Rheological properties and structure of step‐ and chain‐growth gels concentrated above the overlap concentration

Abstract

Cross-linked polymeric gels are widely used in applications ranging from biomaterial scaffolds to additives in enhanced oil recovery. Despite this, fundamental understanding of the effect of polymer concentration and reaction mechanism on the scaffold structure is lacking. We measure scaffold properties and structure during gelation using multiple particle tracking microrheology. To determine the effect of concentration, we measure gelation as polymer interactions are increased in the backbone precursor solution: below, at and above the overlap concentration, c � . To determine structural changes due to the gelation mechanism, we measure gelation between the same polymers undergoing both step-and chain-growth reactions using self-assembling maleimide:thiol and photo-initiated acrylate:thiol chemistries, respectively. We determine the critical relaxation exponent, n, a measure of structure. n decreases with increasing concentration, indicating a change from a percolated (c < c � ) to a tightly cross-linked network (c � < c). The gelation mechanism does not have a measurable effect on the scaffold structure.Time-cure superposition (TCS) is used to analyze dynamic microrheological measurements during gelation to determine Figure 3. Non-Gaussian parameters throughout the gelation of acrylate:thiol and maleimide:thiol gels plotted as a function of normalized time. Scaffold are measured for a range of concentrations of PEG-maleimide backbone, 3 wt % (a and e), 7.5 wt % (b and f), 10 wt % (c and g), and 18 wt % (d and h). [Color figure can be viewed at wileyonlinelibrary.com] AIChE Journal

Structural Changes in Polymeric Gel Scaffolds Around the Overlap Concentration

“…This complexity grows out of the vast array of applications and high demand on gel versatility. Gels are part of everyday life from commonly used personal, fabric and home care products to exotic biomaterials designed to mimic the extracellular matrix (ECM) (Winter and Chambon, 1986; Scanlan and Winter, 1991; West and Hubbell, 1999; Raeber et al, 2005, 2007; Yamaguchi et al, 2007; Benton et al, 2009; Fairbanks et al, 2009b; Zustiak and Leach, 2010; Wylie et al, 2011; Wehrman et al, 2018). Cross-linked gels have also played a significant role in enhanced oil recovery (Moradi-Araghi et al, 1988; Zolfaghari et al, 2006; He et al, 2009; Jung et al, 2013; Tongwa et al, 2013).…”

“…Due to this, polymeric interactions are defined as only physical interactions between the macromolecules in solution. Polymeric solutions have three regimes in the concentration-viscosity curve: dilute, semi-dilute and entangled (Graessley, 1980; Doi and Edwards, 1988; Pavlov et al, 2003; Rubinstein and Colby, 2003; Wehrman et al, 2018). In the dilute regime, polymers do not interact.…”

“…In the semi-dilute regime, the pervaded volume of the polymers begin to overlap. In the entangled regime, polymer molecules physically interact and entangle (Graessley, 1980; Doi and Edwards, 1988; Pavlov et al, 2003; Rubinstein and Colby, 2003; Wehrman et al, 2018). The overlap concentration, c * , is defined in a good solvent as…”

“…Previous work determined the overlap concentration, c * = 13 ± 4 wt%, of the PEG-acrylate backbone using bulk rheology (Wehrman et al, 2018). This previous investigation also characterized the change in material properties and structure when the PEG-acrylate backbone is below (3 wt%) and above (10 and 18 wt%) the overlap concentration.…”

“…This previous investigation also characterized the change in material properties and structure when the PEG-acrylate backbone is below (3 wt%) and above (10 and 18 wt%) the overlap concentration. This work characterized a change in structure to a more tightly cross-linked scaffold as PEG-acrylate concentration is increased (Wehrman et al, 2018). The present work expands upon this work to determine if there is a gradual or step change in the structure of the material below and above c * .…”