Polymer gels PNIPAM

The PNIPAM containing benzo[18]crown-6, described in Sect. 4, which underwent the chemical-induced phase transition in aqueous solution can be applied for constructing chemicals-responsive polymer gels. When the PNIPAM is crosslinked to form a gel network, the conformation change accompanied by the phase transition is expected to result in the volume phase transition. 

Cell-polymer interactions of fluorescent polystyrene (FPS) latex particles coated with thermosensitive PNIPAM and poly(N-vinylcaprolactam) or grafted with poly(ethylene oxide)-macromonomer were analyzed by Vihola et al. [131] by modifying the surface of the FPS particles with the thermosensitive polymer gels or with poly(ethylene oxide)-macromonomer grafts. In all the cases, the FPS were surface-modified by polymerization of both thermosensitive monomers and macromonomer onto the surface of the fluorescent particles. The final surface-coated particles have potential biotechnological applications in the form of either stealth-carrier behavior or enhanced cellular contact. 

Another class of materials are the thermoreversible gels obtained when LCST polymers such as PNIPAm [150-152], PEOP [153], or methylcellulose [154] are grafted on chitosan. Figure 4.4 shows the variation of mechanical moduli to stepwise periodic changes in temperature from 10 to 30 °C. 

The first set of experiments involved fluorescence resonance energy transfer (FRET) between the naphthalene and pyrene-laheled polymers. A 5 1 mixture of PNIPAM-Py to PNIPAM-Na was used. When assembled in micelles, the pyrene acts as a quencher to the naphthalene, leading to high pyrene fluorescence and low naphthalene fluorescence. When the mixture is added to DMPC (liquid phase) or DSPC (gel phase) vesicles at room temperature, naphthalene fluorescence is increased, while pyrene fluorescence is dramatically decreased. This effect is not seen with the PNIPAM-Py-Na polymer, so the reduction in FRET is not due to the hydrophobic environment. This means that the hydrophobic anchors of the PNIPAM-Py and the PNIPAM-Na likely enter the membrane and the dyes are moved apart from one another. The fact that the anchor appeared to insert into the gel-phase DSPC membrane was somewhat surprising. The authors attribute the effect to defects between crystalline domains in the membrane. To test if the LCST transition still occurs when the polymers are anchored to the membrane, differential scanning calorimetry (DFC) was used. The LCST transition of the PNIPAM-Py/PNIPAM-Na mixture in solution was observed in the DFC ttace. When combined with DSPC or DMPC vesicles, the same peak was observed, indicating that the transition does indeed stiU occur, even in the presence of the lipid. 

The sol-gel transition can be induced by various physicochemical parameters, such as the temperature, the concentration in salt in the solution and the pH. Poly(N-isopropylacrylamide) (PNIPAm) exhibits a thermosensitive transition. Below 31 °C, the polymer is hydrophilic and swollen by water. Above this temperature, the polymer is hydrophobic and the network collapses, as shown in Figure 2.12. Such temperature-dependent behaviour can be of great interest. 

Fig. 10.15 Comparison of the phase diagrams of telechelic associating polymers (a) random hydration (cr = 1.0) for telechelic PEO, (b) cooperative hydration (a =0.3) for telechelic PNIPAM. Spinodal lines (solid lines) and sol-gel transition hnes (broken lines) are shown. The various curves correspond to polymers of different molecular weights. Other parameters are fixed at the values obtained from the single-chain study. (Reprinted with permission from Ref. [38].)...

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