Phase transition in swollen gels

Abstract

The swelling and mechanical behaviour of ionized networks of N-isopropylacrylamide with an ionic comonomer, (2-acrylamidoethyl)trimethylammonium chloride (mole fractions x S = 0-0.1), in the presence of a crosslinker, N,N'-methylenebisacrylamide, was investigated in water as a function of temperature and in aqueous NaCl solutions (c NaCl = 10 -5 − 1 M) at 23 °C. On heating, a continuous decrease in the swelling degree in water, Q w , was observed; increasing x S shifts the volume transition temperature, T tr , (from the swollen to collapsed state) to higher temperatures. The expected decrease in the swelling degree, Q, with increasing NaCl concentration in aqueous NaCl solutions was observed and two shrinking regions in ionic gels were found. The decrease in Q w with increasing temperature and the decrease in Q with increasing c NaCl are accompanied by an increase in equilibrium shear modulus of gels, so that the mechanical behaviour of gels is predominantly determined by the swelling degree. The experimental swelling behaviour could be, in the first approximation, described by the theory of polyelectrolyte networks in which repulsion of charges on the chain and finite chain extensibility were considered.

Phase transition in swollen gels : Part 32. Temperature transition in charged poly(N-isopropylmethacrylamide) hydrogels in water and aqueous NaCl solutions

“…Similar results were obtained for PIPAAm gels. 24 As expected, the swelling degree in water at 23 C, Q w , increases with ionic comonomer concentration, x S (Table 1). As we know all network structure parameters (u d ,r,v 1 ), and if we further assume that the degree of ionization i ¼ x S , we can use eqn.…”

“…A similar effect was observed earlier for charged PIPAAm hydrogels. 24 In Fig. 3(c) the dependence of log G r on log Q s for samples measured in aqueous NaCl solutions is also shown.…”

“…While the transparent structure is an ordinary homogeneous threedimensional network, the turbid one is heterogeneous due to microsegregates formed inside the gel. 24 The decrease in log G r with increasing x S in the collapse region suggests that the segregates are formed from association of charges and increasing x S increases the two-phase character of hydrogels. Final decrease in the log G r values, found at the highest temperatures (T > 80 C, Fig.…”

“…Such results were obtained earlier for charged PAAm, PDEAAm, PIPAAm gels. 5,10,13,24 Small-angle neutron scattering behavior in D 2 O…”

Effect of chemical structure on the volume-phase transition in neutral and weakly charged poly(N-alkyl(meth)acrylamide) hydrogels studied by ultrasmall-angle x-ray scattering

“…[23][24][25][26] Elasticity measurements of charged poly͑N-isopropylmethacrylamide͒ gels showed that charges induce hysteresis in swelling, while neutron scattering studies of poly͑N-alkylacrylamide͒ gels have showed coiled and globular conformations below and above the lower critical solution temperature ͑LCST͒. [23][24][25][26] Elasticity measurements of charged poly͑N-isopropylmethacrylamide͒ gels showed that charges induce hysteresis in swelling, while neutron scattering studies of poly͑N-alkylacrylamide͒ gels have showed coiled and globular conformations below and above the lower critical solution temperature ͑LCST͒.…”