Mechanism of microfibrils formation

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. 

The model proposed above accounts for orientation produced at low extension. To account for the c-axis orientation observed at high extension, either c-axis slip, or breakup of the large lamellar crystallites accompanied by rigid rotation and formation of microfibrils must occur. Both mechanisms can occur as elongation progresses, and an example of the latter is shown in Figure... 

Polymerization of the D-glucan chains occurs by way of a multi-subunit, enzyme complex embedded in the plasma membrane an almost simultaneous association, by means of hydrogen bonds, of the newly formed chains results in formation of partially crystalline microfibrils. This mechanism of polymerization and crystallization results in the creation of microfibrils whose chains are oriented parallel (cellulose I). In A. xylinum, the complex is apparently immobile, but, in cells in which cellulose is deposited as a cell-wall constituent, it seems probable that the force generated by polymerization of the relatively rigid microfibrils propels the complex through the fluid-mosaic membrane. The direction of motion may be guided through the influence of microtubules. 

Deposition of the matrix substances and formation of the microfibrils are accompanied by a sequence of related processes that lead to the development and differentiation of the cell wall this sequence includes expansion of the wall, changes in the composition of the polysaccharides, organization and orientation of the different layers, deposition of callose for formation of pores in phloem, lignification (see p. 299), and other processes. Considerable information has been obtained about the mechanism of some of these processes and the factors that affect them this information has been reviewed by leading molecular biologists, and will very briefly be mentioned here because of its relevance to cell-wall formation and to the constitution of cell-wall polysaccharides of interest to carbohydrate chemists. According to the new concepts, the transformations of the cell wall are effected, or are assisted, by the presence of a variety of enzymes, proteins, and, perhaps, even ribonucleic acid to the extent that primary... 

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

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]. 

What could be the reason for these superior mechanical properties of MFCs prepared from blends of condensation polymers and polyolefins It could hardly be the same as in the case of blends of condensation polymers the perfect adhesion between matrix and reinforcing microfibrils are due to the formation of chemical bonds between them as outlined above. Neither could it be related to a better aspect ratio of the reinforcement, since the sizes of microfibrils do not depend that strongly on the chemical composition of the blend. 

The tan(5 curves delineated in Eigure 15.21 indicate a sharp decrease with frequency for all the samples except H5I and H8I. The plateau region in the i nS curve for H8I is an indication of the formation of a physical network between the solid PET fibrils and the molten PP matrix, which hinder the relaxation of the PP component. The microfibrils with low aspect ratio in H2I and HIOI are not capable of affecting the relaxation mechanism of the matrix. This behavior is similar to the observations of Monticciolo et al. [49] where... 

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