(708i) Reduction in d-Spacing and Volume of Microphase Separated Acrylate Block Copolymers during Casting from Solution
Solution casting of block copolymers is a common method for the production of an array of products, such as pressure sensitive adhesives and proton exchange membranes. Three poly(methy methacrylate-b-butyl acrylate-b-methyl methacrylate) triblock copolymers of varying block fractions were cast from solution in toluene to characterize the self-assembly process. The films were cast at approximately 400 microns in thickness and dried down to a thickness of approximately 100 microns. The films were characterized using in-situ small-angle x-ray scattering, and the volume fraction of ordered phase was calculated over time in each film. The scattering profiles indicated compression of microdomains in the vertical direction regardless of phase geometry. This directional compression was attributed to deswelling of the domains during drying. Horizontal compression was unable to occur due to the films being affixed to the substrate. In addition, during drying, the relative volume of ordered phase increased in all samples before decreasing sharply and finally reaching a lowered equilibrium value, indicating ordering followed by disordering in the sample. This equilibrium value for volume of ordered phase was between 2 and 4 orders of magnitude below the maximum volume in the experiment, with the difference between equilibrium value and maximum value increasing with increasing poly(methyl methacrylate) content. This was likely caused by a combination of factors, namely the difference in glass transition temperatures between the two blocks and the geometry of the microphase separated domains. The lower glass transition temperature of the poly(butyl acrylate) blocks when compared with poly(methyl methacrylate) may have been acting to reduce compressive stress on the domains during drying, reducing the amount of disordering. In addition, a change in phase separated geometry from hexagonal cylinders to lamellae as the poly(methyl methacrylate) content increased may have also contributed to disordering. Anisotropic deformation of the microstructure may have altered interfacial curvature between phase separated blocks, increasing interfacial tension. Lamellar structures may have experienced greater changes in interfacial curvature or a greater effect of interfacial curvature on interfacial tension due to their geometry, which may have destabilized the lamellar structures to a larger degree than cylindrical structures.