Cellulosic polymers description

Most of the available commercial microporous membranes such as polysulfone, polyethersulfone, polyamide, cellulose, polyethylene, polypropylene, and polyvinylidene difluoride are prepared by phase inversion processes. The concept of phase inversion in membrane formation was introduced by Resting [75] and can be defined as follows a homogeneous polymer solution is transformed into a two-phase system in which a solidified polymer-rich phase forms the continuous membrane matrix and the polymer lean phase fills the pores. A detailed description of the phase inversion process is beyond the scope of this section as it was widely discussed in Chapters 1 and 2 nevertheless a short introduction of this process will be presented. 

Any formal definition of a gel has been avoided, because its formulation would be difficult on the basis of external properties. By any criterion, a 1 % aqueous solution of agar does form a gel when it is cooled from 95 to 20°, whereas a 1 % solution of sucrose does not but properties may be so continuous between these extremes that any dividing line would be arbitrary. Most of this Chapter is concerned with fairly permanent network structures formed from polymer solutions. As a definition of gels, this description is incomplete. Some cellulose gels, and pastes of gelatinized, starch granules, would be excluded, because they are formed by limited dispersion of solids. In other words, an arbitrary choice will be made to focus on those gels that are dilute with respect to polymer, because the information available about them at the molecular level is more precise than for others. 

During the first decade extensive descriptive studies were carried out on natural polymers of all kinds proteins (wool, silk, and leather), carbohydrates (cellulose, starch, and gums), and other resinous products (shellac, rubber, and gutta-percha). Three large domains of scientific and technical interest came into being ... 

Xylan-based polymers show no convincing evidence for molecular interaction with, or cross-linking of, cellulose under the high dilution fermentation conditions. Whether this is also true in the less dilute cell wall environment remains to be determined. Description of hemicellulose polymers as glucan-binding may therefore not always be accurate. 

The formation of liquid-crystalline phases of cellulosics, especially of the lyotropic kind, has yet to be explained. Certainly the chain stiffness may have to be taken into account as one of the factors in question, but the solvent-polymer interaetion may have to be considered as well. In the next section, models for the description of the pitch as a chiral property and models to... 

As can be seen from the above description, there are many variables involved in the phase-inversion technique. Among others the composition of the polymer solution, the solvent evaporation temperature and evaporation period, the nature and the temperature of the gelation media, and the heat treatment temperature are the primary factors affecting the reverse osmosis performance of the membrane. When polymers other than cellulose acetate are used, solvents and nonsolvent additives appropriate to prepare membranes from the particular polymer must be found. Depending on the combination of variables, membranes of different polymeric materials with different pore sizes can be prepared. 

Role of polymer Adhesive/coating/consoli-dant/moulding material A brief description of the tested conservation application is valuable Cellulose acetate was rejected as a picture varnish... 

Ott, E., H. M. Spurlin and M. W. Grafflin (eds.), Cellulose and Cellulose Derivatives , 2nd edn., parts 1, 2 and 3 Bikales N. M., and L. Segal (eds.), parts 4 and 5, Interscience, New York, 1971. The first edition published in 1943 was considered the most authoritative description of the field in the English language. The second edition covers the important advances in cellulose science and technology High Polym., vol. 5). 

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