Small-angle neutron scattering study of microphase separation in thermoassociative copolymers

The morphology of poly(sodium acrylate)-g-poly(ethylene oxide) graft copolymers (PAA-g-PEO) in semidilute solution has been studied by small-angle neutron scattering using a temperature or salting-out effect as trigger for the phase separation of PEO side chains. As soon as the critical conditions are reached, a scattering peak arising from the correlation between the PEO domains is clearly observed. Beyond the critical conditions, e.g., at higher temperature or salt concentration, the magnitude of the fluctuations keeps on increasing while their wavelength or the periodicity of the microdomains remains constant. At the same time, the asymptotic behavior, studied at higher values of the scattering vector q, indicates that PEO side chains undergo a continuous transition from an homogeneous solution of random coils, in the weak segregation regime, to a microseparated two-phase structure with a sharp boundary in the so-called strong segregation regime (corresponding to high temperature and/or salt concentration). The modeling performed in the strong segregation regime indicates that PEO microdomains behave as polydisperse spherical micelles organized in a simple cubic lattice. Moreover, the absolute fitting of the experimental results with a polydisperse sphere model clearly indicates that the clustering of the PEO grafts, e.g., the temperature of microphase separation, the number of side chains gathered into the microdomains, and the concentration inside these clusters, is totally described, qualitatively and quantitatively, by the phase diagram of PEO. Using various copolymers, differing either by the number of hydrophilic units between the side chains (N-A) or the number of ethylene oxide units (N-B), We show that the aggregation number(N-ag) is mainly fixed by the primary structure of the copolymer. N-ag can be roughly estimated using a scaling relation taking into account its double dependence with the lengths of both hydrophilic and "hydrophobic" units: N-ag similar to N-B(beta)/N-A(alpha).