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addressed in the article in this issue by Peter F. Green and Edward J. Kramer concerning self and mutual diffusion of macromolecules. Here the focus is on testing ideas of S.F. Edwards 2 and P. deGennes 3 on the reptation model of polymer diffusion. Green and Kramer utilize forward recoil spectrometry to monitor the motion of deuterated polymers into a hydrogenous matrix.

Significant improvements in chemical synthesis and a growing collaborative effort between polymer chemists and materials scientists have resulted in the availability of extremely well-defined materials.... The spaghetti picture of the condensed state of noncrystalline polymers needs to be augmented with another image from the kitchen, that of uncooked linguine, to recognize the class of rigid chain polymers. Such macromolecules were first synthesized extensively in the mid 1970s. They have found ever increasing use as novel materials due to their propensity to form anisotropic liquids (liquid crystals) either with certain solvents (lyotropic state) or upon melting (thermotropic state). In this issue, Alan Windle overviews this area of liquid crystalline polymers which, while related to low molar mass liquid crystals, have special features unique to macromolecules. These properties, of course, owe their nature to the inherent molecular shape anisotropy of liquid crystalline polymer molecules. The large shape anisotropy (L/D ratios of 1,000 or more) is accompan i e d by an e n o r m o u s b o n d i n g force a n i s o t r o p y s t e m m i n g from s t r o n g intramolecular covalent bonds with weak intermolecular secondary bonds. The sideby-side packing of the macromolecules to form ordered melts gives rise to easy alignment via flow and the fabrication of articles with ultrahigh molecular orientation. The advent of liquid crystallinity in polymers dates back to Robertson's work with PBLG in the 1950s.4 The commercial success of Dupont's KEVLAR™ fiber, which is spun from a lyotropic solution of the rigid chain molecule in sulfuric acid, derives

from the outstanding mechanical properties of the material and its ability to withstand high temperatures. Indeed, there has grown an intensive interest in the development of high performance fibers, and the article in this issue by W. Wade Adams and Ronald K. Eby updates this subject (including efforts to form ultrao r i e n t e d f i b e r s from f l e x i b l e c h a i n molecules). It is worthwhile to point out to nonpolymer materials scientists (who may consider polymers flimsy cousins to metals and ceramics) the truly impressive physical properties currently achieved with these materials. Tensile moduli and strengths are 1.5-2.0 times those of steel! With densities typically 1.5 g/cm3, specific properties of high performance fibers are over an order of magnitude higher than those of steel! This area of materials research is primarily driven by the aerospace industry with an eye for ever better fiber reinforcements for composite materials. A novel thrust in the direction of reinforced materials is tha