Lower critical solution temperature behaviour

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.)...

Figure 3 The composition-temperature diagram for pure poly-olefins showing UCST (upper critical solution temperature) and LCST (lower critical solution temperature) behaviour (because of the nature of this mixture, the UCST cannot be observed and therefore is dotted)...

Irani C.A., Cozewith C., Lower critical solution temperature behaviour of ethylene propylene copoloymers in multicomponent solvents, J, of Applied Polymer Science, 1986, (31), 1879-1899... 

A similar situation, depicted in Fig. 4, is valid for modifiers such as poly-(ether sulfone) (PES) exhibiting a lower-critical-solution temperature behaviour [21-23]. In this case, the reaction temperature T is located below the initial miscibility gap. 

Fig. 4. Evolution of the miscibility gap with conversion (p) for a modified-thermosetting polymer showing a lower-critical-solution-temperature behaviour, LXIST (< ho = initial volume fi-action of modifier, Pcp = cloud-point conversion)...

Figure 7.2 Phase diagrams of binary blends showing the upper and lower critical solution temperature behaviours (as UCST and LCST, respectively). (Modified from Strobl. )...

The hydrophobic interaction results in the existence of a lower critical solution temperature and in the striking result that raising the temperature reduces the solubility, as can be seen in liquid-liquid phase diagrams (see Figure 5.2a). In general, the solution behaviour of water-soluble polymers... 

Type V fluid phase behaviour shows at temperatures close to 7C-A a three-phase curve hhg which ends at low temperature in a LCEP (h=h)+g and at high temperature in a UCEP (h+h) g The critical curve shows two branches. The branch h=g runs from the critical point of pure component A to the UCEP. The second branch starts in the LCEP and ends in the critical point of pure component B. This branch of the critical curve is at low temperature h=h in nature and at high temperature its character is changed into h=g- The h=h curve is a critical curve which represents lower critical solution temperatures. In Figure 2.2-7 four isothermal P c-sections are shown. 

In type VI phase behaviour a three-phase curve l2hg is found with an LCEP and an UCEP. Both critical endpoints are of the type (l2=li)+g and are connected by a l2=h critical curve which shows a pressure maximum. For this type of phase behaviour at constant pressure closed loop isobaric regions of l2+li equilibria are found with a lower critical solution temperature and an upper critical solution temperature. 

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... 

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. 

The Flory-temperature or theta-temperature (0F) is defined as the temperature where the partial molar free energy due to polymer-solvent interactions is zero, i.e. when y = 0, so that the polymer-solvent systems show ideal solution behaviour. If T = 0F, the molecules can interpenetrate one another freely with no net interactions. For systems with an upper critical solution temperature (UCST) the polymer molecules attract one another at temperatures T < 0F. If the temperature is much below 0F precipitation occurs. On the other hand for systems with a lower critical solution temperature (LOST) the polymer molecules attract one another at temperatures T > F. If the temperature is much above 0F precipitation occurs. Aqueous polymer solutions show this behaviour. Systems with both UCST and LCST are also known (see, e.g. Napper, 1983). 

Finally, it has been shown that in some cases, the use of a lower critical solution temperature colloidal stabiliser can control ink behaviour on the substrate. Above 37°C, the solubility of the stabiliser decreases causing a dramatic increase in viscosity. Lines printed using this approach did not display deviations at their starts and ends, and bulges in the line were prevented. 

From the above table it is clear that the lower critical solution temperature is raised, and the upper critical solution temperature is lowered, by increase of pressure. Under j>res,sure of 830 kgm. per sq. cm. the two critical solution points coincide. Under pressures higher than this, complete miscibility exists at all temperatures. A similar behaviour is found in the case of water and methylethylketone. 

The lower critical solution temperature (LCST) of the pure, weakly cross-linked PNIPAAm gel is 34°C determined by the swelling method (Shibayama et al. 1994). In order to increase the mechanical properties hydrogels with higher cross-linking densities can be prepared. In Fig. 2 the temperature-dependent swelling behaviour... 

The hydrophobic interaction results in the existence of a lower critical solution temperature and in the striking result that raising the temperature reduces the solubility, as can be seen in liquid-liquid phase diagrams (see Figure 5.2a). In general, the solution behaviour of water-soluble polymers represents a balance between the polar and the non-polar components of the molecules, with the result that many water-soluble polymers show closed solubility loops. In such cases, the lower temperature behaviour is due to the hydrophobic effects of the hydrocarbon backbone, while the upper temperature behaviour is due to the swamping effects of the polar (hydrophilic) functional groups. 

GEE Geever, L.M., Devine, D.M., Nugent, M.J.D., Kennedy, J.E., Lyons,J.G., Hanley, A., and Higginbotham, C.L., Lower critical solution temperature control and swelling behaviour of physically ctosslinked thermosensitive copolymers based on A-isopropylacrylamide, Eur. Polym. J., 42, 2540, 2006. 

It is now established both theoretically and experimentally that many thermodynamic variables assume a simple power-law behaviour at or near critical points in both pure and mixed fluids. The actual functional dependence of one variable on another can be characterized by the so-called critical indices a, 5, etc. The critical index j8, for example, defines both the shape of the gas-liquid coexistence curve for a pure fluid and the liquid-liquid coexistence curve of a binary mixture in the vicinity of either an upper or a lower critical solution temperature. The correspondence between critical phenomena in one-, two-,... 

Also, auxiliary compounds can demonstrate very interesting self-aggregative behaviour, which allows controlled interaction with the desired products. We have mentioned already the example of aqueous two-phase systems on the basis of aqueous polymer-polymer, polymer-salt and smfactant-based micellar systems. Exiting developments are achieved with block copolymers composed of two alkyl chains connected by a hydrophilic polymer. Modifleation of the chain lengths of the blocks allows variation in the lower critical solution temperature (LCST - onset to phase separation) from 273 K to 333 K. Typically less then 5 wt% of polymer is required to construct these systems. 

One of the more recently developed material classes is that of thermo-responsive polymers. This accounts mainly for those polymers showing a so-called lower critical solution temperature (LOST) or - to a much less extent - an upper critical solution temperature (UCST), meaning polymers that change their solution status significantly upon temperature changes. In general, this behaviour is the result of a delicate balance of hydrophilic and hydrophobic groups in the polymer. The major... 

Fig. 3a,b. Schematic representation of phase diagrams showing a upper critical solution temperature (UCST) b lower critical solution temperature (LCST) behaviour. Binodals (—) and spinodals (-----) are shown together with UCST( t) and LCST() points... 

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. 

The importance of water soluble polymers such as the polyacrylamides is well established but only now are fundamental data on these systems beginning to accumulate. The unperturbed dimensions of these polymers tend to depend on the lateral substituent, and specific interactions are thought to produce large chain expansions with a corresponding low chain flexibility. In some cases the specific interactions can lead to a system exhibiting a pseudo-lower critical-solution temperature. The characteristic parameter C for polyacrylamide in water has also been reassessed in the belirf that the published value is too high. The excluded volume parameter and unperturbed dimensions have also been measured for poly ( -1,1-dimethyl-3-oxobutylacrylamide) in MEK. Many of the papers mentioned in Table 1 contain additional data on the sedimentation behaviour and thermodynamic parameters. 

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