In situ neutron reflectometry study of the near-surface solvent concentration profile during solution casting

The drying process of solution cast statistical copolymer films is recorded in situ with neutron reflectometry as a function of molecular weight. P(nBA-stat-MA) with a majority of n-butyl acrylate (nBA) and a minority of methyl acrylate (MA) is dissolved in deuterated toluene and the polymer dries out due to the evaporation of toluene. The evolution of the depth dependence of the near-surface solvent concentration profile is monitored in situ. The detected kinetics deviates from a simple case II diffusion. For all probed samples we find an acceleration of the solvent evaporation for the freshly cast films which then slows down towards an equilibrium stage. With increasing molecular weight the maximum evaporation velocity decreases and equilibrium is reached later. The solvent concentration in the near-surface region is constant as a function of distance from the film-air interface except at times near the maximum evaporation speed, where solvent enrichment at the surface is found. The surface roughness depends neither on the solvent content nor on the chain length of the statistical copolymer. In the final dried films, independent of the molecular weight, a very small amount of solvent remains. Gravimetric experiments concerning the weight loss during drying show that the near-surface kinetics is different from its bulk behavior.