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

Abstract

This work is focused on the effect of a smallmolecule/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 ∼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 postassembly 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.

Hydrogen‐Bonded Complexes of Star Polymers

“…Similar to what was previously reported for hydrogen‐bonded complexes of linear PEO and PMAA, [ 24,27,29 ] the enthalpy of IPC formation for star polymers is positive, suggesting that PEOPMAA binding is controlled by the gain in entropy associated with release of water molecules from the hydration shells of associating macromolecules. [ 30 ] The enthalpy of formation of s PEO/PMAA complexes (calculated per PEO unit) is higher than that for l PEO/PMAA complexes (≈50–60% higher at pH 2.5, i.e., ≈1.4 and ≈0.9 kJ mol −1 , respectively), and s PEO/PMAA complexes accommodate more PMAA as compared to l PEO/PMAA complexes (with the ratio of PMAA/PEO units of 3.2 and 2.2 for s PEO/PMAA and l PEO/PMAA complexes, respectively, Figure 1C). The enthalpy for l PEO/PMAA IPCs is approximately threefold smaller than the earlier reported enthalpy for linear PVP/PMAA complexes (0.9 vs 2.5 kJ mol −1[ 30 ] for l PEO/PMAA and PVP/PMAA complexes, respectively), suggesting a significant role of chemistry of the hydrogen‐bonding units.…”

“…[ 20 ] Yet, hydrogen‐bonding complexes are mostly studied for linear polymer partners, including IPCs of linear poly(ethylene oxide) ( l PEO) with linear polyacids. [ 21–29 ] Previous studies of poly(ethylene oxide) (PEO)/poly(methacrylic acid) (PMAA) [ 24,29 ] and other hydrogen‐bonded complexes of linear polymers with polycarboxylic acids [ 24,30,31 ] revealed positive values for the enthalpy of IPC formation. Because the formation of hydrogen‐bonded complexes lacks release of small ions, usually ascribed as the main driving force for association of polyelectrolyte chains, positive enthalpy of the complex formation suggests an entropic gain due to release of hydration water molecules as a driving force for intermolecular association.…”

“…Because the formation of hydrogen‐bonded complexes lacks release of small ions, usually ascribed as the main driving force for association of polyelectrolyte chains, positive enthalpy of the complex formation suggests an entropic gain due to release of hydration water molecules as a driving force for intermolecular association. [ 24,30 ] Our recent study of IPCs of linear poly(vinyl pyrrolidone) (PVP) with PMAA showed that the hydration shell of the polymers can be significantly distorted by introducing low‐molecular‐weight competitors for hydrogen bonding, leading to dramatic changes in thermodynamic parameters of IPC formation. [ 30 ] Studies of hydrogen‐bonded complexes which involve macromolecules of nonlinear architecture are much less common.…”

“…[ 24,30 ] Our recent study of IPCs of linear poly(vinyl pyrrolidone) (PVP) with PMAA showed that the hydration shell of the polymers can be significantly distorted by introducing low‐molecular‐weight competitors for hydrogen bonding, leading to dramatic changes in thermodynamic parameters of IPC formation. [ 30 ] Studies of hydrogen‐bonded complexes which involve macromolecules of nonlinear architecture are much less common. [ 32,33 ] In one known to us example, Song et al.…”

Organic Thermoelectric Multilayers with High Stretchiness

“…Functionalized multilayers can be created with a variety of combinations of materials such as quantum dots, biological molecules, dendrimers, and carbon nanomaterials [31][32][33][34]. LbL assembly is mainly a result of electrostatic interaction in most cases, but other molecular interactions between the LbL materials, including hydrogen bonds, coordination bonds, charge transfer, hydrophobic interactions, and the combined interaction of the above forces have been shown to be driving forces to build up multilayer films [35][36][37]. LbL films have been engineered in a diverse range of applications, such as drug delivery, sensing, self-cleaning, super hydrophobic surfaces, separation membranes, and energy storage [38][39][40][41].…”