Rubber Elasticity of Poly(N-isopropylacrylamide) Gels at Various Charge Densities

Abstract

The mechanical properties and the swelling behavior of a series of N-isopropylacrylamide copolymers with the sodium salt of 2-acrylamido-2-methylpropanesulfonic acid were investigated. The increase of the equilibrium gel volume V eq with the charge density fN was considerably weaker (V eq ≈ fN 0.75) than expected assuming Gaussian statistics (V eq ≈ fN 1.5). This deviation is interpreted as the non-Gaussian elasticity of the hydrogels swollen to equilibrium. The exponent 0.75 found indicates that the elastic free energy Δ G el scales with the linear swelling ratio α as ΔG el ≈ α4, thus deviating from ΔG el ≈ α2 derived using Gaussian chain statistics. This expression for the elastic free energy explains the behavior of elastic modulus data of the equilibrium swollen hydrogels.

Swelling and elasticity of poly (N‐isopropylacrylamide‐co‐4‐vinyl benzene sulfonic acid sodium salt) hydrogels

“…The filled symbols are data points for PNIPA hydrogels prepared at the same crosslinker ratio X and monomer concentration C 0 but by using AMPS as the ionic comonomer. 10,11 V eq increases as the SSA content of the hydrogel x i increases. This is a consequence of the osmotic pressure exerted by the counterions of SSA units in the network chains.…”

“…The detail about the hydrogel preparation was described before. 10 The degree of dilution of PNIPA networks after their preparation was denoted by 2 0 , the volume fraction of crosslinked PNIPA after the gel preparation. To determine 2 0 , the hydrogels after preparation were first swollen in water to extract nonpolymerizable or soluble components and then dried to constant mass.…”

“…This procedure ensured uniformity of the network concentration throughout the gel sample. 10 After a given evaporation time (a few minutes up to a few months), the diameters of partially swollen gels were measured, from which their volumes V were calculated.…”

“…[5][6][7][8][9] Recently, we investigated the elasticity of a series of PNIPA hydrogels with varying amounts of charged units derived from the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid (AMPS). 10,11 It was shown that, even at low charge densities, the increase of the equilibrium gel volume with the gel charge density is considerably weaker than expected assuming Gaussian statistics. The results could be explained with the non-Gaussian form of the rubber elasticity theory.…”

“…The results could be explained with the non-Gaussian form of the rubber elasticity theory. 10,11 In the present work, we used 4-vinylbenzenesulfonic acid sodium salt (styrene sulfonate, SSA) instead of AMPS as the ionic comonomer of NIPA. Our aim was to increase the stiffness of the polymer backbone of ionic PNIPA gels by replacing AMPS with SSA units.…”