Structure of PS−PEO Diblock Copolymers in Solution and the Bulk State Probed Using Dynamic Light-Scattering and Small-Angle Neutron-Scattering and Dynamic Mechanical Measurements

The phase behavior of a low molecular weight ( M W = 4000) diblock copolymer of polystyrene and poly(ethylene oxide), PS−PEO, in the bulk as well in aqueous, D 2 O, solutions has been studied using small-angle neutron-scattering, dynamic and static light-scattering, and rheological methods. At low temperature the pure block copolymer forms a lamellar mesophase driven by the crystallization of PEO. The melting point of PEO, T ≈ 64 °C, is correspondingly accompanied by an order-to-disorder transition. The melt is typified by single-exponential depolarized correlation functions in the dynamic light-scattering spectra, observed in the temperature range 60 to 100 °C. Neutron scattering fails to show concentration fluctuations in the amorphous, disordered phase due to lack of contrast. In aqueous solutions up to roughly 20% polymer concentration, PS−PEO self-associates into spherical micelles with a core size of R c ≈ 56 Å and an interaction radius R hs ≈ 115 Å, corresponding to an aggregation number of 470. The hydrodynamic radius R h = 140 Å obtained from dynamic light scattering is close to but somewhat larger than the interaction radius, as expected. Extended clusters, displaying a small but significant anisotropy, form at above C = 10% and coexist with the single spherical micelles. These clusters may be due to association of spherical micelles or micelles of different form attributed to residual aggregates of PS moities.