Can a single function for χ account for block copolymer and homopolymer blend phase behavior?

Abstract

Most theoretical treatments of polymer-polymer phase behavior assume that homopolymer mixtures and block copolymer melts are controlled by a common segment-segment interaction parameter knows as . This publication describes the results of small-angle neutron scattering ͑SANS͒ experiments conducted as a function of temperature and composition from homogeneous mixtures of poly͑ethylene͒ ͑PE͒ and poly͑ethylenepropylene͒ ͑PEP͒ of equal molecular weight. Analysis of these SANS measurements based on the random phase approximation indicates that PE/PEP is independent of composition and linear in T Ϫ1 . The associated symmetric phase diagram calculated with Flory-Huggins theory contains a stability curve that is consistent with the divergence in single phase susceptibility obtained by SANS. This function PE/PEP (T) is compared with functions for PE-PEP (T) associated with the homologous PE-PEP diblock copolymers, extracted both from the temperature dependence of the disordered state scattering structure factor, and from the N dependence of the order-disorder transition ͑ODT͒ temperature T ODT , where N is the degree-of-polymerization. Analysis of these data using mean-field and fluctuation theories leads to four distinct expressions for PE-PEP (T), none of which is consistent with PE/PEP (T). We attribute this disparity to the effects of polarization and stretching of diblock molecules in the vicinity of the ODT. A simple model calculation that corrects the prediction for (N) ODT based on the experimentally determined shift in the SANS peak location corroborates this deduction. We conclude that current theories cannot account for block copolymer and homopolymer phase behavior based on a single function for , and suggest that this deficiency may be rectified with a more sophisticated block copolymer theory that properly accounts for chain stretching.

Formation and structure of the microemulsion phase in two-dimensional ternary AB+A+B polymeric emulsions

“…Experiments [43] as well as simulations [44] indeed reveal that the peak wavevector q * is ∼ 15 − 20% lower at the order-disorder transition than predicted by SCFT. However, this effect can also be explained by local chain stretching [44,45].…”

Micro-vs.macro-phase separation in binary blends of poly(styrene)-poly(isoprene) and poly(isoprene)-poly(ethylene oxide) diblock copolymers

“…This change points to a macro-phase-separated system, where the relative amount of the phases is changed. Although higher-order reflections are absent, the coexisting phases can be expected both to be lamellar, since the diblock copolymers are symmetric [12].…”

Thermodynamics of Symmetric Diblock Copolymers Containing Poly(styrene-ran-styrenesulfonic acid)

“…Each experiment yields a χ value slightly different than another, and best practice dictates measuring χ in the manner most closely related to that in which it will be used. 11,37 For weakly to intermediately segregated systems with experimentally accessible T ODT s, the relationship χ(T ODT ) = χ ODT SCMFT /N generally correlates well with the form χ(T) = α/T + β, where the parameter α is directly related to the excess enthalpy of mixing and β the excess entropy. 12,37−39 This strategy for measuring χ(T) is ideally suited for use in conjunction with SCMFT for the prediction of phase behavior.…”