Volume phase transition, VPT

Keywords volume phase transition (VPT) PNIPA phenol resorcinol phloroglucinol swelling uptake isotherm dynamic light scattering (DLS) small angle neutron scattering (SANS)... 

The operating principle of such devices relies on materials whose volume changes abruptly, i.e., that display a volume phase transition (VPT), when an appropriate stimulus is applied. Thermosensitive poly(N-isopropylacrylamide) (PNIPA) hydrogel, where the cross-links are chemical, namely peptide bonds, is widely used for this purpose since its VPT in pure water is close to 34°C.1 3... 

In accordance with the reversibility of the colloidal properties of thermally sensitive particles, the adsorption of proteins is also found to be reversible in the same cases. In fact, 90% of adsorbed protein can be desorbed just by lowering the temperature (i.e., from above to below the volume phase transition temperature). The hydration processes of the particles lead to a reduction in adsorption affinity, which favors the desorption process (see Figure 9.27). Furthermore, the desorbed amount of protein can be inaeased by reducing the adsorption affinity through changing the pH and salinity levels. The residual adsorbed (or the non-total desorption) amount is closely related to the adsorption time and to the protein nature. In fact, the more the incubation time (above the Typr) is increased, the more the desorbed amount (below the vpt) is reduced. 

The prepared hydrogel exhibited the dual properties, it shrank under acidic condition and at a temperature above volume phase transition temperature (VPT) of PNIPAAm and swelled in the alkaline condition or temperature below VPT of PNIPAAm. 

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