Thermo-sensitive hydrogels membranes from PAAm networks and entangled PNIPAAm: effect of temperature, cross-linking and PNIPAAm contents on the water uptake and permeability

Abstract

In this work is presented an alternative route for preparation of membranes from thermo-sensitive hydrogels based on polyacrylamide (PAAm), network having poly( N-isopropylacrylamide) (PNIPAAm) entangled. The hydrogels were chemically synthesized by radical polymerisation using sodium persulfate as initiator. The PAAm networks were obtained by reaction of the monomer acrylamide (AAm), the cross-linking agent and co-monomer methylene-bis-acrylamide (MBAAm), the N, N, N ′, N ′-tetramethylethylenediamine (TEMED), which acts as accelerator, and sodium persulfate (SP) operating as initiator. The membranes were characterized through their water uptake capability and permeability to Orange II, at temperatures ranging from 25 to 40 °C. The physical cross-linking entanglements on the PNIPAAm chains trapped on PAAm network contribute to a decreasing of water uptake and the effect becomes more intense at temperatures above 32 °C, the lower critical solution temperature (LCST) of PNIPAAm in water. It was suggested that, above 32 °C the PAAm network mechanically supports the collapsed PNIPAAm chains. This fact could raise the average mesh size of the hydrogel, thus, enhancing the diffusion of Orange II molecules through the membrane. The activation energy for the permeability to Orange II is lowered when the temperature of semi-IPN is above 32 °C. Below this temperature the PNIPAAm chains are randomly distributed and the dye requires more energy to permeate through the membrane. In addition, as more concentrated in PNIPAAm and cross-linked is the hydrogel, more difficulty is the diffusion of the dye through the membrane. The hydrogels obtained in this work behave in the same way that analogous photo-polymerised ones, using gamma rays instead and showing that, in this case, these polymerisation pathways do not influence the properties of such hydrogels.