Chain-Length Dependence of Relaxation and Dynamics in Poly(methyl methacrylate) from Oligomers to Polymers

A study of local relaxation is presented in a series of poly(methyl methacrylate)s (PMMAs) of varying molar mass from oligomers to polymers. Decoupling phenomena from the viscous flow in the rotational relaxation at temperature TC and violations of the Debye−Stokes−Einstein relation characterize the rotational dynamics of the series, also evidencing the influence exerted by the chain length on segmental relaxation. We discuss different dynamics regimes, coupling degrees of local relaxation to viscosity, and step-like behaviors with mass. The latter is a recurrent result in rotational relaxation and its dynamic parameters, discriminating the molecular-like behavior of the oligomers from a more specific polymeric response. Particular attention has been devoted to the onset of the Rouse dynamics in polymer samples. Scaling laws of the relaxation times with the mass are considered in the different temperature regions. Above TC the results agree with the forecasts of the Rouse theory. Below TC, the scaling exponent could be accounted for by considering the Guenza theory of the cooperative interchain dynamics.