Structure and Dynamics of Hydrogel Interfaces

Hydrogels are broadly described as three-dimensional water-borne networks composed of thermally-fluctuating hydrophilic polymer chains linked by chemical crosslinks and/or physical entanglements. For semi-dilute hydrogels composed of flexible polymers at the critical overlap concentration, c*, the characteristic mesh size, ξ, or correlation length between all molecular pairs in the hydrogel network, is similar to the average distance between chemical crosslinks. The average mesh size, determined from small-angle X-ray scattering (SAXS) of bulk hydrogels, controls the elastic modulus (ξ ~ E ^-3) and permeability (ξ ~ k ²) of hydrogels, and has been correlated with friction coefficient in self-mated or "Gemini" interfaces (ξ ~ μ ^-1). When free-radical polymerization occurs near oxygen-rich interfaces, depth-wise gradients in polymer density emerge. These near-surface regions affected by oxygen-inhibited polymerization are difficult to examine with bulk characterization techniques like SAXS and require surface-sensitive approaches. Recent efforts by the groups of Spencer, Tran, Sawyer, Dunn, and others have implemented optical microscopy, indentation, and neutron reflectometry, but each of these measurements require bulk hydrogel samples to remain stationary or quasi-static, frustrating efforts to connect structure and dynamics. This work describes the development of a first-of-its-kind sample environment (linear reciprocating tribometer) designed to measure friction of hydrogel interfaces on the horizontal liquids reflectometer at the Oak Ridge National Laboratory Spallation Neutron Source. This instrument was used to quantify near-surface polymer density of covalently-crosslinked and highly entangled hydrogels during dynamic applications of compressive stresses and frictional shear stresses across hydrogel-silicon interfaces while fully submerged in D₂O. Our results show that polymer density increases with compressive stresses, in good agreement with previous investigations by the Spencer and Tran groups. We also demonstrate polymer density is sensitive to frictional shear stresses imposed across different sliding velocities. Our efforts can inform the design of hydrogel-based medical devices and coatings that require structural inhomogeneity to mitigate friction.