Nonuniversal Behavior of the Thermodynamic Interaction Parameter in Blends of Star and Linear Polybutadiene

Values of the bulk thermodynamic interaction parameter, χ eff, for blends of anionically polymerized star (number of arms = 4, 6, 8, 12) and linear polybutadienes (PB) of well-defined architecture and molecular weight were measured as a function of temperature using small-angle neutron scattering. Comparison of these measured values of χ eff with results from comparable polystyrene (PS) blends suggests the existence of nonuniversal aspects in the thermodynamic interaction due to entropic contributions, χ ε, arising from architectural differences in chains. While the value of χ ε for PS star/linear blends increases monotonically with number of arms in the star, the value of χ ε in the PB star/linear blends does not, a result which cannot be anticipated by the Gaussian field theory (GFT) of Fredrickson et al . 1 An important discrepancy between theory and experiment is also found for the variation of χ ε with linear chain length. Theory anticipates the value of χ ε should decrease with increasing linear chain size, but in fact it increases. Qualitative agreement with the GFT is found on two counts: χ ε decreases with increasing concentration of star (when assuming χ isotopic for linear/linear blends is constant with concentration), and χ ε decreases with increasing length of the star arm. In general, the value of χ ε for a PB blend of star and linear components is larger than the value of χ ε for a comparable PS blend. Indeed, phase separation is observed in one particular PB blend of a six-arm star with a sufficiently large linear chain.