LCST Phase Behaviour

However, the relationship between LCST and 1,2-unit content of BR was exclusively dependent upon the researchers. This was inferred to be due to the difference in molecular weight of the rubbers, despite the fact that the IR and BR used in the previous works were high molecular weight polymers. To investigate the effect of molecular weight on miscibility and LCST phase behaviour, it is necessary to use NR and BR with 32.3% 1,2-unit content (BR(32.3)). 

However, the blend underwent phase separation at 333 K, 343 K and 358 K, as reflected by two T gS. Hence, the LCST phase behaviour was also conflrmed for the blend of NR-gel having three-dimensional network structures with BR(32.3). 

NR-sol and NR-gel were miscible with BR(32.3) at 298 K, despite the ultra-high molecular weight and three-dimensional network structure that are present in the two rubbers, respectively. The LCST phase behaviour was found for the NR-sol/BR(32.3) and NR-gel/BR(32.3) blends by Tg measurements using DSC after quenching the annealed blends. The LCST of the NR-sol/BR(32.3) and NR-gel/BR(32.3) blends was 328 K, being identical to the LCST of the IR/ BR(32.3) blend. It is concluded that the difference in molecular weight plays no significant role in the LCST of the NR-sol/BR(32.3), NR-gel/BR(32.3) and IR/ BR(32.3) blends. 

Fig. 10 Phase diagrams of dPS/PnPMA blends, o UCST at ambient pressure, LCST at ambient pressure. Closed-loop phase behaviours are observed at higher pressure A 97 bar 117 bar 138 bar 166 bar 186 bar. Dashed line Prediction of Tg, blend by Fox equation at ambient pressure. From [59]. Copyright 2004 American Chemical Society... 

The phase behaviour of many polymer-solvent systems is similar to type IV and type HI phase behaviour in the classification of van Konynenburg and Scott [5]. In the first case, the most important feature is the presence of an Upper Critical Solution Temperature (UCST) and a Lower Critical Solution Temperature (LCST). The UCST is the temperature at which two liquid phases become identical (critical) if the temperature is isobarically increased. The LCST is the temperature at which two liquid phases critically merge if the system temperature is isobarically reduced. At temperatures between the UCST and the LCST a single-phase region is found, while at temperatures lower than the UCST and higher than the LCST a liquid-liquid equilibrium occurs. Both the UCST and the LCST loci end in a critical endpoint, the point of intersection of the critical curve and the liquid liquid vapour (hhg) equilibrium line. In the two intersection points the two liquid phases become critical in the presence of a... 

The thermodynamic definition of the spinodal, binodal and critical point were given earlier by Eqs. (9), (7) and (8) respectively. The variation of AG with temperature and composition and the resulting phase diagram for a UCST behaviour were illustrated in Fig. 1. It is well known that the classical Flory-Huggins theory is incapable of predicting an LCST phase boundary. If has, however, been used by several authors to deal with ternary phase diagrams Other workers have extensively used a modified version of the classical model to explain binary UCST or ternary phase boundaries The more advanced equation-of-state theories, such as the theory... 

PEO shows UCST behaviour (Tq, = 115°C) above its LCST (Tcp = 106°C) under pressure of 3 MPa, between 106 and 115°C PEO demixes (immiscibiUty island) phase behaviour studied using SANS [395]... 

The polymer architecture affects the demixing behaviour of thermoresponsive polymers [562], On the basis of theoretical studies it is expected that, as a rule, branched macromolecules are more soluble than their linear analogues [563-565]. This prediction was confirmed experimentally in the case of a solution of star-like polystyrene in cyclohexane (an UCST-type phase separation) for which an increase in the degree of branching resulted in a decrease in the temperature of demixing [566, 567], On the basis of a review of water-soluble polymers of various shapes by Aoshima and Kanaoka [30], it appears that water-soluble polymers do not offer a uniform tendency in their LCST-type phase behaviour. 

Some (ionic liquid + water) systems do not exhibit the usual UCST behaviour, but show a lower critical solution temperature (LCST) behaviour instead. Here, the phase separation takes place at higher temperatures and mutual solubilities increase upon cooling until a homogeneous solution is reached. Only a small change in ionic liquid structure can already change the type of phase behaviour (UCST or LCST). For example, the tetrabutyl-phos-phonium-based ionic liquid with trans 2-butenedione (fumarate) as anion shows UCST behaviour, whereas the similar ionic liquid with the cis 2-bute-nedione (maleate) shows LCST behaviour after mixing with water. ... 

Both, natural and synthetic polymers with associative properties arising from hydrophobic interactions give aqueous solutions with LCST. Among the most known systems having LCST behaviour should be mentioned polyethylene glycol-water and aqueous solutions of methyl cellulose. Also, in poly(methacrylic) acid, LCST phase diagrams were determined from the change in shear modulus and turbidity. For alkali chitin, the main key role played by hydrophobic interactions in LCST is evident from the decrease in the fluorescence ratio observed in Fig. 3b. 

In this case, a moderately water-soluble amphiphilic N-vinylcaprolaclam (NVC1) played the role of a fl-unit, and a well-water-compatible N-vinyl-imidazole (NVIAz) served as a P-unil. The polymerization was carried out in a medium of 10% aqueous dimethylsulfoxide (DMSO). The addition of DMSO to the reaction solvent was necessary because of insufficient NVC1 solubility in pure water. It was also shown that in this solvent mixture, the NVCl-homopolymers and NVCl/NVIAz-copolymers retained their LCST-behaviour [26,28]. Hence, the DMSO in the reaction solvent did not significantly suppress the hydrophobic interactions of the NVC1 units. The polymerization was initiated by the redox system (N,N,N, N -tetramethylethylenediamine (TMEDA) + ammonium persulphate (APS)) and was carried out at 65 °C (1st step). This condition was very important, since admittedly the temperature was higher than the phase separation threshold of the reaction bulk when the polymeric products were formed that is, under these thermal conditions, hydrophobically-induced folding as the NVCl-blocks appear was ensured. After completion of the reaction, the... 

In a blend of immiscible homopolymers, macrophase separation is favoured on decreasing the temperature in a blend with an upper critical solution temperature (UCST) or on increasing the temperature in a blend with a lower critical solution temperature (LCST). Addition of a block copolymer leads to competition between this macrophase separation and microphase separation of the copolymer. From a practical viewpoint, addition of a block copolymer can be used to suppress phase separation or to compatibilize the homopolymers. Indeed, this is one of the main applications of block copolymers. The compatibilization results from the reduction of interfacial tension that accompanies the segregation of block copolymers to the interface. From a more fundamental viewpoint, the competing effects of macrophase and microphase separation lead to a rich critical phenomenology. In addition to the ordinary critical points of macrophase separation, tricritical points exist where critical lines for the ternary system meet. A Lifshitz point is defined along the line of critical transitions, at the crossover between regimes of macrophase separation and microphase separation. This critical behaviour is discussed in more depth in Chapter 6. 

Figure 8.2 Schematic phase diagrams for thermoplastic-epoxy monomer (diglycidyl ether of bisphenol A) blends, (CPC = cloud point curve, and VC = vitrification curve), (a) and (b) UCST (upper critical solution temperature) behaviour for PPE and PEI (respectively) - DGEBA n = 0.15 (c) LCST (lower critical solution temperature) behaviour for PES-DGEBA y n = 0.15. (Pascault and Williams, 2000 - Copyright 2001. Reprinted by permission of John Wiley Sons Inc.)...

An alternative approach to using a rubber such as CTBN as the disperse phase is to use an engineering thermoplastic such as polyether sulfone or a polyimide in which the mechanical properties and Tg are more closely matched to those of the resin. These systems exhibit LCST behaviour, so the cure tempemture is located below the initial miscibility gap... 

Temperature-sensitive hydrogels with actuator properties show aLCST behaviour. They are swollen at low temperatures and shrink by exceeding of the volume phase transition temperature T. The best known hydrogel with LCST behaviour is PNIPAAm (Fig. 9). 

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