COALESCENCE IN AN INTERFACE-MODIFIED POLYMER BLEND AS STUDIED BY LIGHT-SCATTERING MEASUREMENTS

The influence of B-B diblock copolymers on coalescence in A:B blends h as been studied by rheo-optical measurements and electron microscopy. Divergent criteria and experimental evidence appear in the literature on the block copolymer (BC) molecular weight (M(W)) and volume fractio n (phi(bc)) when the requirement is the BC to reside at the interface. In the present study the block chain lengths were chosen shorter than the corresponding homopolymers as a starting point. For selected mode l systems it was found that symmetrical diblock copolymers with phi(bc ) greater than or equal to 1% were most effective for inhibiting coale scence. However, rheo-optical measurements revealed that the stabiliza tion effect is not unconditional during the flow; coalescence is preve nted for a time which decreases with increasing shear rate due to remo val of the BC away from the interface. The origin of the observed beha viour is discussed based on various mechanisms: shear-induced mutual c ompatibility between components, squeeze-out/drainage of the interfaci al layer, frictional pull-out of BC chains, collision-induced entrapme nt of BC between interfaces, encapsulation of the BC based on the conc ept of elastic interfacial curvature.