Effects of Solubility Difference of Tackifier to Respective Components of Block Copolymers on Microphase-Separated Structures in Coated Layers of Pressure-Sensitive Adhesive Prepared by Solution Coating Process

Effects of solubility difference of tackifier to respective components of block copolymers on microphase-separated structures were examined using coated layers of the polystyrene-block-polyisoprene-block-polystyrene triblock copolymer and polystyrene-block-polyisoprene diblock copolymer blend, which forms spherical microdomains of polystyrene (PS) in the polyisoprene (PI) matrix, mixed with three types of tackifiers: aliphatic (C5) resin, aliphatic–aromatic (C5–C9) resin, and rosin ester (RE) resin. We performed small-angle X-ray scattering (SAXS) measurements to evaluate d spacing of the (110) planes of body-centered cubic (bcc) lattice and the average radius (R) of the PS spheres. Then, it was found that both d and R decreased with an increase in the tackifier content for all types of tackifiers. This decreasing tendency can be explained by considering that the added tackifier plays the role of solvent to swell the PI block chains to meet the wet-brush condition, resulting in a decrease in the aggregation number of the PS chains in a sphere. Thus, the PS sphere is smaller compared to the case without tackifier. The volume fraction of PS calculated from d and R values by assuming the bcc lattice was decreased with the tackifier content for the case of C5 and C5–C9 resins, while it was constant for the case of the RE resin. These results ensure that the RE resin plays the role of the neutral solvent while the other tackifiers play roles of PI-selective solvent. To discuss the long-term durability of nanostructures, gradual temporal changes in the nanostructures were also examined up to 30 days at 50 °C. As a consequence, the specimen mixed with the RE resin is considered to be suitable because the change in the nanostructures was the least for this case.