Upper critical solution temperature thermodynamics

Only a few sterns are available which meet these conditions. The first degradation studies investigating the interrelation of thermodynamics and chain scission were done using polystyrene (PS) in various solvents The system studied most extensively so far is given by trons-decalin/PS (TD/PS) It exhibits upper critical solution temperature and a theta temperature of 21 °C Figure 5 shows the corresponding cloud point curves determined for narrowly distributed PS samples. 

Binary mixtures of non-aromatic fluorocarbons with hydrocarbons are characterized by large positive values of the major thermodynamic excess functions G , the excess Gibbs function, JT , the excess enthalpy, 5 , the excess entropy, and F , the excess volume. In many cases these large positive deviations from ideality result in the mixture forming two liquid phases at temperatures below rSpper. an upper critical solution temperature. Experimental values of the excess functions and of Tapper for a representative sample of such binary mixtures are given in Table 1. 

NIE Nies, E., Li, T., Berghmans, H., Heenan, R.K., and King, S.M., Upper critical solution temperature phase behavior, composition fluctuations, and complex formation in poly(vinyl methyl ether)/D20 solutions Small-angle neutron-scattering experiments and Wertheim lattice thermodynamic perturbation theory predictions, J. 

Cite the following fact to illustrate the difficulties in the phase analysis of polymer systems. For the poly(vinyl alcohol)- -water system, some researchers propose a state diagram of amorphous phase separation with an upper critical solution temperature, others — ainor]>hons separation with a lower critical solution temperature about 100 C there are some who think that there is no region of amorphous separation below 150 -instead, they observe liquid-crystal phase separation. Such are the discrepancies on the basic question of thermodynamics ... 

There are ample books in the literature considering in detail the thermodynamics of polymer solutions, i.e. the state of the P-I-LMWL system above the 9 point (for systems with an upper critical solution temperature) (see the bibliography). With the exception of riory (J953) and Tompa (1956), the other authors cither did not deal with phase separation or mentioned it only in its relation to fractionation. A need has, therefore, arisen to look into the questions of liquid-liquid phase separation (including multiphase separation) as c.arefully as possible, the more so that many applications of these problems can be introduced into the technology of polymer materials. 

Phase equilibria are strongly affected by temperature raising the temperature of a solution may increase the miscibility of the two components or, less frequently, may result in a decrease of their miscibility. In the first case, as depicted in the phase diagram in Figure 18, the two components are totally miscible above a point known as the upper critical solution temperature (UCST). At any temperature below the UCST, compositions lying outside the curve constitute homogeneous phases, while compositions inside the curve are thermodynamically unstable and will phase-separate. When the miscibility of a polymer in a solvent decreases as the temperature increases, the mixture possesses a lower critical solution temperature (LCST Fig. 18). For temperatures below this point, the two components are totally miscible. 

Liquid—Liquid Phase Separation, in contrast to solid-liquid phase separation, lowering temperature can induce liquid-liquid phase separation of a polymer solution with an upper critical solution temperature and when the crystallization temperature of the solvent is sufficiently lower than the phase separation temperature. In an equilibrium phase diagram of a polymer solution, the spin-odal curve divides the liquid-liquid phase separation region into two regions a thermodynamically metastable region (between the binodal and spinodal) and a thermodynamically unstable region (enclosed by the spinodal) (Fig. 11). Above the... 

When polymers undergo phase separation in thin films, the kinetic and thermodynamic effects are expected to be pronounced. The phase separation process can be controlled to effect desired morphologies. Under suitable conditions a film deposition process can lead to pattern replication. Demixing of polymer blends can lead to structure formation. The phase separation process can be characterized by the binodal and spinodal curves. UCST is the upper critical solution temperature, which is the temperature above which the blend constituents are completely miscible in each other in all proportions. LUST behavior is not found as often in systems other than among polymers. LUST is the lower critical solution temperature. This is the... 

However for intermediate compositions (32-50% in CHDM) they formed homogeneous mixtures above an upper critical solution temperature (UCST) that rendered a miscible metastable phase upon quenching. A thermodynamic analysis of the USCT-type behavior demonstrated that the bare interaction energy for each pair of blends, was positive and increased with the content of 1,4-CHDM units in the copolymer (140). Commercial PEj.C -T/ PC blends (Ektar DA series, Eastman Kodak) have been used in lawn and garden equipment, floor care appliance parts, sterilizable medical equipment, etc. In these applications, a beneficial combination of clarity, toughness, chemical resistance, heat, UV and gamma radiation resistance has been profited. Molded parts made of these blends generally showed excellent surface finish and hence molded-in-color could be used (141). 

Schwahn et al [175] were the first to report a transition from mean-field to non-mean-field behavior in polymer mixtures on the basis of SANS results obtained from a PS-poly(vinyl methyl ether) (PVME) mixture. Subsequently, Bates et al. [176] quantitatively verified these conclusions using a model polyisoprene-poly(ethylene-propylene) mixture above the upper critical solution temperature (Tc = 38 °C), which revealed a transition from y = 2v = 1 (mean-field behavior) toy = 1.26 (non-mean-field behavior) approximately 30 °C above the critical temperature. These SANS crossover studies established the limitations of mean-field theory, which has been used extensively for evaluating polymer-qx)lymer thermodynamics, and similar crossover phenomena have been investigated via SANS for polymers in small-molecule solvents (e.g. polystyrene in cyclohexane) and supercritical media (e.g. polydimethylsiloxane in CO2), as described in Section 7.6.4... 

In polymer solutions, liquid-liquid (L-L) demixing is another common phase transition besides crystallization. The thermodynamic boundary conditions for both of them behave as the functions of polymer concentrations and temperatures, demonstrated as phase diagrams. The schematic L-L binodal and liquid-solid (L-S) coexistence curves in polymer solutions and their interception are shown in Figure 13.2. The illustrated L-L binodal contains an upper critical solution temperature. Some other solutions also contain binodals with a lower critical solution temperature. When the L-S curve intersects with the L-L curve in the overlapping temperature windows, both curves are terminated at the intersection point, which is referred to as the monotectic triple point. 

Because of thermodynamic criteria, the majority of existing homopolymers form immiscible mixtures constituted of two or more phases. Some couples form complex mixtures exhibiting partial and conditional miscibility. They display lower critical solution temperature (LCST) or upper critical solution temperature (UCST). Indeed, depending on temperature and composition (i.e., position in the phase diagram), the blend can be miscible or immiscible. In monophase blends, the final macroscopic properties are most often intermediate between those of the corresponding parent homopolymers. In contrast, additive properties of immiscible blends can only be obtained if a good level of interfacial adhesion, and appropriate particle size, shape, and distribution of the dispersed phase are reached via adequate compatibilization routes. 

The first attempt to describe theoretically the processes of phase separation during the reaction of formation of semi-IPNs has been done in the works [296,297]. Semi-IPNs based on PS and a reactive epoxy monomer based on DGEBA with a stoichiometric amount of 4,4 -methylenebis(2,6-diethylaniline) were studied experimentally. Thermodynamic analysis of the phase separation proceeding during the curing reaction was performed that considered the composition dependence of the interaction parameter x(T, 2) (where T is the temperature and 2 is the voliune fraction of PS) and the polydispersity of both polymers. The latter is especially important, hi this analysis, x(T,)]. For the initial mixture (before the reaction) the cloud point curves showed upper critical solution temperature behavior and the dependence x(r, >2) on the composition was determined from the threshold point. 

Several recent theories for polymer solution thermodynamics include entropic, enthalpie and free-volume contributions to the free energy of mixing. The free-volume eontributions modify the entropic components, and have the opposite temperature dependence from the combinatorial term, helping to explain the lower critical solution temperature. Since Eq. (B 18) assumes a constant value for the entropic component, it may not be valid over large temperature ranges. However, Eq. (B18) will predict the upper critical solution temperature, and, from this standpoint, is adequate for a number of phase equilibria applications near this condition. 

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