Low-temperature gelation

In some aqueous polymer solutions, hydration is noncompetitive with association. For instance, in solutions of telechelic polymers, main chain hydration only indirectly affects the end-chain association. There is interference only in the region very close to the chain end. Dehydration and chain collapse start near the core of the flower micelles in the form of heterogeneous nucleation. The solutions with such coexisting hydration and association turn into gels on cooling (low-temperature gelation), while they phase separate at high temperatures. 

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. 

Pour point depressants The crystallization of paraffin wax in the base oil can lead to a gelation of the lubricant at low temperature. Pour point depressants cannot prevent crystallization but change the shape of the crystals from a needle-like to a densely packed, rounded one. This leads to a much better flow behavior. 

Pluronic copolymers with molecular weight of 11,500 have been used as a viscosity modifier and form a clear gel at 20 to 25% w/w in water. A solution of Pluronic undergoes a thermal gelation process in which a mixture of the polymer and water forms a clear solution at low temperature and then becomes a gel when raised to room temperature (above 21°C). A blend of Pluronic and lecithin is sold as Organogel for topical gel formulations. 

Versteeg et al. (50) have isolated various multiple forms of pectin esterases in oranges and shown that they differ in affinity to pectins and pectates and in heat stability and therefore may play different roles in cloud loss phenomena. One form (about 5% of the total PE activity) was found to be much more heat stable than the other forms it was also active at low pH (2.5) and at low temperatures. Another form did not cause self-clarification for this enzyme a similar mode of attack was proposed as for fungal pectin esterases which produce low-ester pectins less sensitive to calcium. Multiple forms of pectin esterases are present in the fruits of all of the orange varieties and citrus species tested. The two isoenzymes known to be responsible for cloud loss and gelation in citrus products were found to occur in all of the component parts of the orange fruit (51). In the French cider industry the endogenous PE of apple is used for the self-clarification of apple juice (52). 

With values of Ej/Bm and FJF, it is a simple matter to calculate Pyj, at any value of Tg (= Te ), and then determine the time to vitrification from an assumed kinetic rate law. Using first order kinetics, which seemed to fit the extent of conversion vs. time data, the temperature of cure vs. the times to gelation and vitrification are shown in Fig. 15. The model fits the data well at low temperatures but appears to... 

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