Microfibril surfaces

Various attempts to interpret the accessibility data as related to surface area of microfibrils of measured dimensions have been made. If 10% of the microfibrils are accessible because they contain a disordered cellulose lattice, indicated by rapid hydrolysis in acid, the remaining 20-25% of the accessible OH groups should be located on microfibril surfaces. 

The fact that the compatibilizer is distributed just on the phase boundary between the blend components in the MFCs has been proved by two different and independent methods using the PP/PET system. No transcrystalline zones were found on the microfibrils surface in the compatibilized blends [56] whereas the small-angle X-ray scattering (SAXS) studies using synchrotron radiation [58] showed shorter, wider, and less pronounced holes at the interface boundaries for PP/PET/C blends (compared to needle-shaped holes for the material without the compatibilizer). The results from the SAXS and TEM studies have demonstrated unambiguously the existence of compatibilizer on the phase boundary between PP and PET in the studied MFCs. 

Based on the discussion above, it is clear that the amount and distribution of the CB particles on the surface of microfibrils are very critical factors to determine the percolation threshold of i-CB/PET/PE composite. When the CB content in the PET phase is just beyond the percolation threshold of CB/PET compound, the continuous network of CB particles may be formed inside the microfibrils. However, there are hardly any CB particles on the surface of the CB/PET microfibrils and there exists a pure polymer layer below the surface of CB/PET microfibrils. This results in a high contact resistance among the microfibrils. The whole system exhibits an insulator state though the electrically conductive microfibrils may form a network. As the CB content in the CB/PET microfibrils reaches max> fhe number of CB paxticles on the surface evidently increases. Conduction pathways axe formed between some contact points in the microfibril network. With a further increase of CB content, the amount of CB particles on the microfibril surface increased significantly, and the electrically conductive contact points also increased. When the number of contact points is large enough to form a network to sustain the electron transmission in the whole... 

NMR has also been used to obtain information about the chemical composition of microfibril surfaces [39, 50, 58, 61]. As with FT-IR, the method is sensitive to the bulk properties of the sample, and the low signal-to-noise ratio make identification of the chemical structure of the surface difficult. 

Probably the most suitable technique for evaluation of cellulose microfibril surface chemistry is X-ray photoelectron spectroscopy (XPS) [62]. The method provides a full elemental analysis, except for hydrogen, of the topmost 1-20 nm of the sample surface. High-resolution XPS Cls measurements give quantitative information on carbon bonding chemistry [63], XPS has been used extensively to quantify the changes of the chemical composition in the microfibril surface during derivatiza-tion [25-27, 60, 64]. 

The isocyanate-coated MFC in dry THF could be immediately coated with bisamino-propyl amine or 3-ethylenamino-l-propylamine, thus introducing positive charges to the microfibril surfaces. 

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