Negatively charged poly(N-isopropyl acrylamide-co-methacrylic acid)/polyacrylamide semi-IPN hydrogels: Correlation between swelling and compressive elasticity

A series of thermoresponsive semi-interpenetrating polymer network (semi-IPN) based on linear polyacrylamide (PAAm), N-isopropylacrylamide (NIPA) and methacrylic acid (MA) were synthesized by varying linear polymer content via free radical polymerization. An interpenetrated network of P(NIPA-MA)/PAAm was designed to modulate elasticity by varying inner composition and to improve rate of swelling-deswelling phenomenon. Correlation between swelling and compression elasticity was demonstrated. Inclusion of linear PAAm chains with high molecular weight (Mw = 9.639 x 105 g/mol) into P(NIPA-MA) network induced physical entanglements, increased apparent crosslinking density nu eand enlarged compressive elasticity. Dependence of nu eon linear polymer content was expressed as a cubic polynomial function. Extent of swelling of semi-IPN P(NIPA-MA)/ PAAm gels was sensitive to presence of linear PAAm chains. Existence of linear polymer decreased apparent ionic group density and increased crosslinking density compared to that of copolymer P(NIPA-MA) network which in turn decreased equilibrium swelling. A significant increase in swelling/shrinking rate was observed in the presence of linear PAAm. Due to ionization of carboxylic acid groups in P(NIPA-MA) network, semi-IPN P(NIPAMA)/PAAm gels showed different degrees of swelling depending on linear PAAm content and temperature of swelling medium. Semi-IPNs exhibited phase transition temperatures shifted higher temperature, suggesting physical entanglements between P(NIPA-MA) network and linear PAAm. Increasing swelling temperature resulted in an increase in Flory-Huggins interaction parameter. Entropic contribution chi Sincreased and enthalpic contribution chi H decreased with PAAm content. The results showed that the prepared semi-IPNs are a suitable adsorbent for adsorption of the cationic dye Methyl violet (MV). Adsorption kinetics followed a pseudo-secondorder model and exhibited a two-stage intra-particle diffusion. A detailed understanding of physicochemical properties of negatively charged thermoresponsive semi-IPNs in relation to structural architecture is an important criterion in many biomedical and pharmaceutical applications.

Automated summary

This study synthesized thermoresponsive semi-interpenetrating polymer networks (semi-IPNs) by varying the content of linear polyacrylamide (PAAm), N-isopropylacrylamide (NIPA), and methacrylic acid (MA). The inclusion of linear PAAm chains increased the crosslinking density and compressive elasticity of the P(NIPA-MA) network. The swelling behavior of the semi-IPN gels was sensitive to the presence of linear PAAm chains, and their equilibrium swelling was decreased compared to the copolymer P(NIPA-MA) network. The prepared semi-IPNs exhibited good adsorption properties for the cationic dye Methyl violet (MV).