NMR AND DSC INVESTIGATIONS OF THE MISCIBILITY OF BLENDS OF CIS-1,4-POLYISOPRENE WITH POLYBUTADIENES OF DIFFERENT MICROSTRUCTURES

Differential scanning calorimetry and torsional braid analysis investi gations of the phase diagram of cis-1,4-polyisoprene/polybutadiene ble nds as a function of the polybutadiene microstructure were reported by several authors. Polybutadienes containing a high vinyl content were shown to be miscible with cis-1,4-polyisoprene on the DSC spatial scal e, whereas polybutadienes containing a low vinyl content were immiscib le. In this article, we used variable-temperature determinations of IH NMR free induction decays and low-temperature, high-resolution solid- state C-13 NMR measurements of proton spin-lattice relaxation times in the rotating frame to probe the phase behavior of the cis-1,4-polyiso prene/polybutadiene blends at the smaller spatial scale of the NMR tec hnique. Blends of cis-1,4-polyisoprene with a polybutadiene having a l arge number of vinyl 1,2 linkages appeared to be miscible on the molec ular scale, in spite of small regions in which the polybutadiene compo nent is not uniformily dispersed in the other polymer. On the contrary , blends in which the polybutadiene has a low content of vinyl 1,2 seq uences were phase separated over the whole temperature range considere d and no intermixed regions could be detected. The limiting case was o bserved with the polybutadiene containing 33 wt % vinyl 1,2 units, for which miscibility on a molecular scale is highly dependent on the ble nd composition. (C) 1995 John Wiley and Sons, Inc.