Phase separation gelation

Fig. 13. TXT cure diagram temperature of cure vs. the times to phase separation (doud point), gelation and vitrification for a neat and two rubber-modified systems. of the neat system is also included. The systems studied were DER331/TMAB O, gelation , vitrificaticm modified with IS parts rubber per hundred parts epoxy 1) pr eacted carboxyl-terminated butadiene-acrylonitrile (CTBN) copolymer containing 17% acrylonitrile (K-293, Spencer Kellog Co.) A, phase separation , gelation , vitrification, and 2) polytetramethylene oxide terminated with anmiatic amine (ODA2000, Polaroid Corp.) A. phase separation O, gelation O, vitrification. (DER331/TMAB/ K-293 data from Ref. )...

These equations can be used to calculate the reaction rate and activation energy of the process. The equations are simple and cannot account for the complexity in each reaction stage nor for all of the physical processes such as phase separation, gelation, or vitrification which determine the outcome of reaction. Several other variations of these equations have been developed to deal better with these complexities. 

Here mass transport is of minor importance compared to the changes in relaxation times induced by phase changes such as crystallization of water or lipid, phase separation, gelation and/or macromolecule aggregation and denatur-ation. Since relaxation times are sensitive to temperature, relaxation time maps might also be used to follow temperature changes and heat transport. 

A TXT cure diagram was developed by Kim et al. for the system poly(ether sulfone)-mter-net-epoxy semi-11 IPN [Kim et al., 1993]. They showed that the transformation of the monomers to the polymers involved five steps onset of phase separation, gelation of the epoxy component, a fixation of the domain size and shape morphology, end of phase separation, and (in this case) vitrification of the epoxy component. In their system, the onset of phase separation preceded gelation. 

Fig. 8. Temperature-conversion transformation diagram lowing phase separation, gelation and vitrification for a castor oil-modiiSed epoxy system (< > , = 0.176) at different temperatures (Reprinted from Polymer International, 30, R.A. Ruseckaite, L. Hu, C.C. Riccardi, RJ.J. Williams, Castor-oil-modified epoxy resins as model systems of rubber-modified thermosets. 2 Influence of cure conditions on morphologies generated, 287-295, Copyright (1993), with kind permission from the Society of Chemical Industry, London, UK)...

Fig. 9. Temperature-conversion transformation diagram showing phase separation, gelation and vitrification for a rubber (R) and a thermoplastic (TP)-modified cyanate ester system (15 wt% of modifier) (Reprinted from Chemistry and Technology of Cyanate Ester Resins (I. Hamerton, ed.), J.P. Pascault, J. Galy, F. Mechin, 112 ISO, Copyright (1994), with kind permission from Chapman Hall, London, UK)...

Fig. 2. Alkali chitin in aqueous solution at 20°C and after phase separation-gelation upon heating to 70°C...

In the case of the segmented polymers, the domains that form during phase separation will lead to the rapid buildup of viscosity and gelation, much like a crosslinking urethane, although these polymers are linear. An an-ologous expression for the viscosity rise in these systems is given by [36] ... 

Concentration of TEOS in all these cases has been restricted up to 50 wt% with respect to the mbber. Beyond 50 wt%, all the hybrids show phase separation which may be due to higher amount of water condensate that is continuously generated and acts as nonsolvent for the mbbers. This is easily understood from the visual appearance of the samples phase-separated composites slowly turn opaque in the course of gelation. 

Contrary to the phase separation curve, the sol/gel transition is very sensitive to the temperature more cations are required to get a gel phase when the temperature increases and thus the extension of the gel phase decreases [8]. The sol/gel transition as determined above is well reproducible but overestimates the real amount of cation at the transition. Gelation is a transition from liquid to solid during which the polymeric systems suffers dramatic modifications on their macroscopic viscoelastic behavior. The whole phenomenon can be thus followed by the evolution of the mechanical properties through dynamic experiments. The behaviour of the complex shear modulus G (o)) reflects the distribution of the relaxation time of the growing clusters. At the gel point the broad distribution of... 

In the domain A, 7red increases by increasing cg and in the domain B the gel formation has been confirmed by measurements of elastic modulus. This phenomenon is due to the predominant presence of polynuclear ions of Al (see Figure 1). The pH range where gelation occurs is very narrow and at pH 4 and 6 only phase separations are observed. 

Temperature not only plays a critical role with the thermodynamics, but also with the kinetics of the polymerization. Once phase separation occurs, the polymer phase will start to settle out of solution since it is denser than the aqueous phase. Chirlia noted this phenomenon by stating that in some reactions, a water layer was evident over the polymer sponge layer (Chirila et al., 1993). Temperature can reduce this settle-out by ramping up polymerization rate, and forcing gelation to occur sooner. 

The information available on aqueous polymer blends is qualitative in nature because of the lack of a suitable theory to interpret the experimental observations. Mixed gels can be comprised of an interpenetrating network, a coupled network (as discussed above), or a phase-separated network [2]. The latter is the most common as the blends have a tendency to form two phases during gelation. In such cases the miscibility and thermodynamic stability have to be empirically investigated and proper conditions for miscible blends identified. This involves a phase diagram study as is described in [3]. 

A second reason for the turn-over in the osmotic modulus may arise from a decrease in A2 until it becomes zero or even negative. This would be the classical situation for a phase separation. The reason why in a good solvent such a phase separation should occur has not yet been elucidated and remains to be answered by a fundamental theory. In one case the reason seems to be clear. This is that of starches where the branched amylopectin coexists with a certain fraction of the linear amylose. Amylose is well known to form no stable solution in water. In its amorphous stage it can be brought into solution, but it then quickly undergoes a liquid-solid transition. Thus in starches the amylose content makes the amylopectin solution unstable and finally causes gelation that actually is a kinetically inhibited phase transition [166]. Because of the not yet fully clarified situation this turn-over will be not discussed any further. 

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