SLOWING-DOWN OF THE DIFFUSION PROCESS IN BINARY POLYMER MIXTURES OF NONCRITICAL COMPOSITION APPROACHING THE COEXISTENCE CURVE

The diffusion process in dPS (M(w) = 1.06 x 10(6) g mol-1)/hPS (M(w) = 2.89 x 10(6) g mol-1) mixtures at constant temperature below the crit ical temperature slows down approaching the coexistence curve from the homogeneous single fluid phase region of the phase diagram. This effe ct can be understood in terms of the composition dependence of the the rmodynamic factor partial derivativemu(dPS)/partial derivativePHI(dPS) T,P in combination with the Flory-Huggins theory of polymer melts. The theoretically expected decrease of the mutual diffusion coefficient w ith increasing temperature along the coexistence curve approaching the critical temperature is not observed in the temperature range studied (5 K < (T(c) - T) < 22 K). The temperature dependence of the diffusio n coefficient D is dominated by the temperature dependence of the phen omenological transport coefficient L. The apparent activation energy o f the diffusion coefficient of the polymer melts studied is larger by a factor of about 10 than that of binary mixtures of low molar mass co mponents.