicted by the Rouse model. On the other hand, the dependence off, on n deviates progressively with increasing density from the Rouse model sinusoidal prediction. The results reveal that the broadening of the spectra is a result of changes in the distribution of eigenmodes (f(p)), and not in the relaxation time span gamma. Our. findings are consistent with the experimental observations, clearly demonstrating the adequacy of simulations for investigating the dynamic behavior of macromolecular systems.