Chain and local dynamics of polyisoprene as probed by experiments and computer simulations

The dynamics of designed short polyisoprene (PI) chains in the melt is investigated on a wide temperature window using dielectric relaxation spectroscopy and pulsed field gradient nuclear magnetic resonance (NMR). At high temperatures, molecular dynamics (MD) simulations performed using two different models (an explicit atom model and a united atom one) capture very well the dynamic properties documented experimentally. Structures pre-equilibrated with end-bridging Monte Carlo are used as initial configurations for MD runs at different temperatures, providing predictions for the temperature dependence of the dynamics of this bulk PI. Local dynamics is unique, independently of the probe (dielectric relaxation, dynamic light scattering, nuclear magnetic resonance, neutron scattering), although mean correlation times are significantly affected, to different extents, by librations. Chain dynamics over the molecular weight and temperature range studied can be described well by the Rouse model, as shown by both experimental data and a normal mode analysis on simulation trajectories. Deviations from the Rouse model emerge for the high modes at short times; still, this model offers a rather accurate picture. (C) 2003 American Institute of Physics.