Miscible polymers solution temperatures

To understand the mechanism of polyblending, experiments have been carried out with polymeric solution. W. Borchard and G. Rehage mixed two partially miscible polymer solutions, measured the temperature dependence of the viscosity, and determined the critical point of precipitation. When two incompatible polymers, dissolved in a common solvent, are intimately mixed, a polymeric oil-in-oil emulsion is formed. Droplet size of the dispersed phase and its surface chemistry, along with viscosity of the continuous phase, determine the stability of the emulsion. Droplet deformation arising from agitation has been measured on a dispersion of a polyurethane solution with a polyacrylonitrile solution by H. L. Doppert and W. S. Overdiep, who calculated the relationship between viscosity and composition. 

KUM Kumaki, J., Hashimoto, T., and Granick, S., Temperature gradients induce phase separation in a miscible polymer solution, Phys. Rev. Lett., 77, 1990, 1996. 

Reciprocals of the critical temperatures, i.e., the maxima in curves such as those in Fig. 121, are plotted in Fig. 122 against the function l/x +l/2x, which is very nearly 1/x when x is large. The upper line represents polystyrene in cyclohexane and the lower one polyisobutylene in diisobutyl ketone. Both are accurately linear within experimental error. This is typical of polymer-solvent systems exhibiting limited miscibility. The intercepts represent 0. Values obtained in this manner agree within experimental error (<1°) with those derived from osmotic measurements, taking 0 to be the temperature at which A2 is zero (see Chap. XII). Precipitation measurements carried out on a series of fractions offer a relatively simple method for accurate determination of this critical temperature, which occupies an important role in the treatment of various polymer solution properties. 

The lower critical solution temperature is another crucial polymer property, which, together with the Upper Critical Solution Temperature (UCST), defines fhe fwo solubility boundaries of polymers in solution. Typically, systems are completely miscible below the LCST but only partially miscible above the LCST and completely immiscible above the UCST. 

The instance we have considered here, that of a polymer in a poor solvent, results in an upper critical solution temperature (UCST) as shown in Figure 2.33. This occurs due to (a) decreased attractive forces between like molecules at higher temperatures and (b) increased solubility. For some systems, however, a decrease in solubility can occur, and the corresponding critical temperature is located at the minimum of the miscibility curve, resulting in a lower critical solution temperature (LCST). This situation is illustrated in Figure 2.34. 

The volume change in these gels is not due to ionic effects, but rather to a thermodynamic phenomenon a lower critical solution temperature (LCST). The uncrosslinked polymer which makes up the gel is completely miscible with water below the LCST above the LCST, water-rich and polymer-rich phases are formed. Similarly, the gel swells to the limit of its crosslinks below the LCST, and collapses above the LCST to form a dense polymer-rich phase. Hence, the kinetics of swelling and collapse are determined mostly by the rate of water diffusion in the gel, but also by the heat transfer rate to the gel. 

The above equation also explains why two different polymers are seldom miscible. Both solute and solvent are now polymeric and thus both suffer from the entropy decreases described above. It also explains why it is necessary to heat a mixture in which the solute does not dissolve at room temperature. This increase in T increases the magnitude of the last term in equation (23), and this is the term that generally makes the free energy change negative. 

The opposite behavior as sketched before was detected for solutions of PS in DOP [112], Again, the critical temperature (an UCST at Tc = 12 °C in the quiescent state) turned out to be a function of the shear rate to which the solution is subjected. But, in contrast to solutions of PS and PB in DOP, here enhancements of the UCST as large as 28 °C were recorded at a shear rate of 220 s-1. Similar results have been found for PS solutions in di(2-ethyl hexyl)phthalate or in a mixture of cis- and frans-decalin [113], The solutions demixed in a converging flow from a reservoir into a capillary tube. It has been observed that an increase in the deformation rate raised the UCST or reduced the region of miscibility. In both of these studies an increase of the cloud point temperature of the polymer solutions was used as an indication of phase separation. 

Thin polymer films composed of two layers with different composition have been used for almost two decades to determine the diffusion coefficient [12,78-82] on the basis of observed broadening of their initial profiles ( >(z). When the two layers are built of two fully miscible phases (T>TC regime for blends with upper critical solution temperature UCST), a free interdiffusion takes place with the interface growing with time t as w1/2°=t1/2. This process proceeds without limits and results in a single homogeneous phase. 

In this process phase inversion is introduced by lowering the temperature of the polymer solution. A polymer is mixed with a substance that acts as a solvent at a high temperature and the polymer solution is cast into a film. When the solution is cooled, it enters into an immiscible region due to the loss of solvent power. Liquid-liquid demixing occurs and the solution is separated into two phases, i.e., the polymer-lean phase is dispersed as droplets in the polymer-rich phase. Further, cooling causes gelation of polymer. Because the solvent is usually nonvolatile, it must be removed with a liquid that is miscible with the solvent but not miscible with the polymer. The membranes made by the TIPS method have pore sizes in the range of 0.1 and 1 pm and the pore structure is uniform in the depth direction. ... 

The most basic question when considering a polymer blend concerns the thermodynamic miscibility. Many polymer pairs are now known to be miscible or partially miscible, and many have become commercially Important. Considerable attention has been focussed on the origins of miscibility and binary polymer/polymer phase diagrams. In the latter case, it has usually been observed that high molar mass polymer pairs showing partial miscibility usually exhibit phase diagrams with lower critical solution temperatures (LCST). 

Three different techniques are used for the preparation of state of the art synthetic polymeric membranes by phase inversion 1. thermogelation of, a two or more component mixture, 2. evaporation of a volatile solvent from a two or more component mixture and 3. addition of a nonsolvent to a homogeneous polymer solution. All three procedures may result in symmetric microporous structures or in asymmetric structures with a more or less dense skin at one or both surfaces suitable for reverse osmosis, ultrafiltration or microfiltration. The only thermodynamic presumption for all three preparation procedures is that the free energy of mixing of the polymer system under certain conditions of temperature and composition is negative that is, the system must have a miscibility gap over a defined concentration and temperature range (4). 

Figure 1. Schematic diagram showing the formation of a microporous membrane by thermal gelation of a polymer solution exhibiting a miscibility gap at certain conditions of temperature and composition.

Polymer blends typically show a decrease in miscibility with increasing temperature. [27] McMaster has used a modified Flory equation of state thermodynamic model to show that the existence of a lower critical solution temperature (LCST) is caused mainly by differences in the pure component thermal expansion coefficients. 

In a miscible system of two liquids it is not expected that they will remain as a single phase over all ranges of composition and temperature. Instead there will be cloud points where two phases occur and the composition of the separated material will be close to that of the original pure components. Even in so-called two-phase systems there will be a partitioning of one component in the other corresponding to a small solubility at the particular temperature. An example of a phase diagram that may occur for two liquids, a polymer solution or a polymer blend is shown in Figure 1.28. 

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