deposition occurs, and the mode of growth of the deposit. Under some conditions the deposit has a single crystal structure and appears as a continuation of the seed crystal. Under other conditions the deposit may start as a single crystal and gradually become polycrystalline with time, or—as a consequence of topographic features-it may form as a single crystal with boundaries parallel to the direction of growth.

Preliminary experiments with extremely short pulses of current show that copper deposits from a sulfate bath at high current efficiency with current pulses of one microsecond, but that efficiency decreases with shorter pulses. During a single pulse of about one microsecond duration, at a high current density, a deposit of copper in the form of a dendrite was formed. An extended study of the characteristics of deposition of metal with short pulses of current is expected to contribute information on the mechanism of electrode reactions and deposit formation.

Electrodeposition from Organic Solutions. Aqueous solutions are not suitable for electroplating on chemically reactive metals such as uranium if the coating is to be strongly adherent to the substrate. A study, partially supported by the Atomic Energy Commission, was therefore undertaken to investigate the electrodeposition of metals from organic solutions. Although the problem of coating adhesion was not completely solved, sound coherent coatings of zinc and of copper were obtained from solutions of their respective salts in formamide. Under suitable conditions, coatings could be easily built up to several hundred microns in thickness. It was also found that tin could be readily electrodeposited from dimethylformamide solutions of stannous chloride. None of these deposits, however, formed as smooth coatings. Instead, they grew in the form of large ductile crystals which were shown to be single crystals by X-ray diffraction.

2.2.5 ORGANIC AND FIBROUS MATERIALS

Research on the properties of organic and fibrous materials covers both natural and synthetic polymeric structures. To gain a better understanding of the relationships between the composition and the properties of polymers, studies are being conducted on rubbers, textiles, papers, leathers, plastics, dental resins, and related materials. The properties of polymers depend upon the size, shape, distribution, and flexibility of their molecules, and on the interactions of the molecules with each other. A more basic knowledge of their fundamental properties and improved measurement techniques are necessary for the development and efficient utilization of these materials. During the past year fundamental studies were made of the kinetics of crystallization, the melting temperatures of crystallites, the nature of free radicals formed in gamma-irradiated polymers, the thermal stability of polymers exposed to a temperature of 1,200 °C, and the polymerization induced by ionizing radiation of monomers confined under high pressure. The propagation of strain waves in fibers subjected to high-velocity tensileimpact loading was investigated as well as the energy-erosion relationships

Model Compounds Used in Vulcanization Studies.

Natural rubber reacts with sulfur and organic accelerators in the normal vulcanization process, but details of the reaction are obscure. Apparently double bonds are necessary, since the reaction does not occur with hydrogenated rubber. Studies of the reaction of sulfur, hydrogen sulfide, and accelerators with two simple model compounds, one a propylene containing one double bond, and the other, a butadiene containing two double bonds, showed the formation of sulfides, disulfides, and carbon-to-carbon bonds. Butadiene with its conjugated double bonds served as a model compound for the conjugated system formed in the vulcanization of rubber. A dithiocarbamate accelerator was found to facilitate the formation of hydrogen sulfide and then to promote the reaction of hydrogen sulfide with propylene or butadiene. Some reactions included the formation of compounds from free radicals originally present. Appreciable portions of the products were nonvolatile.

Crosslinks Determined in Anisotropic Fibers. The Flory-Rehner theory of isotropic swelling of rubber crosslinked in the dry state was extended to an anisotropic system crosslinked (short crosslinks) in the dry, oriented state. The new parameters introduced into the equation were readily determined from dimensional changes of the fiber in a suitable solvent using a photomicrographic technique. With this technique swelling equilibrium can be attained within 30 minutes. In the study, surprisingly good agreement was found between the equivalents of crosslinks calculated from swelling measurements and from chemical analyses.

Impact Loading of Fibers. In many civilian and military applications the performance of textile materials subjected to high rates of strain by impact loading is not well understood. An investigation was therefore made of the behavior of representative textile yarns subjected to impact loading at velocities of 150 ft/sec in which Von Karman's concept of critical velocity (that velocity at which a filament breaks immediately upon impact in tension) was extended and applied. In the study, critical velocity estimates were arrived at, ranging from 400 ft/sec for glass fibers to 950 ft/sec for nylon and some high strength rayons.

Mechanism of Retannage Studied. A combined chrome-vegetable tannage not only converts hides more rapidly into leather than a single tannage but the combination improves abrasive wear and tannage stability. Recent work showed that a reaction, probably of the chelate type, occurs between chrome and the vegetable tannage, and that the affinity of the substrate for tannin is at a peak when 2.5-3.5 percent of a chromium complex (percentage calculated as chromic oxide) is present. Experiments indicated that a copper compound gave results similar to those obtained with chromium. It therefore appears that other metallic salts may be developed for future uses in combination tannages.

Synthetic Fibers Structurally Modified. To provide basic information for the development of structurally modified fibers that will resist extreme environmental conditions, the Bureau conducts fundamental research on the properties of synthetic fibers. In this program, efforts are made to correlate the chemical properties, heat and radiation resistance, and polyelectrolytic

Polymers have been produced from materials, such as carbon disulfide and nitrogen, which do not form polymers under ordinary conditions. A combination of high pressure and subsequent exposure to intense gamma radiation was used to induce polymerization (page 104).

behavior of structurally modified fibers with size, number, and kind of crosslinks and grafts in the polymer fibers.

In preliminary investigations, unexpected crimping and helical coiling occurred when solid nylon-6 homofibers (homogeneous one-component fibers with round cross sections) are treated with a swelling agent and disulfide and poly(methylene sulfide) crosslinks inserted while the fibers are in the swollen state. Subsequent crimping in the dry state is attributed to heterogeneous crosslinking. Helical coiling occurs when the crimped fibers are treated with a strong swelling agent which completely destroys the remaining crystallites. Polymer Crystallization Studied. Although thermodynamic equilibria can be applied to the crystallization and melting of polymeric materials, these large molecules behave somewhat differently from small ones, partly because they never become completely crystallized. If crystallization of