RESEARCH
I U P A C symposium sheds light on folded p o l y m e r chains, n e w concept of polymer crystals idLow polymer chains c a n b e packed into a crystal form still isn't completely understood—and it's a subject on which iome very highly qualified opinions differ. At the International Union of Pure and Applied Chemistry's symposium on macromolecules in Wiesbaden, Germany, explanations of work on several different facets of this problem seemed to bring it closer to solution. One of the b i g questions has been low polymer chains are arranged with a »eries of regular, very sharp folds, as proposed by Dr. Andrew Keller of the university r.V Bristol in England. The : oIded chain concept was brought out )y Keller a couple of years ago, al:hough it had been suggested as far j a c k as 1938. T h e problem is to explain how long polymer molecules can fît into very :hin crystal layers a n d still remain Derpendicular to the flat crystal surraces. T h e crystals of polyethylene, for example, are only about 100 A. thick, vhile the molecules are m u c h longer h a n that. However, x-ray diffraction ;hows that the chains a r e oriented so h a t they must lie in this short space. The theory of chains sharply folded at egular intervals was Dr. Keller's explanation. H e a n d Dr. A. O'Connor, also of Bristol University, found b y making :-ray studies of a series of straight chain îydrocarbons (ranging from waxes of nolccular weighstr of a b o u t 10OO to inear polyethylene of molecular veight about 100,000) that as the hyIrocarbon chains become larger a stage s reached w h e r e they d o not remain traight in the crystal, b u t fold back on hemselves. This means there is a continuity between paraffins and polyners. T h e experiments also showed hat fold length does not change with ham length, even when molecular «/eight changes by a factor of about 00. T h e best w a y to have done this xperiment, Dr. Keller says, would have ieen to start with paraffins a n d syntheize the higher members of tîi^ series 11 the w a y up t o the long-chain polyîers. Lacking facilities for doing this, e degraded linear polyethylene ( Mar-
lex 50) by heating and separated the fractions in solvents, from which pure crystals were formed. T h e fractionated samples ranged from a hard polymer to soft waxes. X-ray studies showed spacings in the range of 1O0 to 120 A. for most samples, decreasing gradually to about 40 A. for the softest wax. Just w h y the fold l e n g t h should be regular still hasn't been proved. T h e fold length (or periodicity) will vary when samples a r e crystallized at different temperatures, w h i c h indicates that there is n o regularly recurring chemical difference along the hydrocarbon chain which w o u l d cause it to fold. Thermodynamic and kinetic explanations have been offered, but not proved. • Amorphous Crystals. A n e w concept of crystallinity in polymers was advanced by Dr. V. A. ICargin of the Academy of Sciences of the U.S.S.R. in Moscow. He told t h e symposium that the classical methods of x-ray diffraction analysis give t h e polymer chemist a very narrow v i e w of a polymer—a view that is V 1 0 to V 1 0 0 of the length of a polymer c h a i n . Electron microscopy and electron
beam studies in recent years h a v e m a d e it possible to determine the over-all form a n d arrangement of polymer molecules. Dr. Kargin has found that sometimes a polymer may form particles which have regular geometric forms like crystals, b u t which seem to be amorphous when examined b y x-ray diffraction. A number of observations of this sort have led him to conclude that there are two types of elementary structural units from which more complicated structures can be built in a polymer. One type of unit is the globule, which consists of a few irregularly twisted polymer molecules. T h e other type of unit is t h e packet of a few polymer chains, stretched so that they have an ordered arrangement. Of these two, only the packets can serve as the starting points for growth of true crystals. These packets are formed only by straight molecules, such as linear polyethylene. Formation of globules precludes growth of true crystals. However, globules, and also packets, can arrange themselves into aggregates having regular geometric forms. But their interference patterns and thermodynamic properties are similar to those of amorphous substances, and they are not true crystals in t h e usual sense of the word. Their molecules apparently are ordered to a certain degree, but they are not as highly ordered as those of a true crystal.
MEETING OF MINDS. D r . Malcolm Dole (left) of Northwestern University talks with D r . Andrew Keller of the University of Bristol, E n g l a n d , between sessions of the IUPAC s y m p o s i u m on macromolecules at Wiesbaden's Rhein-Main-Halle NOV.
2,
19 59
C&EN
39
