Diameter step transition

Briefly, two immiscible polymers with different melting points (T ) and good simultaneous drawability are blended in their molten state and extruded into a thick cable that is approximately 1 to 2 mm in diameter (Step 1). In a following drawing process that is below the melting temperatmes but above the glass transition temperatures (Tg)... 

For the mainly oil-soluble Span 20 siufactant, however, the lifetimes are much less and films rupture prematurely, in line with predictions based on Bancroft s rule. At concentrations well above the CMC where the effective volume fraction of micelles is significant (>5 vol%), thin liquid films may drain in a stepwise fashion by stratification. This phenomenon, seen initially with foam films, was explained by the formation of periodic colloidal structures inside the film that results in layering of the micelles. At a step-transition, a layer of micelles leaves the film and the film thickness decreases by approximately the effective micellar diameter. It can also occur in emulsion films shown recently for hexadecane-aqueous sodium case-inate-hexadecane systems. The step-height seen of around 20 nm is very close to the measured diameter of the casein micelles of between 20 and 25 nm. The layering ultimately increases the lifetime of a film, but a critical film area exists below which step transitions are inhibited such thick films containing layers of micelles are even more stable. 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

The time required for a solute to flow past a given particle of stationary phase whose diameter is 5 pm is (5 pm)/(2.4 mm/s) = 2.1 ms. The stochastic theory predicts that the fraction of time that a molecule in the mobile phase will travel less than distance d is 1 — e <,/T" = 1 — e- 2-1 msW(3-5ms) = 0.55. That is, approximately half of the time, a solute molecule does not travel as far as the next particle of stationary phase before becoming adsorbed again to the same particle from which it just desorbed. If we lined up spherical particles of stationary phase, it would take 30 000 particles to cover the 15-cm length of the column. Each solute molecule binds —17 000 times as it transits the column, and half of those binding steps are to the same particle from which it just desorbed. 

The results of experimental studies of the sorption and diffusion of light hydrocarbons and some other simple nonpolar molecules in type-A zeolites are summarized and compared with reported data for similar molecules in H-chabazite. Henry s law constants and equilibrium isotherms for both zeolites are interpreted in terms of a simple theoretical model. Zeolitic diffusivitiesy measured over small differential concentration steps, show a pronounced increase with sorbate concentration. This effect can be accounted for by the nonlinearity of the isotherms and the intrinsic mobilities are essentially independent of concentration. Activation energies for diffusion, calculated from the temperature dependence of the intrinsic mobilitieSy show a clear correlation with critical diameter. For the simpler moleculeSy transition state theory gives a quantitative prediction of the experimental diffusivity. 

N2 adsorption-desorption isotherms and pore size distribution of sample II-IV are shown in Fig. 4. Its isotherm in Fig. 4a corresponds to a reversible type IV isotherm which is typical for mesoporous solids. Two definite steps occur at p/po = 0.18, and 0.3, which indicates the filling of the bimodal mesopores. Using the BJH procedure with the desorption isotherm, the pore diameter in Fig. 4a is approximately 1.74, and 2.5 nm. Furthermore, with the increasing of synthesis time, the isotherm in Fig. 4c presents the silicalite-1 material related to a reversible type I isotherm and mesoporous solids related to type IV isotherm, simultaneously. These isotherms reveals the gradual transition from type IV to type I. In addition, with the increase of microwave irradiation time, Fig. 4c shows a hysteresis loop indicating a partial disintegration of the mesopore structure. These results seem to show a gradual transformation... 

The sharp steps in the isotherms at a relative pressure p/po = 0.28 in case of the pristine MCM-48 silica and 0.23 for the transition metal containing silica reveal the high order of the mesoporous systems. The hysteresis starting at a relative pressure of p/po = 0.47 arises from a second porous system, which will be discussed elsewhere. Applying the BJH theory to the adsorption isotherms of all three samples the pore diameter distributions can be calculated. 

We will next consider the case of a lew silica content co-gel. A 5% silica-content silica-alumina was prepared by precipitation of aluminum isdsutoxide and tetraethoxv-silane as described for the silica-free gel. After gelation water was added just sufficient to fill the pore voids of the gel. The added water led to formation of a boehmite-rich hase during recrystallization. After drying at 120 and calcination at 500 0 for 16 hours, a transitional alumina hase is formed with a surface area of 410 m /g and a pore volume of 1.9 oc/g. This silica-alumina had an average pore diameter of 18 nm, similar to the silica-free material discussed previously. Steam treatment of this 18 nm pore diameter silica-alumina at 870°C (1600 ) in 90% H20-10% N2 for 16 hours resulted in a material with surface area of 196 m /g. This surface area is much hi er than expected for an amori ous gel and is consistent with silica enrichment of the outer surface during the recrystallization step vhere water was added to the pores of the amoridious gel. Silica stabilization of bodunite alumina by formation of a surface Aiase complex has been reported in recent work (9). ESCA analysis also indicates silica surface enrichment vhen compared to the amori ous gel. 

The production of carbon fibers or filaments by decomposing a hydrocarbon gas over a transition metal catalyst has been the subject of extensive research. The product consists of filaments with diameters in the range of 1-100 pm and lengths up to 100 mm. In microstructure, it is different from traditional carbon fibers, resulting in a sword and sheath fracture mode without catastrophic failure. Since, in addition, these fibers are produced in a single step with no really expensive processing, they are attractive candidates for reinforcing composites. 

In light of the above discnssion, the optical absorption spectrum of a quantum film is expected to consist of a series of steps, with the position of these steps corresponding to the transitions between heavy or light hole quantum states and electron quantum states following the selection rule An = 0. Furthermore, since the widths of the wells are commonly smaller than the calculated diameter of an exciton, the exciton binding... 

The surface area and pore volume of the support both increased with the first increments of phosphate added. They went through an optimum at a P/Al atomic ratio of about 0.2. Pore volume was high in this series because all the samples were washed in n-propanol before the final drying step. The surface area dropped considerably as P/Al in the precipitate approached the stoichiometric 1.0, because of the formation of crystalline AIPO4. This transition caused a large increase in the pore diameter as P/A1 increased. 

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