Effects of drying temperature in solution coating process on microphase-separated structures in coated layers of pressure-sensitive adhesive composed of di- and triblock copolymer blends as revealed by small-angle X-ray scattering

Abstract

An acrylic block copolymer system is a promising candidate for the functional adhesives bearing good weatherability as well as high transparency. Coated layers of the acrylic block copolymers were prepared by the solution coating method using a polymethylmethacrylate-block-poly(n-butylacrylate)-block-polymethylmethacrylate triblock copolymer (MAM) and a polymethylmethacrylate-block-poly(n-butylacrylate) diblock copolymer (MA). The thickness of the prepared specimens was 50 μm, which is typical of the pressure-sensitive adhesives. Atomic force microscope (AFM) observations and two-dimensional small-angle X-ray scattering (2d-SAXS) measurements were performed to analyze the nano structures and to discuss the effects of the drying temperature on the nano structures. Note that better physical properties were obtained in the specimen prepared at the lower drying temperature. The AFM observations revealed cylindrical morphology for the annealed MAM neat specimen. On the other hand, the 2d-SAXS measurements revealed spherical morphology for all of the specimens including MAM neat and MA/MAM blends. The effects of the drying temperature on the ordering regularity of spheres, d spacing, and radius of sphere has been examined quantitatively. It was found that the ordering regularity of spheres was more improved for the higher drying temperature. As for the behaviors of d and R, the experimental results exhibited very complicated behaviors as a function of the drying temperature. Namely, d and R increased as a function of the drying temperature, which is completely opposite to the general behaviors of the block copolymer microdomain structure as d or R ∝ T−1/3 for the upper critical solution temperature system where the interaction between two kinds of the constituent block chains is decreased with an increase in temperature. The effect of structure freezing during the solvent evaporation was taken into account and such complicated behaviors of experimental results (d and R) could be almost perfectly explained. It is concluded by this study that the poor regularity of the sphere ordering provides softer property with larger elongation at break for the coated film, which is more suitable as the pressure-sensitive adhesives.