Fiber crystallinity procedure

Microscopy (qv) plays a key role in examining trace evidence owing to the small size of the evidence and a desire to use nondestmctive testing (qv) techniques whenever possible. Polarizing light microscopy (43,44) is a method of choice for crystalline materials. Microscopy and microchemical analysis techniques (45,46) work well on small samples, are relatively nondestmctive, and are fast. Evidence such as sod, minerals, synthetic fibers, explosive debris, foodstuff, cosmetics (qv), and the like, lend themselves to this technique as do comparison microscopy, refractive index, and density comparisons with known specimens. Other microscopic procedures involving infrared, visible, and ultraviolet spectroscopy (qv) also are used to examine many types of trace evidence. 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

Crystal ins. Major water-soluble structural proteins found in fiber cells of vertebrate eye lenses account for the transparency of the lens. Heterogeneous family composed of (our groups a, 0. y, S which have been separated on the basis of size, charge, immunological properties and source. isolation procedures A. Spector. Invest. 

This is used when polymers are readily melted without degradation and the molten polymer is forced through a spiimeret comprising 50 to 1000 fine holes. On emerging from the holes, the threads sohdify, often in an amorphous glassy state, and are wound into a yam. Orientation and crystallinity are important requirements in fibers, and the yam is subjected to a drawing procedure that orients the chains and strengthens the fiber. This technique is applied to polyesters, polyamides, and polyolefins. 

The degree of crystallinity of polyamide fibers may be estimated from density determinations, calorimetric measurements, and infrared and x-ray data. Although not an absolute method, assessment of the degree of crystallinity from the density is a very facile, rapid, and precise procedure. It is independent of orientation or geometry of the sample, but requires dry samples that are free of voids and pigments. This method is based on the assumption that the density p or its reciprocal value and the specific volume V are represented by Equation 2.85 and Equation 2.86, respectively. 

In native cellulose, the structure develops under conditions of thermodynamic equilibrium and occurs very slowly. For regenerated cellulose, however, not only must the structure be formed rapidly, but also the organization of the macromolecules by crystallization is constrained by the extent of tangling present in the solution. It was suggested by Baker [261] that the structure of cellulose derivatives could be represented by a continuous range of states of local molecular order rather than by definite polymorphic forms of cellulose. This view is supported by the observation that the x-ray diffraction pattern of rayon often reveals both cellulose II and IV components to an extent, depending on the conditions used to make the fiber. Hindeleh and Johnson [262] have described an x-ray diffraction procedure to measure crystallinity and crystallite size in cellulose fibers by which the relative proportions of cellulose II and IV in rayon can be determined. 

X-ray diffraction patterns of fibers spun from liquid crystalline melts of p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) show a high degree of axial orientation. Several meridional maxima are detected which are aperiodic and also change in position and number with the monomer composition. The positions of these maxima can be predicted by calculating the theoretical scattering of random copolymer chains, in which the residues are represented by points separated by the monomer lengths. Both peak positions and intensities are reproduced when intraresidue interferences are allowed for in an atomic model for the random chains. This procedure also allows determination of the stiff-chain persistence (or correlation) length from the breadth of the maximum at d=2.lA which increases from 9 to 13 residues as the HBA content is increased from 25 to 75%. 

The effect of chemical modification of cellulose nanofibers on the properties of PLA nanocomposites has been investigated (44). Ac-etylated nanofibers, with a degree of substitution 1.07, were isolated from acetylated kenaf fibers by a mechanical procedure. The acet-ylated nanofibers were found to be more hydrophobic. Both crystallinity and thermal stability of acetylated nanofibers were lower in comparison to the non-acetylated nanofibers. 

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