Graft fibrous cellulose

By using this technique acrylamide, acrylonitrile, and methyl acrylate were grafted onto cellulose [20]. In this case, oxidative depolymerization of cellulose also occurs and could yield short-lived intermediates [21]. They [21] reported an electron spin resonance spectroscopy study of the affects of different parameters on the rates of formation and decay of free radicals in microcrystalline cellulose and in purified fibrous cotton cellulose. From the results they obtained, they suggested that ceric ions form a chelate with the cellulose molecule, possibly, through the C2 and C3 hydroxyls of the anhy-droglucose unit. Transfer of electrons from the cellulose molecule to Ce(IV) would follow, leading to its reduction... 

An effective method of NVF chemical modification is graft copolymerization [34,35]. This reaction is initiated by free radicals of the cellulose molecule. The cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. Then, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with a suitable solution (compatible with the polymer matrix), for example vinyl monomer [35] acrylonitrile [34], methyl methacrylate [47], polystyrene [41]. The resulting copolymer possesses properties characteristic of both fibrous cellulose and grafted polymer. 

Properties of Graft and Block Copolymers of Fibrous Cellulose... 

Graft and block copolymers of cotton cellulose, in fiber, yam, and fabric forms, were prepared by free-radical initiated copolymerization reactions of vinyl monomers with cellulose. The properties of the fibrous cellulose-polyvinyl copolymers were evaluated by solubility, ESR, and infrared spectroscopy, light, electron, and scanning electron microscopy, fractional separation, thermal analysis, and physical properties, including textile properties. Generally, the textile properties of the fibrous copolymers were improved as compared with the properties of cotton products. 

Solubility of cellulose in the products has also been used to indicate the presence of covalent bonds between cellulose and the polyvinyl copolymer. Thin sections of fibrous cellulosic copolymers, as prepared for electron photomicrography, are examined both before and after successive extractions with solvents for cellulose and for the polymer. The presence of undissolved cellulose and polymer in the extracted thin section is interpreted as evidence for the presence of covalent bonds and for grafting (52). 

Table X. Effects of Degree of Polymerization of the Irradiated Cellulose and Extent of Grafting on the Elastic Recovery Properties of Fibrous Cellulose—Poly (ethyl acrylate) at 25°C...

An Interpretative review of the reactions initiated by macrocellulosic free radicals with vinyl monomers to yield block and graft copolymers of fibrous cellulose was made. Macrocellulosic radicals are usually formed by interactions with radiation or chemical redox systems. Important factors in these heterogeneous reactions are lifetimes and accessibilities of the radicals and interactions of solutions of monomer with fibrous cellulose. Changes in organochemical, macromolecular, and morphological structures in cellulosic fibers through formation of copolymers are made. 

Figure 8. Degree of grafting for cellulose and cellulose derivatives initiated by UV light of > 280 nm. Key a, fibrous carboxymethylcellulose b, cellulose c, hydroxyethylcellulose d, ethylcellulose e, methylcellulose f, powdered carboxymethylcellulose g, acetylcellulose.

Graft copolymerization reactions of fibrous cellulose with vinyl monomers were initiated at free radical sites formed on the cellulose molecule by interaction with radiation, by reaction with Ce4+ ions in acidic solution, or by H abstraction by OH radicals formed by reaction of Fe2+ ions with H202 in aqueous solution. The effects of experimental conditions on the location of these sites on the cellulose molecule and on the reactions were studied by ESR spectroscopy. The molecular weights of the grafted copolymers and the distribution of the polymers within the fibrous cellu-losic structure were determined. Some of the properties of the copolymers are discussed. 

Ionizing Radiation. When the activation reaction and copolymerization reaction were conducted simultaneously, the fibrous cellulose was first treated with monomer solution and then irradiated. Usually some homopolymer was formed and was removed from the cellulose graft copolymer by extraction with solvent for the polymer. When the activation reaction and copolymerization reaction, were conducted consecutively, the fibrous cellulose was first irradiated and then treated with monomer solution. As compared with the simultaneous procedure, formation of homopolymer was very low.. The changes in the ESR spectra for irra-... 

The morphology of the fibrous cellulose graft copolymers depended on the method of initiation of free radical formation, experimental conditions during the copolymerization, chemical modification of the cellulose before reaction, and the type of monomer used (60). Variations in the shape of the fibrous cross section, in layering effects in the fiber, and in the location and distribution of the grafted copolymer in the fiber were observed by electron microscopy (61). Cotton cellulose—poly (acrylonitrile) copolymer was selected to show the possible variations in location and distribution of the grafted copolymer in the fiber. 

This and the following subsection will compare heterogeneous and homogeneous synthetic methods, with emphasis on cellulosics. Fibrous cellulosic graft copolymers, prepared by radiation-induced free-radical copolymerization reactions of vinyl monomers with cellulose, retain some... 

The morphological or supermolecular structure of fibrous cellulose is easily altered by formation of graft copolymers. The distribution of copolymer within the fibrous cellulose primarily depends on... 

The apparent distributions of grafted copolymers in fibrous cellulose are indicated in Figures 18 and 19. Differential solubility tests have been made on electron-microphotograph cross-sections to define these distributions more clearly. Covalently linked cellulose and polymer are less soluble in selected solvents than in either solvents for cellulose or polymer. ... 

The modification of the properties of fibrous cotton cellulose through free-radical initiated copolymerization reactions with vinyl monomers has been investigated at the Southern Laboratory for a number of years. Both graft and block copolymers are formed. Under some experimental conditions the molecular weight of the polyvinyl polymer, covalently... 

After four hours, the degree of grafting for various samples are in the order of fibrous carboxymethyl cellulose > cellulose... 

It should be noted that wool, the other major fibrous polymer of natural origin, has also been used as a substrate for grafting. Indeed, materials of commerce are often grafted in a manner analogous to that used for the cellulosic fibers (Rebenfeld, 1971). 

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