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, M w . The asymptotic Rouse behavior is achieved when the chain begins to display Gaussian behavior. Surprisingly, the molecular weight dependence of physical properties such as T g and density also level off at the same molecular weight. A comparison of PDMS and polystyrene (PS) demonstrates that neither molecular weight between entanglement ( M e ) nor Kuhn segment length l K is important for T g and density. We speculate that there may be a third important parameter of molecular weight, in addition to M e and l K, which characterizes chain statistics and determines the molecular weight dependence of some physical properties. We introduce the molecular weight of a random step, m R, as such a parameter. m R is estimated to be ∼560 for PDMS and ∼5100 for PS. The molecular weight dependence of T g in PDMS and PS appears to be the same when presented as a function of M w / m R . Moreover, our analysis shows that a molecule becomes a polymer (approaches Gaussian coil) at M w ∼ 10−20 m R .