Solution Crystallized

Wunderlich et al. have also reported the preparation and sepeiration of extended chain crystals of polyethylene from the melt at 510 K and 500 MN m and separating them by etching with fuming nitric acid. The product was, of course, an a,(o-dicarboxylic acid with a molecular length of about 900 —CH — units. Cavello et have grown crystals of random isotactic copolymers of propylene and but-l-ene from isoamyl acetate solution. By comparison with melt-crystallized materials they concluded that both morphologies, and hence crystallizations, were fundamentally identical, and that the co-monomer unit co-crystallizes. There is a depression of the melting point over that of the homopolymer and a eutectic is observed at 48% butene content. 

Macroscopic crystals have been prepared in the polymerization of diphenyl-silylene with an extended chain configuration. However, these crystals do not reform from the melt. Poly(phenylacetylene) macroscopic crystals continue to attract a great deal of interest, particularly from the point of view of their electrical conductivity.  

Melt Crystallized.—Spherulites are observed to consist of radiating fibrils or lamellae which in melt-crystallized polymers are 100—500 A thick, depending on crystallization temperature, and /um in breadth. The c axis, i.e, the chain direction, coincides with the thickness of the lamellae and, by analogy to single crystal morphology, the molecular chain folds backwards into the lamellae. Amorphous polymer fills the regions between lamellae and can account for a substantial amount of the content. Painter et al. have shown that these inter-lamellar regions are very similar to melt-crystallized polymer in their i.r. spectrum. 

There can be little doubt that these conclusions are correct for quenched crystallized polyethylene, but slow-crystallized material can reject non-crystal-lizable high molecular weight material, as was first shown by Keith and Padden, and considerable rearrangement must be possible within the time scale of crystallization. Unfortunately, to date, slow crystallization cannot be studied by neutron scattering experiments due to molecular aggregation. 

Keller et have attributed similar observations to kinetic effects, arguing that the nucleation density is sensitive to the pre-heating temperature and duration, and that the density essentially determines rate of crystallization and so crystallization temperature, the lamellae thickness then being solely determined by crystallization temperature. 

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Solution crystalli tion (adiabatic evaporation (vacuum cooling))... 

Secondary nucleation is crystal formation through a mechanism involving the solute crystals crystals of the solute must be present for secondary nucleation to occur. Thorough reviews have been given (8,9). 

Crystallization from Solution. Crystallization techniques are related to the methods used to iaduce a driving force for soflds formation and to the medium from which crystals are obtained. Several approaches are defined ia the foUowiag discussion. 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

Rawlings, J.B., Miller, S.M. and Witkowski, W.R., 1993. Model identification and control of solution crystallization processes A review. Industrial and Engineering Chemistry Research, 32, 1275-1296. 

Add a little caustic potash solution. Crystals of potassium oxalate are deposited. The ester is hydrolysed. 

To a suspension containing 4.86 parts of 4-methylbenzenesulfonyl urethane (MP 80° to 82°C) and 36 parts of anhydrous toluene there are rapidly added 2.5 parts of N-amino-3-azabicyclo(3.3.0)octane (BP/18 mm = 86°C). The reaction mixture is heated under reflux for 1 hour. The resulting ciear solution crystallizes on cooling. The crystals are filtered, washed with 2 parts of toluene, then recrystallized from anhydrous ethanol. There are obtained 3.8 parts of the desired product, MP 180° to 182°C. 

For PVDF, several experiments have shown that at room temperature the p form is thermodynamically the most stable, while the ot form is kinetically the most advantageous. For instance, by solution crystallization (casting from polar hexamethylphosphamide solutions) p form, y form and a form crystals are obtained for low, intermediate and high evaporation rates, respectively [15, 66]. 

A lot of emphasis is laid on three possible basic interferences during the application process (1) single constituents of the sample solution crystallize out, (2) the property of the solvent used for dissolution, and (3) the concentration of the solution generating an imacceptably broad application zone. Such interferences could be avoided with proper planning before application. 

Two kinds of solid solutions crystallize, a solution of metal 1 in metal 2 and vice versa (limited miscibility). 

Describe each of the solutions indicated as saturated, unsaturated, supersaturated, or impossible to tell, (a) More solute is added to a solution of that solute, and the additional solute all dissolves. Describe the original solution. (6) More solute is added to a solution of that solute, and the additional solute does not all dissolve. Describe the final solution, (c) A solution is left standing, and some of the solvent evaporates. After a time, some solute crystallizes out. Describe the final solution, (d) A hot saturated solution is cooled slowly, and no solid crystallizes out. Describe the cold solution, (e) A hot solution is cooled slowly, and after a time some solid crystallizes out. Describe the cold solution. (/) A hot saturated solution is cooled slowly, and no solid crystallizes out. The solute is a solid that is more soluble hot than cold. Describe the cold solution. 

Nadagouda, M.N. and Varma, R.S. (2008) Microwave-assisted shape-controlled bulk synthesis of Ag and Fe nanorods in polyfethylene glycol) solutions. Crystal Growth and Design, 8, 291-295. 

Very shock-sensitive and explodes readily when precipitated from aqueous solution. Crystals obtained by slow evaporation of a carbon tetrachloride extract were less sensitive. 

The residual amine in the filtrate may be isolated in the form of the hydrochloride. The combined solutions are evaporated on a steam bath, 50 ml. of concentrated hydrochloric acid is added, and heating is continued for 2 hours. On cooling, the syrupy solution crystallizes. It is triturated with 50 ml. of ethanol, and the 4-amino-l,2,4-triazole hydrochloride is filtered, washed with a little ethanol, and dried. The yield of the hydrochloride is 10-18 g. (8-15%) the salt melts at 147-148° and may be recrystallized from 95% ethanol, using 10 ml. per gram the melting point is thus raised to 151-152°. 

Fig. 5 Initial fold length L against undercooling AT = To - T for both melt- and solution-crystallized polyethylene, from different solvents [16]. Dashed line gives previous calculations (Model A [9], old model in Fig. 4), solid line shows the results from the new model in Fig. 4 after re-adjusting the energy of fusion per - CH2 - unit from E= 1.07 to E- 1.42 kcal/mol...

The d,L-arabitol pentaacetate was hydrolyzed by refluxing it for three hours with methanol containing an excess (7 moles) of hydrogen chloride. After concentration to one-half its volume the solution crystallized spontaneously. By collecting the product before crystallization was too far advanced and washing it with a little fresh methanol, crystals melting at 105-106° were obtained. This is the melting point of d,L-arabitol as first recorded by Ruff.1 ... 

The triselenadiborolanes 3,5-R2-l,2,4,3,5-Se3B2 R=Et (33), Pr readily formed coordination adducts with two equivalents of pyridine, 3,5-dime-thylpyridine, and 3-chloropyridine.168 With one equivalent of base, only one of the B atoms became coordinated, and surprisingly, the system was not fluxional at room temperature.168 The addition of two equivalents of pyrazole to 33 (Scheme 7) resulted in a brown suspension and a yellow solution. Crystals of a B2N4Se2-bicyclo[2.2.2]octane were formed upon cooling this solution to —80 °C. With bulkier pyrazole derivatives (phenyl-pyrazole), the B2N4Se-bicyclo[2.2.1] heptanes were formed.169... 

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