Architecture‐driven thermodynamic interactions in blends of star‐branched and linear poly(methyl methacrylate)

Abstract

In blends of two polymer chains of identical architecture, the bulk thermodynamic interaction can vary with differences in blend concentration, microstructure, tacticity, and isotopic labeling. The strength of this interaction is typically expressed in terms of an effective segment–segment-interaction parameter, eff, the value of which varies with all these molecular particulars. The fluctuation theory of Fredrickson et al. suggests that if one component in a polymer blend is linear and the other has a long-chain branched architecture, the bulk interaction will increase over that in the case of an analogous blend with two linear components. This increase in the bulk interaction should manifest itself in an architectural contribution to the value of the eff parameter derived from small-angle neutron scattering (SANS) data, although the architectural effect is intrinsically a nonlocal effect. Recently, the magnitude of the architectural contribution to the bulk thermodynamic interaction has been probed experimentally in star/linear polystyrene (PS) blends and star/linear polybutadiene (PB) blends. Greenberg and coworkers found that values of eff for blends of star and linear PS measured with SANS increased monotonically with an increasing number of arms of the star, although the overall molecular weight of the stars was not kept rigorously constant and two different types of arm linking were used. Estimates of the thermodynamic interaction resulting from architectural effects alone, , were made by measuring separately the magnitude of the interaction because of isotopic labeling and subtracting this from the value of the overall interaction, eff. increased with the number of arms of the star as expected from Fredrickson’s theory, but there was near-quantitative agreement only for the case of a four-arm star. The rate of change in the value of with the number of star arms, p, was much larger for a change in p from 4 to 8 than when p increased from 14 to 21. Martter et al. measured the value of eff for blends of star and linear PB with SANS and found that eff and varied nonmonotonically with an increase in the number of arms from 4 to 12. This result differed from the expectation from theory that a monotonic increase in with the number of arms of the star should be universally observed regardless of the chemical structure of the repeat unit. In this work, a brief study of star/linear blend for a third type of repeat unit was undertaken to further test the universality of the predictions of Fredrickson et al. Poly(methyl methacrylate) (PMMA) was chosen because methods exist for the preparation of well-defined star polymers by atom transfer radical polymerization (ATRP). However, two other differences between the stars studied here and those examined previously must be noted. First, the star core chemistries differ from those used for the PS or PB stars as necessitated by the PMMA star synthesis. Although linear PMMA can be polymerized anionThe Supplementary Material referred to in this article can be found at http://www.interscience.wiley.com/jpages/08876266/suppmat/0202017B.html Correspondence to: M. D. Foster (E-mail: foster@ polymer.uakron.edu)