RESEARCH
Radiation Boosts Reactivity of Cellulosics Beta-radiation produces cross-linking in nonionic, water soluble cellulose compounds; gamma-rays induce styrene grafting High-energy radiation unexpectedly produces cross-linking in some cellulosic compounds, according to Dow Chemical's Dr. Frederick C. Leavitt. Though cellulose or its derivatives usually degrade when exposed to highenergy radiation, nonionic water soluble cellulose ethers form gels. This occurs when polymeric free radicals couple as a result of indirect effects of radiation on the aqueous solution. Generally, a wide range of highenergy radiation causes cellulosics to undergo chain scission, oxidation, or cleavage of substituent groups, Dr. Leavitt told the Third Cellulose Symposium of State College of Forestry at Syracuse University. In these cases, he says, no cross-linking is observed. Even low dose rates—from y-radiation, for instance—produce no cross-links, though they, too, cause degradation. Dr. Leavitt, working at Dow's eastern research laboratory at Framingham, Mass., finds that viscosity of the system being irradiated plays a big part in determining whether cross-linking does take place. Highly viscous solutions give limited freedom of motion to polymer chains. High-energy radiation degrades these systems until viscosity decreases enough to permit free birnolecular coupling reactions. At this point, he observes that the system gels immediately. Physical means such as high-speed mixing won't disperse these gels. Cellulosics are inherently sensitive to radiation, Dr. Leavitt points out. To most cellulose chemists, this means chain scission. But, he explains, scission reactions may well follow atom abstraction by free radicals in solution. He proposes that the actual cross-linking in his studies involves an indirect effect of radiation. For example, hydroxyl radicals produced by the electron beam may then abstract a hydrogen atom from anhydroglucose rings. The polymeric free radicals first formed could couple directly, or after rearranging to more stable forms. This 52
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could then lead to cross-linked systems. Cross-linking Is Free Radical Process. Cross-linking, he feels, must be a free radical process. For one thing, the gel sets up too fast during radia-
tion with high-energy electrons for it to be caused by intermolecular condensations through oxidized sites. For example, when mobile polymer solutions ( 1 % by weight) receive 0.25
Measures Ultrafow Pressures Photomultiplier ion gage developed at Westinghouse by Dr. W. J. Lange (above) and co-workers measures pressures over a range of 10~3 to 10~10 mm. Hg, the company says. Ultraviolet light is beamed onto a metal surface to release electrons, which are then multiplied by being guided onto a series of similar surfaces. The electrons form ions, which are collected and counted; the rate at which ions form is a measure of pressure. The unit differs from conventional low-pressure measuring devices by doing away with a heated filament. The usual methods measure low pressures by placing an electrical charge on gas particles remaining in a vacuum system and counting the rate at which ions form. Normally, the charges come from electrons which are "boiled off" the surface of a hot tungsten filament. But the gas often interacts with the hot filament surface, which breaks down the gas and converts it to another substance.
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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