Phase Transition in Swollen Gels XXIII. Effect of the Positive Charge Concentration on the Collapse and Mechanical Behavior of Poly(1-vinyl-2-pyrrolidone) Gels

Abstract

ABSTRACT:The swelling and mechanical behavior of ionized networks of copolymers of 1-vinyl-2-pyrrolidone, ionic comonomer, N,N-dimethyl-N,N-diallylammonium chloride (molar fraction of salt Xs = 0--0.27) and crosslinker, 3,3'-ethylidenebis(l-vinyl-2-pyrrolidone) was investigated in water-acetone mixtures and aqueous NaCl solutions. The networks were prepared by a radiation copolymerization method. At xs2:0.038, the first-order phase transition (collapse) was observed. While the critical acetone concentration in the mixture at collapse, ac = 76 vol%, is independent of x, the extent of the collapse (jumpwise change in the gel volume), .1, increases with increasing salt concentration. The jumpwise change in the gel volume is accompanied by a similar change in equilibrium modulus. The swelling and mechanical behavior of ionized networks in aqueous NaCl solutions were also measured. The expected decrease in the swelling and the increase in the modulus with increasing electrolyte concentration CNaC1 was found. The theory of swelling equilibria of polyelectrolyte networks, in which the effect of electrostatic interactions of the charges on the chain and a limited chain extensibility were included, semiquantitatively describes the swelling data provided an effective concentration of the charges (lower than xs) was introduced. Mechanical behavior of networks is predominantly determined by their degree of swelling.KEY WORDS Vinylpyrrolidone Networks / Polyelectrolytes / Swelling Equilibria / Mechanical Behavior / First-Order Phase Transition / Collapse Phenomenon / Swollen polymer networks carrying low amounts of charges on the chain (1-10 mol %), when subjected to a change in the external parameters (temperature, composition of the mixed solvent, electric field and the like), undergo a first-order phase transition (collapse), at which the jumpwise change in volume of the gel can be from 10 to 1000 times. 1 -4 SANS experiments showed that in the expanded state, the chains have a coil shape, while in the collapsed state they are in globular form.5 It has been shown,2 both experimentally and theoretically, that the conditions of network formation (concentration of cross linker and diluent) and of the charge concentration have a great influence on the appearance and extent of the collapse. The jump in volume is accompanied by a jump in other physical properties, such as shear modulus, refractive index, dielectric permittivity, and the like. 2 Due to these changes, these systems are considered for use in various devices, sensors, mechanochemical actuators, 6, 7 etc. Most experimental results were obtained with charged poly(acrylamide) (PAAm), poly(N-isopropylacrylamide) (PIPAAm), and poly(N,N-diethylacrylamide) (PDEAAm) hydrogels. 1 -3 ,8 While in PAAm gels, the collapse can be brought about by the changes in the solvent composition, in PIPAAm and PDEAAm gels the transition can be induced in water, by a temperature change. With all these systems, the networks had to be prepared at high dilution (the volume fraction of polymer ...

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

“…A continuous universal increase in w on v 2 regardless of x S was found; this universal dependence suggests that assumption of lower effective ionization than the sodium salt concentration is probably correct. The increase in w with v 2 was observed also earlier 5,24,30,31 and is related to the decreasing solvent quality with increasing temperature (as in the uncharged network). As was discussed previously, 13,27 the van der Waals loop in the dependence of w on v 2 is a necessary condition for the appearance of a jump in the gel volume (1st-order phase transition).…”

“…This theory semiquantitatively describes collapse in water/acetone mixtures and aqueous NaCl solutions of various charged gels. 5,30,31 The theory predicts swelling pressure P as the sum of four terms 27…”

“…Eqn. (13) quantitatively described the transition in weakly charged polyacrylamide networks 5,13,30,31 in dependence on the degree of ionization, and amount of crosslinker and diluent at network formation (using molecular parameters obtained independently from the experiments).…”

Phase transition in swollen gels

“…(6) from Ref. [13], which was based on Donnan equilibrium analysis. The values c Na + , together with other network parameters (v d , ρ, v 0 ), χ = 0.496 and α* = φα = φx S , were used in Eq.…”