470
JOH?; H . L . WA’I’SOX
ELECTROS 1IICKOSCOPY O F RADIATIOS‘ POI,T!hIERLZXTIOS PRODUCTS1
The main eniphasib in this 11orb is related to the polymerization of acetylene, since this problem TI ab of most importance to the dhawnigan Chemicals Ltd. under whose auspice3 it \\ab begun. Interest \\ hrbt directed ton ard a study of the catalytic polymer lino\\n as cuprene, arid this led to an invebtigation of the polymers formed from acetylene under dplia-ray bombardment and corona discharge. This in turn led to preliminary nark vith polymers of hydrogen cyanide and cyanogen. References for some of the observationb iecordecl liere may be found in previously published papers (1, 4). Prior to the publication of these inxestigations the acetylene polymer, regardless of the method of production, \\-as aln ays referred to as “cuprene” and \\a$ assumed to po\besb the same structure and t o be the same substance. I n electron micrographs each polymer type studied is found to be quite different from the others; therefore there seems to be reason for suggesting that for clarity in the discusion each should be referred to by a different name. The names C ~ ~ N L I L alprene, L , and coprent nil1 be used for the catalytic, alpha-ray, and corona types, respectively. It is to be noted that there are, in addition, mrious forms of. cuprene itself 71-hich are recognizably different in electron micrographs, although no attempt has been made as yet to classify them. The particular catalyst appears to be the vaiiable factor nhich introduces the differenceq. 1,ikewise there may be recognizably different forms of both alprene and coprene, further investigation of controlled samples Jvill settle this point. The so-called “gunks” nhich occur in reaction tubes and in ionization or radiation chambers are undoubtedly polymerization products whose microphysical properties \I ould also be north electron-microscope examination. Cuprene is formed chemically at about 300°C. and is a brown cork-like mass J\ ith all the characteristics of a light, dry solid. Xlprene is a soft bron n ponder formed :it normal temperature and piessure by alpha-ray bombardment. Coprene, which \\-as a~ailablein the form of solid brittle sheets, is formed \\hen acetylene gas is acted upon by a corona dischaige. In electron micrograph, cbiiprene 1s *ecn t o be a fibrous material. The polymeiization has started upon the catalyst surfave and a hollon- fiber has g o \ \ n from it. On the other hand, the polymerization process initiated by alpha-ray bombardment results in round particles joined one to the other in haphazard arrangement by relxtisrely short, thicL necks. The action of a corona discharge upon acetylene is t o form an almost structureless film uhich looks the bame in electron 1 I’iesc%ntedat the, P?mposiuni o n I < d l a t i o n (‘hcnnst!r and 1’hotoctlcilllst1\ \ \ h l e h \ \ a s I ~ c ~ lat lt thc 1 7 n ip~i 3 i t y of Y o t i v I ) a i n c > Yotii. l)a~ncs Iritliniia .Junc 14-27 lrI4i
POLYMERIZATIOS
OF ACETI-LESE
ny
RLDI-LTIOS
4 i1
micrographs as it appears to the eye? except of course that it is seen on a submicroscopic scale. The cuprene fiber has both a longitudinal and a transverse structure and is allrays a tube. The cross section of this tube is variable and many shapes have been observed, including ribbon-like, square, rectangular, oval, round, and even hexagonal ones. It is theorized that a combination of catalyst a c t i v e b n t e r properties and molecular dimensions determines the resultant fiber shapes. An interesting experiment, important from the point of view of understanding the catalytic m e c h d m , involves growing cuprene fibers on microscope specimen screens for A matter of seconds before stopping the xction abruptly. In this way small, newly formed cuprene filaments can be examined. Uhually each is observed to carry a small piece of heavy material near its tip. Since copper is alii-ays found in a cuprene gron-th even in those areas which are completely removed from the original catalyst, it is suggested that these small bodies are pieces of copper, reduced from the oxide by a hydrogen atmosphere Trhich, carried along by the fiber. continue to catalyze the reaction. Electron-diffraction studies detect a faint presence of copper but no recognizable structure. In a recent private communication from Dr. A . T.Mottlau of the &so Laboratories the usual,oorganic three-halo pattern of grating constants of approximately 4, 3, and 2 A. i: claimed for cuprene.' The alprene particles are rather large. Measured statistical constants arc given in table 1 for a number of samples of alprenc, along with those for some other polymerization products. A number of comments are in order respecting the (lata for the alprene samples in this table. ( 1 ) It is seen that there is considerable variety in the data from sample to sample. ( 2 ) In all but one case (sample S o . 3) very till?- particles (less than 300 nip) are conspicuous b>- their absence, a n d it7 no cow is /hew a corztit~iiousriist~ibutiontlotci, to f h c /i?nifof rcsolufioir . !.ji l'hc ,standard deviations are relatively high, ayeraging about 35.5 per cent of the m w n diameter for these five alprene samples. ( 4 ) I n sample 3 there is definite ei.idencr i n the results for the presence of a double distribution, which ~\-oultlaccount in this case for the much higher value of standard devi a t ion. ' In electron micrographs the particles of alprcne appear t o l i p perfectly rowid, but actually from shadoiv-casting experiments they are found to be flattened sonitwhat under their on-n w i g h t as they lie on the supporting wi,facP. In addition, they distort or flow spontaneously around holes in :I >upporting film :ind therefore h a w the properties of a very viscous liquid. Thc joining neck> ai^ 50-100 nip in length and are not artifacts 1101' electron optical illusion.;. T o demonstrate the physical presencc of the necks they ha\-e lwcn strctchcd, :i Dr. 1Iottl:ilL filids further t h a t n-licn brown cupi'cnc is cvalrincyl in 2 .Idtied i n p w ( i f : nitrogen at 700°F. t h e rlectron-tliffiactioii p a t t e r n becomcs soincn-hat sharper a n d c>lrwly approaclics that normwlly g i w n by acetylcnc black (3). This confirms t h e roi,lic,r rlaiins of t h c a u t h o r (4) t h a t when this polymer is h a t e d t o abovc 300°C. a partial tlisintcyi~ntion t o carbon blark occurs. CoIisequently i t appcnrs doubtful t o :issum(* t h s t tliffrnctioii pattcrxis of this material arise from t h c polymer alonc. for it is rqually possihlr t h a t disintegration products contribute.
472
JOHX H. L. TVATSOX
procoss which not only lengthens the necks but distorts the particle shapes permanently in a manner similar to that achieyed by stretching tar. I n electron micrographs the alprene loolis very similar to normal liquid latex except for the presence of the neclis, which in stereoscopic pairs give it the appearance of molecular models. This type of alprene throws no x-ray or electron diffraction pattern, a fact accounted for by its quasi-liquid nature. It is very questionable, of course, that alprene particles can be identified absolutely with Lind's cluster-ions; a t any rate many more than nineteen or twenty neutral acetylene molecules are contained in clusters the size of these particles, and from photographic contrast in the micrographs m know that all of these
ME IN
'
D I 4"JETER
bTANDARD DEY14TI0,
I
, APPROXIMATELY 65 PER CENT OF TEE PARTICLES LIE BLTTEES
mi.l
130 210 220 1x0
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its in this case are again structureless films which contaminate all surfaces inside the instrument, including those of thr1 specimen itielf. I t should be noted that the brown atains \\-hidl alu :tys o c ~ u ron metal surfaces under electron bombardment in L U C I L Oare bignificant only to the extent that they indicate contamination by hydrocarbon polymerization. Electron microscopy of samples of electron and corona radiation polymerization product- formed in controlled experiments may thron. considerable light not only on aome of the problems of radiatioil chemistry but wl>o on the subject of contamination in electron microscopes. One fact concerning the coiit:iminatioii phenomenon IS particularly ivorth noting here. The depositiori of contaminant is alnayb greatly enhanced at or near a conducting, electrically grounded surf:ice, indicating that the process of polyrnerization and deposition i.; electrical and that probably a negative free radical or ion iz involved. -ilthough the polymerization mechanism for tlit. ioirnation of hydi ogen cyanide polymer ib probably similar to that for dprrne, tlic appearance in micrographs The ihapes of the particle5 much more rariable, i b soinell-hat different. necessitating somc choice of the particle shape in order to secure the statistical constants giren iii the tablci. Sirice the major portion of the material is compo,ed of round particle., theae alonc are measured to give the tabled values. Some :ire wtremely huge :uid Lire :ible to :~bsorbsufficient energy fiom the electron beam to aypeai' to become ley> vi$cou- :tiid oacillatc like suspended liquid droplets. Often one large particle so bombarded \\ill separate into t n o or three. All of the sample material is composed of smooth rounded units, a fact nhich indicates its semiliquid natuie. There is some evidence for joining necks between the particles, but these are not as obvious as they are in alprene. The cyanogen polymers formed uiidei- alpha-ray bonibnrdment h a w a much reduced mean particle size. -111 of the groups seen in the micrographs appear t o be aggregates of small p i t i c l w of mean diameter about 130 m,u. These particles are often straight-eclged, do not :ippew to be joined together by necks, :md from their appearance are proba1)lj- solid. From the samples studied t o d:Lte we may conclude: ( I ) that the alpha-ray polymers deposit us fairly laige, definite particles Jvhose size and shape are recognizable; (2) that the electron or coron:i polymers deposit as films possessing extremely fine structure, (,?) that catalytically grov 11 polymerq are formed as fibers; ( 4 ) that the appearance of these fibers varies according to the catalyst used; and (2)that polyrneis forinrd from the same source substance, but polymerized in different n z i p , are diBerent rnaterials and should be notated as such. These results are necessarily preliminary and qualitative or qemiquantitative, but they ma\- sert'e to incrcase the grouing bocly of data nhich uill be used to clarify our knoil-ledge of the mechanisms of photochemical and radiation-chemical
474
CH.1HLES 1IOSESBLI.M
phenomena. Possible future applications of electron microscopy in this subject may involve: (1) examination of polymerization products from controlled slowelectron reactions; ( 2 ) classification of the microphysical properties of a large number of substances polymerized by a \vide yariety of types of radiat'ion; and ( 3 ) examination of the polymerization products which probably OCCUT to *'kill" Geiger-counter tubes. 1t E: FF: It P X CXE: s ( I ) \ Y A T ~ O S .J . 11. I..: , J , I'tiys. ('olloid ('hem. 61,654 (I!IiT , (21 \Y.~Tso~Y.J . H. I,,: J . .lpplieil I'hgs. 18, 153 (1!117~, (3) WATSON,J. H. I,.: Thc Elcrt rorhcmirnl S o v i r t y, I'repriiit 92-4, h u t u r n n tnccting, 194i. ( 4 1 ~ ' A T S O S . J. 13. I,.. .\xi> llC)C"EMICXL POT,T~IERTZ_ITIOSOF A~C'I
