Molecular Weight Dependence of Spreading Rates of Ultrathin Polymeric Films

TL;DR

This study experimentally investigates how the molecular weight of polymers influences the spreading rates of ultrathin films, revealing a dependence modulated by surface properties and humidity, supported by theoretical explanations.

Contribution

It demonstrates the molecular weight dependence of spreading rates and introduces a model explaining how surface friction and humidity affect this relationship.

Findings

Spreading radius grows as R(t) = (D_{exp} t)^{1/2}.

D_{exp} scales with molecular weight as M^{- ext{gamma}}.

Gamma varies with humidity, approximately 1 at low RH and near zero at high RH.

Abstract

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(t) of the film grows with time as R(t) = (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 RH gets progressively increased. We propose simple theoretical arguments which explain the observed behavior in the limits of low and high RH.