SURFACE SEGREGATION IN BLENDS OF STYRENE-ACRYLONITRILE COPOLYMERS

We investigated the kinetic as well as thermodynamic aspects of surfac e segregation in blends of random copolymers of styrene and acrylonitr ile (SAN) with different acrylonitrile contents (22.5 and 27.2 wt%) an nealed at 163 degrees C. A comparison was made with equilibrium data c ollected by Mansfield et al. on the same system using neutron reflecti vity. The lower-AN-content SAN, which is labelled with deuterium (d-SA N), was observed to segregate to the vacuum/polymer interface. Time-of -flight and conventional forward recoil spectrometry were used to dete rmine the depth profile of the deuterated component. The apparent diff usion coefficient D-app controlling the segregation kinetics at a bulk volume fraction phi of d-SAN of 0.34 was obtained. It was smaller by a factor of approximately 1.5 than the mutual diffusion coefficient ($ ) over tilde D at phi=0.34 calculated without taking the expected ther modynamic slowing of diffusion into account. However, D-app was larger than the measured D ($) over tilde at phi=0.34 by a factor of approxi mately 3. The mean-field theory of Schmidt and Binder was employed to extract the Flory interaction parameter chi, which was found to be sma ller than but close to its critical value chi(c). The value of ($) ove r tilde D computed using this chi value was in good agreement with the independently measured ($) over tilde D at phi=0.34.