Quantitative evaluation of evaporation rate during spin-coating of polymer blend films: Control of film structure through defined-atmosphere solvent-casting

Abstract

Thin films of polymer mixtures made by spin-coating can phase separate in two ways: by forming lateral domains, or by separating into distinct layers. The latter situation (self-stratification or vertical phase separation) could be advantageous in a number of practical applications, such as polymer optoelectronics. We demonstrate that, by controlling the evaporation rate during the spin-coating process, we can obtain either self-stratification or lateral phase separation in the same system, and we relate this to a previously hypothesised mechanism for phase separation during spin-coating in thin films, according to which a transient wetting layer breaks up due to a Marangoni-type instability driven by a concentration gradient of solvent within the drying film. Our results show that rapid evaporation leads to a laterally phase-separated structure, while reducing the evaporation rate suppresses the interfacial instability and leads to a self-stratified final film.

Fundamentals of Phase Separation in Polymer Blend Thin Films

“…Although a Marangoni-like instability has been suggested to explain the distortion of the bilayer interface prior to lateral phase separation [62,61,41], in a manner like in figure 4.1, I have shown that the intrinsic instability of the bilayer and surface bifurcation is sufficient to cause distortion of the interface in a similar if not near-identical way. Therefore we should consider that the phase equilibria of polymer films and the surface bifurcation mechanism I have presented here might be responsible for the film evolution seen in many experiments.…”

“…The pioneering work of Walheim highlighted the possibility of lateral phase separation via a transient wetting layer 23 [59]: the initial formation of a transient (temporary) wetting layer that forms as the blend initially vertically phase separates, due to preferential attraction by the film surface(s), which subsequently breaks up due to an instability. This subject has received more recent attention [60,41,61,62], with a fair amount of evidence now suggesting that, for spin cast films, before the blend laterally phase separates into a laterally segregated film (which is usually 19 Vertical Phase Separation: phase separation into vertically layered phases, e.g. a bilayer, usually caused by preferential surface attraction.…”

“…a transient wetting layer); see below for description of spin coating. The most posited reason for the breakup of the bilayer is a Marangoni-like instability caused by solvent gradients in the film as solvent evaporates from the top surface [60,62]. This may be what causes the bilayer to develop a sinusoidal distortion which leads to the breakup of the bilayer, resulting in a laterally segregated film.…”

“…This may be what causes the bilayer to develop a sinusoidal distortion which leads to the breakup of the bilayer, resulting in a laterally segregated film. Control of the evaporation rate, and therefore the steepness of the solvent gradient in the film, has been shown to be a mechanism by which the lateral phase separation can be prevented, allowing the final morphology to be either vertically or laterally segregated [62]. A schematic showing the process of the breakup of a bilayer is shown in figure 4.1.…”

Effect of including a gas layer on the gel formation process during the drying of a polymer solution

“…The drying of liquid films of polymer solutions by evaporation of the solvent is an important process which is considered in numerous industrial applications including painting [1][2][3], coating [4][5][6], ink-jet printing [7][8][9], production of electronic-devices [10] and so on. The building of a basis for these technologies necessitates understanding the underlying physics of drying phenomena, and especially requires quantitative analysis of solvent evaporation in thin films.…”