Microphase Separation Kinetics in Block Copolymers during Film Drying

Solution casting is a commonly used technique to produce block copolymer films for various applications such as the production of membranes and lithography. The morphology and overall long range ordering are known to be affected by the rate of solvent removal in the system, but the effects of the drying rate on the phase separation kinetics in the system are not currently well understood. We studied several different block copolymer systems in a neutral solvent, toluene. These included poly(styrene-b-isoprene) triblock copolymer, poly(methyl methacrylate-b-butyl acrylate) triblock copolymer with varied block fractions, and poly(styrene-b-butadiene) diblock copolymer. Small-angle X-ray scattering (SAXS) was performed in-situ to characterize and quantify the volume fraction of ordered microstructure in the films during solvent removal, and dynamic mechanical analysis was performed at individual bulk concentrations. Films were cast from solution to an initial thickness of 400μm, drying down to a final thickness of less than 100μm. The rate of solvent removal was controlled via a sweep gas running underneath the sample stage to regulate the solvent vapor concentration in the drying chamber, and solvent concentration in the film was tracked gravimetrically. Solvent partitioning between the microdomains was estimated using the solvent concentration in the film. This allowed for calculation of the volume fraction of ordered phase in the films over time.  A mass transfer model was fitted to the drying data in order to determine the concentration profile in the film and to more accurately account for the solvent partitioning between the microdomains.