Microfibrils formation

The al and a2 chains of type VI collagen are similar in size and contain one N-terminal NC domain and a C-terminal NC domain with two subdomains. Cl and C2. In contrast, the a3 chain is much longer, and the N-terminal NC domain contains 10 subdomains, Nl-NlO, and 5 C-terminal NC subdomains, Cl-C5. The protomer is an ala2a3 heterotrimer. Deletion studies have demonstrated that whereas the C5 subdomain of the a3 chain is required for the extracellular microfibril formation, the Cl subdomain, in all chains, is sufficient for chain recognition and protomer assembly... 

Attempts to examine the process of cellulose crystallization have frequently involved culturing Acetobacter in the presence of fluorescent brighteners, direct dyes, carboxy-methyl-cellulose, or other agents which compete for interchain hydrogen bond sites, thereby disrupting microfibril formation... 

Tobacco primary cell wall and normal bacterial Acetobacter xylinum cellulose formation produced a 36.8 3A triple-stranded left-hand helical microfibril in freeze-dried Pt-C replicas and in negatively stained preparations for transmission electron microscopy (TEM). A. xylinum growth in the presence of 0.25 mM Tinopal disrupted cellulose microfibril formation and produced a... 

Reinhardt, D. P., Gambee, J. E., Ono, R. N., Bachinger, H. P., and Sakai, L. Y. (2000b). Initial steps in assembly of microfibrils. Formation of disulfide-cross-linked multi-mers containing fibrillin-1./. Biol. Chem. 275, 2205-2210. 

B. A Model of Cellulose-Fibril Deposition During Secondary-wall Formation in Microsterias.7 [Each rosette is believed to form one 5-nm microfibril. A row of rosettes forms a set of 5-nm microfibrils which aggregate laterally to form the larger fibrils of the secondary wall. Above side view. The stippled area in the center of a rosette represents the presumptive site of microfibril formation, although details of its structure, composition, and enzymic activity remain unclear. Below surface view, with expanded, cross-sectional view of cellulose fibrils.]... 

X-Ray diffraction studies on the (1 -> 3)- 3-D-glucan, curdlan, from Alcaligenes faecalis have shown the existence of microfibril formation, and a triple helical structure is proposed for the crystalline form of this polysaccharide. ... 

Tunicates, the only animals known to produce cellulose, are a family of sea animals that have a mantle consisting of cellulose microfibrils embedded in a protein matrix [Fig. 13.2). The thick leathery mantle in their mature phase is used as a new source of cellulose. Most research has used a class of tunicates that are commonly known as "sea squirts" [Ascldlacea), marine invertebrate filter feeders [9]. The structure and properties of tunicate cellulose [TC) are expected to be comparable between species, but there may be small differences in the cellulose microfibril formation process [10]. 

Cellulose Imeta-stable cellulose I converts to cellulose Ip on aimealing. 

Figure 9.9 Schematic of the microfibril formation mechanism in polymer blends during cold drawing (transformation of the spherical particles into microfibrils via...

Fakirov, S., Bhattacharyya, D., Lin, R.J.T., Fuchs, C., and Friedrich, K. (2007) Contribution of coalescence to microfibril formation in polymer blends during cold drawing. /. MacromoL ScL Part Phys., B46, 183-193. 

At the early stage of research on in situ composites, most work has been conducted on the LCP microfibril formation connected with the melt viscosity or viscosity ratio, composition, and flow mode, etc. These results have been summarized and reviewed by several researchers [11-17]. In the aspect of flb-rillation, the current work is only a supplement to those reviews, with additional new insights and results. We will summarize and review the recent work on the compatibility of LCP blends in this chapter. 

Herth W. (1983). Arrays of plasma-membrane rosettes involved in cellulose microfibril formation of Spirogyra. Planta, 159, 347-356. 

Mechanism of microfibril formation in polymer blends and effect of the compatibilizers on this process... 

The observed contradiction regarding the microfibril lengths and the spheres diameters can be explained by assuming the following mechanism of microfibrils formation starting from the spherical particles. Considering the fact that the spheres are rather densely pop-... 

The first indirect experimental result in favor of this idea came from the finding that the mechanical properties of the compatibilized PP/PET blends with MFC structure were quite poor, i.e., almost no reinforcing effect of the PET microfibrUs was observed [50]. At the same time, SEM analysis of the PP/PET/C samples indicated that in the compatibilized blends the microfibrils were much shorter [48,50]. This last observation led to the necessity of checking systematically the validity of the coalescence hypothesis regarding the mechanism of microfibrils formation. 

An additional support in favor of the coalescence hypothesis of the microfibrils formation mechanism can be found in a study by Perilla and Jana [59] where they investigated the coalescence of droplets during the flow of a polymer blend through a capillary die. The formation of fibrils by coalescing droplets has been reported for both shear flows, e.g., in parallel disk devices [60]) and in capillary flow [61] and extensional flows e.g., in post-extrusion stretching) along the machine direction [62-65]. 

The microfibril reinforced polymer-polymer composites are prepared as follows, as shown in Figure 12.1. First, the two thermoplastics with different melt temperatures are melt-mixed in a single-screw extruder with a slit die to ensure uniform deformation. The extrudate is then hot stretched by a take-up device with two pinching rolls to facilitate the microfibril formation. Different hot stretch ratios (HSR, i.e., the area of the transverse section of the die to that of the transverse of the extrudate) are obtained by adjusting the speed of the take-up device. Subsequently, the extrudate is immediately quenched in cold water (20 °C) after stretching to preserve the formed microfibrils, and finally a thin ribbon is obtained. 

Li Z M, Yang M B, Huang R, Yang W and Feng J M (2002) Poly(ethylene terephthalate)/poly-ethylene composite based on in situ microfibril formation, Polym Plast Technol Eng 41 10-32. 

Notes ED = extruder die The values in parentheses are for oriented blends after the selective extraction of the HDPE matrix The estimations of the microfibriis lengths are made assuming that one PAG globule at ED produces one microfibril in the final UDP MFC. The role of coalescence for microfibril formation in the HDPE/PA blends is considered further in the text. 

HerthW. 1980. CalcofluorwhiteandCongo red inhibitchitinmicrofibril assembly of Poterioochromo a5 Evidence for a gap between polymerization and microfibril formation. J Cell Biol 87 442-450. 

Tunicates are a family of sea animals which is known to produce cellulose microfibrils in large amount. These animals are having a thick, leathery mantle, which is a good source of cellulose. Large numbers of tunicate species are available in namre and the properties of cellulose obtained can vary from species to species. The cellulose microfibril strucmre and properties obtained from different species are often comparable, but small differences in the cellulose microfibril formation process may affect the final properties of microfibrils. 

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