Phase separation on heating

The latter prediction appears to be well substantiated by experiment. On the other hand, it appears, as first shown by Freeman and Rowlinson (1960), that most, if not all, polymer solutions phase separate not only on cooling but also on heating. This constitutes a serious contravention of the predictions of the Flory-Huggins theory. In what follows, it will become apparent that the Flory-Huggins theory correctly identifies the driving force behind the mixing of polymer and solvent but overlooks a major factor that disfavours mixing. 

The binary phase diagram for typical polymer-solvent systems is shown schematically in Fig. 3.8. Nonpolar polymer solutions usually display an upper and a lower critical solution temperature. In the limit of infinite polymer molecular weight, these would correspond to an upper (6,) and lower (dj) theta-temperature. This can be readily seen as follows. 

For a polymer of infinite molecular weight, 2 in equation (3.20) is zero so that the chemical potential of the solvent swelling the polymer is given by 

Solution of these equations for Vj leads to only one acceptable value for Xi-This is Xi =i, a value corresponding to the attainment of theta-conditions (whether dv or 6i). 

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In aqueous solutions of Cm-(EO)n amphiphilic molecules, two interesting features are observed. First, isotropic micellar solutions undergo phase separation on heating. Such behavior is typical of hydrophobic interaction and is also observed for several water-soluble polymers. Hydrophobic interaction results from a change of order in the water structure [54]. Second, at high concentration, liquid crystalline phase behavior is observed with several structures [55]. 

PMMA is typical of many polymer pairs, for which the parameter is positive and of order 0.01, making only low molar mass polymers form miscible blends. PVME/PS, PS/PPO, and PS/TMPC have a strongly negative x parameter over a wide range of temperatures (of order — 0.01) but since >0 and Bblends phase separate on heating. PEO/ PMMA, PP/hhPP and PlB/hhPP, all represent blends with very weak interactions between components (x = 0). 

In order to explain phase separation on heating, I.e., LCST behavior, the effect of volume changes on mixing must be considered. This effect Is described by equation-of-state theories such as that developed by Flory and co-workers (30). The free volume contributions to the free energy are unfavorable and increase with temperature. 

Although the Flory-Huggins theory is sound in principle, several experimental results cannot be accounted for. For example, it was found that the x parameter depends on the polymer concentration in solution. Most serious is the fact that many polymer solutions (e.g., PEO) show phase separation on heating, when theory predicts that this should occur only on coohng. Another complication arises from specific interactions with the solvent, for example hydrogen bonding between the polymer and solvent molecules (e.g. with PEO and PVA in water). Aggregation in solution (a lack of complete dissolution) may also present another problem. 

One of the main features of nonionic water-soluble cellulose derivatives is that they exhibit, like some other polyethers, an inverse solubility-temperature behavior, i.e. there is phase separation on heating above the so-called lower critical solution temperature (LCST). The temperature at which a polymer-rich phase separates is normally referred to as the cloud point (CP). For ideal solutions, this temperature corresponds to the theta-temperature. Actually, for some derivatives, the cloud point may be preceded, if the concentration is not too low, by a sol-gel transformation with an increase in viscosity and possibly formation of liquid crystals (see Sect. 3.5). As it will be seen later, this reversible thermotropic behavior may be detrimental to the performance of the derivatives or can be advantageneously utilized to develop applications. 

The free volume dissimilarity provides one of the important conceptual features that is missing from the Flory-Huggins theory. It rationalizes (i) the observed phase separation on heating (ii) the strong entropic contribution to X that opposes mixing and (iii) the observed increase in x with volume fraction of polymer in certain systems. Qualitatively, we can write for the mixing of polymer and solvent at room temperature ... 

A variety of phase behaviors have been observed in binary homopolymer blends. Some blends phase separate on heating while others phase separate on cooling. This depends on whether x increases or decreases with temperature. Blends in which x changes nonmonotonically with temperature exhibit more complex phase diagrams. The... 

Blends which are miscible at low temperature may undergo phase separation on heating if the temperature to which they are heated exceeds the LOST of the phase diagram and if the compositions fall within the binodal. The failure of simple theory in this regard has its origins in defects in the simple lattice model. The lattice mo del assumes a lattice site, of some specific size, can be occupied by a unit of either component. In reaUty, mismatches in sizes of lattice sites and units can lead to an uneven distribution of free volume which can result in LOST behavioiu . A second cause of LCST behaviour can arise from specific (exothermic) interactions, such as hydrogen-bonds, between the components. Theoretical calculations using equations of state theory can predict LCST behaviour [39]. 

In the central region of the miscibility window blends are miscible up to the thermal decomposition temperature of PCL and phase separation on heating is not observed. However, at the edges of the miscibility window the shape of the window is such that blends which are miscible at low temperatures show LCST... 

Random copolymerization can be employed in another useful way to control the phase behavior of blends. Many miscible blends exhibit reversible phase separation on heating, i.e., lower critical solution temperature or LCST behavior. In some cases, the... 

It had been known that PMMA was compatible with PVC under some conditions, but contrary to earlier reports it has recently been found that a wide range of polyacrylates and polymethacrylates show compatibility with PVC. Such polymers are believed to be compatible due to a specific interaction between the carbonyl group in the ester and the hydrogen or halogen in the PVC. Similarly, it was known that poly(vinylidene fluoride) was compatible with PMMA and poly(ethyl methacrylate), it has now also been shown to be compatible with poly(vlnyl acetate), poly(vinyl methyl ketone), and polyacrylates. This work has also been extended to show the effect of tacticity on the compatibility of poly(ethyl methacrylate) where ail isomers are compatible but the isotactic form phase separates on heating."... 

In the last few years, high performance miscible polymer blends have attracted attention in the search for new materials at lower costs. Recently, two new families of high performance blends consisting of an aromatic polybenzimidazole and aromatic polyimides [1-3] and aromatic polyimides and polyethersulfone [4] have been reported. However, even though these blends appear to be miscible over the entire range of compositions, phase separation on heating above Tg is irreversible. This contribution describes the miscibility behavior in polyether-sulfone/polyimide PI 2080 and polyethersulfone/ polyimide XU 218 blends with and without solvents. 

A phase diagram relative to a polymer solution that phase separates on heating is shown in Figure 4. The solid line in this figure is called the binodal and it separates the stable from the metastable regions of the phase diagram. The dashed line is the spinodal curve, which separates the metastable and unstable regions. The spinodal touches the binodal at the critical point, Tc, which for a polsTner solution is defined by equations 17 and 18. 

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