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

Abstract

Swelling and mechanical behavior of ionised networks of copolymers of N-isopropylmethacrylamide with an ionic comonomer, sodium methacrylate (mole fractions x S ¼ 0-0.1), and a crosslinker, methylenebisacrylamide, was investigated in water as a function of temperature and in aqueous NaCl solutions (c NaCl ¼ 10 À5 -1 M) at 23 C; small-angle neutron scattering (SANS) was used for structure investigation. On heating, a continuous decrease in the swelling degree in water, Q, was observed; increasing x S shifts the temperature of transition from the swollen to collapsed state, T tr , to higher values. On subsequent cooling, the temperature T tr was independent of x S . This means that in the collapsed state clusters of ions are formed and these microsegregates remain stable at low temperatures; the presence of clusters is supported by SANS results. The expected decrease in the swelling in aqueous NaCl solutions, Q s , with increasing NaCl concentrations, c NaCl , was observed. The decrease in Q with T and the decrease in Q s with c NaCl are accompanied by an increase in equilibrium shear modulus of gels. The experimental swelling behavior was analyzed using the theory of polyelectrolyte networks in which repulsion of charges on the chain and finite chain extensibility were considered.

In-situ characterization of the temperature-sensitive swelling behavior of poly(N-isopropylacrylamide) brushes by infrared and visible ellipsometry

“…(N A = Avogadro constant and ϱ = 1.103 g/cm 3 = bulk density of PNIPAAm [22]), were calculated to have similar values of σ Sample 1 = 0.08 nm −2 and σ Sample 2 = 0.09 nm −2 . Surface homogeneity of the brushes was confirmed by atomicforce-microscopy measurements, which showed root-mean-square roughness values smaller than 1 nm.…”

Modeling Investigation of Hydrogel Volume Transition

“…Additional contribution of electrostatic interaction to the free energy for ionizable hydrogels was taken into account by Fomenko et al [28] Kramarento et al expressed the free energy of weakly charged hydrogel as a sum of six terms: the elastic free energy, the free energy of interaction of the monomer units, the free energy gained from ion pairing, the free energy related to the translational entropy of mobile counterions inside the hydrogel and in the outer solution, the free energy of Coulombic interaction, and the free energy related to the combinatorial entropy of distribution of counterions between three possible states. [29] The free energy was also described in terms of probability distribution function of different configurations.…”

Synthesis, characterization, and swelling studies of pH‐ and thermosensitive hydrogels for specialty applications

“…10 Swelling and mechanical behavior of the ionized networks of poly(sodium methacrylate)-cl-N,N-MBAAm was reported as a function of temperature in water and NaCl solution. 11 Apart from the effect of the copolymeric composition and also of the external environmental factors; surfactant-polymer interactions, especially, at the critical micelle concentration (CMC) of the surfactant affects swelling behavior of the hydrogels in a significant manner. [12][13][14][15][16] The presence of surfactants affects swelling behavior of hydrogels by micelle formation.…”