Poly phase transition occurs

At present, we believe that the jump transitions observed in many of the gels studied here represent first order phase transitions. If this is the case, then the gels studied here are among the first found so far in which a first order phase transition occurs near room temperature in pure aqueous solvent with substantial added salt. Early studies by Tanaka s group with poly(acrylamide) based gels required that hydrophobic solvents such as acetone be added for a discontinuous phase transition to be observed near room temperature [6-10]. The more recently studied gels based on poly(n-isopropylacrylamide) [11, 12] and other lower critical solution temperature polymers show discrete phase transitions in water with no salt [11], but the swelling transitions become continuous when moderate amounts of salt are added [12],... 

Since Pelton and Chibante reported the synthesis of the first temperature-sensitive microgel from iV-isopropylacrylamide (NIPAM) and crosslinker in 1986 [74], responsive microgels have attracted numerous attempts to explore their potential application in many fields, such as sensing and drug delivery [75]. Now the most extensively studied respruisive microgels are prepared with poly(fV-isopropyla-crylamide) (PNIPAM) [2], It is weU known that the phase transition temperature of PNIPAM is about 32°C [76], When r< LCST, the PNIPAM chain is soluble in water as a random cod. When r LCST, the subtle balance between PNIPAM and water is broken and phase transition occurs [77]. However, the single-molecule level mechanism of the phase transition of PNIPAM had not been proposed until very recently. 

Asymmetric substituted poly(silylenes), such as Ri = butyl and R2 = hexyl, show similar behavior.(177) In many cases the anisotropic mesophase is present at room temperature so that the typical first-order phase transition occurs on cooling below ambient temperature.(178)... 

The prototypical smart polymer is poly(N-isopropyl acrylamide) (P(NIPAM)), which exhibits an inverse temperature solubility profile in water, that is it is water-soluble below 32 °C but precipitates above 32 °C. The temperature at which this coil-to-globule phase transition occurs is known as the Lower Critical Solution Temperature (LCST), and conveniently this can be modified in P(NIPAM) by incorporation into the polymer chain of more hydrophobic or hydrophilic monomers. Owing to the fact that the LCST is close to body temperature and can readily be modified to just below or just above 37 °C through this co-monomer addition, P(NIPAM) polymers have been widely exploited in biomedical applications. The chemistries and applications of P(NIPAM) have been extensively reviewed elsewhere, [75-81] but even 15 years after one particularly well-cited review, many research groups are working with this remarkably versatile polymer [82-87]. 

A thermoresponsive polymer hydrogel can be synthesized by copolymerizing these acrylamide-type monomers and a difunctional monomer such as methylenebisacrylamide. Hirokawa and other researchers studied the temperature dependence of the degree of swelling of a thermoresponsive poly(N-isopropylacrylamide) gel in pure water [18]. It was observed that a discontinuous volumetric phase transition occurred around the LCST of the polymer and the volume change was 8 times. 

The -dependence of the decay rate F of tracer polystyrene in polymethylmethacrylate benzene was measured by Numasawa, et a/.(60). The value of V/q increases at large q, as also seen for dilute polymers in simple solvents as discussed in Chapter 11. The effect of a phase transition on Ds was observed by Russo, et al, who examined poly(y-benzyl-a,L-glutamate) pyridine(61). An isotropic-cholesteric liquid crystal phase transition occurs for this rodlike polymer at elevated c. The value of Ds c) increases dramatically at the transition, but on both sides of the transition Ds c) decreases as c is increased. 

Solutions of poly(l,4-phenylene-2,6-benzobisthiazole), PBT, exhibit an isotropic to nematic phase transition in a variety of solvents including methane sulfonic acid, MSA, chlorosulfonic acid, CSA, and poly phosphoric acid, PPA (1-4). In the latter case the transition occurs over a range of water -P2O5 compositions. In these acids the polymer, with repeating unit... 

The polymerizability of a monomer is also influenced by the physical state of the polymerization. For example, crystallization of poly(oxy methylene) provides the driving force for trioxane polymerization. In this case, propagation occurs at active sites on the crystal lattice rather than in solution, and AGP includes the change in free energy of the phase transition as well as that of the solution polymerization [Eq. (19)]. 

An extreme case that has received attention is that of a cooperative conformational transition examplified by the coil-helix (c h) transition that occurs in poly-a-aminoacids. Whereas the polypeptide chain is quite flexible when it exists as a random coil, the rigid helical form may bring about formation of a liquid crystalline phase, as discussed above, if its concentration is sufficient. The conformational transition and the phase transition may therefore be coupled. The helix-coil transition may then acquire the character of a first-order phase transition, owing to generation of the liquid crystalline phase. 

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