PH-responsive gels

Castelletto, V. et al, 2004. Micro structure and physical properties of a pH-responsive gel based on a novel biocompatible ABA-type triblock copolymer. Langmuir, 20(10), 4306-4309. 

The pH responsive gel used in the project is based on the monomer methacrylic acid (MAA) The gels are synthesised via a free-radical mechanism. The synthesis was carried out in an aqueous environment using 2,2 -azobis (2-methylpropionamidine) dihydrochloride (AMPA) and N,N -methylene bisacrylamide (MBA) which are water-soluble free radical initiator and crosslink agent respectively (Scheme 1). The polymerizations were carried out in water (20%w/v MAA 0.03%w/v MBA 0.06%w/v AMPA) under a nitrogen atmosphere. The polymerization was initiated thermally at 70°C, and gelation occurred in less than one hour. Residual monomer was removed from the gels by alternate soaking in water and methanol. 

The CT reaction was chosen as a model system for coupling with pH-responsive hydrogel because (i) pH difference between the reacted and the unreacted state is very large (ApH 8) so that it can readily induce volume changes in conventional pH-responsive gels ( ) its chemical state depends on the size of the system, so that volume changes might introduce a feedback on the chemistry (Hi) it leads to oscillatory and excitability phenomena in a gel under fixed boundary conditions. 

Spontaneous Deformations of pH-responsive Gels Coupled to the CT Reaction... 

The coupling is experimentally achieved by immersing a cylinder of pH-responsive gel in a CSTR where the CT reaction is operated (Figure 1). The CSTR contents is kept in the F (alkaline) state so that it differs only slightly from the input composition. We use the value of the sodium hy-droxyde concentration [OH ]o in the total input flow as control parameter. When a piece of gel previously soaked in a neutral or an alkaline solution is introduced in the CSTR, it remains clear and swollen, whatever the value of the control parameter. To start the experiment, a cylinder of gel is soaked into an acidic solution (pH =2) outside the CSTR, which makes it... 

Figure 3. Turbid trigger wave in a pH responsive gel cylinder. The photographs are tilted by 9(P. Snapshots from top to bottom at 0, 110, 220 minutes. Experimental conditions d=lmm, [OH ]o=5.3xlO M. White scale bar=3mm.

Figure 6, Non equilibrium phase diagram of the pH responsive gels coupled to the CT reaction, o swollen state x trigger waves A volume oscillations collapsed state A transition of the CSTR into the T state. The parameter d is the inner diameter of the mold.

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