Critical solution temperature, effect upper

Interactions between different distant parts of the molecule tend to expand it, so that in the absence of other effects a would be greater than unity, but in solution in poor solvents interactions with the solvent tend to contract it. According to Flory s theory (18) these two tendencies will just balance so that a — 1 at a particular temperature T—0 (the theta temperature ), and at this temperature A2 =0 and further this temperature is the limit as Mn- go of the upper critical solution temperature for the polymer-solvent system in question. Quantities relating to T=0 will be denoted by subscript 0. Flory s theory implies that ... 

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. 

Orzechowski K (1999) Electric field effect on the upper critical solution temperature. Chem Phys 240 275-281... 

Define the upper and lower critical solution temperature. What is the effect of impurities on them (Meerut 2004)... 

Cowie, J. M. G. Maconnachie, A. Ranson, R. J., "Phase Equilibria in Cellulose Acetate-Acetone Solutions. The Effect of the Degree of Substitution and Molecular Weight on Upper and Lower Critical Solution Temperatures," Macromolecules, 4, 57 (1971). 

Zeman, L. Patterson, D., "Pressure Effects in Polymer Solution Phase Equilibria. II. Systems Showing Upper and Lower Critical Solution Temperatures," J. Phys. Chem., 76, 1214 (1972). 

Chen et al. [67,68] further extended the study of binary blends of ESI over the full range of copolymer styrene contents for both amorphous and semicrystalline blend components. The transition from miscible to immiscible blend behavior and the determination of upper critical solution temperature (UCST) for blends could be uniquely evaluated by atomic force microscopy (AFM) techniques via the small but significant modulus differences between the respective ESI used as blend components. The effects of molecular weight and molecular weight distribution on blend miscibility were also described. 

Although the biphasic properties of fluorous-organic systems are desirable for separations, monophasic conditions would favour enhanced reaction rates. Therefore, it is important to know the general miscibilities of fluorous solvents and the effect of temperature (Tables 7.2 and 7.3). In Table 7.2, the temperature given for the phase separation is a consulate or upper critical solution temperature. However, these temperatures should only be taken as a guide, as... 

Consider diffusion in a binary liquid mixture exhibiting an upper critical solution temperature (UCST) or lower critical solution temperature (LCST) (see Fig. 3.1). Let us take a mixture at the critical composition x at point A just above the UCST. Any concentration fluctuation at A will tend to be smeared out due to the effects of diffusion in this homogeneous mixture. On the other hand, any fluctuation of a system at point B, infinitesimally below the UCST, will lead to separation in two phases. Similarly, the mixture at point D, just below the LCST is stable whereas the mixture at point C, just above the LCST is unstable and will separate into two phases. 

Zeman, L., and D. Patterson. 1972. Pressure effects in polymer solution phase equilibria. II. Systems showing upper and lower critical solution temperatures. J. 

It is claimed that although measurements of the lower limit are reproducible determination of the upper limit is quite a problem with errors of 2-3 nm in the calculations. Apparently above this limit solvent cast films appear to be immiscible whatever the polymer tacticity. It is suggested that changes in the donor/acceptor emissions with tacticity are simply due to an effect on the chain conformation of the probability of intermolecular interactions. As a final criticism it is shown that heating a monophase blend above the lower critical solution temperature does not actually result in a significant enough change in the donor/acceptor emission ratio to be able to detect phase separation. However, it should be pointed out that the studies of... 

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. 

Certain principles mnst be obeyed for experiments where liquid-liquid equilibrium is observed in polymer-solvent (or snpercritical flnid) systems. To understand the results of LLE experiments in polymer solutions, one has to take into acconnt the strong influence of polymer distribution functions on LLE, because fractionation occnrs dnring demixing. Fractionation takes place with respect to molar mass distribution as well as to chemical distribution if copolymers are involved. Fractionation during dentixing 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 mnlticomponent system. Clond-point cnrves are measnred instead of binodals and per each individnal feed concentration of the mixtnre, two parts of a coexistence cnrve occnr below (for upper critical solution temperatnre, UCST, behavior) or above the clond-point cnrve (for lower critical solution temperature, LCST, behavior), i.e., produce an infinite nnmber of coexistence data. 

More recently Young has measured the upper critical solution temperatures of mixtures of n-C7Fi6 with a series of n-alkanes (n-Cs to n-Qs) and has shown that, although solubility-parameter theory is unable to predict the experimental critical temperatures with any accuracy, a modification of this theory due to Reed is more successful. Reed s improved theory takes into account the effect of the size and ionization potential diffidences between the two component molecules and is thus similar in many respects to the theory of Hudson and McCoubrey discussed in the next Section. 

The solubility of macromolecules as a rule improves with the rising temperature. Solvent - polymer mixtures usually exhibit the upper consolute temperature or upper critical solution temperature, UCST, with a maximum on the plot of system concentration versus temperature. Above the critical solution temperature, polymer is fully soluble at any concentration. For practical work, the systems with UCST below ambient temperature are welcome. There are, however numerous polymer - solvent systems, in which the solvent quality decreases with increasing temperature. The plot of system concentration versus temperature exhibits a minimum. The phenomenon is called lower consolute temperature or lower critical solution temperature, LCST Polymer is only partially soluble or even insoluble above lower critical solution temperature. This unexpected behavior can be explained by the dominating effect of entropy in case of the stiff polymer chains or by the strong solvent - solvent interactions. The possible adverse effect of rising temperature on polymer solubility must be kept in mind when woiking with low solubility polymers and with multicomponent mobile phases. It may lead to the unforeseen results especially in the polymer HPLC techniques that combine exclusion and interaction retention mechanisms, in coupled methods of polymer HPLC (see section 11.8, Coupled Methods of Polymer HPLC). 

While most of the work described in this monograph emphasizes the two-phased nature of IPNs and related materials, it is interesting to explore more deeply the characteristics of phase separation in polymer/polymer systems. Of key importance, McMaste/ " showed that most polymer/polymer phase diagrams are expected to exhibit a lower critical solution temperature (LOST). This means that as the temperature is raised the polymer pair becomes less mutually soluble, and phase separates. This effect is not immediately predicted by equations (2.1) and (2.2), which suggest the usual phenomenon of an upper critical solution temperature (UCST). 

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