Molecular Weight Dependence of Spreading Rates of Ultrathin Polymeric Films

We study experimentally the molecular weight $M$ dependence of spreading rates of molecularly thin precursor films, growing at the bottom of droplets of polymer liquids. In accord with previous observations, we find that the radial extension $R\left(t\right)$ of the film grows with time as $R\left(t\right)=(D_{\exp }t{)}^{1/2}$. Our data substantiate the $M$ dependence of $D_{\exp }$; we show that it follows $D_{\exp }\sim M^{-\gamma }$, where the exponent $\gamma$ is dependent on the chemical composition of the solid surface, determining its frictional properties with respect to the molecular transport. In the specific case of hydrophilic substrates, the frictional properties can be modified by the change of the relative humidity (RH). We find that $\gamma \approx 1$ at low RH and tends to zero when the RH gets progressively increased.