Tailoring polymer brushes to act as responsive coatings requires knowledge of brush solvation thermodynamics throughout the grafting density continuum, as the extent of chain-chain interactions alters brush sensitivity to its environment. Current brush models each focus on a limited grafting density range; these restricted windows do not compare differences across grafting regimes or quantify where regime transitions occur. Furthermore, these models do not address solvation in the vapor phase, which is desirable for brush applications such as gaseous membranes and sensors. Measurements that provide a quantitative comparison of brushes across grafting densities and solvent phases will yield accurate maps of brush thermodynamics to harness these next-generation thin films.<br/><br/>We have previously demonstrated that polymer-solvent interaction parameters (χ) within a dense polymer brush differ greatly from solution, as brush χ is dependent on solvent concentration. In this study, polymer brushes with systematically-varied grafting densities were exposed to solvent vapor to compare brush solvation across grafting density regimes. Timed UV irradiation controlled surface initiator concentration. The brushes grafted from those initiators extended into the cross-over regime, which neither of the two primary thermodynamic models accurately capture. In situ x-ray reflectivity measurements were conducted on all brushes to measure film thickness and scattering length density as a function of solvent activity. The polymer-solvent interaction parameter as a function of grafting density, χ(σ), was determined using modified mean field theory. At low activity, brushes in the moderate density regimes absorbed more solvent than the high density brushes; this absorption behavior is similar to thin film gels, where chains have a higher degree of mobility.

Comparison of polymer brush solvation across grafting density regimes via vapor absorption measurements

251st National Meeting (2016)

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