Rod-like crystallites

The exact length of the cellulose molecule is not known but it is variable and very large. Cellulose threads possess micellar structure and consist(according to x-ray diagrams) of numerous rod-like crystallites, which are oriented with their long axis parallel to the thread axis, forming a fiber... 

Aqueous suspensions of cellulose microcrystalhtes obtained by acid hydrolysis of native cellulose fibers can also produce a cholesteric mesophase [ 194]. Sulfuric acid, usually employed for the hydrolysis, sulfates the surface of the micro crystallites and therefore they are actually negatively charged. Dong et al. performed some basic studies on the ordered-phase formation in colloidal suspensions of such charged rod-like cellulose crystallites (from cotton filter paper) to evaluate the effects of addition of electrolytes [195,196]. One of their findings was a decrease in the chiral nematic pitch P of the anisotropic phase, with an increase in concentration of the trace electrolyte (KC1, NaCl, or HC1 of < 2.5 mM) added. They assumed that the electric double layer on... 

It may be ccmsidered that the molecular cluster in which rod-like molecules align nearly parallel to the cluster axis, itself forms a crystallite. The intensity of the X-ray diffraction for the incident beam that is perpendicular to the magnetically oriented fflm of polypeptide, /, is well expresred in the form (38) ... 

The order parameter s finks R to the mosaic distribution of the azimuthal rotation angle about the surface normal (). The former is measured using optical absorption spectroscopy whereas the latter is measured separately for each crystallite types using GIXRD. In this task it has been assumed that the rod-like molecules are always parallel to the (0yz) plane (i.e., perfectly planar alignment) and a two-dimensional order parameter can be given as... 

Solution properties indicate that the metal-poly(yne)s exist in a rod-like structure. They display lyotropic nematic mesophases [30] and form crystallites with a diameter of up to 50 nm [31]. Absorption and luminescence spectra of the polymers show that 7t-electron conjugation is expanded over the whole polymer chain, and third-order non-linear optical properties are exhibited. For polymer 7 good photocurrents were found in sandwich-diodes. 

As previously mentioned, natural fibres present a multi-level organization and consist of several cells formed out of semi-crystalline oriented cellulose micro fibrils. Each microfibril can be considered as a string of cellulose crystallites, linked along the chain axis by amorphous domains (Fig. 19.10) and having a modulus close to the theoretical limit for cellulose. They are biosynthesized by enzymes and deposited in a continuous fashion. A similar structure is reported for chitin, as discussed in Chapter 25. Nanoscale dimensions and impressive mechanical properties make polysaccharide nanocrystals, particularly when occurring as high aspect ratio rod-like nanoparticles, ideal candidates to improve the mechanical properties of the host material. These properties are profitably exploited by Mother Nature. 

Stable aqueous suspensions of polysaccharide nanocrystals can be prepared by the acid hydrolysis of vegetable biomass. Different descriptors of the resulting colloidal suspended particles are used, including whiskers, mono-crystals and nanocrystals. The designation whiskers is used to describe elongated rod-like nanoparticles. These crystallites have also often been referred in the literature as microfibrils, microcrystals or microcrystallites, despite their nanoscale dimensions. Most of the studies reported in the literature refer to cellulose nanocrystals. A recent review described the properties and applications of cellulose whiskers in nanocomposites [31]. 

Ljungberg N, Cavaille J-Y, Heux L (2006) Nanocomposites of isotactic polypropylene reinforced with rod-like cellulose whiskers. Polymer 47 6285-6292 Lu Y, Weng L, Cao X (2005) Biocomposites of plasticized starch reinforced with cellulose crystallites from cottonseed linter. Macromol Biosci 5 1101-1107 Lu J, Wang T, Drzal LT (2008) Preparation and properties of microfibrillated cellulose polyvinyl alcohol composite materials. Compos Part A 39A 738-746 Magalhaes WLE, Cao X, Lucia LA (2009) Cellulose nanocrystals/cellulose core-in-shell nanocomposite assemblies. Langmuir. doi 10.1021Aa901928j Malainine ME, Mahrouz M, Dufresne A (2005) Thermoplastic nanocomposites based on cellulose microfibrils from Opuntiaficus-indica parenchyma cell. Compos Sci Technol 65 1520-1526 Marchessault RH, Sundararajan PR (1983) Cellulose. In Aspinall GO (ed) The polysaccharides. Academic, New York... 

Optically, a nematic phase can be uniaxial or biaxial. The latter is formed by elongated lath-like molecules. Conventional nematic liquid crystals formed by rod-like molecules constitute a uniaxial medium with nonpolar symmetry. The constituent molecules rotate (freely or hindered) around both their short and long axes. Nonoriented samples are analogous in some measure to polycrystalline powders they consist of individual (liquid) crystallites, each of which has as a definite feature, a directed optical axis, which... 

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