When Does a Molecule Become a Polymer

Using light scattering spectroscopy, we demonstrate that a transition from small molecular behavior to Rouse dynamics in poly(dimethylsiloxane) (PDMS) is a smooth function of molecular weight, Mw. The asymptotic Rouse behavior is achieved when the chain begins to display Gaussian behavior. Surprisingly, the molecular weight dependence of physical properties such as Tg and density also level off at the same molecular weight. A comparison of PDMS and polystyrene (PS) demonstrates that neither molecular weight between entanglement (Me) nor Kuhn segment length lK is important for Tg and density. We speculate that there may be a third important parameter of molecular weight, in addition to Me and lK, which characterizes chain statistics and determines the molecular weight dependence of some physical properties. We introduce the molecular weight of a random step, mR, as such a parameter. mR is estimated to be ∼560 for PDMS and ∼5100 for PS. The molecular weight dependence of Tg in PDMS and PS appears to be the ...