Polymer-solvent-precipitant systems

The experimental procedures described in this paper are mainly concentrated on the preparation of membranes from various polymer-solvent systems by precipitation in a nonsolvent, generally water. The membranes are then characterized in terms of their transport properties and structures. Furthermore, the three-component phase diagrams are determined for various polymer-solvent-precipitant systems. 

The Ternary Phase Diagram. The ternary phase diagrams of various polymer-solvent-precipitant systems have been determined. The results shown in Figure 11 indicate that the phase diagrams determined for the systems... 

The Effect of the Polymer-Solvent-Precipitant System on Membrane Structure and Properties. The effect of the polymer on membrane structure and properties is closely related to the solvent used in the casting solution and the precipitant. The solvent and the precipitant used in membrane preparation determine both the activity coefficient of the polymer in the solvent-precipitant mixture and the concentration of the polymer at the point of precipitation and solidification. The polymer-solvent-precipitant interaction can be approximately correlated in terms of the disparity of the solubility parameter of polymer and solvent. 

KLE Klein, J. and Schiedermaier, E., Temperature dependent precipitation behavior in polymer-solvent-precipitant systems (Ger.), Kolloid-Z. 1. Polym., 236, 118,1970. 

The Effect of the Solvent-Precipitant System on Membrane Structure and Properties. The precipitant and the solvent used in membrane preparation determine both the activity coefficient of the polymer in the solvent-precipitant mixture and the concentration of polymer at the point of precipitation and solidification. 

A representative plot of binodal and spinodal curves for ternary polymer/ monomer/precipitant systems (which is similar to that of a polymer/solvent/ non-solvent system) is shown in Fig. 1.1.2 at constant temperature and pressure. The phase envelope pertains to the region encompassed by the binodal curves, in which there exist two phases at equilibrium. Outside the phase envelope is the single-phase region. The so-called tie lines are straight lines that join the binodal compositions at equilibrium. If the system has an LOST, then the size of the phase envelope increases with increasing temperature. If the system has a UCST, then the size of the phase envelope decreases with increasing temperature. 

In this subsection, experimental efforts are presented to determine parameters for phase separation kinetics of polymer/solvent and polymer/monomer/precipitant systems. It is necessary to first apply these methods for polymer/solvent systems, since theoretical bases of experimental methods are well established for such systems. Later, they are applied to relevant ternary systems. 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

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. 

Fractional Precipitation of Cellulose Triacetate. The reported partial or non-fractionation of cellulose triacetate from chlorinated hydrocarbons or acetic acid may be explained in terms of the polymer-solvent Interaction parameter x (1-11) The x values for cellulose triacetate-tetrachloroethane and cellulose triacetate-chloroform systems are reported (10,21) as 0.29 and 0.34 respectively. The lower values of x for such systems will result in a smaller or negative heat of mixing (AHm) and therefore partial or non-fractionation of the polymer in question results. 

Considering the rather complicated processes that take place during dissolution it is not surprising that some systems show peculiar behavior. For example, while solubility generally increases with temperature, there are also polymers that exhibit a negative temperature coefficient of solubility in certain solvents. Thus, poly(ethylene oxide), poly(N-isopropylacrylamide), or poly(methyl vinyl ether) dissolve in water at room temperature but precipitate upon warming. This behavior is found for all polymer-solvent systems showing a lower critical solution temperature (LCST). It can be explained by the temperature-dependent... 

At temperature lower then 50°C, polymerization proceeds very slowly, whereas at 50°C, 4-vinylpyridine and 2-vinylpyridine polymerize very rapidly without any initiator. Using DMF as solvent, the system remains homogeneous through the polymerization time, whereas in acetone precipitates polymer mixture after a few minutes. It is also... 

The organic solvent is the most important variable as it controls partition and diffusion of the reactants between the two immiscible phases, the reaction rate, solubility, and swelling of permeability of the growing polymer. The solvent should be of such composition so as to prevent precipitation of the polymer before a high molecular weight has been attained. The final polymer should not dissolve in the solvent. The type of solvent will influence the characteristics of the physical state of the final polymer. Solvents such as chlorinated or aromatic hydrocarbons make useful solvents in this system. 

An example of such a catalyst system is racemic isopropylene bis(l-indenyl) zirconium dichloride in combination with an alumi-noxane (21). The reaction is carried out in hydrocarbon solvents, e.g., toluene. A solution of norbornene in toluene with the catalyst is degassed and then pressurized with ethene. The polymerization is carried out while stirring at 70°C under constant ethylene pressure at 18 bar. After completion, the polymer is precipitated in acetone and filtered (21). The cycloolefin copolymers obtained in this way have a high thermal shape stability and it is possible to use the polymers as thermoplastic molding compositions. 

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). 

The microstructure of the multiphase media is often the product of phase transitions, e.g. (i) capillary condensation in the porous media, (ii) phase separation in polymer/polymer and polymer/solvent systems, (iii) nucleation and growth of bubbles in the porous media, (iv) solidification of the melt with a temporal three-phase microstructure (solid, melt, gas), and (v) dissolution, crystallization or precipitation. The subject of our interest is not only the topology of the resulting microstructured media, but also the dynamics of its evolution involving the formation and/or growth of new phases. 

Hydride polyaddition of divinyl-containing compounds was carried out for various lengths of a,co-dihydridedimethylsiloxanes. The reaction run was searched by a decrease of active =Si-H groups concentration. It was found that for rhodium acetylacetonatedicarbonyl as a catalyst, copolymers soluble in organic solvents were obtained, which were structured after some time. This may be explained by the fact that in spite of polymers re-precipitated from toluene solution by methyl alcohol, rhodium catalyst remains in polymeric systems, which decompose and induce structuring (cross-linking) of copolymers. 

The effect of introduction of regularly disposed cyclic fragments into the macromolecular backbone on conformational and hydrodynamic parameters is also studied [30], For this purpose, copolymers 3 and 4 (Table 1) were fractionated into twelve fractions from the benzene (solvent) - methanol (precipitator) system. The influence of cyclic groups, introduced into the polymer backbone, on rigi-dity parameters was determined by direct computer simulation of macromolecular coil with the help of the Monte-Carlo method. 

For ternary polymer-polymer-solvent systems, the compositions of the equilibrium phases may be determined using a variety of microanalytical methods depending upon the chemical nature of the polymers (Dobry and Boyer-Kawenoki, 1947). Each of the phases is sampled, weighed, and dried to determine the solvent concentration. If the two polymers are sufficiently different chemically, microanalytical determination of carbon and hydrogen may be used. In systems containing polystyrene, the proportion of polystyrene may be determined by precipitating it with acetic acid and weighing the precipitate. Other microanalytical methods have also been used to determine phase compositions. 

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