Hydrogen-Bonded Complexes of Star Polymers.
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The effect of molecular architecture, star versus linear, poly(ethylene oxide) (PEO) on the formation of hydrogen-bonded complexes with linear poly(methacrylic acid) (PMAA) is investigated experimentally and rationalized theoretically. Isothermal titration calorimetry reveals that at pH 2.5 interpolymer complexes (IPCs) of PMMA with a 6-arm star PEO (sPEO) contains 50% more polyacid than IPCs formed with linear PEO (lPEO). While the enthalpy of IPC formation is positive in both cases, its magnitude is 50% larger for sPEO/PMAA complexes that exhibit a lower dissociation constant than lPEO/polyacid complexes. These results are rationalized based on a higher localized density of hydrogen bonds formed between sPEO and the polyacid which prevents penetration of star molecules into PMAA coils. Accordingly, Fourier transform infrared results indicate approximately twofold excess of self-associated >COOH units over intermolecularly bonded >COOH units in sPEO-containing complexes. The excess of PMAA chains in IPCs and the percentage of self-associated carboxylic groups in sPEO/PMAA complexes both increase with polyacid molecular weight. Other findings, including a positive entropy, hysteresis in composition at strongly acidic pH, and progressive equilibration of IPCs at increased pH are consistent with the critical role of charge and release of water molecules in the formation of sPEO/PMAA and lPEO/PMAA complexes.