Micro fibrils formation

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

Figure 5.8 shows the electron micrographs o( PTOX irradiated with various doses. The micro-fibrils of PTOX change into a mosaic-like stack of crystallites by irradiation, and the larger the dose the more imiform the observable stacking. These patterns as well as the SAXS patterns in Fig. 5.1 indicate the formation of a layer-like void perpendicular to the o-axis of fibrillair crystals with a period of about 1000 A. 

The template-assisted synthetic strategies outlined above produce micro- or mesoporous stmetures in which amorphous or crystalline polymers can form around the organic template ligands (174). Another approach is the use of restricted spaces (eg, pores of membranes, cavities in zeolites, etc.) which direct the formation of functional nanomaterials within thek cavities, resulting in the production of ultrasmaU particles (or dots) and one-dimensional stmetures (or wkes) (178). For example, in the case of polypyrrole and poly(3-methylthiophene), a solution of monomer is separated from a ferric salt polymerization agent by a Nucleopore membrane (linear cylindrical pores with diameter as small as 30 nm) (179—181). Nascent polymer chains adsorb on the pore walls, yielding a thin polymer film which thickens with time to eventually yield a completely filled pore. De-encapsulation by dissolving the membrane in yields wkes wherein the polymer chains in the narrowest fibrils are preferentially oriented parallel to the cjlinder axes of the fibrils. 

This phenomenon was discussed in terms of the possible direct correlations between stepwise creep and fiber morphology, namely, of micro-shear displacements of various fibrillar elements in a stick-slip mode. It was shown that the fibrillar units were weakly connected and loosely packed in these fibers interfibrillar regions contained pores and a small number of tie molecules. The length of microfibrils has been estimated to be microns, whereas the length of macrofibrils reached lOOpm and more. These sizes correlate satisfactorily with the observed deformation steps. Of course, this approach (slippage of fibrils upon creep) did not exclude the participation of the intracrystalline slip events and the process of scission of overstressed interfibrillar tie molecules in jump-like creep. Submicro- and microcrack formation could also contribute, to some extent, to creep heterogeneity and the total deformation of fibers [314]. 

Figure 2.1 Scanning electron microscopy (SEM) images of Gluconacetobacter xylinus and BC network of micro and nano fibrils and schematic description of the formation of bacterial cellulose. Reproduced with permission from [7].

In conclusion, the appearance of the micrometer-sized structures formed by the xylans seems to be related to the morphology of the cellulose substrate rather than the degree of crystallinity. The crystal structure of the cellulose substrate (cellulose I or II) is not unimportant but its influence is indirect, through the absence of fibril-like surface features that can induce the formation of xylan structures on the regenerated substrates (cellulose II). On a nanometer scale, the xylan layer looks similar on all of the cellulose substrates, supporting the conclusion that the cellulose surfaces studied are different on the micro scale but quite similar on the nano and molecular levels. 

Damage under stress can produce cracks in the material. Crazing concerns the areas constituted of vacuum and highly oriented fibrils in the polymer. These can lead to the formation of a micro-cracks and cracks. 

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