megarad of /^-radiation from a Van de Graaff generator % at beam current of 225 microamperes for several seconds, the gel forms as rapidly as he can ex amine the sample (less than 30 sec onds). He observes no post-radiation gelling. For a second piece of evidence favoring a free radical reaction, Dr. Leavitt points to the fact that free radical scavengers lower or completely inhibit the cross-linking sections. Vinyl monomers and chain transfer agents are examples of these inhibi tors; and they also reduce oxidation. Co-solvents such as methanol seem to reduce coupling, too. The effects of varying dosage rates serve as another argument favoring the free radical process. High dose rates produce network structures due to high concentrations of radicals in solu tion, thus leading to high rates of bimolecular couplings, Dr. Leavitt says. Equivalent total dosages at low rates lead entirely to scission and form no network structure, he finds. To show the importance of mobility, chain scission occurs in frozen solu tions or highly viscous media (contain ing high molecular weight polymers), where lack of mobility prohibits crosslinking. The same concentration of lower molecular weight polymer allows greater freedom of motion, Dr. Leavitt says, permitting combination of two radicals before degradative scission lakes place. At very low concentra tions, however, not enough radicals are formed to cross-link before degra dation becomes predominant. He emphasizes that the several different reactions (cross-linking, scission, oxi dation, cleavage) are occurring simul taneously during irradiation. High-energy radiation does generate cross-linking in such compounds as methyl cellulose, hydroxyethyl cellu lose, methyl hydroxypropyl cellulose, and methyl hydroxybutyl cellulose. Carboxylated cellulosics, containing highly polar groups, show interchain repulsion. With no intimate polymer/ polymer interaction possible, crosslinking can't occur, Dr. Leavitt con cludes. γ-Rays Induce Grafting. Graft polymerization of cellulose with sty rène is induced with radiation, Dr. Yoshio Kobayashi of Textile Research Institute, Toyo Spinning Co., Ltd., Japan, said in a paper read by Dr. Vivian T. Stannett of State College of Forestry. Preirradiation (under water) of untwisted viscose rayon yarn
by cobalt-60's γ-rays for 24 hours makes the cellulose reactive with styrene monomer. Grafting takes place at 50° C , Dr. Kobayashi says. Weight of the yarn increases with time, with about one third of the increase being due to grafting; the rest is homopolymer, im pregnated into the fiber. The reaction system is a monomer solution com posed of 20% (by volume) styrene, 72% methanol, and 8% water. Preirradiation in hydrogen peroxide instead of water gives much faster rates of subsequent grafting. Dr. Kobayashi considers decomposition of the hydroperoxide as the rate deter mining step. He calculates that about 7% of the hydroperoxides or peroxides act as grafting sites. This graft copolymer is insoluble in any solvent, he finds. But subsequent acetylation, which is faster and easier than with cellulose alone, makes it soluble in methylene chloride and other solvents.
Single Crystals of Cellulose Have Well Ordered Structure Single crystals of pure cellulose, said to have never before existed, can now be reproducibly prepared, says Dr. Bengt G. Rânby of Empire State Paper Research Institute at State College of Forestry. Lamellar particles crystallized from aqueous solution possess a mercerized-cellulose lattice with goor1 order and well defined morpï'-^iogy. The structure of these crystals resembles that of pyramidal, screw-dislocation single crystals of linear polyolefîns, Dr. Rânby told the Third Cellulose Symposium at Syracuse University. Considering their polymolecularity, the compact crystals contain folded cellulose chains, he believes. These would be oriented perpendicular to the plane of the basic plate structure, as is believed to be the case in polyethylene single crystals formed slowly from solution. Until recently, according to Dr. Rânby, it has been felt that the structure of cellulose consists of a vast number of submicroscopic crystallite domains, or micelles. These would be separated but bonded to each other, and embedded in an amorphous matrix. Dr. Rânby and co-worker Ralph W. Noe find that such cellulose derivatives as cellulose triacetate and triethyl cellulose will crystallize from solution in the same type of lamellar
SINGLE CRYSTALS OF CELLULOSE. Photomicrograph of cellulose crystals shows pyramidal spiral structure
form as linear polyethylene, stereoregular polypropylene, and linear polyamides. Right Conditions Give Crystals. Pure cellulose has been known only in the "native" form of partly crystalline microfibrils (derived from bacterial membranes or plant fibers). But Dr. Rânby and Mr. Noe say that proper conditions and a proper solvent system allow controlled cellulose crystallization. They use a water soluble cellulose acetate (16.5% acetyl groups, 0.74 acetyl group per glucose unit). A fraction with a low degree of polymerization (about 50) mixed with an aqueous buffer solution saponifies when heated. Concentration of the cellulose acetate solutions should be about 0.5 mg. per ml., with buffer capacity sufficient to hold pH to 8.0±0.2 units during the reaction at 90° C. In the early stages of crystallization, lamellar particles (frequently diamond-shaped) dominate, Dr. Rânby and Mr. Noe say. These crystals, 10 to 25 microns in diameter, show a weak double refraction in polarized light. As the isothermal crystallization proceeds, smaller crystals (1 to 2 microns) appear. Usually elongated, these show stronger double refraction. Next step in these studies, says Dr. Rânby, will be to find new liquid systems, or co-solvent combinations. These should permit the cellulose crystals to grow larger and will allow the use of cellulose having higher molecular weights. This means more material for detailed studies of the crystalline cellulose should be available. NOV.
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