Preparation of MFC

Physical or chemical modification of commercial polymers is one of the most promising routes for obtaining polymer materials with tailored properties. The blending of two or more homopolymers is a typical example of physical modification. The homopolymer compatibihty in the blends is of prime importance for their final properties. Compatibility is directly related to the chemical nature and crystallization ability of homopolymers as well as to the conditions of preparation and processing of the blends. Since polymers are in general incompatible, there is a constant search for means of improving their compatibility and/or suppression of incompatibility. 

Since the compatibUization issue plays an essential role seriously affecting the performance of polymer blends, it seems appropriate to list some basic considerations. 

The manufacture of MFCs involves blending of thermodynamically immiscible polymeric partners, differing in their melting temperatures, T.. The essential stages of MFC preparation are as follows  

During the drawing step, the blend components are oriented, and nano- or microfibrils are created fibrillation step). In the subsequent thermal treatment stage, when melting of the lower-melting component occurs isotropization step), the oriented fibrillar structure 

This process can be divided into three distinct steps, each vital to the successful creation of a microfibril reinforced composites  

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Evidences of the described physical processes occuring during the preparation of MFC as well as some of the steps of the interfacial chemical Interactions can be seen in the WAXS patterns displayed in Fig. 1... 

As mentioned above, the suggested approach for the preparation of MFCs and NFCs offers the possibility to isolate micro- or nanofibrils as a separate material, which have many opportunities for biomedical and technical applications. In the latter case, as mentioned in the preceding paragraph, the preparation of gas and/or liquid nanofilters as nonwoven textiles is a possibility. 

The preparation of MFCs is quite different from that of the conventional composites, insofar as the reinforcing micro- or nanofibrils are created in situ during processing, as is the relaxed, isotropic thermoplastic matrix. The MFC technology can, therefore, be contrasted with the electro-spinning methods used to produce nano-sized materials mainly in the form of nonwoven fibers with colloidal length scales, i.e., diameters mostly of tens to hundreds of nanometers [57]. 

This chapter summarizes present knowledge about the preparation of MFC directly from wood, and its surface and colloidal properties. In most of the more advanced applications suggested for this material, chemical modification of the surfaces of the fibrils is of major importance, and methods to achieve such modification are described in some detail. Finally, a brief survey of already demonstrated applications of MFC is given. 

A multistep reaction pathway leads to polymers 43 and 44 with phosphatidylcholine moieties in the main chain and long alkyl groups in the side chain [122]. These polymers exhibit thermotropic liquid-crystalline behavior. Polyamides 45 were obtained by interfacial polycondensation they are insoluble in any normal solvent [123]. Poly-MPC capped with cholesteryl moieties at one or both polymer ends was prepared by the radical polymerization of MFC initiated with 4,4 -azobis[(3-cholesteryl)-4-cyanopentanoate] in the presence of a chain transfer agent [124]. The self-organization of these polymers was analyzed with fluorescence and NMR measurements. 

The material of PtRu alloy exhibits good properties for CO tolerance in polymer electrolyte membrane fuel cells (PEMFC) [68] and has been studied extensively in recent years [69]. Particular interest has been focused on the application of the PtRu alloy materials as anodes in methanol fuel cells (MFC) for electric vehicles [70]. The most convenient way to alter the surface composition of a PtRu alloy is to employ the electrochemical co-deposition method in the preparation of the alloy. Richcharz and co-workers have studied the surface composition of a series of PtRu alloys using X-ray photoelectron spectroscopy (XPS) and low-energy ion spectroscopy (LFIS)... 

It will be clear that in actual practice, the optimal operating temperature depends, apart from problems of microbial contamination which becomes increasingly significant at temperatures below 60°C, on the relative costs of the enzyme preparation per unit of column volume and the operating costs per unit volimie. Another important limitation may be the total amount of MFCS to be produced with a given capacity in terms of column volume, this will to a large extent be determined by the demand for MFCS. At times of low demand the optimal production temperature will tend to be lower than in times of high demand. 

Christian et al. [109] studied the oxygen permeability of MFC films at different relative humidity (RH). At low RH (0%), the MFC films showed very low oxygen permeabihty as compared with films prepared from plasticized starch, whey protein and arabinoxylan, and values in the same range as that of conventional synthetic films, e.g., ethylene vinyl alcohol. At higher RH s, the oxygen permeabihty increased exponentially, presumably due to the plasticizing and swelling of the carboxymethylated nanofibers... 

The MFC Concept for Preparation of Polymer-Polymer Composites with Superior Mechanical Properties... 

Henriksson M, Henriksson G, Berglund LA, Lindstro m T et al (2007) An environmentally friendly method for enzyme assisted preparation of microfibrillated cellulose (MFC) nanofibres. Eur Polym J 43 3434—3441... 

The main aim of this contribution is to present various approaches to the preparation of nanocellulosic materials from plant sources. The focus is on the extraction and investigation of microfibrillated cellulose (MFC) in particular however, to put this topic in context, cellulose whiskers and bacterial cellulose are also discussed in particular sections of the text and applications of nanocellu-losics in the animal body for the development of medical devices such as artificial blood vessels, and the application of bacterial nanocellulose as animal woimd dressings and cosmetic tissues. Therefore, this chapter has brought together a variety of areas from chemistry, medicine, and biotechnology. 

Crystallinity of PLA has a strong impact on its mechanical properties. Suryanegara et al. have prepared PLA/MFC nanocomposites in both fully amorphous and crystallized states. The tensile modulus and strength of pristine PLA were improved with an increase of MFC content in both amorphous and crystallized states. Dynamic mechanical analysis (DMA) has been used to study the effect of MFC reinforcement on the thermomechanical properties of PLA in both states and the results are shown in Figure 9.5. In the amorphous state, the storage modulus of pristine PLA below Tg is almost constant at around 3 GPa. Above Tg, the modulus drops to 4 MPa at 80 °C, and then increases to 200 MPa at 100 °C owing to the cold... 

Peculiarities of MFCs prepared from blends of condensation polymers... 

The previous notes on compatibility in polymer blends are important for the better understanding of one of the main advantages of MFCs prepared from condensation polymers. Dealing with such p>ol5rmers, in addition to isotropization dirring short (several hours) thermal treatment, chemical reactions additional condensation and transreacUons) between condensation p>olymers in the melt [78] as well as in the sohd state [79] can take place at the interfaces, as schematically shown below [67] ... 

The described peculiarities of MFCs can be expected and realized when the blend partners are condensation polymers or, at least, functionalized polyolefins. The latter restriction however, concerns only the self-compatibilization effect but by no means the basic principles of MFC manufacturing, i.e., MFCs can be prepared also from polyolefin partners provided the basic temperature requirements for their preparation are satisfied. 

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