PMAA hydrogels

Another promising class of hydrogels that exhibit responsive behavior is complexing hydrogels. Osada studied complex formation in PMAA hydrogels (Osada, 1980). In acidic media, the PMAA membranes collapsed in the presence of linear PEG chains due to the formation of interpolymer complexes between the PMAA and PEG. The gels swelled when placed in... 

Thomas et al. [16] used CSLM to observe in situ the formation of polyamide membranes and the measurements were used to study polymer precipitation kinetics. Turner and Cheng [17] applied CSLM and hydrophilic fluorescent probes of varying molecular weights to image the size distribution of poly(methacrylic acid) (PMAA) hydrogel domains in polydimethylsiloxane (PDMS)-PMAA interpenetrating polymer networks. The combination of CSLM with AFM, SEM and X-ray spectroscopy allowed characterization of the structure of stimuli-responsive polymeric composite membranes [18]. 

Hydrogels that have the ability to respond to pH changes have been studied extensively over the years. These gels typically contain side ioni-zable side groups such as carboxylic acids or amine groups (Oppermann, 1992 Scranton et al., 1995). The most commonly studied ionic polymers include polyacrlyamide (PAAm), poly(acrylic acid) (PAA), poly(metha-crylic acid) (PMAA), poly (diethylaminoethyl methacrylate) (PDEAEMA), and poly(dimethylaminoethyl methacrylate) (PDMAEMA). 

In this work, we generated new PMAA/PNIPAAm IPN hydrogels with both pH and temperature sensitivities by the interpenetration of the pH-sensitive and temperature-sensitive polymer networks. 

DSC were conducted on PMAA/PNIPAAm IPN hydrogels swollen at different pH values. The results show that the difference in pH has great influence on the LCST transitions of the IPN hydrogels, as shown in Figure 5. 

At pH 4.3, there is no significant transition detected around 32°C. Transition temperatures are detected and increase as pH increases. This is because, at low pH, the aggregation of PMAA decreases the mobility of the PNIPAAm network as well as the water uptake of the IPN, resulting in drastically lowering the temperature sensitivity of the IPN hydrogel. However, at higher pH value, the swollen PMAA allows the PNIPAAm to have a higher mobility, which makes the IPN more temperature sensitive. 

The large contraction change shown by curve 2 is due to polymer dehydration or precipitation induced by complexation between PEO and PMAA. It is evident that the dehydration or precipitation is enhanced by complexation compared to curve 1. Furthermore, curve 2 indicates a reversible contraction/ recovery change, suggesting that complexation and dissociation occur reversibly with temperature changes within the PMAA gel. This drastic contraction change can be applied to temperature sensitive hydrogels as well. 

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