SMALL-ANGLE NEUTRON-SCATTERING STUDY OF MICROPHASE SEPARATION IN THERMOASSOCIATIVE COPOLYMERS

The morphology of poly(sodium acrylate)-g-poly(ethylene oxide) graft c opolymers (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 c ritical conditions are reached, a scattering peak arising from the cor relation between the PEO domains is clearly observed. Beyond the criti cal conditions, e.g., at higher temperature or salt concentration, the magnitude of the fluctuations keeps on increasing while their wavelen gth or the periodicity of the microdomains remains constant. At the sa me time, the asymptotic behavior, studied at higher values of the scat tering vector q, indicates that PEO side chains undergo a continuous t ransition from an homogeneous solution of random coils, in the weak se gregation 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 i n the strong segregation regime indicates that PEO microdomains behave as polydisperse spherical micelles organized in a simple cubic lattic e. Moreover, the absolute fitting of the experimental results with a p olydisperse 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 quantit atively, by the phase diagram of PEO. Using various copolymers, differ ing 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 ag gregation number(N-ag) is mainly fixed by the primary structure of the copolymer. N-ag can be roughly estimated using a scaling relation tak ing into account its double dependence with the lengths of both hydrop hilic and ''hydrophobic'' units: N-ag similar to N-B(beta)/N-A(alpha).