A rheo-optical study on the linear viscoelasticity and molecular dynamics of block copolymer solutions forming hexagonal close-packed cylindrical domains

Abstract

The linear viscoelastic properties of block copolymer solutions forming hexagonal close-packed cylindrical domains and their relaxation dynamics were investigated by using a rheo-optical method based on simultaneous measurements of stress and strain-induced birefringence. The sample solution is composed of a PS-b-PI diblock copolymer and a mixed solvent of DOP and SALACOS 913, which are a neutral solvent for both PS and PI and a highly selective solvent for PI, respectively. The copolymer concentration was fixed, and the DOP fraction of the mixed solvent was increased to change the domain structure from a hexagonal close-packed cylindrical structure to a spherical structure. The sign of the complex strain-optical coefficient, which is the complex ratio of the oscillatory strain-induced birefringence to the strain, varied with frequency, indicating that the viscoelastic relaxation spectra include multiple relaxation modes. Four relaxation modes were successfully estimated with the aid of the modified stress-optical rule. The four modes were assigned to (1) reorientation of corona chains, (2) reorientation of core chains, (3) deformation of domains, and (4) reorientation of grains. The intrinsic birefringence of the cylindrical structure was found to be 1.7 × 10−5. By assuming entropic stress of grain reorientation, the grain size was estimated to be ~0.2 μm from the viscoelastic intensity. The linear viscoelastic properties of block copolymer solutions forming hexagonal close-packed cylindrical domains were investigated by using a rheo-optical method. Based on the modified stress-optical rule, the complex shear modulus was separated to four relaxation modes. Considering the sign and magnitude of the stress-optical coefficient associated with each mode, the four modes were assigned to (1) reorientation of corona chains, (2) reorientation of core chains, (3) deformation of domains, and (4) reorientation of grains. The results were compared with those for spherical domains.