Crystallinity of natural

Kerkam, K., Viney, C., Kaplan, D., and Lombardi, S. (1991). Liquid crystallinity of natural silk secretions. Nature 349, 596-598. 

The degree of molecular orientation, that is, degree of crystallinity, of natural products is also dependent on the source of the products. For example, cellulose in cotton fiber has a degree of crystallinity of about 80 percent. Celluloses from other sources are less molecularly oriented. The degree of molecular o-rientation in cellulose as well as crystal lattice type... 

The effect of surface treatment on the crystallinity of natural fiber has been explored by means of X-ray diffraction analysis [134]. ft was reported that the crystaUinity index of untreated hemp fiber was 84.7%. The value was increased to 89.6% for NaOH-treated hemp and 89.1% for Na0H/Na2S03-treated hemp. 

Different atmospheric conditions also result in different microstructures. For example, in the case of a planetary mill where the deformation forces are mainly shear in nature, mechanically induced oxidation on the surface, probably along the edges of the graphene planes, suppresses the fracture rate and preserves the crystallinity of natural graphite milled in oxygen. 

The way the chemist knows that she has methylamine and not ammonium chloride is that she compares the look of the two types of crystals. Ammonium chloride crystals that come from this reaction are white, tiny and fuzzy. The methylamine hydrochloride crystals are longer, more crystalline in nature and are a lot more sparkly. The chemist leaves the methylamine crystals in the Buchner funnel of the vacuum filtration apparatus and returns the filtrate to the distillation set up so it can be reduced one last time to afford a second crop. The combined methylamine hydrochloride filter cake is washed with a little chloroform, scraped into a beaker of hot ethanol and chilled. The methylamine hydrochloride that recrystallizes in the cold ethanol is vacuum filtered to afford clean, happy product (yield=50%). 

The qua si-crystalline stmcture of natural starch granules causes them to be insoluble in water at normal room temperature and gives them relative resistance to carbohydrases other than a-amylase and glucoamylase unless the granules become swollen. Three-dimensional arrangements of crystalline and amorphous zones in starch granules have been suggested (2). 

AH graphite has crystal stmcture but only certain kinds and sizes of natural graphites are commercially classified as crystalline, a term used for import duty purposes. Throughout this article reference is made separately to dake, vein (lump or high crystalline), and amorphous forms, all of which are essentially the same crystalline form of carbon. However, fine stmctured graphites (cryptocrystalline (2)) have been classified as amorphous. 

Molecular sieves are an adsorbent that is produced by the dehydration of naturally occurring or synthetic zeolites (crystalline alkali-metal aluminosilicates). The dehydration leaves inter-crystalline cavities into which normal paraffin molecules are selectively retained and other molecules are excluded. This process is used to remove normal paraffins from gasoline fuels for improved combustion. Molecular sieves are used to manufacture high-purity solvents. 

There is a world shortage of natural graphite which is particularly marked in North America and Europe. As a result, prices have risen steeply they vary widely in the range 500-1500 per tonne (1989) depending on crystalline quality amorphous graphite is 220-440 per tonne. The annual world production of 649 ktonnes was distributed as follows in 1988 China 200kt, South Korea 108, the former Soviet Union 84, India 52, Mexico 42, Brazil 32, North Korea 25, Czechoslovakia 25, Others 81 kt. 

Some of them were obtained for the first time by an enzymatic procedure which, of course, can result only in the aza analogs of natural nucleosides, i.e., ribofuranosyl-6-azauracil (6-azauridine) (75) and 2 -deoxyribofuranosyl-6-azathymine (6-azathymidine). The first of these was prepared by Skoda et ai. and a modification of their procedure was used by Handschumacher, In this way it is possible to obtain the crystalline nucleoside on the large scale. 

Indeed, great emphasis was placed on the presentation of compounds in crystalline form for many years, early chromatographic procedures for the separation of natural substances were criticized because the products were not crystalline. None the less, the invention by Tswett (3) of chromatographic separation by continuous adsorption/desorption on open columns as applied to plant extracts was taken up by a number of natural product researchers in the 1930s, notably by Karrer (4) and by Swab and lockers (5). An early example (6) of hyphenation was the use of fluorescence spectroscopy to identify benzo[a]pyrene separated from shale oil by adsorption chromatography on alumina. 

One may now ask whether natural systems have the necessary structural evolution needed to incorporate high-performance properties. An attempt is made here to compare the structure of some of the advanced polymers with a few natural polymers. Figure 1 gives the cross-sectional microstructure of a liquid crystalline (LC) copolyester, an advanced polymer with high-performance applications [33]. A hierarchically ordered arrangement of fibrils can be seen. This is compared with the microstructure of a tendon [5] (Fig. 2). The complexity and higher order of molecular arrangement of natural materi-... 

TPEs from blends of rubber and plastics constitute an important category of TPEs. These can be prepared either by the melt mixing of plastics and rubbers in an internal mixer or by solvent casting from a suitable solvent. The commonly used plastics and rubbers include polypropylene (PP), polyethylene (PE), polystyrene (PS), nylon, ethylene propylene diene monomer rubber (EPDM), natural rubber (NR), butyl rubber, nitrile rubber, etc. TPEs from blends of rubbers and plastics have certain typical advantages over the other TPEs. In this case, the required properties can easily be achieved by the proper selection of rubbers and plastics and by the proper change in their ratios. The overall performance of the resultant TPEs can be improved by changing the phase structure and crystallinity of plastics and also by the proper incorporation of suitable fillers, crosslinkers, and interfacial agents. 

Compatibility and various other properties such as morphology, crystalline behavior, structure, mechanical properties of natural rubber-polyethylene blends were investigated by Qin et al. [39]. Polyethylene-b-polyiso-prene acts as a successful compatibilizer here. Mechanical properties of the blends were improved upon the addition of the block copolymer (Table 12). The copolymer locates at the interface, and, thus, reduces the interfacial tension that is reflected in the mechanical properties. As the amount of graft copolymer increases, tensile strength and elongation at break increase and reach a leveling off. 

The crystallinity of artificial graphite can generally be controlled by the heat-treatment temperature, but it is lower than that of natural graphite. The Lc of artificial graphite is less than 1000 A and the d value is more than 3.36 A. 

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