Spatially-Resolved Network Dynamics of Poly(vinyl alcohol) Gels Measured with Dynamic Small Angle Light Scattering

Abstract

Hydrogels are cross-linked polymer networks swollen in water. The large solvent content enables hydrogels to have unique physical properties and allows them to be used in diverse applications such as tissue engineering, drug delivery, and absorbents. Gel properties are linked to internal dynamics. While bulk gel dynamics have been studied extensively, how gel networks respond locally to deformation has yet to be understood. Here, poly(vinyl alcohol) (PVA) gels have been stretched to study the effects of deformation on gel dynamics parallel and perpendicular to the stretching direction using dynamic small angle light scattering (DSALS). The implementation of DSALS is described and compared to traditional DLS for PVA gels with different crosslink densities, ranging from 0.75-2%. Despite the orders of magnitude difference in the scattering vector, q, range of the techniques, the dynamics match, and the apparent elastic diffusion coefficient, D_A increases linearly with the crosslink density for unstretched gels at a constant 2 wt% concentration. We observe that the elastic motion depends on the direction of stretch, decreasing perpendicular to stretching and increasing at parallel direction. Using DSALS can therefore be an effective tool to evaluate local hydrogel response to deformation.

Keywords: gel dynamics; poly(vinyl alcohol); spatially-resolved.

Figure 1

A schematic representation of a polymer gel, in which the gel strands (blue) are covalently linked by crosslinks (red). Since the crosslinking is random, the network topology can be described by an average mesh size, ξ, and regions of higher crosslinking density, separated by a distance b.

Figure 2

PVA dynamics determined from DSALS and DLS as a function of crosslinking density. (a) DLS intensity correlations at θ = 60°. The black line is a double exponential fit to the data. (b) DSALS intensity correlations at q = 1.423 µm⁻¹. The black line is a double exponential fit to the data. (c) The dominant fast relaxation rate Γ determined from the exponential fit vs. q² as a function of crosslink density determined from DLS. (d) The dominant fast relaxation rate Γ determined from the exponential fit vs. q² as a function of crosslink density determined from DSALS.

Figure 3

Comparison of PVA gel dynamics determined from DLS and DSALS. (a) A schematic illustrating the difference in q-vectors for DLS and DSALS where an average mesh size is, ξ, and the distance between the regions of higher crosslinks, b. (b) The fast relaxation rates plotted across the whole q² range from both techniques. (c) The apparent diffusion coefficient D_A as a function of crosslinking %. The values from DLS and DSALS are within error. Error bars represent standard error from the linear fit to Γ vs. q² trends.

Figure 4

Apparent elastic diffusion coefficient as a function of the spatial azimuthal angle for an unstretched 2 wt% 1% crosslink PVA gel.

Figure 5

(a) Intensity autocorrelation function at q = 2.245 µm⁻¹ for a stretched gel at ψ = 0° (perpendicular to stretching) and 90° (parallel to stretching) compared to an unstretched gel at 0°. (b) A schematic representation of local gel deformation at ~3 µm length scale.

Figure 6

DSALS optical design. (a) Ray diagram of scattered light from the sample S, at an angle θ, as it is focused by L1 through a beamstop BS, through L2 to a CMOS detector D. (b) A photo of the benchtop setup. (c) Typical scattering patterns at the detector, at a frame rate of 480 fps, the intensity of which can be autocorrelated as a function of θ and the spatial azimuthal angle ψ.