Hydrogen‐Bonded Complexes of Star Polymers

Abstract

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.

Impact of Star Polyacid Branching on Polymer Diffusion within Multilayer Films

“…This result can be rationalized as more compact star PAA forming ionic pairs with PDMAEMA and likely preferentially interacting with the peripheral units of the star polymers. Note that this result is different from what was observed in hydrogen-bonded complexes of linear poly­(methacrylic acid) (PMAA) with star poly­(ethylene oxide) (PEO), where star PEO provided a higher density of hydrogen-bonded sites leading to higher enthalpy and higher content of PMAA within its complexes with star PEO . In the system described here and in the selected conditions of our experiments of pH 2.5, IPCs were stabilized by electrostatic rather than hydrogen-bonding interactions, while protonation of PAA units competed with PDMAEMA-induced ionization and possibly supported dimerization of carboxylic groups in the higher density environment of the star polymers.…”

“…Note that this result is different from what was observed in hydrogen-bonded complexes of linear poly(methacrylic acid) (PMAA) with star poly(ethylene oxide) (PEO), where star PEO provided a higher density of hydrogen-bonded sites leading to higher enthalpy and higher content of PMAA within its complexes with star PEO. 65 In the system described here and in the selected conditions of our experiments of pH 2.5, IPCs were stabilized by electrostatic rather than hydrogen-bonding interactions, while protonation of PAA units competed with PDMAEMA-induced ionization and possibly supported dimerization of carboxylic groups in the higher density environment of the star polymers. In agreement with a smaller number of ionic contacts being formed by star PAAs with PDMAEMA, the dissociation constant (K D ) calculated from the ITC results was up to one order of magnitude higher for star PAAs.…”

“…Deposition of LbL films can be correlated with the formation of interpolymer complexes (IPC) in solution. − Thus, differences in ionic paring within L PAA/PDMAEMA and star PAA/PDMAEMA systems were further explored using ITC. This technique was previously used to study correlations between the enthalpy of intermolecular binding and the growth mode of electrostatic LbL films, as well as the role of a hydrogen-bonding competitor, polymer branching, and a binding partner on interpolymer hydrogen bonding. − Figure A and S12 show raw ITC data for the titration of linear and star PAAs, respectively, with PDMAEMA at pH 2.5. Interestingly, in all cases, the enthalpies of polymer–polymer interactions (Δ H int ) after saturation of PAA with PDMAEMA were similar (∼ −5.5 kJ per mole of PDMAEMA units) (Figure B).…”