Thermal inversion process

G.T. Caneba and D.S. Soong, Polymer Membrane Formation Through the Thermal-inversion Process, Macromolecules 18, 2538 (1985). 

The polymer used in the thermal inversion process of making membranes was initially dissolved and maintained at a temperature above the critical point of the binary system under consideration. Solutions of 10wt% polystyrene in cyclohexanol were prepared at 90°C and then placed in an oven at a temperature of 115-120°C. Note that the upper critical solution temperature of polystyrene/cyclohexanol system is about 82°C (Shultz and Flory, 1953). Also, the critical composition is at a polymer volume fraction of about 0.03, while the polymer-rich binodal composition is at a volume fraction of about 0.20. Thus, the cast solutions are expected to coagulate inside the spinodal curve, as we have verified using a diode-array time-resolved light scattering system similar to that used by Hashimoto and his coworkers (Inaba et al., 1986). Solid membranes were then made within 24 h, after the solutions were prepared. These membranes were then sputter-coated with gold-palladium and observed under the microscope. 

Fig. 1,4.31 Schematic diagram of the quenching system used in the formation of membrane-type structures through the thermal inversion process...

Figure 1.4.36-1.4.38 shows SEM micrographs of the PMAA membranes formed via thermal inversion process starting with a 7%/3%/90% solution of PMAA/MAA/water at room temperature and quenched at 80°C. The micrographs show fibrous, interconnected network, with open pores, indicative of spinodal decomposition. As phase separation time increases to about 2 min, a cellular... 

CAN Caneba, G.T. and Soong, D.S., Polymer membrane formation through the thermal-inversion process. 1. Experimental study of membrane stmcture formation. 

Both thermal- and acid-induced equilibrations of 3,3-disubstituted thietane oxides were very slow (K 10-5 s-1)194. The results suggest that thietane oxides are similar to the various acyclic sulfoxides with respect to the rates of thermally induced pyramidal inversion at sulfur238, and that this inversion process, therefore, does not interfere significantly in the above exchange/racemization studies. 

Examples of shifts of higher order than [1, 5] are relatively rare. But an example of a thermal suprafacial process involving [1, T shift and inversion of the migrating group is afforded by bicyclononatrienes. 

For cycloheptatriene and a series of its derivatives various thermal unimolecular processes, namely conformational ring inversions, valence tautomerism, [1,5]-hydrogen and [l,5]-carbon shifts, are known. An example of such multiple transformations was described65 which can provide a facile approach to new polycyclic structures by a one-step effective synthesis (yields up to 83%) of the two unique ketones 156 and 157. The thermolysis of the neat ether 151 at 200 °C for 24 h gives initially the isomeric allyl vinyl... 

Although planar structures for 111 and 112 were not attained, it is still likely that these novel diazabiaryls can serve as chiral ligands with C2 symmetry. In support of this likelihood, the resolution of 112 was accomplished recently on swollen, microcrystalline triacetylcellulose by Jan Sandstrom. The free energy barrier to ring inversion of 112 was found to be about 101 kJ/mol, through a thermal racemization process using chiral 112. ... 

The basic differences in electron delocalization between the homoaromatic homotropenylium and homocyclopropenium ions and the bicyclo[3.1. Ojhexenyl cations result in fundamentally different reactions of these cations. As was noted earlier, the homotropenylium and homocyclopropenium ions undergo a characteristic ring-inversion process which interconverts the exo and endo substituents on the methylene bridge. With 61 and its derivatives no such reaction occurs. Rather, two different types of thermal isomerization occur. The first of these is the irreversible rearrangement to the cyclo-hexadienyl ions mentioned above. The second thermal isomerization involves a circum-ambulation of the methylene group around the periphery of the five-membered ring,37-143.,45.152. 

Because Dcr is proportional to the content of Ag nanocrystals and Ai/2 is in inverse proportion to their mean size d [77, 78], one may conclude that the state and amount of Ag nanocrystals were not affected by the cryopolymerization. Sharp growth of DCI at heating of obtained metal-polymer films specifies that the main part of Ag at 77 K is in a form of small noncrystalline Ag clusters, which aggregate with formation of nanocrystals under action of thermal relaxation processes in polymer matrix. According to data in work [79], in UV vis spectra of PPX films on a background of the PPX absorption only absorption bands of Ag with n> 15 could be observed in open range of PPX spectrum at X >320 nm. Because in this spectral range... 

The classical inversion mechanism is a thermally activated process 2>, activation energies being determined from the variation of inversion rates with temperature. The corresponding rates for passage over the barrier may be calculated from the absolute reaction rate theory 2>. The rate constant is given by the Eyring rate equation ... 

The thermal diffusion factor a is proportional to the mass difference, (mi — mo)/(mi + m2). The thermal diffusion process depends on the transport of momentum in collisions between unlike molecules. The momentum transport vanishes for Maxwellian molecules, particles which repel one another with a force which falls off as the inverse fifth power of the distance between them. If the repulsive force between the molecules falls off more rapidly than the fifth power of the distance, then the light molecule will concentrate in the high temperature region of the space, while the heavy molecule concentrates in the cold temperature region. When the force law falls off less rapidly than the fifth power of the distance, then the thermal diffusion separation occurs in the opposite sense. The theory of the thermal diffusion factor a is as yet incomplete even for classical molecules. A summary of the theory has been given by Jones and Furry 15) and by Hirschfelder, Curtiss, and Bird 14), Since the thermal diffusion factor a for isotope mixtures is small, of the order of 10", it remained for Clusius and Dickel (8) to develop an elegant countercurrent system which could multiply the elementary effect. 

Figure 9 also plots the orbital correlations as the change progresses 123,124). It can be seen that for d complexes, not only must a spin be reversed, but an electron must move from one orbital to another. Thus, the transition is forbidden on the grounds of both spin and orbital symmetry. The ground state of the tetrahedron correlates with an excited state of the square plane 125). This in turn means that square-planar isomerizations and tetrahedral inversions, each of which depend on the operation of two of these steps, are not thermally allowed processes. (They are, however, photochemically allowed.)... 

Sol 2 is present either when one phase separates into two phases or when two phases are prevented from recombining into a single phase. It is expedient to entitle this factor inoompatibilityt and to discuss the various phase inversion processes in terms of the reasons for incompatibility. In the sections to follow four phase inversion processes are discussed a dry process, a wet process, a thermal process and a polymer assisted phase inversion process. 

The thermal process is perhaps the most universally applicable of all the phase inversion processes because it can be utilized over the widest range of both polar and nonpolar polymers. However, its commercial use for membrane applications will probably be restricted to polyolefins, particularly polypropylene. A large number of the substances can function as latent solvents (Table X). They usually consist of one or two hydrocarbon chains terminated by a polar hydrophilic end group. Therefore, they exhibit surface activity which may explain their ability to form the emulsion-like Sol 2 micelles at elevated temperatures. One latent solvent which is worthy of special mention because of its broad applicability is N-Tallowdiethanolamlne (TDEA). 

The stereochemical course of the photoinduced walk rearrangement sensitized by benzophenone was examined for the optically active ester (-)-47 and nitrile (+)-48 (76). Similarly, as in the corresponding thermal rearrangement (Figure 5), the inversion process is preferred in both systems (stereoselectivity for 47 >92%, for 48 >76%). In nitrile 48 an additional racemization made of the starting material due to a one-center epimerization at C-7 competes with the rearrangement. Stereoselective diradical processes of the triplet states were proposed to explain these results. 

Recombination reactions are the inverse of unimolecular dissociation processes, and the associated rate coefficients correspondingly exhibit also a pressure dependence. Like thermal decomposition processes, recombination reactions require an energy transfer by collision. The pressure dependence results from the change in efficiency with which the excess energy is removed from the incipient product molecule by the third body M. The situation can be made clearer by writing the reaction as a sequence of two steps... 

Thermal-Phase-Inversion Process. Still another approach to forming porous membranes out of polymers not soluble at room temperature is to elevate the temperature until they dissolve in a selected solvent. The resulting solution is then cooled in a controlled way until the polymer precipitates around the solvent which serves as the "pore-former" at room temperature. The process is called "thermal-phase-inversion."... 

Figure 2.5 Photomicrograph of polypropylene capillary membrane (made by ENKA S thermal-phase-inversion process).

---