RESEARCH
• With aromatic hydrocarbons, T C N E will form intensely colored complexes in a new way to characterize aromatic hydrocarbons—yellow with benzene, orange with toluene, a n d red with xylenes. H o w m a n y of the n e w chemicals will eventually b e c o m e commercial is uncertain, but D u Pont will likely explore the more promising prospects. H o w ever, T C N E is only m a d e in lab amounts n o w , and there a r e n o firm plans to commercialize it y e t .
This photomicrograph shows spherulite surface structure on polyethylene films. A great deal of motion occurs w h e n a polymer crystallizes
New Light on Crystallinity Proposed mechanism for spherulite growth may b e basis for mathematical t r e a t ment o f crystallization To u n d e r s t a n d how semicrystal.NATIONAL line polymers bef MEETING have often m e a n s NATIONAL l i n e polymers be_ _ ^Pèlynier _ MEETING have often m e a n s ? you h a v e to know Chemistry how spherulites grow. Various mechanisms have explain this growth, been proposed t o explain this growth, b u t all have t h e same flaw: They fit only specific cases or narrowly-defined systems. Now, however, C, F . H a m m e r a n d P. H . Geil, Jr., of D u Pont, have come u p with a n e w general mechanism for growth of spherulites and crystallites (spherulites are aggregates of submi-
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croscopic crystalline regions, or crystallites, e m b e d d e d in an amorphous matrix—the so-called fii$e structure of a polymer). Geil told the Division of Polymer Chemistry's Symposium on the Crystalline State t h a t the proposed mechanism is "a first step" toward a general theory because only linear polymers without bulky side groups have been investigated. But he expects the new mechanism will furnish a basis not available before for mathematical treatment of crystallization. The mechanism, says Geil, is based on these hypotheses: Molecular segments in t h e melt are highly mobile; there is a critical crystallite size that d e p e n d s on temperature. In a mass of molten polymer 2 0 ° to 3 0 ° C. above t h e melting point, segmental motion is rapid; jumps occur more often than 10 6 per second. These jumps produce spontaneous order in small regions for very short intervals. A t some temperature, Geil says, the critical crystallite size will be the same as chat of t h e ordered regions produced b y the segmental jumps. At this point, nucleation begins, and a stable crystallite forms a n d grows. The crystallite can grow in two ways: alignment of additional molecule segments already in the nucleus; addition of new molecule segments. W h i l e the first crystallite grows, other nearby regions, subcritical in size, will become stable and grow ( n u c l e a t e ) . These crystallites in turn will grow a n d induce further nucleation. According to Geil, spherulite growth results from a growing crystallite inducing nucleation in its vicinity. This theory, Geil says, has wide application, and m a n y experimental observations can b e explained on this basis. For example, spherulite growth r a t e increases as temperature decreases. T h e proposed mechanism explains it this way: Growth rate depends on rapid chain motion and the probability of secondary nucleation. This probability increases as the critical crystallite size decreases with decreasing temperature. • More Theories. In polyethylene, molecular weight has a direct effect on t h e degree of crystallinity, D o w Chemical's L. H. T u n g and S. Buckser told t h e symposium. By studying fractions of low-pressure-process polyethylene, relatively free of chain branches, T u n g finds that high molecular weight poly-
mers have a lower degree of crystallinity than low molecular weight polymers. For example, at a given temperature, a polymer with a molecular weight of 110,000 is 70% crystalline, while a polymer with a molecular weight of 2100 is 8 5 % crystalline. Tung uses a dilatometer to measure specific volume of the polymer fractions at a series of temperatures approaching the melting point, and calculates per cent crystallinity from these data. The relation between molecular weight and crystallinity holds until temperature is close to t h e melting point. Furthermore, as t h e temperature is raised, differences in crystallinity between polymer fractions of different molecular weight change very little. O n this basis, T u n g concludes that the low crystallinity of high molecular weight samples is not d u e to a r a t e effect. Wide differences in melting point of the poly-oj-olefins are apparently due to changes in chain flexibility, F . P. Reding a n d his coworkers at Union Carbide Chemicals told the symposium. Polypropylene h a s a higher melting point t h a n polyethylene. Reason for this, Reding says, is that t h e pendent methyl g r o u p on every other carbon atom decreases chain flexibility and raises the melting point. However, the next member of the series, polybutene, has a lower melting point. Reason for this, says Reding, is that the longer side chain is more flexible. This decrease in melting point continues in the series until ^polymers higher t h a n polyhexene a r e reached. Here the trend reverses itself, and melting points go up for higher members of the series. According to Reding, in this region the main chain no longer crystallizes; instead, the side chains crystallize in a normal polyethylene unit cell. Using stereospecifîc catalysts, Reding has p r e p a r e d a whole host of crystalline polymers from branched «-olefin monomers. These polymers can be arranged in homologous series, with a different series for each terminal group on the side chain. Each member of the series differs from the next in the number of methylene groups between the terminal group a n d the main chain. When arranged in this kind of series, the first member has the highest melting point, Reding says. Successively higher members have melting points 30° t o 6 0 ° C. lower until the point is reached where side chain crystallization takes over. APRIL
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