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Conformational properties of isolated macromolecules.

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Die Angewandte Makromolekulare Chemie 22 (1972) 1-2 ( N r . 323)
From the Department of Chemistry, Dartmouth College, Hanover, New Hampshire,
03755. USA
Conformational Properties of isolated
(Eingegangen am 15. April 1971)
The properties of many kinds of solid material (e. g., ionic crystals, quartz, etc.)
appear to have only a tenuous correlation with those of the isolated individual atoms
or molecules of which they are made. I n macromolecular substances, however, thanks
primarily to the great strength of covalent bonds, such correlations are strong and
indeed essential to fundamental understanding of structure-property relationships.
With certain exceptions, such as polypeptides and polynucleotides with their remarkable helix-coil transformations, the conformational properties of macromolecules in liquid or amorphous environment are only secondarily dependent on the nature or concentration of diluents; and in many cases the molecular conformation
observed in the crystalline state is also the most stable conformation of the isolated
chain. There are also strong correlations between the rates of conformational change
for isolated macromolecules in dilute solution and various non-equilibrium properties
of polymeric substances in bulk.
During the past decade, great progress has been made in the analysis and prediction of macromolecular conformations. Detailed accounts may be found in the books
and by FLORY.
By combination of several types of experimental evidence with semi-empirical and theoretical considerations, it has been possible to construct fairly reliable potential energies governing
the conformations of a wide variety of chain molecules. An important simplifying
feature of the results is that for many purposes it is sufficient to focus attention on
the various minima of the potential energy, corresponding to the so-called rotationalisomeric states. Another essential feature is that as a rule one finds strong interdependence of the rotational states of adjoining bonds in the chain backbone; a simple
example is found in the “pentane effect” for parafiin chains, which renders highly
improbable the occurence of two successive gauche rotations of opposite sense. An
important exception is seen in polyamides, where the essentially rigid planarity of
On request of the author only the summary is published, as the lecture has been
a general review and no original paper.
Presented at the ScientificSymposium “The Physics and Chemistry of Fibre Materials”, Miinchen 3rd and 4th June 1971.
the amide link allows independence of the rotations of the immediately adjacent
bonds. Inclusion of other rigid or semi-rigid elements in the chain backbone, such as
the rings in cellulose and in terephthalate polyesters, leads to similar effects, but in
detail each chain is sui gene&. The conformations of chains bearing asymmetric
centers (e. g., PVC, polypropylene) depend strongly on tacticity and stereoregularity.
For the discussion of non-equilibrium properties a t a fundamental level, the maxima (barriers between minima) of the conformational potential energy also play an
essential role, but at present most theories of viscoelasticity, for example, are based
on less realistic models, such as beads-and-springs, random-flight chain, KRATKY
wormlike chain, etc. The elaboration of these models and their relation to the
detailed molecular structure is currently an active field of study.
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conformational, properties, macromolecules, isolated
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