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43683-G7
Effects of Particle Additions to Hydrogels

Valeria Milam, Georgia Institute of Technology

Overview

         Hydrogels are water-swollen interconnected polymer networks that can be covalently or physically cross-linked.  Polyacrylamide-based hydrogels have been extensively studied for biomaterials applications such as cellular substrates.  While their high liquid content mimics the permeability of the extracellular matrix surrounding a cell, the liquid component of hydrogels also results in low mechanical stiffness values (modulus on the order of kPa) compared to that of biological tissues (modulus of MPa – GPa).  In the present study we examined the effects of composition and volume fraction on the shear storage and loss moduli for a hydrogel system comprised of polyacrylamide copolymerized with either polyacrylic acid or polysodium acrylate. While we expect the polyacrylamide to maintain a neutral charge, the polyacrylic acid and especially polyacrylate are expected to impart a net negative charge of to the hydrogel via their carboxylic acid groups.  Current and future work involves investigating the strengthening effects of interactive particle filler additions.

  Rheological Results  

In order to examine the effects of hydrogel composition on shear storage modulus values, frequency (data not shown) and strain amplitude sweeps were performed for hydrogels with a fixed total polymer volume fraction of 0.1, but varying volume percentage ratios of polyacrylic acid to polyacrylamide (pAAc/pAAm) and polyacrylate to polyacrylamide (pNaAc/pAAm). Figure 1(a) shows representative strain sweeps for the polyacrylic acid-based system in which the storage modulus generally increases with increasing percentage of polyacrylamide.  The rise in G' at intermediate strain values (~ 10² – 10³ % strain) occurs at approximately the same strain regardless of composition.  This behavior is in contrast to the pNaAc/pAAm hydrogels (Figure 1(b)) which did not exhibit a rise in G' values prior to a steep drop at larger strain values.  Unlike pAAc-based hydrogels, this steep drop occurs at progressively lower strain values for compositions with higher pAAm percentages and subsequently higher G' values.  Thus, pNaAc-based hydrogels with greater pAAm percentages exhibit more brittle behavior compared to the pAAc-based hydrogels.  It is worth noting that within the linear viscoelastic regime, less than 10% strain, pure polyacrylate hydrogels have low shear storage modulus values (~1.0 x 10³ Pa at 0.1 polymer volume fraction) in comparison to pure polyacrylic acid hydrogels (~ 8.0 x 10³ Pa at 0.1 polymer volume fraction).  Thus, the addition of polyacrylamide has a greater effect on the mechanical properties of polyacrylate-based hydrogels and may explain the larger degree of brittle behavior observed in the pNaAc/pAAm system.  

Next, we examined the effects of the total hydrogel volume fraction on the shear elastic modulus for varying hydrogel compositions.  Three hydrogel compositions were chosen for the polyacrylic acid-based system, 25/75, 50/50 and 100/0.  These compositions were tested across a range of polymer volume fractions (Ф_p) from 0.02 to 0.25 using strain amplitude sweeps ranging from 0.001% to 4500% at a fixed frequency of 6 rad/s.  Figure 2(a) shows representative strain sweeps of a 50/50 pAAc/pAAm composition.  An increase in total hydrogel volume fraction results in a higher solids content and a corresponding increase in the shear storage modulus.  It is also important to note that similar to the G' behavior seen in Figure 1(a), all polymer volume fractions show similar “critical strain” values corresponding to a decrease in G'.  

Four compositions of the polyacrylate-based hydrogels were chosen for volume fraction studies:  0/100, 15/85, 25/75, and 100/0.  Again, the total polymer volume fractions (Ф_p) ranged from 0.02 to 0.25. Strain amplitude sweeps were used to measure the storage modulus and ranged from 0.001% - 3000% at a fixed frequency of 6 rad/sec. Figure 2(b) depicts a representative strain sweep of a 50/50 pNaAc/pAAm composition.  As expected, an increase in total polymer volume fraction results in an increase in shear storage modulus.  This is explained by the increase in number of elastically active polymer chains with increasing total polymer volume fraction.  Unlike pAAc-based hydrogels, however, the “critical strain” before the steep downturn in G' occurs at lower and lower strains for hydrogels with greater polymer volume fractions.  This trend is similar to the behavior observed in Figure 1(b) in which more brittle hydrogels showed a steep drop in G' at lower strain values.

Figure 1.  Representative strain sweeps showing shear storage modulus as a function of percent strain for hydrogels with a fixed total polymer volume fraction of 0.1 and varying volume percentage ratios of (a) polyacrylic acid to polyacrylamide (pAAc/pAAm) or (b) polyacrylate to polyacrylamide (pNaAc/pAAm).  Frequency was fixed at 6 rad/sec.

Figure 2.  Representative strain sweep measurements showing shear storage modulus as a function of percent strain for hydrogels at varying total polymer volume fraction, Ф_p and a fixed composition of (a) 50/50 pAAc/pAAm or (b) 50/50 pNaAc/pAAm.  The frequency was fixed at 6 rad/sec.

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