Fiber-forming microcrystalline

As mentioned earlier, poly[bis(trifluoroethoxy)phosphazene] is a fiber-forming, microcrystalline polymer that can be fabricated into microfibers by solution extrusion from THF into a nonsolvent such as heptane. Alternatively, it can be electrospun to nanofibers from a solution in THF. The structure-property relationships for this polymer are fairly well understood, and these provide a basis for expanding the understanding to polymers with other side groups. 

Microcrystalline, fllm-and fiber-forming polymer stable to water, hydro-phobic)... 

A number of microcrystalline homopolymers, such as [NP(OCH2CF3)2]n [NP(OC H5)2]n> t>e solution -fabricated into fibers. These do not burn. However, the higher cost of most poly(organophosphazenes) compared to the well-known organic fiber-forming pol)miers may restrict their use to specialized, high-performance applications. 

Solid cellulose forms a microcrystalline structure with regions of high order, i.e., crystalline regions, and regions of low order that are amorphous. Naturally occurring cellulose (cellulose I) crystallizes monoclinic sphenodic. The molecular chains lay in the fiber direction ... 

Both types are hydrophobic materials that, depending on the side group arrangements, can exist as elastomers or as microcrystalline fiber- or film-forming materials. Preliminary studies have suggested that these two classes of polyphosphazenes are inert and biocompatible in subcutaneous tissue implantation experiments. 

A spider s orb-web is formed by extrusion of a concentrated protein solution and stretching of the resulting fiber. The cross-strands, which are stronger than steel, resemble silkworm silk. The molecules contain microcrystalline p sheet domains that are rich in Gly-Ala repeats as well as polyalanine segments. The capture spiral is formed from much more elastic molecules that contain many -tum-forming sequences. These assume a springlike p spiral. See Box 2-B. 

Another form of cellulose, microcrystalline cellulose, has been used for pharmaceutical applications. Microcrystalline cellulose is a nonhbrous form of cellulose in which the cellulose fibers are fragmented into particle forms ranging in size from... 

Cellulose reacts with isocyanates in anhydrous pyridine or with urea and substituted ureas at relatively high temperature to yield carbamates. The optimum carbamation reaction of microcrystalline cellulose with urea in a dry solid mixture has been studied [51]. In addition, a preferentially C6-modified cellulose carbamate derivative has been obtained [52]. Heating of cellulose with thiourea at 180°C yielded cellulose thiocarbamate [53]. Heat treatment of cellulose isocyanate products has been utilized for the production of urethanes [54]. When ceUuIose was treated with phenylisocyanate at 100 C in DMF in the presence of dibutyltin dilaurate and triethylenediamine, celiuiose bisphenylcarbamate was formed [55]. Treatment of cellulose with urea at temperatures at or above the latter s melting point (where urea decomposes into isocyanic acid and ammonia) has been employed for the production of cellulose carbamates fibers [56]. The advantages and disadvantages of using urea as an intermediate for production of fiber have been discussed [57]. 

Whereas cotton represents the purest form of cellulose, wood contributes the vast majority of it. Depending on end use, paper, board or chemical grade, wood conversion to some type of cellulose-rich fiber amounts to about 200 x 10 t/a a worldwide [29]. This compares to about 15 X 10 t/a cotton [13]. Only a minor amount, ca. 7 x 10 t/a, of high-purity cellulose is used for chemical purposes, mainly regenerated fibers (viscose rayon, lyocell) and derivatives (esters, ethers). Smaller amounts are also used for hydrocolloids [microcrystalline cellulose (MCC)]. 

The OH groups can be reacted with other groups such as acetyl and carboxymethyl to produce modified celluloses and vary the activity. A 0.1 mm layer on a plate usually will expand to 0.25 mm when wetted. Binders usually are not required with cellulose. Microcrystalline cellulose has been cut, so there are no long fibers to form a preferred direction of flow. 

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