Influence of Conformational Asymmetry on Polymer−Polymer Interactions:  An Entropic or Enthalpic Effect?

The order-to-disorder transition (ODT) of 21 compositionally symmetric diblock copolymers consisting of various combinations of poly(ethylene) (PE), poly(ethyl ethylene) (PEE), poly(ethylene−propylene) (PEP), poly(ethylene oxide) (PEO), and poly(dimethylsiloxane) (PDMS) was used as a measure of the strength of the interaction between the polymers. Conformational asymmetry makes a contribution to polymer−polymer thermodynamics that is not explainable within existing simple theories. For a series of PDMS−hydrocarbon diblock copolymers the observed phase behavior can be rationalized through a self-consistent set of solubility parameters. However, this model as well as other models breaks down when the phase behavior of a series of PEO−hydrocarbon block copolymers is included. We suggest that conformational asymmetry leads to an enthalpic contribution to the Flory−Huggins parameter χ through an effective coordination number which is maximized when both of the blocks have large conformational symmetry parameters, β2.