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 ∼ 10-20m_R.