Crystallinity detection

This chapter discusses the topics of crystallinity detection and measurement, the morphology of polymer crystalline aggregates at different levels of size and the kinetics of melt-crystal phase transformation. Chapter 3 presents thermodynamic aspects of crystallinity in polymers. The treatment is intended to provide a broad... 

NH4][ON(NO)C6Hj]. a reagent originally suggested for use in the detection of Cu but now used for the separation of Fe/Ti and Zr which it precipitates from acid solutions. Cupferron is a brownish-while crystalline substance, soluble in water. 

Nitro compounds, when hquid, have characteristic odours, are insoluble in water, highly refractive and with a density greater than unity. Many are crystalline sohds. Most nitro compounds are slightly coloured, generally yellow the intensity of the colour increases with the number of nitro groups. The following reactions will assist in their detection. 

Aromatic carboxylic acids are usually crystalline sohds, bum with a smoky flame, and are generally sparingly soluble in water. They may be detected and characterised as already described under Aliphatic Carboxylic Acids (Section 111,85). 

NH3 Ag+ Br- H+ I- SiOj (amorphous) Detection of Br Detection of Ag Detection of I and Br Differentiation of crystalline and amorphous Si02 (with CrO )... 

In milk fat, cholesterol is associated with Hpoproteins in the milk fat globule. It is also a component of animal membranes and controls rigidity and permeabihty of the membranes. Cholesterol has interesting surface properties and can occur in Hquid crystalline forms. Plants contain sterols such as P-sitosterol [83-46-5] (4b) or stigmasterol [83-48-7] (4c). Their functions in plant metaboHsm are not yet well understood. Analysis of sterols has proven useful for detection of adulteration of edible fats (9). 

Transmission electron microscopy (tern) is used to analyze the stmcture of crystals, such as distinguishing between amorphous siUcon dioxide and crystalline quartz. The technique is based on the phenomenon that crystalline materials are ordered arrays that scatter waves coherently. A crystalline material diffracts a beam in such a way that discrete spots can be detected on a photographic plate, whereas an amorphous substrate produces diffuse rings. Tern is also used in an imaging mode to produce images of substrate grain stmctures. Tern requires samples that are very thin (10—50 nm) sections, and is a destmctive as well as time-consuming method of analysis. 

Sohd ammonium nitrate occurs in five different crystalline forms (19) (Table 6) detectable by time—temperature cooling curves. Because all phase changes involve either shrinkage or expansion of the crystals, there can be a considerable effect on the physical condition of the sohd material. This is particularly tme of the 32.3°C transition point which is so close to normal storage temperature during hot weather. 

Acesulfame-K is a white crystalline powder having a long (six years or more) shelf life. It readily dissolves in water (270 g/L at 20°C). Like saccharin, acesulfame-K is stable to heat over a wide range of pH. At higher concentrations, there is a detectable bitter and metallic off-taste similar to saccharin. Use of the sodium salt of feruHc acid [437-98-4] (FEMA no. 3812) to reduce the bitter aftertaste of acesulfame-K has been described (64). The sweetness potency of acesulfame-K (100 to 200x, depending on the matching sucrose concentration) (63) is considered to be about half that of saccharin, which is about the same as that of aspartame. 

Copolymerisation also affects morphology under other crystallisation conditions. Copolymers ia the form of cast or molded sheets are much more transparent because of the small spheruHte size. In extreme cases, crystallinity cannot be detected optically, but its effect on mechanical properties is pronounced. Before crystallisation, films are soft and mbbery, with low modulus and high elongation. After crystallisation, they are leathery and tough, with higher modulus and lower elongation. 

In crystallizing fatty acids, solvent polarity does not influence crystal form as much as temperature and concentration (9). Infrared (9,10) and wide-line nmr spectra (11) as well as x-ray methods (12,13) can be used to detect the various crystalline forms. 

Polymerization. Chloroprene is normally polymerized with free-radical catalysts in aqueous emulsion, limiting the conversion of monomer to avoid formation of cross-linked insoluble polymer. At a typical temperature of 40°C, the polymer is largely head-to-taH in orientation and trans in configuration, but modest amounts of head-to-head, cis, 1,2, and 3,4 addition units can also be detected. A much more regular and highly crystalline polymer can be made at low temperature (11). Chloroprene can also be polymerized with cationic polymerization catalysts, giving a polymer with... 

Effects of Impurities nd Solvent. The presence of impurities usually decreases the growth rates of crystalline materials, and problems associated with the production of crystals smaller than desired are commonly attributed to contamination of feed solutions. Strict protocols should be followed in operating units upstream from a crystallizer to minimize the possibiUty of such occurrences. Equally important is monitoring the composition of recycle streams so as to detect possible accumulation of impurities. Furthermore, crystalliza tion kinetics used in scaleup should be obtained from experiments on solutions as similar as possible to those expected in the full-scale process. 

Crystallinity in ECH and ECH—EO finished products increases over time, and may be detected by x-ray analysis or differential scanning calorimetry. In synthesizing ECH—EO, the process is designed to maximize random monomer sequence and minimize crystallinity. The ECH—EO molecular ratio in these products ranges from approximately 3 1 to 1 1. 

The detection of Hquid crystal is based primarily on anisotropic optical properties. This means that a sample of this phase looks radiant when viewed against a light source placed between crossed polarizers. An isotropic solution is black under such conditions (Fig. 12). Optical microscopy may also detect the Hquid crystal in an emulsion. The Hquid crystal is conspicuous from its radiance in polarized light (Fig. 13). The stmcture of the Hquid crystalline phase is also most easily identified by optical microscopy. Lamellar Hquid crystals have a pattern of oil streaks and Maltese crosses (Fig. 14a), whereas ones with hexagonal arrays of cylinders give a different optical pattern (Fig. 14b). 

Peroxide Formation. Except for the methyl alkyl ethers, most ethers tend to absorb and react with oxygen from the air to form unstable peroxides that may detonate with extreme violence when concentrated by evaporation or distillation, when combined with other compounds that give a detonable mixture, or when disturbed by heat, shock, or friction. Appreciable quantities of crystalline soHds have been observed as gross evidence for the formation of peroxides, and peroxides may form a viscous Hquid in the bottom of ether-fiHed containers. If viscous Hquids or crystalline soHds are observed in ethers, no further tests for the detection of peroxides are recommended. Several chemical and physical methods for detecting and estimating peroxide concentrations have been described. Most of the quaHtative tests for peroxides are readily performed and strongly recommended when any doubt is present (20). 

Analytical applications of pyrazolones have been reviewed by Busev et al. (65RCR237). Organic bases are easily characterized by formation of highly crystalline salts with picrolonic acid (l-(4-nitrophenyl-3-methyl-4-nitro-5-hydroxypyrazole). The last-named compound is used as a reagent for alkaloids, tryptophan, phenylalanine and for the detection and estimation of calcium (B-76MI40404). 

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