Microfibril diameter, average

This surface energy component is apparently inversely proportional to the average microfibril diameter, it is suggested that a maximum fibril diameter limit exists such that, above this limit, hard elastic behavior will not occur. 

Figure 1. Freeze-dried gel of A. xylinum cellulose ribbons deposited during normal growth. The arrows point to triple-stranded left-hand helical microfibrils averaging 36.8 3A in diameter (1). The sample was replicated with 17.3A Pt-C and backed with 90.2A of carbon. 

The maximum energy released during the primary opening of such a microcrack with average dimensions about 100 A (average diameter of microfibril) is about 10" erg if one takes for the chain E =... 

Hence, a maximum fibril diameter limit exists where "microfibrillar" behavior occurs. By microfibrillar behavior, we refer to their high elasticity [12] and large surface energy component in their stress. It is our belief that hard elastic behavior is a bulk manifestation of the mechanical properties of microfibrils. Thus, above a certain average fibril diameter level, hard elastic behavior will not occur. This bulk-to-microfibril transition diameter is, no doubt, characteristic of the particular polymer. Determination of this critical fibril diameter in the future will lead to greater understanding of the structural criterion for hard elastic behavior. 

Hard elastic behavior is a manifestation of a bulk-microfibril superstructure. A substantial surface energy component of the stress exists in these materials, independent of strain at high tension. As a result, significant changes in the equilibrium stress occurs when the polymers, under load, are subjected to changes in environmental surface tension. An apparent requirement for this surface tension component is load bearing microfibrils with sufficiently small radii. Evidently, a maximum average fibril diameter exists whereby hard elastic behavior may occur in polymers with these structures. 

Uniaxially oriented isotactic polypropylene (iPP) was imaged using afm, showing microfibrils and macromolecules. Fibrils with an average diameter of 150 nm were observed. Individual polymer chains with 1.17 nm chain-chain distance were seen. The authors propose that the (110) crystal plane was being resolved with this work (90). Other workers, who were able to clearly resolve right-and left-handed helices (Fig. 13) with pendant methyl groups visible, accomplished atomic scale resolution of iPP (91). 

Microfibrillar cellulose refers to isolated cellulose microfibrils or microfibril bundles that are derived from a cellulose raw material. Microfibrils typically have a high aspect ratio. The niunber average diameter is typically below 200 nm. The smallest microfibrils are similar to so-called elementary fibrils, which are typically 2-12 nm in diameter. 

Cellulose in cotton is synthesized by the condensation of glucose monomers at enzyme complexes. Each enzyme complex produces 30 cellulose chains, which lie in the same direction to crystallize into long microfibrils. The microfibrils have an average diameter of several nanometers. From this aspect, the cellulose in cotton is considered to be 100% ciystalhne since all cellulose chains contribute to the formation of microfibrils. However, the density of cotton is lower than the crystal density, and experimental measirrements using diffraction, spectroscopic, thermal and other techniques show cotton is about 60% crystalline. This can be explained by the imperfect packing when the microfibrils are formed. Microfibrils consisting of aligned cellirlose chairts are the main structural components of cotton fibers. 

The cortical layer cells are composed of densely located cylindrical, thread-like macrofibrils of about 0.05-0.2 pm in diameter. Macrofibrils of the cortical layer are composed of microfibrils with the average diameter of 7-7.5 run [26, 27]. 

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