Delayed rapid crystallization

The crystalliza tion resistance of vulcaniza tes can be measured by following hardness or compression set at low temperature over a period of time. The stress in a compression set test accelerates crystallization. Often the curve of compression set with time has an S shape, exhibiting a period of nucleation followed by rapid crystallization (Fig. 3). The mercaptan modified homopolymer, Du Pont Type W, is the fastest crystallizing, a sulfur modified homopolymer, GN, somewhat slower, and a sulfur modified low 2,3-dichlorobutadiene copolymer, GRT, and a mercaptan modified high dichlorobutadiene copolymer, WRT, are the slowest. The test is often mn near the temperature of maximum crystallization rate of —12° C (99). Crystallization is accelerated by polyester plasticizers and delayed with hydrocarbon oil plasticizers. Blending with hydrocarbon diene mbbers may retard crystallization and improve low temperature britdeness (100). 

The rapid crystallization rate of homopolymer SPS reduces the practical benefit of cast film production. However, copolymerization of the SPS structures helps to delay the onset of crystallization during cast film production until a secondary biaxial orientation operation. SPS crystallites improve the dimensional stability and chemical resistance of films and have also been investigated for unique optical properties. 

Prepare two solutions, one containing i g. of diphenylamine in 8 ml. of warm ethanol, and the other containing 0-5 g. of sodium nitrite in i ml. of water, and cool each solution in ice-water until the temperature falls to 5°. Now add o 8 ml. of concentrated hydrochloric acid steadily with stirring to the diphenylamine solution, and then without delay (otherwise diphenylamine hydrochloride may crystallise out) pour the sodium nitrite solution rapidly into the weil-stirred mixture. The temperature rises at once and the diphenylnitrosoamine rapidly crystallises out. Allow the mixture to stand in the ice-water tor 15 minutes, and then filter off the crystals at the pump, drain thoroughly, wash with water to remove sodium chloride, and then drain again. Recrystallise from methylated spirit. Diphenylnitrosoamine is thus obtained as very pale yellow crystals, m.p. 67 68° yield, 0 9-1 o g. 

Dissolve 13 g. of sodium in 30 ml. of absolute ethanol in a 250 ml. flask carrying a reflux condenser, then add 10 g. (9 5 ml.) of redistilled ethyl malonate, and place the flask on a boiling water-bath. Without delay, add a solution of 5 3 g. of thiourea in a minimum of boiling absolute ethanol (about 100 ml.). The sodium salt of thiobarbituric acid rapidly begins to separate. Fit the water-condenser with a calcium chloride guard-tube (Fig. 61, p. 105), and boil the mixture on the water-bath for 1 hour. Cool the mixture, filter off the sodium salt at the pump and wash it with a small quantity of cold acetone. Dissolve the salt in warm water and liberate the acid by the addition of 30 ml. of concentrated hydrochloric acid diluted with 30 ml. of water. Cool the mixture, filter off the thiobarbituric acid, and recrystallise it from hot water. Colourless crystals, m.p. 245 with decomposition (immersed at 230°). Yield, 3 5 -4 0 g. 

Lente insulin is a mixture of 30% semilente (an amorphous precipitate of insulin with zinc ions in acetate buffer that has a relatively rapid onset of action) with 70% ultralente insulin (a poorly soluble crystal of zinc insulin that has a delayed onset and prolonged duration of action). These two components provide a combination of relatively rapid absorption with sustained long action, making lente insulin a useful therapeutic agent. As with regular insulin, the time of onset, time to peak, and duration of action are dose-dependent. 

However, in the PSCO system, the delayed disruption ( blistering ) of the interface with large crystals appeared to be the primary source of destabilization. Interfacial blistering was also observed in the lard droplets, although much later than in the emulsified PSCO. This visual observation coincided with SFC and particle size measurements that indicated the SFC of the PSCO droplets had stabilized prior to the rapid increase in droplet size and destabilization. 

All of the above data refer to dilute solutions the effect of solute concentration on the phosphorescence emission yield and lifetime is very marked. At concentrations of IM and above, a decrease in Xp and the x ratio has been reported for EPA (214), EOA (144,152), methylcyclohexane (219), and cyclohexane (207). Phosphorescence is negligible in both pure benzene crystals (225) and polycrystalline powder (144,214). This has been attributed to a rapid triplet-triplet annihilation process, an explanation apparently confirmed by Cundall and Pereira (219), who detected solution some slight rather diffuse phosphorescence together with delayed fluorescence from pure benzene. 

The ettringite crystals on the surface of the cement particles are so fine that they cannot bridge the gap between the cement particles and therefore do not form a solid structure. This is the basis of the delay in solidification upon adding gypsum to cement. Without gypsum the tricalcium aluminate immediately reacts with water to calcium aluminate hydrate, which fills the space between the cement particles with its large crystals and leads to very rapid solidification of the cement slurry. 

Melhaeholinc chloride occurs as colorlc.ss or while crystals or as a while cry.slalline powder. It is rxiorless or has a. slight odor and is very delique.secnl. It is freely soluble in water, alcohol, or chloroform, and its aqueous solution is neutral to litmus and biller. It is hydrolyzed rapidly in alkaline solutions. Solutions arc relatively. stable to heal and will keep for at least 3 or 3 weeks when refrigerated to delay growth of molds. 

A mechanism for oiling out can be postulated as follows When supersaturation is achieved rapidly such that the concentration is beyond the upper metastable limit—as can often be the case in a nucleation-based process—the substrate is forced to separate into a second phase by the creation of the resulting high solution concentration. However, crystallization is delayed by a slow crystallization rate. This combination may result in the creation of a nonstructured oil or possibly an amoiphous solid. The rates of phase separation and nucleation are relative to each other such that slow nucleation implies only that nucleation was not fast enough to create discrete particles before oil separation. 

Spin-lattice relaxation times, nTj, for different phases of various nylon samples are listed in Table 6. The a crystal form of both nylon-6 and nylon-11 exhibit a single exponential decay function in a plot of signal intensity vs delay x, with corresponding nTt values in the 103 s range. The sample prepared in situ has the shortest Tj relaxation time which indicates more rapid motions due either to plasticization by residual caprolactam or differences in the crystalline regions. 

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