Chemistry Reference
In-Depth Information
produces fewer nuclei and larger spherulites. The polymeric structures produced
under such conditions are more likely to be brittle than if they were produced by
faster cooling from the melt. This is because there will be fewer interspherulitic
tie molecules and because low-molecular-weight uncrystallizable matter will have
had more opportunity to diffuse together and produce weak boundaries between
spherulites.
The supermolecular structures developed on fast cooling of crystallizable
polymers change with time because of secondary crystallization. A parallel phe-
nomenon is the progressive segregation of mobile uncrystallizable low-molecular-
weight material at storage temperatures between
T
g
and
T
m
. This will also result
in a gradual embrittlement of the matrix polymer. A useful way to estimate
whether an additive at a given loading can potentially cause such problems over
the lifetime of a finished article is to accelerate the segregation process by delib-
erately producing some test specimens under conditions that facilitate slow and
extensive crystallization.
The type of nucleation that produces spherulitic supercrystalline structures
from quiescent melts is not the same as that which occurs more typically in the
industrial fabrication of semicrystalline polymer structures. The polymer mole-
cules are under stress as they crystallize in such processes as extrusion, fiber spin-
ning, and injection molding. The orientation of chain segments in flow under
stress results in the formation of elongated crystals that are aligned in the flow
direction. These are not folded chain crystallites. The overall orientation of the
macromolecules in these structures is along the long crystal axis rather than trans-
verse to it as in lamellae produced during static crystallization. Such elongated
chain fibrils are probably small in volume, but they serve as a nucleus for the
growth of a plurality of folded chain lamellae, which develop with their molecular
axes parallel to the parent fibril and their long axes initially at right angles to the
long direction of the nucleus. These features are called
row structures
,or
row-
nucleated structures
, as distinguished from spherulites. Row-nucleated micro-
structures are as complex as spherulites and include tie molecules, amorphous
regions, and imperfect crystallites. The relative amounts and detailed natures of
row-nucleated and spherulitic supercrystalline structures in a particular sample of
polymer are determined by the processing conditions used to form the sample.
The type or organization that is produced influences many physical properties.
Other supercrystalline structures can be produced under certain conditions. A
fibrillar morphology is developed when a crystallizable polymer is stretched at
temperatures between
T
g
and
T
m
. (This is the orientation operation mentioned in
Section 1.4). Similar fibrillar regions are produced when a spherulitically crystal-
lized specimen is stretched. In both cases, lamellae are broken up into folded-
chain blocks that are connected together in microfibrils whose widths are usually
between 60 and 200
10
2
8
cm. In each microfibril, folded-chain blocks alternate
with amorphous sections that contain chain ends, chain folds, and tie molecules.
The tie molecules connecting crystalline blocks along the fiber axis direction are
principally responsible for the strength of the structure. Microfibrils of this type
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