Effect of shear history on the morphology and coarsening behaviour of polycarbonate/poly(styrene-co-acrylonitrile) blend

The morphology and coarsening behaviour of polycarbonate (PC) blend with styrene-co-acrylonitrile (SAN) random copolymer of 25 wt% AN have been investigated as a function of shear history using transmission electron microscope (TEM) and time resolved light scattering techniques. Simple shear apparatus of two parallel plates geometry was used to generate different shear rate values depending on the different distances from the centre of the sample disk. The morphology of PC/SAN-25=70/30 blend showed that the dispersed phase of SAN was elongated and broken-up in the direction of flow with weaker contrast at high shear rate values. The shear rate was found to suppress the concentration fluctuations and enhance the miscibility of SAN (dispersed phase) in the PC matrix to a great extent. The average of the dispersed particle diameter was evaluated as a function of different shear memories at 240 °C for different time intervals based on the Debye-Bueche theory. The obtained data were found to be shear memory dependent i.e. the average particle diameter decreases with increasing shear memory. This result indicated that the coarsening process is greatly suppressed by shear memory and the shear could produce a permanent morphological change (irreversible change) over the time scale of the measurement. This behaviour was attributed to the very high melt viscosities of the blend components, which in turn led to a very long relaxation time and consequently a high difficulty to erase the effect of shear at the experimental temperature. Furthermore, the coarsening process for all the measured samples followed the general power law, R¯3(t)=R¯3(0)+t, regardless of the shear memory of the blend. This behaviour implied that the shear could only retard the rate of domain growth without any effect on the coarsening mechanism.