Molecular mixed phases

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

Glaser and Litt (G4) have proposed, in an extension of the above study, a model for gas-liquid flow through a b d of porous particles. The bed is assumed to consist of two basic structures which influence the fluid flow patterns (1) Void channels external to the packing, with which are associated dead-ended pockets that can hold stagnant pools of liquid and (2) pore channels and pockets, i.e., continuous and dead-ended pockets in the interior of the particles. On this basis, a theoretical model of liquid-phase dispersion in mixed-phase flow is developed. The model uses three bed parameters for the description of axial dispersion (1) Dispersion due to the mixing of streams from various channels of different residence times (2) dispersion from axial diffusion in the void channels and (3) dispersion from diffusion into the pores. The model is not applicable to turbulent flow nor to such low flow rates that molecular diffusion is comparable to Taylor diffusion. The latter region is unlikely to be of practical interest. The model predicts that the reciprocal Peclet number should be directly proportional to nominal liquid velocity, a prediction that has been confirmed by a few determinations of residence-time distribution for a wax desulfurization pilot reactor of 1-in. diameter packed with 10-14 mesh particles. 

Photochemical response of these liposomes is different from each other. With progression of trans - cis photoisomerization of azobenzene, ICD at the absorption band of the trans isomer decreases. As shown in Figure 4, depression in ICD is almost proportional to the amount of photoisomerization for the phase separated system. Photoisomerization in the domain of azobenzene aggregate proceeds independently from the rest of DPPC aggregate so that the depression in ICD corresponds to the concentration of remaining transazobenzene. When the two components are molecularly mixed, change of... 

Molecular mixing via dynamic mechanical spectroscopy. While electron microscopy yields the phase size, shape, etc., as delineated above, dynamic mechanical spectroscopy (DMS) yields the composition within each phase. The DMS measurements employed a Rheovibron direct reading viscoelastometer model DDV-II (manufactured by Toyo Measuring Instruments Co., Ltd., Tokyo, Japan). The measurements were taken over a temperature range from -120°C to 140°C using a frequency of 110 Hz and a heating rate of about 1°C/ min. Sample dimensions were about 0.03 x 0.15 x 2 cms. 

Each of the SIN s examined showed two glass transitions, one for each phase. In general, the transitions were shifted inward, suggesting small but significant extents of molecular mixing. 

The volume fraction of each phase was taken from the fractional area in the transmission electron micrographs. Combined with the values shown in Table 1, the compositions within each phase were calculated and are shown in Table 2. Overall, the results suggest variably 0-20% actual molecular mixing. Noting the probable errors in estimating the experimental Tg s, the and W2 values are probably correct to within +0.05. Thus mixing plays an important role in interpenetration and influences the reinforcement within each phase. 

The scanning electron micrographs (SEM) for both IPN-I and IPN-II revealed a one phase morphology which implied that IPN coatings with molecular mixing were obtained by introducing similar structures into both polymer systems and using polymers with low MW and broad MWD at distribution. 

Finally, we should mention the phenomenon of incompatibility of mixtures of polymer solutions. It applies to nearly all combinations of polymer solutions when the homogeneous solutions of two different polymers in the same solvent are mixed, phase separation occurs. For example, 10% solutions of polystyrene and poly(vinyl acetate), each in benzene, form two separated phases upon mixing. One phase contains mainly the first polymer, the other phase mainly the second polymer, but in both phases there is a certain amount of the other polymer present. This limited compatibility of polymer mixtures can be explained thermodynamically and depends on various factors, such as the structure of the macromolecule, the molecular weight, the mixing ratio, the overall polymer concentration, and the temperature. 

There are several interesting families of inorganic mixed-valence compounds that we have not discussed here (see Yvon, 1979 McCarley, 1982). For example, there are metal-cluster compounds such as the Chevrel phases, M,jMo6X8(X = S or Se) and condensed metal-cluster chain compounds such as TlMojScj, TijTe, NaMo O and M PtjO. TTF halides and TTF-TCNQ complexes (Section 1.9) constitute molecular mixed-valent systems in which the mixed valency is associated with an entire molecule the charge on TTF in such compounds is nonintegral. The structure of TTF-Br(, 79 and... 

Fig. Ell.la The in-phase G and out-of-phase G" moduli of the PET/TGIC samples, one molecularly mixed (solution) and the other made of compressed and initially segregated PET. As expected, the homogeneous sample, after the required time for thermal equilibrium, shows the expected response of first-order kinetics. [Reprinted by permission from R. Dhavalkikar and M. Xanthos, Monitoring the Evolution of PET Branching Through Chemorheology, Polym. Eng. Sci., 44, 474 (2004).]...

In order to avoid over-crowding of the alkyl chains, but retain the advantages of molecular self-assembly, mixed phases of long and short chain alkyls have been prepared. As one of such possible combinations, C3 chains have been mixed with... 

Because the components must initially form miscible solutions or swollen networks a degree of affinity between the reacting components is needed. Therefore, most of the investigations into epoxy IPNs have involved the use of partially miscible components such as thermoplastic urethanes (TPU) with polystyrenes [57], acrylates [58-61] or esters which form loose hydrogen-bound mixtures during fabrication [62-71 ]. Epoxy has also been modified with polyetherketones [72],polyether sulfones [5] and even polyetherimides [66] to help improve fracture behavior. These systems, due to immiscibility, tend to be polymer blends with distinct macromolecular phase morphologies and not molecularly mixed compounds. 

Yet more important was the publication by Schottky and Wagner (1930) of their classical paper on the statistical thermodynamics of real crystals (41). This clarified the role of intrinsic lattice disorder as the equilibrium state of the stoichiometric crystal above 0° K. and led logically to the deduction that equilibrium between the crystal of an ordered mixed phase—i.e., a binary compound of ionic, covalent, or metallic type—and its components was statistical, not unique and determinate as is that of a molecular compound. As the consequence of a statistical thermodynamic theorem this proposition should be generally valid. The stoichiometrically ideal crystal has no special status, but the extent to which different substances may display a detectable variability of composition must depend on the energetics of each case—in particular, on the energetics of lattice disorder and of valence change. This point is taken up below, for it is fundamental to the problems that have to be considered. 

Polymer reactors can often be a complex combination of many different physical phenomena (reaction, mixing, phase transfer, heat and mass transfer, etc.). Reactor design then becomes crucial to ensure that we have enough manipulators to achieve partial control of the dominant variables affecting the desired polymer properties. The new features for polymer reactors are typically composition, molecular weight, and molecular weight distribution. 

Dependent Variables. Electron microscopy revealed the size and shape of the phase domains, and DMS gave complementary information about the extent of molecular mixing and phase continuity. The stress-strain and impact studies revealed the extent of toughness of the materials. 

Aerosol-assisted CVD introduces rapid evaporation of the precursor and short delivery time of vapor precursor to the reaction zone. The small diffusion distance between the reactant and intermediates leads to higher deposition rates at relatively low temperatures. Single precursors are more inclined to be used in AACVD therefore, due to good molecular mixing of precursors, the stoichiometry in the synthesis of multicomponent materials can be well controlled. In addition, AACVD can be preformed in an open atmosphere to produce thin or thick oxide films, hence its cost is low compared to sophisticated vacuum systems. CVD methods have also been modified and developed to deposit solid phase from gaseous precursors on highly porous substrates or inside porous media. The two most used deposition methods are known as electrochemical vapor deposition (EVD) and chemical vapor infiltration (CVI). 

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