Novel Method to Monitor the De-Swelling of PNIPAAm Hydrogels of Different Cross-Link Density

The effect of cross-linker (methylene-bis-acrylamide) (MBA) on the volume phase transition, mechanical properties and de-swelling of Poly(N-isopropyl acrylamide-co-methylene-bis-acrylamide) hydrogel (PNIPAAm/MBA hydrogel) was investigated. A new method, namely isothermal thermo-gravimetry was developed for monitoring de-swelling of PNIPAAm/MBA hydrogel. Monomer/ Cross-linker ratio of the initial monomer composition R = moleNIPAAm/moleMBA was introduced. It has been proven earlier that initial monomer composition is close to the copolymer composition; hence R values may be used to express cross-link density. Hydrogels from R10 to R150 were investigated. The results of DSC analysis revealed that the less the cross-linker ratio in the gel (from R10 to R150) the more sharp the temperature range of volume phase transition and the higher its enthalpy. Cross-link density, namely increasing cross-linker content in the copolymer (R from 150 to 10) does not significantly affect the temperature range of volume phase transition. It sets on at 33°C - 34°C, and ends between 35?C and 38?C. Cross-link density has significant effect on compression modulus. By decreasing the ratio of cross-linker (by increasing R from 10 to 150), the compression modulus increases, goes through a maximum, and then decreases. The highest compression modulus was measured for PNIPAAm/MBA hydrogel R20. Hydrogels with cross-linker content between R100 and 30 are strong enough and have their thermoresponsivity. Isothermal thermograms of de-swelling are of similar character for all the gels with different cross-linker content. During the initial stage of de-swelling for gels with higher cross-linker content (R10 - R15) the solute release is quicker than for gels R20 - 150 and the thermograms are drawn out. In the initial stage of de-swelling, i.e. during the first 40 minutes the rate of solute release is the highest for gels R70 - 150. The cross-linker content effects solute release, especially for gels with high cross-linker content. It is noteworthy that gels R10 - 15 release solute quicker than gels R30 - 50 and their rate of de-swelling is comparable to that of gels R100 - 150. The novel thermogravimetric method enables the selection of gels based on the rate of solute release and it can also be applied for other cross-linked gels.