Chemistry Reference
In-Depth Information
temperature
T
m
is expressed in K degrees.) The impact resistance and other
mechanical characteristics of semicrystalline polymers are dependent on crystalli-
zation conditions. The influence of fabrication conditions on the quality of articles
is much more pronounced with semicrystalline polymers than with metals or other
materials of construction, as a consequence.
4.3.2.3
Morphology of Semicrystalline Polymers
The morphology of a crystallizable polymer is a description of the forms that
result from crystallization and the aggregation of crystallites. The various mor-
phological features that occur in bulk crystallized polymers are reviewed in this
section.
Crystalline lamellae are the basic units in the microstructures of solid semi-
crystalline polymers. The lamellae are observed to be organized into two types of
larger structural features depending on the conditions of the bulk solidification
process.
The major feature of polymers that have been bulk crystallized under quies-
cent conditions are polycrystalline structures called
spherulites
. These are roughly
spherical supercrystalline structures which exhibit Maltese cross extinction pat-
terns when examined under polarized light in an optical microscope. Spherulites
are characteristic of semicrystalline polymers and are also observed in low-
molecular-weight materials that have been crystallized from viscous media.
Spherulites are aggregates of lamellar crystallites. They are not single crystals
and include some disordered material within their boundaries. The sizes of spher-
ulites may vary from somewhat greater than a crystallite to dimensions visible to
the naked eye.
A spherulite is built up of lamellar subunits that grow outward from a com-
mon nucleus. As this growth advances into the uncrystallized polymer, local inho-
mogeneities in concentrations of crystallizable segments will be encountered. The
folded chain fibril will inevitably twist and branch. At some early stage in its
development the spherulite will resemble a sheaf of wheat, as shown schemati-
cally in
Fig. 4.5a
. Branching and fanning out of the growing lamellae tend to cre-
ate a spherical shape, but neighboring spherulites will impinge on each other in
bulk crystallized polymers and prevent the development of true spherical symme-
try. The main structural units involved in a spherulite include branched, twisted
lamellae with polymer chain directions largely perpendicular to their long axes
and interfibrillar material, which is essentially uncrystallized. This is sketched in
Fig. 4.6
.
The growth of polymer spherulites involves the segregation of noncrystalliz-
able material into the regions between the lamellar ribbons. The components that
are not incorporated into the crystallites include additives like oxidation stabili-
zers, catalyst residues, and so on, as well as comonomer units or branches. The
spherulite structures and interspherulitic boundaries are held together primarily by
polymer molecules which run between the twisted lamellar subunits and the
spherulites themselves. Slow crystallization at
low degrees of supercooling
