Hydrogen-bonded assembly of star and linear polymers.

We report on the effect of the molecular architecture of poly(ethylene oxide), PEO on its hydrogen-bonding assembly with poly(methacrylic acid (PMAA). Isothermal titration calorimetry (ITC) revealed the entropic nature of binding between 6-arm star polymer (sPEO), or its linear counterpart (lPEO) of matched molecular weight with linear PMAA. While in both cases interpolymer complexes (IPCs) formed between pH 2.5 and 4.0, significant hysteresis of endothermic heats absorbed occurred as a function of mixing order of polymer solutions at pH 2.5, revealing non-equilibrium conformations. At the same time, the mixing hysteresis disappeared at pH 4 when PMAA became more ionized. The enthalpy of IPC formation was dependent on pH and molecular architecture of PEO, with about two-fold higher endothermic values for sPEO/PMAA complexes as compare to lPEO/PMAA complexes, probably due to differences in hydration and structure of these IPCs. The differences in the IPC composition were further confirmed by the dependence of IPC stoichiometry on PEO chain architecture. In particular, the ratio of PMAA-to-PEO repeat units was 1.8 and 1 for PMAA/sPEO and PMAA/lPEO complexes, respectively, suggesting the existence of PMAA loops surrounding star-like PEO molecules.