Two DSC Glass Transitions in Miscible Blends of Polyisoprene/Poly(4-tert-butylstyrene)

Conventional and temperature-modulated differential scanning calorimetry (DSC) experiments have been carried out oil miscible blends of polyisoprene (PI) and poly(4-tert-butylstyrene) (P4tBS) over a broad composition range. This system is characterized by all extraordinarily large T-g difference (similar to 215 K) between the two homopolymers. Two distinct calorimetric glass transitions were observed in blends of intermediate compositions(25%-50% PI) by both conventional and temperature-modulated DSC. Good agreement was found between the component T-g values measured by the two methods. Fitting of the component T-g values to the Lodge-McLeish model gives a phi(self) of around 0.63 for PI in this blend and 0.03 for P4tBS. The extracted phi(self) for PI is comparable to reported values for PEO in blends with PMMA and is significantly larger than values reported for PI in other blends with smaller homopolymer T-g differences. This observation is consistent with the presence of a nonequilibrium or confinement effect in PI/P4tBS blends, which results in enhanced dynamics of the fast component below the T-g of the slow component.