UCST behavior

Miscible blends of high molecular weight polymers often exhibit LOST behavior (3) blends that are miscible only because of relatively low molecular weights may show UCST behavior (11). The cloud-point temperatures associated with Hquid—Hquid phase separation can often be adequately determined by simple visual observations (39) nevertheless, instmmented light transmission or scattering measurements frequendy are used (49). The cloud point observed maybe a sensitive function of the rate of temperature change used, owing to the kinetics of the phase-separation process (39). 

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. 

Fig. 1. Phase diagram of a polymer solution. The upper curve shows LOST and the lower curve UCST behavior. Solid lines binodal, dotted lines spinodal T. temperature, x polymer concentration...

Figures 8.2a and b describe a UCST behavior, while Fig. 8.2c represents an LCST behavior. It is interesting to note that in the case of an UCST behavior the cloud-point curve will usually intersect the vitrification curve, while this may not be the case for an LCST behavior.

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).

In Fig. 8.4 an UCST behavior is represented. The corresponding situation for an LCST behavior is a shift of the cloud-point curve to lower temperatures as conversion increases. A similar description of the phase-separation process is valid also for the LCST case. 

Before discussing theoretical approaches let us review some experimental results on the influence of flow on the phase behavior of polymer solutions and blends. Pioneering work on shear-induced phase changes in polymer solutions was carried out by Silberberg and Kuhn [108] on a polymer mixture of polystyrene (PS) and ethyl cellulose dissolved in benzene a system which displays UCST behavior. They observed shear-dependent depressions of the critical point of as much as 13 K under steady-state shear at rates up to 270 s Similar results on shear-induced homogenization were reported on a 50/50 blend solution of PS and poly(butadiene) (PB) with dioctyl phthalate (DOP) as a solvent under steady-state Couette flow [109, 110], A semi-dilute solution of the mixture containing 3 wt% of total polymer was prepared. The quiescent... 

Recent attempts to prepare 26 by RAFT, however, failed [153]. Double hydrophilic block copolymers of NIPAM and 23e [154], as well as of N,N-diethylacrylamide and 23b [155], were prepared with the CTA benzyl dithiobenzoate, and exhibit LCST and UCST behavior in water. The new polymer 51 is also part of amphiphiUc di- and triblock copolymers [152]. Diblock copolymers with poly(ethylene glycol) methyl ether acrylate, dimethylacry-lamide, or 4-vinylstyrene sulfonate are macrosurfactants with a switch-able hydrophobic block. Triblock copolymers containing additionally 4-vinylbenzoic acid differ in the nature of the hydrophilic part [152]. Near-monodisperse block copolymers of N,N-dimethacrylamide and 49a were synthesized in different ways via macro-CTAs of both monomers as the first step. Such sulfobetaine block polymers form aggregates in pure water but are molecularly dissolved after addition of salt [152,156,157]. 

The temperature dependence of the total interaction parameter shows that there exists an optimum condition for the composition at a given temperature (Fig. 3). Binary blends of PEO/PS and PEO/PAA are immiscible and miscible, respectively, at room temperature. The shape of curves implies that the homopol-ymer/homopolymer blends will exhibit UCST behaviors. A drastic effect of the sequence distribution on the miscibility can be found in Fig. 4. As the AA content in SAA increases from 5 mol% (Fig. 4a) to 7 mol% (Fig. 4b) to 10mol% (Fig. 4c), the blend becomes more miscible. The blend with random copolymers becomes miscible at a composition between 5 and 7 mol%, which agrees well with the experimental results [15]. At 7 mol%, the blend with block copolymers shows positive x> while the blend with random copolymers has negative y. This is very interesting because the miscibility could be controlled only by the change of copolymer sequence distributions. 

All the blends have an upper critical solubility temperature (UCST) behavior. It is evident that the CE monomer is a better solvent of rubbers than the epoxy prepolymer. 

Fig. 10 Dependence of the cloud points of aqueous (PDMAEMAno) solutions (0.1 gL 1 in buffer of pH 8 + 0.1 MNaCl) on the [Co (CN)6]3 concentration (upper curve assigns LCST-type cloud points, bottom curve refers to cloud points of the UCST-behavior) [81]. Reprinted by permission of ACS...

Fig. 10). The UCST-type cloud points shift to higher temperatures with the increased concentration of added trivalent counterions, whereas the LCST-type cloud points hardly change. Thus, the LCST-type transition can be adjusted by pH, while the UCST-type cloud points can be adjusted by the concentration of trivalent counterions. It is also possible to switch the UCST behavior of the PDMAEMA by using the light-sensitivity of hexacyanocobaltate(III) as the trivalent counterions. Under UV illumination, the trivalent counterions are turned into a mixture of divalent and monovalent counterions and the UCST-behavior disappears [81].

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. 

Phase diagrams have been proposed for blends of polyethylenes where the first component is linear, or less branched, and the second component is more branched. The method involves quench cooling each blend composition from the melt at various temperatures, so that there is insufficient time for liquid-liquid phase separation. Separated blends are considered to be separated at the particular temperature of the melt prior to quenching. TEM and DSC are used to characterize the quenched blends. The phase diagrams exhibit UCST behavior, depending on the difference in branching content of the component polyethylenes. Branch length has been found not to be important since it is the branch points that are excluded from the crystals (20,21). 

The polyethylenes with higher functionality were soluble in epoxy resin and required lower temperamre and time for forming homogeneous blend systems. The miscibility of the polymers was dependent on the type of epoxy resin also. Cycloaliphatic epoxy resin showed more miscibility with the polymers compared to DGEBA resin and phase separation occurred in these blend systems as a result of crystallization of PE. Upper critical solution temperature (UCST) behavior was... 

In Figure 15.1a, only UCST behavior is illustrated. Vapor-liquid equilibrium as well as vapor-liquid-liquid equilibrium, where two liquid phases are in equilibrium with one vapor phase, can be distinguished. In FigureI5.Ib, one can recognize the two LLE regions in the T-w plain. With increasing pressure, the two-phase regions become smaller. Usually, the slope of the UCST line is steeper than that for LCST. 

Type of data cloud points (UCST-behavior)... 

The trend is seen even more cleary with At/A = dlnA/dT listed in the last column of Table 2. In general, blends exhibiting an LCST behavior have either a negative or a small positive A value at low temperature, which increases rapidly with increasing temperature. On the other hand, blends exhibiting a UCST behavior have a positive A value at low tenperature, which in general decreases as the temperature is increased. The data presented in... 

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