Microfibrils

The microfibrils iu vegetable fibers are spiral and parallel to one another iu the cell wall. The spiral angles iu flax, hemp, ramie, and other bast fibers are lower than cotton, which accounts for the low extensibiUty of bast fibers. 

As a result, leather is made up of interlaced bundles of coUagen fibers (Fig. 1). A schematic model of coUagen bundles in leather is shown in Figure 2 (4). A coUagen bundle (about 80 )Tm in diameter) is made up of coUagen fibers (1—4 pm), composed of microfibrils (0.08—0.1 pm). Furthermore, a microfibril consists of many protofibrils (about 1.5 nm), which consist of several bundles of polypeptide chains. 

Microfibrils are laid down as ribbons having their flat faces parallel to the cell periphery. HemiceUuloses and lignin are deposited between the microfibrils... 

Fig. 3. A cross-section of a nearly square cellulose microfibril, with the individual molecular chains shown as rectangles. Also shown are the one- and two-chain unit cells of la and ip. This view of the microfibril is parallel to the long axis. The chains are arranged so that the edges of the crystal correspond...

The superimposition of diffraction spots from both phases gives the previously reported pattern that was thought to require an eight-chain unit cell. In the la stmcture, because of its one-chain unit cell, all chains must have parallel packing. Since the la and ip stmctures exist in the same microfibril of cellulose, the chains in the ip stmcture should also be parallel. 

A theoretical model whereby maximum peak capacity could be achieved by the use of 3-D planar chromatographic separation was proposed by Guiochon and coworkers (23-27). Unfortunately, until now, because of technical problems, this idea could not be realized in practice. Very recently, however, a special stationary phase, namely Empore silica TLC sheets, has now become available for realization of 3-D PC. This stationary phase, developed as a new separation medium for planar chromatography, contains silica entrapped in an inert matrix of polytetrafluoroethy-lene (PTFE) microfibrils. It has been established that the separating power is only ca. 60% of that of conventional TLC (28) this has been attributed to the very slow solvent migration velocity resulting from capillary action. 

Figure 6 shows SEM micrographs of the fracture surface after tensile test. For the immiscible Ultem/Vec-tra B blends, numerous microfibrils are observed to be pulled out from the surface (Fig. 6A), Many matrix voids generated by the pull-out of microfibrils reveal the poor...

Figure 13 Higher magnification of SEM photographs of nylon 6-TLCP-elastomer blend (x 10,000) (A) Dispersed fine microfibrils of TLCP, (B) cluster of TLCP microfibrils. Source Ref. 56.

Table 5 compares the tensile properties of Vectra A950 in the form of dispersed fibers and droplets in the matrix by injection molding, microfibril by extrusion and drawing [28], injection molded pure thick sample and pure thin sample, and the pure drawn strand [28]. As exhibited, our calculated fiber modulus with its average of 24 GPa is much higher than that of the thick and thin pure TLCP samples injection molded. It can be explained that in cases of pure TLCP samples the material may only be fibrillated in a very thin skin layer owing to the excellent flow behavior in comparison with that in the blends. However, this modulus value is lower than that of the extruded and drawn pure strand. This can be... 

The filaments of all plant fibers consist of several cells. These cells form crystalline microfibrils (cellulose), which are connected together into a complete layer by amorphous lignin and hemi-cellulose. Multiple layers stick together to form multiple layer composites, filaments. A single cell is subdivided into several concentric layers, one primary and three secondary layers. Figure 5 shows a jute cell. The cell walls differ in their composition and in the orientation of the cellulose microfibrils whereby the characteristic values change from one natural fiber to another. 

Noncrystalline separating layers of microfibrils are formed from two crystallite surface layers that are side by side in the microfibril and from a central part called the intermediate zones. The latter are mainly made up of tie molecules among which are distinct straightened... 

Figure 1 The structure of a microfibril. C - crystallite S -separating layer SL - surface layer IZ - intermediate zone mF - border of microfibril k - crystallite length U - separating layer length L - mean long period (spacing).

Microfibrils are formed in PET fibers during the stretching of a solidified polymer stream. With an increase in the draw ratio, microfibrils become increasingly slender. The microfibril length is assumed to be proportional to the draw ratio (R), their lateral dimension proportional to Jr, while the length-to-lateral dimension ratio is proportional to... 

Another type of fibril substructure in PET fibers, besides the microfibrillar type already discussed, is the lamellar substructure, also referred to as the lateral substructure. The basic structural unit of this kind of substructure is the crystalline lamella. Formation of crystalline lamellae is a result of lateral adjustment of crystalline blocks occurring in neighboring microfibrils on the same level. Particular lamellae are placed laterally in relation to the axis of the fibrils, which explains the name—lateral substructure. The principle of the lamellar substructure is shown in Fig. 2. 

The distinction between the types of substructures of the microfibrils occurring in the PET fiber is possible based on the value of the substructure parameter (yl). 

Figure 2 The lamellar substructure of a fibril. (a) Reciprocal positions of crystalline lamellae as a result of fiber annealing. (b) The situation after relaxation of stress affecting TTM. ai.2 - average angle of orientation of TTM CL - crystalline lamellae CB - crystalline blocks (crystallites) mF -border of microfibrils and F - fibril. In order to simplify it was assumed that (1) there are the taut tie molecules (TTM) only in the separating layers, (2) the axis of the fibril is parallel to the fiber axis.

The mesomorphous phase, also called an intermediate phase or a mesophase, is formed by molecules occurring in surface layers of the crystallites. It can be assumed that the mesophase is made up largely by regularly adjacent reentry folds. However, it cannot be excluded that the mesophase is also composed of some irregular chain folds, which are characterized by a long length and run near the crystal face in the direction perpendicular to the microfibril axis. 

From the foregoing it is clear that indentation anisotropy is a consequence of high molecular orientation within highly oriented fibrils and microfibrils coupled with a preferential local elastic recovery of these rigid structures. We wish to show next that the influence of crystal thickness on AMH is negligible. The latter quantity is independent on 1 and is only correlated to the number of tie molecules and inter-crystalline bridges of the oriented molecular network. 

---