Upper critical solution temperature -type phase diagram

Figure 14.10 The five types of (fluid + fluid) phase diagrams according to the Scott and van Konynenburg classification. The circles represent the critical points of pure components, while the triangles represent an upper critical solution temperature (u) or a lower critical solution temperature (1). The solid lines represent the (vapor + liquid) equilibrium lines for the pure substances. The dashed lines represent different types of critical loci. (l) [Ar + CH4], (2) [C02 + N20], (3) [C3H8 + H2S],...

This figure clearly shows the temperature and composition windows where it is either a two-phase system or a single-phase system. The characteristic features of an upper critical solution temperature (UCST) and a lower critical solution temperature (LCST) corresponding to the phase transition are identified. For a particular composition of two immiscible polymers, if the temperature is increased, the UCST is the highest temperature at which two phases may co-exist in the blend. There is then a window of miscibility as the temperature is increased further, followed by phase separation again at the LCST. This type of diagram is often seen for polymer solutions, e.g. polystyrene in cyclohexane. Often polymer blends show... 

According to the type of T versus q> diagram (Fig. 25.4), the binary solution can exhibit an upper critical solution temperature (UCST), a lower critical solution temperature (LCST), or both (close-loop phase behavior). Above the UCST or below the LCST the system is completely miscible in all proportions [82], Below the UCST and above LCST a two-phase liquid can be observed between cp and cp". The two-phase liquid can be subdivided into unstable (spontaneous phase separation) and metastable (phase separation takes some time). These two kinds of mixtures are separated by a spinodal, which is outlined by joining the inflexion points (d AGIdcp ) of successive AG versus cp phase diagrams, obtained at different temperatures (Fig. 25.3b). Thus, the binodal and spinodal touch each other at the critical points cp and T. ... 

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 most important defect in simple theory, from a practical point of view, is its failure to describe the temperature dependence of polymer-polymer miscibility with temperature. Simple theory predicts miscibility at high temperature, irrespective of the value of AH , since TAS inevitably dominates at sufficiently high temperature. This behaviour is characterised by a phase diagram of the type shown schematically in Fig. 3a and is typical of small molecule systems. Such systems are characterised by an upper-critical-solution temperature (UCST). In contrast, polymer-polymer systems (if not immiscible at all accessible temperatures) are usually characterised by a lower-critical-solution temperature (LOST), Fig. 3b, and are more likely to be miscible at low temperatures. 

As shown in Figs. 1 and 2, there are two generic types of phase diagrams. The conditions for the existence of these phase diagrams are obtained by comparing the NIT temperature T i of pure liquid crystals and the upper critical solution temperature T<. in the isotropic phase. 

Critical Solution Temperature n The liquid-liquid critical point of a solution (solvent-polymer or other) denotes the limit of the two -phase region of the phase diagram (temperature vs. composition). This is point at which an infinitesimal change such as temperature or pressure will lead to separation of the mixture into two distinct separate phases. Two types of liquid-liquid critical points are the upper critical solution temperature (UCST) which denotes the warmest point at which cooling will induce phase separation and, and the lower critical solution temperature (LCST), which denotes the coolest point at which heating will induce phase separation... 

A binary mixture of deuterated polybutadiene (DPB) and polybutadiene (PB) was used to investigate the phase-separation processes of SD. Detailed information about the DPB/PB mixture, e.g., molecular characteristics, phase diagram, and sample preparation, can be found elsewhere [72]. Note that the DPB/PB mixture used here has an upper critical solution temperature (UCST)-type phase diagram and it requires homogenization by mechanical mixing [73] in order to bring the system to the single-phase state. The thickness of the mixture, D, was 200 fim. 

Figure 7.1 Schematic of various types of phase diagrams in binary polymer mixtures (a) upper critical solution behavior (UCST) (b) lower critical solution behavior (LCST) (c) and (d) mixed phase diagrams with both upper and lower critical temperatures.

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