Interchain electron donor–acceptor complexes. Determination of equilibrium constant and thermodynamic parameters in the solid state

The interpolymeric electron donor–acceptor (EDA) complex of donor poly[( N ‐ethylcarbazol‐3‐yl)methyl methacrylate] (PHMCM‐2) with acceptor poly‐(2‐[(3,5‐dinitrobenzoyl)oxy]ethyl methacrylate) (PDNBM‐2) presents a single glass transition temperature and a decomplexation endotherm on differential scanning calorimetric (DSC) thermograms. This system is considered a “polymer blend model” which exhibits a lower critical solution temperature (LCST). Phase separation of this blend is kinetically controlled and positive deviations of the glass transition temperatures from weight average values suggest that it behaves as a thermally reversible crosslinked network. Calorimetric methods to determine the heats of mixing of small molecule complexes in solution were adapted for this solid state blend to estimate the equilibrium constant ( K eq ) and other thermodynamic parameters. Applying a computer iterative procedure and assuming 1 : 1 stoichiometry, a least‐squares fit was found for several different donor molecular weights with three different high molecular weight acceptors. At moderate molecular weights, K eq rises to represent saturation fractions near unity as found in biological systems. K eq decreases for higher molecular weights, possibly due to trapped chain entanglements. These results are supported by a composition‐independent, “horizontal line” phase diagram, thus resembling the completely complexed/denaturation process in DNA.