Robert A. Mendelson

Surface segregation in blends of styrene-acrylonitrile copolymers

We investigated the kinetic as well as thermodynamic aspects of surface segregation in blends of random copolymers of styrene and acrylonitrile (SAN) with different acrylonitrile contents (22.5 and 27.2 wt%) annealed at 163°C. A comparison was made with equilibrium data collected by Mansfield et al. on the same system using neutron reflectivity. The lower-AN-content SAN, which is labelled with deuterium (d-SAN), was observed to segregate to the vacuum/polymer interface. Time-of-flight and conventional forward recoil spectrometry were used to determine the depth profile of the deuterated component. The apparent diffusion coefficient D app controlling the segregation kinetics at a bulk volume fraction ω of d-SAN of 0.34 was obtained. It was smaller by a factor of approximately 1.5 than the mutual diffusion coefficient D app at =0.34 calculated without taking the expected thermodynamic slowing of diffusion into account. However, D app was larger than the measured D at =0.34 by a factor of approximately 3. The mean-field theory of Schmidt and Binder was employed to extract the Flory interaction parameter χ , which was found to be smaller than but close to its critical value χ c . The value of D computed using this χ value was in good agreement with the independently measured D at= 0.34.