Effect of a Competitive Solvent on Binding Enthalpy and Chain Intermixing in Hydrogen-Bonded Layer-by-Layer Films

This work is focused on the effect of a small-molecule/hydrogen-bonding competitor on the heat of complexation of hydrogen-bonding polymers in aqueous solutions, as well as on deposition of these molecules within layer-by-layer (LbL) films. Specifically, binding between poly(methacrylic acid) (PMAA) and poly(vinylpyrrolidone) (PVP) under acidic conditions in the presence of dimethyl sulfoxide (DMSO) has been explored. Isothermal titration calorimetry showed that with increasing concentration of DMSO in aqueous solution, the positive enthalpy of formation of PMAA/PVP complexes decreased and switched sign at similar to 25 vol % DMSO. This change is interpreted as a transition from endothermal association between PMAA and PVP in water, driven by release of solvation water molecules, to exothermic association driven by the enthalpy of interpolymer hydrogen bonding. This switch is interpreted as destruction of polymer hydration shells by DMSO molecules. The content of DMSO also strongly impacted the growth of LbL films. With an increase in DMSO concentration from 0 to 60 vol %, polymer mass deposited within the bilayer increased, while larger DMSO concentrations suppressed film deposition. The effect of DMSO was also revealed in neutron reflectometry measurements, which showed enhanced interpenetration of deuterated PMAA chains within a hydrogenated LbL matrix at increasing concentration of DMSO. The dependence of film intermixing on DMSO content was strongly history dependent. While assembly of LbL films in aqueous solutions followed by exposure to a mixed solvent post-assembly did not significantly influence the internal film structure, the effect of DMSO on film intermixing was strongest when DMSO was present in the film deposition solutions.