RESEARCH
amount of tritium incorporated is greater. With cholesterol, for example, Wilzbach gets specific activities a hun dredfold greater than b y the recoil triton method. Labeling, of course, is more or less random, and anyone waaung a label at a particular point in a molecule will still need to turn to syntheses. Where specific orientation is n o t required, »~Ι*\\\Τ£Λ-%Τ£*Ύ·
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prove more than satisfactory. It's still too early t o predict all facets of the method, bat so far Wilz bach has noted ring compounds are easier to label than straight-chain ones. He's also tried enough different com pounds to b e reasonably confident, however, that his method will b e generally applicable. Biochemists should fbid the tech nique useful, since they c a n label mate rials of complex or even unknown struc ture. Such materials having a high tritium content will be especially suit able where complex substances are ad ministered in small amounts and be come highly diluted in the organism. Chemists will find tritium labeling with self-radiation an effective tool for radiation chemistry studies. Since products from such irradiations are labeled with tritium, they can be de-
tected by carrier techniques even when there's not enough for detection b y physical methods. Wilzbach himself has already taken one step in this direc tion in irradia tin?? iolnf*nf*: H e finds tritium-labeled benzene and xylenes in the reaction products. This indicates methyl group transfer during irradia tion, previously unobserved with less sensitive techniques.
Labeling with tritium hasn't
spectacular in the past, but tritium is now in a position where future use can surpass that of any other radioactive isotope. Its low cost—currently $ 2 per curie—and the fact that as little as 10~ 10 curie can be analyzed b y simple methods make it attractive for largescale industrial application, as in petroleum exploration and engine test ing. With a short half-life ( 1 2 . 4 years) and the high isotopic purity pos sible, h i g h specific activities can b e o b tained and dilutions as high as a mil lion million fold are possible. A n d finally, its low radiation energy mini mizes health hazards since its radiation is completely absorbed in less than 10~3 centimeter of liquids or solids.
Probing Teflon Du Pont finds polytetrafluoroethylene undergoes several transitions a t room tempera tures What's "^NSIONAL MEETING \
Polymer Chemistry
behind
t h e
structural changes of poly-
* tetrafluoroethylene (Teflon) at room tempera ture? X-ray dif fraction studies o n the polymer per formed by E . S. Clark and collabo rators at D u Pont indicate many of Teflon's unusual physical properties appear influenced b y structure changes; understanding these changes will per mit utilizing the properties to best ad vantage, says Clark. T h e practical value of knowing how Teflon's crystal structure changes with temperature may help answer problems i n handling the polymer and show how tempera K. E . Wilzbach of Argonne National ture influences fabrication processes Laboratory transfers tritium gas into and procedures. Polytetrafluoroethylene's crystalline a reaction vessel containing an or ganic compound to produce a tri structure transitions were considered tium-labeled compound. This proc by Clark by treating chain configura ess, he told the Division of Organic tion and chain packing of the polymer Chemistry, was developed in a pro separately and through a concept of a gram on applications of tritium in helical stucture for the chains. Transi tions occur at about 20° C , 30° C , and research and industry 4618
C&EN
SEPT.
24, 19 56
at higher temperatures but below the 327° C. melting point. Extensively in vestigating those at 20° and 30° C , Clark told the Division of Polymer volves changes both in chain configu ration and in chain packing while the 3 0 c C . transition brings a change in chain packing. • Helical Structure Concept. Work ers in the field of protein molecule structure devised a theoretical method for determining a chain configuration which involves atoms regularly spaced on helices. Clark utilizes this helical structure concept to interpret x-ray diffraction data of Teflon. Below 20° C , the helical configuration was that of an ethylenic zig-zag having a 180 degree twist for every 13 carbon atoms. T h e crystal unit cell of this form is triclinic. Although hysteresis makes determin ing t h e exact temperature difficult, at about 20° C. a transition occurs in Teflon. T h e unit cell changes from triclinic to hexagonal with a slight un twisting of the helical configuration. Clark finds that a 180 degree twist in a crystal unit cell contains 1 5 carbon atoms above 20° C. Other diffraction measurements indicate a change in packing occurs during this transition as well as a configuration change. • rinding Packing Changes. T h e method used b y Clark to determine changes in chain packing is based on the character of Bragg planes i n the diffraction pattern produced b y re flection from atoms i n the crystals. Using data obtained with a Geiger counter diffractorneter, Clark found the helical structure shifted from 13 t o 15 carbon atoms per 1 8 0 degree twist dur ing t h e 20° C. transition. W h e n a change i n chain packing oc curs, the Bragg plane relationships are disturbed and areas of diffuseness ap pear instead of a sharp diffraction pat tern. Clark interprets the chffraction diffuseness as arising from shifting of the chain axes. T h e disorder involves either a random linear translation of the chains or possibly a random mixture of right- and left-handed helical forms— or perhaps a combination of both. The configuration and packing changes are distinctly different. At about 30° C , a second transition occurs. X-ray diffraction studies o n this transition show again the precise Bragg plane relationship in the hexag onal crystal disappears, showing a change in chain packing. Higher tem perature transitions produce diffraction patterns suggesting further uncoiling of the helical configuration, says Clark. However, the packing remains hexago nal to the melting point. •
