Kinetics of Irreversible Adsorption of Polymer Melts onto Solid Substrates

Abstract

Polymer films prepared by spincoating are intrinsically non-equilibrium systems. Differently than in bulk melts, equilibration is not achieved upon relaxation of the chains on the timescale of the reptation time. Prolonged annealing above the glass transition temperature induces, in fact, the formation of interfacial layers irreversibly adsorbed onto the supporting substrate, which affect several properties of the confined system. Although a large experimental evidence demonstrated a strong correlation between the presence of these adsorbed layers and the deviation from bulk behavior, the mechanisms permitting pinning onto a solid substrate are not well known in the case of polymer melts . In this chapter, after reviewing current theoretical models on adsorption of macromolecules, we will present recent experimental results on polystyrene, a flexible polymer, and the outcome of molecular dynamics simulations on a similar model system. In particular we show that at short annealing times the adsorbed amount (experimentally determined as the thickness of the irreversibly adsorbed layer) increases linearly with time. This first order reaction mechanism is inhibited at longer annealing times, where the space available for pinning of new chains is strongly limited by previously adsorbed chains. This condition is achieved at a molecular weight independent crossover time, and at adsorbed amount scaling as N^1/2, where N is the polymerization degree. In this regime, following models treating adsorption via the formation of  loops, we show that the growth of the adsorbed amount becomes logarithmic in time. We developed a series of analytical expressions permitting to follow the kinetics of irreversible adsorption, in line with the outcome of our experiments and simulations.