Upper critical solution temperature UCST behavior

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).

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. 

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... 

The X parameters of a large number of polymer blends exhibit this kind of temperature dependence. An example of this is the SPB(88)/JSPB(78) blend [system 27a], and the temperature dependence of x is shown in Fig. 19.1(a). Increasing temperature in such blends leads to increased miscibility. This behavior is often referred to as upper critical solution temperature (UCST) behavior. A typical phase diagram obtained from such systems is shown in Fig. 19.1(b). The spinodal and binodal curves were calculated for a SPB(88)/JSPB(78) blend with N = 2,000. A 50/ 50 mixture of these polymers is predicted to be two phase at room temperature but single phase at temperatures above 105 °C. The qualitative features of the phase diagrams obtained from all type I blends will be similar to Fig. 19.1(b). Of course the locations of the phase boundaries will depend on A, B, and N. 

The majority of compositionally different poly(meth)acrylates are immiscible. Nevertheless, there are some examples noted in the literature of miscible combinations. One of the most interesting and well-studied cases involves isotactic PMMA (iPMMA) with syndiotactic PMMA (sPMMA).[149] The miscibility observed may be expected, but the formation of a stereocomplex offered an interesting system to study. The tacticity of PMMA blends with poly(vinyl pyrrolidone) (PVP) affected the blend phase behavior as atatic and syndiotactic PMMA were miscible with PVP, but isotactic PMMA was phase sepa-rated.[150,151] Miscibility with upper critical solution temperature (UCST) behavior was observed for PnBA blends with poly(propylene glycol) (linear and three arm star) oligomers.[152] Poly(vinyl buty-ral)/PMMA blends were found to be phase separated at high MW PMMA.[153] At low MW PMMA (2000-5000 g/mol), miscibility with UCST behavior was observed. The PMAA/PVAc miscibility was... 

With the pre-selected polymer concentrations, isobaric critical lines can be constructed from the isopleths in Figure 1. The thus obtained critical lines from 100 bar to 800 bar, respectively, show in Figure 2 on a T- plane. In Figure 2 the coexistence boundaries are described on a T- plane at the indicated pressures (bar)(the boundary under 1 bar was extrapolated from Figure 1). The shape of the coexistence curve depends only slightly on pressure, which can be recognized by comparing the curve at lOObar with the curve at 800 bar, and this system shows that it is an upper critical solution temperature (UCST) behavior system... 

DLS is also useful for determining phase behavior and micelle formation in block copolymers dissolved in ILs. The work by Lee et al. with poly(ethylene oxide)-h-poly(A-isopropylacrylamide) (PEO-h-PNIPAM) in [C2CjIM][BF ] and [C CjIM][BFJ shows how the lower critical solution temperature (LCST) behavior of PEO and the upper critical solution temperature (UCST) behavior of PNIPAM can be exploited to form PNIPAM-core micelles at low temperature and PEO-core micelles at high temperature. As can be seen in Figure 2.15, this system is thermally reversible allowing for potential applications as thermosensitive materials [8]. 

Polymers with upper critical solution temperature (UCST) behavior... 

In general, and for polymers that exhibit a miscibility gap at lower temperatures (blends that show upper critical solution temperature, UCST, behavior), interfacial tension is found to decrease linearly with increasing temperature, with temperature coefficients of the order of 10 dyn/(cm C) [10]. This is about one half of the values observed for the temperature coefficients of polymer surface tension [10,120,176]. 

Certain principles must be obeyed for experiments where liquid-hquid equilibrium is observed in polymer-solvent (or supercritical fluid) systems. To understand the results of LLE experiments in polymer solutions, one has to take into account the strong influence of polymer distribution functions on LLE, because fractionation occurs during demixing. Fractionation takes place with respect to molar mass distribution as well as to chemical distribution if copolymers are involved. Fractionation during demixing leads to some effects by which the LLE phase behavior differs from that of an ordinary, strictly binary mixture, because a common polymer solution is a multicomponent system. Cloud-point curves are meastrred instead of binodals and per each individual feed concentration of the mixture, two parts of a coexistence curve occur below (for upper critical solution temperature, UCST, behavior) or above the cloud-point curve (for lower critical solution temperature, LCST, behavior), i.e., produce an infrnite number of coexistence data. 

The three systems show liquid-liquid partial miscibility by decreasing the temperature at constant pressure, and thus present upper critical solution temperature (UCST) behavior as depicted in Figure 20.4.11. As can be observed from Figure 20.4.12, the addition of oleic acid, which is a naturally occurring olive oil constituent, significantly improves the mutual solubility. The presence of 11.60 wt% of oleic acid in olive oil decreases the UCST 11.2K (see Tables 20.4.8 and 20.4.9). 

Two-component block copolymers commonly display upper critical solution temperature (UCST) behavior [7]. They form ordered, microphase-separated morphologies at lower temperatures but ean be heated to temperatures where the discrete heterogeneity is lost. The transition point from a heterogeneous microstructure to a compositionally homogeneous state is termed the order-to-disorder transition (Eqdt)- At any given degree of polymerization N, the highest Eqdt exists in systems with equal volumes of the two components, at... 

The morphology and micro-domain structure is dependent on temperature and process conditions. Hashimoto and Balsara have shown that the order-order and order-disorder transitions are profoundly influenced by sample preparation and thermal history . In block copolymers showing upper critical solution temperature (UCST) behavior, it has been observed that the d-spacing scales as d where, T is absolute temperature. This effect is attributed to the change in interaction parameter (j) (i.e. X decreases with increase in temperature) Process conditions such as sample preparation, annealing and quench depth (during melt processing) affect the... 

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