Polyisoprene/polystyrene (PI/PS) blends have been prepared and investigated for compatibility using dielectric and calorimetric measurements. Various blends were prepared from polystyrene (number average molecular weight, and polyisoprene with values of 13 800, 40 500 M n \ 160 000 g mol~1) M n and 130 000 g mol~1. Dielectric measurements have been carried out over a wide frequency range (10~2Γ106 Hz) and in the temperature range of the glass and normal mode processes ([70 to ]70Β‘C). The glass transition, as well as the corresponding relaxation process, of polyisoprenes were shifted to higher temperatures in the di β erent blends, indicating compatibility. The blends showed a lower critical solution temperature (LCST) at temperatures above 105Β‘C. It was surprising to Γnd that blending of polyisoprene with polystyrene led to a great shift to higher values in the relaxation frequency of the normal mode process for the isoprene segments. The measurements showed that the relaxation time of the normal mode process in the blends was longer than that of the glass process by a constant factor (3Γ2 decades), regardless of the molecular weight of the polyisoprenes used in the blends. This Γnding implied that the domain length responsible for the compatibility of the two polymers was consistent regardless of the molecular weight used (where the critical molecular weight). In M n
[ M c , view of the results obtained, and by using a molecular model, it was possible to determine the size of the structural domains responsible for the compatibility. The value obtained (16Γ7 nm) is very similar to that suggested to be responsible for the glass transition in pure polymers.