Origin of internal viscosity effects in flexible polymers: A comparative neutron spin-echo and light scattering study on poly(dimethylsiloxane) and polyisobutylene

We present a comparative neutron spin-echo and dynamic light scattering stu dy of the chain dynamics of the dynamically very flexible poly(dimethylsilo xane) (PDMS) with the orientationally hindered polyisobutylene (PIB). Both polymers exhibit the same static rigidity. In the melt PDMS follows the Rou se dynamics up to momentum transfers of Q = 0.4 Angstrom (-1), while PIB di splays a strong influence of local dynamics already above Q = 0.15 Angstrom (-1). In dilute solution the dynamic structure factors and the diffusion c oefficients of both polymers were studied over a wide temperature and & ran ge. A comparative evaluation of the PIE intrachain dynamics on the basis of PDMS results, which are taken to reflect as in the melt "ideal" relaxation s, shows that intrachain viscosity effects are the leading mechanism displa yed by the Rouse and Zimm models. The characteristic relaxation time of the intrachain viscosity to agrees well with the rotational barriers in PIB, c orroborating the underlying physical idea of a delayed redistribution of co nformational states to be at the origin of internal viscosity effects.