Upper critical solution temperature behavior

Ougizawa, T. and Walsh, D.J. 1993. Upper critical solution temperature behavior in polystyrene/polyfmethyl... 

KOA Koak, N., Loos, Th.W. de, and Heidemann, R.A., Upper-critical-solution-temperature behavior of the system polystyrene + methylcyclohexane. Influence of CO2 on the liquid-liqttideqttilibria, / / / //., 145, 311, 1998. 

Fig. 6. Schematic representation of possible phase diagrams for binary mixtures the shaded areas indicate two phase regions (a) upper critical solution temperature behavior (UCST) (b) lower critical solution temperature behavior (LOST) (c) combination of LOST and UCST, mostly observed in nonpolar polymer solutions (d) phase diagram showing upper, lower, and quasilower critical phase boundaries (e) immiscibility loop and (f) hourglass-shaped phase boundary obtained by convergence of upper and lower critical boundaries.

Svoboda, P., Kressler, J., Chiba, T., and Inoue, T. (1994) Light-scattering and TEM analyses of virtual upper critical solution temperature behavior in PCL/SAN Blends. Macromolecules, 27, 1154-1159. 

The first attempt to describe theoretically the processes of phase separation during the reaction of formation of semi-IPNs has been done in the works [296,297]. Semi-IPNs based on PS and a reactive epoxy monomer based on DGEBA with a stoichiometric amount of 4,4 -methylenebis(2,6-diethylaniline) were studied experimentally. Thermodynamic analysis of the phase separation proceeding during the curing reaction was performed that considered the composition dependence of the interaction parameter x(T, 2) (where T is the temperature and 2 is the voliune fraction of PS) and the polydispersity of both polymers. The latter is especially important, hi this analysis, x(T,)]. For the initial mixture (before the reaction) the cloud point curves showed upper critical solution temperature behavior and the dependence x(r, >2) on the composition was determined from the threshold point. 

Even with negative values of AGm, negative values of eqn [2] will yield an area of the phase diagram where the polymer-solvent mixture will separate into a polymer-rich phase and a solvent-rich phase. Temperature changes can result in phase separation as well as nonsolvent addition. Although polymer-polymer blends often exhibit lest (lower aitical solution temperature) behavior, polymer-solvent systems usually exhibit ucst (upper critical solution temperature) behavior. 

The Class I binary diagram is the simplest case (see Fig. 6a). The P—T diagram consists of a vapor—pressure curve (soHd line) for each pure component, ending at the pure component critical point. The loci of critical points for the binary mixtures (shown by the dashed curve) are continuous from the critical point of component one, C , to the critical point of component two,Cp . Additional binary mixtures that exhibit Class I behavior are CO2—/ -hexane and CO2—benzene. More compHcated behavior exists for other classes, including the appearance of upper critical solution temperature (UCST) lines, two-phase (Hquid—Hquid) immiscihility lines, and even three-phase (Hquid—Hquid—gas) immiscihility lines. More complete discussions are available (1,4,22). Additional simple binary system examples for Class III include CO2—hexadecane and CO2—H2O Class IV, CO2—nitrobenzene Class V, ethane—/ -propanol and Class VI, H2O—/ -butanol. 

Figure 8.17 Vapor fugacity for component 2 in a liquid mixture. At temperature T, large positive deviations from Raoult s law occur. At a lower temperature, the vapor fugacity curve goes through a point of inflection (point c), which becomes a critical point known as the upper critical end point (UCEP). The temperature Tc at which this happens is known as the upper critical solution temperature (UCST). At temperatures less than Tc, the mixture separates into two phases with compositions given by points a and b. Component 1 would show similar behavior, with a point of inflection in the f against X2 curve at Tc, and a discontinuity at 7V...

Using the estimated interaction parameters phase equilibrium computations were performed. It was found that the EoS is able to represent the VL2E behavior of the methane-n-hexane system in the temperature range of 198.05 to 444.25 K reasonably well. Typical results together with the experimental data at 273.16 and 444.25 K are shown in Figures 14.14 and 14.15 respectively. However, the EoS was found to be unable to correlate the entire phase behavior in the temperature range of 195.91 K (Upper Critical Solution Temperature) and 182.46K (Lower Critical Solution Temperature). 

Temperature-sensitive polymers, depending on polymer structure and polymer-polymer interactions, generally exhibit two behaviors, lower critical solution temperature (LCST) [31] and upper critical solution temperature (UCST). Phase diagrams for these behaviors are presented in Figure 9. 

Fig. 3a,b. Schematic phase diagrams displaying a upper critical solution temperature (UCST) behavior b lower critical solution temperature (LOST) behavior... 

The cloud point curves of the epoxy monomer/PEI blend and BPACY monomer/PEI blend exhibited an upper critical solution temperature (UCST) behavior, whereas partially cured epoxy/PEI blend and BPACY/PEI blend showed bimodal UCST curves with two critical compositions, ft is attributed to the fact that, at lower conversion, thermoset resin has a bimodal distribution of molecular weight in which unreacted thermoset monomer and partially reacted thermoset dimer or trimer exist simultaneously. The rubber/epoxy systems that shows bimodal UCST behavior have been reported in previous papers [40,46]. Figure 3.7 shows the cloud point curve of epoxy/PEI system. With the increase in conversion (molecular weight) of epoxy resin, the bimodal UCST curve shifts to higher temperature region. 

Figure 8.4 Temperature vs. composition transformation diagram for a modified thermoset with an upper critical solution temperature (UCST) behavior (Crit = critical point for a and p see text).

Chen et al. [67,68] further extended the study of binary blends of ESI over the full range of copolymer styrene contents for both amorphous and semicrystalline blend components. The transition from miscible to immiscible blend behavior and the determination of upper critical solution temperature (UCST) for blends could be uniquely evaluated by atomic force microscopy (AFM) techniques via the small but significant modulus differences between the respective ESI used as blend components. The effects of molecular weight and molecular weight distribution on blend miscibility were also described. 

The aqueous solutions of aromatic and aliphatic polyzwitterions exhibit significantly different phase behavior while 26 has an upper critical solution temperature (UCST) at 286 K, 23b shows both an UCST at 306 K and an apparent inverted lower critical solution temperature (LCST) at 289 K [196, 200]. Compound 25 is insoluble over the whole temperature range between 273 and 373 K. Thus, 23b is considered to be in a collapsed coil in water below the UCST due to intra- and/or interchain association. 

In general, the miscibility of a pair of polymers depends on temperature and composition. Figure 10.1 schematically shows three typical phase diagrams. The ordinate and the abscissa axes represent temperature and composition, respectively. The solid line in Fig. 10.1(a), below which the blend becomes immiscible (two-phase), is referred to as an upper critical solution temperature (UCST). However, Fig. 10.1(b) shows a lower critical solution temperature (LCST) behavior. Some polymer pairs display both UCST and LCST as shown in Fig. 10.1(c). As will be shown in the following, UCST is rarely observed for a polymer blend. 

PMMA/PVF2. They compared the crystalline fraction obtained by DSC and the isolated fraction of PVF2 measured by NMR as a function of the annealing time. They observed different crystallization behaviors depending on annealing temperatures, and suggest an upper critical solution temperature (UCST) for PMMA/PVF2. 

If the binodal and spinodal points are determined at various temperatures and are plotted together, a phase diagram such as the one shown in Figure 6.1 b may result. The temperature at which the binodal and spinodal curves merge together is the critical temperature. The phase diagram shown illustrates a case in which the miscibility gap occurs at temperatures above the critical temperature, and the system is said to exhibit a lower critical solution temperature (LCST) behavior. A system, on the other hand, may display an upper critical solution temperature (UCST) behavior, in which the miscibility gap occurs below the critical temperature. 

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