Two DSC Glass Transitions in Miscible Blends of Polyisoprene / Poly(4-\textit{tert}-butyl styrene)

Conventional and temperature modulated differential scanning calorimetry experiments have been carried out on miscible blends of polyisoprene (PI) and poly(4-\textit{tert}-butyl styrene) (P4tBS) over a broad composition range. This system is characterized by an extraordinarily large component T$_{g}$ difference ($\sim $215 K) between the two homopolymers. Two distinct calorimetric T$_{g}$s were observed in blends with an intermediate composition range (25{\%}$\sim $50{\%} PI) by both conventional and temperature modulated DSC. Good agreement was found between the T$_{g }$values measured by the two methods. Fitting of the measured T$_{g}$s to the Lodge-McLeish model gives a $\phi _{self}$ of 0.62$\sim $0.64 for PI in this blend and 0.02$\sim $0.05 for P4tBS. The extracted $\phi _{self}$ for PI$_{ }$is comparable to reported values for PEO in blends with PMMA and is significantly larger than those reported for other PI blends with smaller component T$_{g}$ differences. This observation suggests the presence of a confinement effect in PI/P4tBS blends, which results in enhanced fast component dynamics below the effective T$_{g}$ of the slow component.