Abstract
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.
Highlights
Tanaka discovered in 1978 the phenomenon that poly(acryl amide) gel swollen in water replies to a change of the environment [1]
We developed a novel method to monitor de-swelling by isothermal thermogravimetry
The results of DSC analysis of PNIPAAm/MBA gels with different cross-linker content are represented in Figure 3, the evaluation of peak temperatures and the enthalpy of phase transition is given in Table 3 and Figure 4
Summary
Tanaka discovered in 1978 the phenomenon that poly(acryl amide) gel swollen in water replies to a change of the environment (temperature, mix composition etc.) [1]. Nowadays there are special journals on intelligent materials such as the “Journal of Intelligent Material Systems and Structures” or “Smart Materials and Structures”. Solids retain their form, and they can be deformed. Gels in solid state retain their form, even if they take up high quantity of liquid, and they are deformable. Due to their high liquid content their physico-chemical properties are similar to those of solutions [2]
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