DIELECTRIC INVESTIGATION OF MOLECULAR-DYNAMICS OF BLENDS .4. EFFECT OF BLENDING ON THE NORMAL-MODE PROCESS OF POLYISOPRENE POLYSTYRENE BLENDS/

Polyisoprene/polystyrene (PI/PS) blends have been prepared and investi gated for compatibility using dielectric and calorimetric measurements . Various blends were prepared from polystyrene (number average molecu lar weight, (M) over bar(n) = 160 000 gmol(-1)) and polyisoprene with (M) over bar(n) values of 13 800, 40 500 and 130 000 gmol(-1). Dielect ric measurements have been carried out over a wide frequency range (10 (-2)-10(6) Hz) and in the temperature range of the glass and normal mo de processes (-70 to +70 degrees C). The glass transition, as well as the corresponding relaxation process, of polyisoprenes were shifted to higher temperatures in the different blends, indicating compatibility . The blends showed a lower critical solution temperature (LCST) at te mperatures above 105 degrees C. It was surprising to find that blendin g of polyisoprene with polystyrene led to a great shift to higher valu es in the relaxation frequency of the normal mode process for the isop rene segments. The measurements showed that the relaxation time of the normal mode process in the blends was longer than that of the glass p rocess by a constant factor (3.2 decades), regardless of the molecular weight of the polyisoprenes used in the blends. This finding implied that the domain length responsible for the compatibility of the two po lymers was consistent regardless of the molecular weight used (where ( M) over bar(n) > (M) over bar(c), the critical molecular weight), In v iew of the results obtained, and by using a molecular model, it was po ssible to determine the size of the structural domains responsible for the compatibility. The value obtained (16.7 nm) is very similar to th at suggested to be responsible for the glass transition in pure polyme rs.