The thickness dependence of the free energy of a polymeric film is calculated for monodisperse melts on grafted polymer layers of the same composition. For monodisperse grafted layers, a comparison is made between the full equilibrium (constant chemical potential) case and the restricted equilibrium (constant grafting density) case. Results for the two situations are nearly identical, indicating that previous results obtained for full equilibrium conditions are valid for restricted equilibrium conditions as well. A monodisperse polymer on a polydisperse grafted layer, consisting of loops and tails of different lengths, is also considered. A simple statistical analysis indicates that a layer produced by uniform adsorption of all segments of the polymer chain will be dominated by relatively long tails. Calculations for representative loop and tail distributions give a monotonically decreasing free energy with increasing film thickness. The absence of an attractive minimum at a finite film thickness means that thin films of all thicknesses are stable, with no thermodynamic driving force for dewetting. The situation changes if both ends of the adsorbed molecules are grafted to the surface, giving an adsorbed layer consisting only of loops. In this case, the stabilizing influence of the tails is eliminated, and dewetting is predicted even for a polydisperse distribution of loops.
ASJC Scopus subject areas
- Materials Chemistry