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 on miscible blends of polyisoprene (PI) and poly(4- tert -butylstyrene) (P4tBS) over a broad composition range. This system is characterized by an extraordinarily large T g difference (∼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 ϕ self of around 0.63 for PI in this blend and 0.03 for P4tBS. The extracted ϕ 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.