Structure and dynamics of symmetric diblock copolymers

Abstract

Symmetric polystyrenepolybutadiene diblock copolymers in the bulk undergo a phase transistion from a disordered melt to a lamellar structure upon variation of temperature or chain length. The interplay between structure and dynamics and the chain conformation has been studied using various scattering techniques. A homologous series of ten symmetric polystyrene-polybutadiene diblock copolymers was synthesized using living anionic polymerization under inert atmosphere. The samples were characterised using NMR, DSC and chromatography and, among others, it was found that the samples had narrow molar-mass distributions. The order-disorder transition (ODT) temperatures of low molar-mass samples were determined in dynamic mechanical measurements. In this way, the Flory-Huggins segment-segment interaction parameter, ,y, could be estimated. Different sample preparation methods were used in order to test the existence of nonequilibrium structures in the ordered state: annealing, solvent-casting and shear alignment. No significant difference in the lamellar thickness according to the preparation method has been observed, indicating that the samples are in thermal equilibrium. The scaling of the characteristic lengthscale, D, with chain length, N, was studied using smallangle X-ray and neutron scattering. Model calculations of the scattering from lamellar structures were performed. In the strong-segregation limit (xN II 30-loo), the characteristic lengthscale scales like D 0: Nos. In the intermediate-segregation regime around the order-disorder transition (xN N 5 - 30), D cc Nos was found. The finding of a crossover from the intermediate-segregation regime to the strong-segregation limit is in accordance with theories. A temperature study of one sample around the ODT temperature using small-angle neutron scattering showed that the characteristic lengthscale does not change in a discontinuous way at the ODT temperature. In contrast, the height and width of the observed peak change discontinuously at the ODT temperature. Using dynamic light scattering, the dynamic processes (modes) in three low molar-mass samples were studied. A new data analysis technique was applied in order to reveal modes of relatively low amplitude. Four modes have been identified in the disordered state close to the ODT: cluster diffusion, single-chain diffusion due to heterogeneity in composition from chain to chain, a mode related to chain stretching and orientation, and the segmental reorientational dynamics of the polystyrene blocks. The two latter processes give rise to depolarized scattering. In the ordered state, all modes give rise to depolarized scattering, which is attributed to the anisotropy of the lamellar structure. In the disordered state, the stretching mode was observed to vanish at XN N 5. This is interpreted to be due to the transition from Gaussian to stretched chain conformation. The main result of the Ph.D. work is that the intermediate-segregation regime is localized between XN N 5 and 30. This report is submitted in partial fulfillment of the requirements for a Ph.D. degree at Roskilde University. The supervisors are Dorthe Posselt and Bent C. Jorgensen at IMFUFA (Institute of Mathematics and Physics), Roskilde University and Kristoffer Almda1 at Rise National Laboratory.