Polymer morphology
Polymer
morphology generally describes the arrangement and microscale ordering of
polymer chains in space.
Crystallinity
When
applied to polymers, the term crystalline has a somewhat ambiguous usage. In
some cases, the term crystalline finds identical usage to that used in
conventional crystallography. For example, the structure of a crystalline
protein or polynucleotide, such as a sample prepared for x-ray crystallography,
may be defined in terms of a conventional unit cell composed of one or more
polymer molecules with cell dimensions of hundreds of angstroms or more.
A
synthetic polymer may be lightly described as crystalline if it contains
regions of three- dimensional ordering on atomic (rather than macromolecular)
length scales, usually arising from intramolecular folding and/or stacking of
adjacent chains. Synthetic polymers may consist of both crystalline and
amorphous regions; the degree of crystallinity may be expressed in terms of a
weight fraction or volume fraction of crystalline material. Few synthetic
polymers are entirely crystalline.
The
crystallinity of polymers is characterized by their degree of crystallinity,
ranging from zero for a completely non-crystalline polymer to one for a
theoretical completely crystalline polymer. Polymers with microcrystalline
regions are generally tougher (can be bent more without breaking) and more
impact-resistant than totally amorphous polymers.
Polymers
with a degree of crystallinity approaching zero or one will tend to be
transparent, while polymers with intermediate degrees of crystallinity will
tend to be opaque due to light scattering by crystalline or glassy regions.
Thus for many polymers, reduced crystallinity may also be associated with
increased transparency.
Chain conformation
The space
occupied by a polymer molecule is generally expressed in terms of radius of
gyration, which is an average distance from the center of mass of the chain to
the chain itself. Alternatively, it may be expressed in terms of pervaded
volume, which is the volume of solution spanned by the polymer chain and scales
with the cube of the radius of gyration.
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