Effect of degree of cross-linking on spatial inhomogeneity in charged gels. I. Theoretical predictions and light scattering study

Abstract

Analysis of the Rabin-Panyukov theory of scattering from polyelectrolyte gels shows that the theory predicts an anomalous cross-link density ͑CD͒ dependence of amplitude of spatial inhomogeneity in charged polymer networks. By increasing the interaction parameter or by decreasing the degree of ionization, one can pass from a regime in which the calculated structure factor is a monotonically increasing function of CD, to one in which an inflection in the CD dependence is predicted. These theoretical predictions were experimentally verified by light scattering experiments on a series of weakly charged polymer gels of N-isopropylacrylamide-acrylic acid copolymer. The ensemble average scattered intensity, a measure of the spatial inhomogeneity, increased with CD for Ͻ c and decreased with CD for Ͼ c , where and c are the interaction parameter and the critical value of , respectively.

Swelling, Elasticity and Spatial Inhomogeneity of Poly(N,N‐dimethylacrylamide) Hydrogels Formed at Various Polymer Concentrations

“…[39] In spite of the limitations in the applicability of the PR theory, it succeeded in predicting qualitatively the dependence of the spatial inhomogeneity in gels on the gel synthesis parameters. [24,25,32] The PR theory takes into account the effect of the network structure at preparation on the structure factor S(q) under the conditions of the measurements. The structure factor of this theory consists of two contributions, one from the thermal fluctuations G(q) and the other from the static inhomogeneities C(q):…”

“…[15,16] Since the gel inhomogeneity is closely connected to the spatial concentration fluctuations, scattering methods such as light scattering, small-angle X-ray scattering and small-angle neutron scattering have been employed to investigate the spatial inhomogeneities. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] The gel inhomogeneity can be manifested by comparing the scattering intensities from the gel and from a semi-dilute solution of the same polymer at the same concentration. The scattering intensity from gels is always larger than that from the polymer solution.…”

Spatial inhomogeneity in poly(acrylic acid) hydrogels

“…Since the gel inhomogeneity results in static concentration fluctuations in gels, scattering methods have been employed to investigate the spatial inhomogeneities [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Scattering measurements show an important scattering in gels at low scattering vectors q, corresponding to concentration fluctuations at length scale between 10 0 and 10 2 nm.…”

Universality and Specificity of Polymer Gels Viewed by Scattering Methods

“…Experimentally, small-angle X-ray scattering (SAXS) 28 and small-angle neutron scattering (SANS) have been extensively employed to elucidate the inhomogeneities in polymer gels. [29][30][31][32][33][34] As a matter of fact, inhomogeneities are one of the most interesting topics in SANS of polymer gels. 7,35 These methods provide information on static scattering, i.e., the ensemble average of concentration.…”

“…On the other hand, the decrease in hIi E with C AAc is ascribed to an increase in osmotic pressure by ionization. However, effects of charges on gel inhomogeneities are complicated issues as extensively investigated by Ikkai et al 23,30,52,53 Inhomogeneities also vary by changing monomer concentration, C NIPA . Figure 12 shows C NIPA dependence of hIi E .…”