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

Abstract

AbstractThe 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 (Keq) 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, Keq rises to represent saturation fractions near unity as found in biological systems. Keq 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.