Abstract
Perhaps no other class of organic molecule has been the subject of quantitative electron crystallographic structure analyses than the linear polymers. Given the real restrictions to crystallization of these materials, the reason for this is very clear, since one can at least obtain single crystal electron diffraction patterns from the chain-folded lamellae for determination of unit cell dimensions and symmetry and for measurement of diffracted intensities. The usual methodology for crystal structure analysis with the observed intensity data is based on the procedure used in fiber X-ray analyses. That is, one determines the X-ray crystal structures of representative monomer and/or oligomer segments of the polymer chain to see what part of the repeat can be held conformationally rigid. These rigid units are concatenated through “linkage bonds”, around which conformational twists are allowed and the search for the stable chain structure is based on the simultaneous minimization of the crystallographic R-factor and an internal energy calculated from nonbonded atomatom potential functions.
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