Chemistry Reference
In-Depth Information
breaking. These limitations are overcome industrially by so-called gel spinning.
In this operation a mixture of the polymer and diluent is extruded and stretched,
the diluent is removed, and the product is given a final stretch. Use of a diluent,
such as a low-molecular-weight hydrocarbon in the case of polyethylene, facili-
tates slippage of entanglements and high elongations. Full extension of all the
macromolecules in a sample requires that the ratio of the stretched to unstretched
fiber lengths (draw ratio) exceeds the ratio of contour length to random coil end-
to-end distance (Section 1.14.2.1). For a polyethylene of molecular weight 10
5
and degree of polymerization about 3600, this ratio would be 60 if the molecules
behaved like fully oriented chains. When allowance is made for the effects of
fixed bond angles and restricted rotational freedom on the random coil dimen-
sions and the contour length, this ratio is calculated to be about 27. This corre-
sponds more or less to the degrees of stretch that are achieved in the production
of “superdrawn fibers” of thermoplastics, although not all the macromolecules
need to be fully extended to achieve optimum properties in such materials. These
products have stiffnesses and tensile strengths that approach those of glass or steel
fibers. The crystal superstructures of fibers of the rodlike macromolecules men-
tioned in
Section 4.6
are similar to those of superdrawn thermoplastics. The for-
mer do not require high draw ratios to be strong, however, because their
molecules are already in a liquid crystalline order even in solution.
During high-speed extrusion processes such as those in fiber and film
manufacturing processes, crystallization occurs under high gradients of pressure
or temperature. The molecules in the polymer melts become elongated and ori-
ented under these conditions, and this reduces their entropy and hence the entropy
change
H
m
is not affected, the
equilibrium crystallization temperature is increased (
Eq. 4-1
) and nucleation and
crystallization start at higher temperatures and proceed faster in such processes
than in melts that are cooled under low stress or quiescent conditions.
In addition to the various morphological features listed, intermediate supermo-
lecular structures and mixtures of these entities will be observed. The mechanical
properties of finished articles will depend on the structural state of a semicrystal-
line polymer, and this in turn is a function of the molecular structure of the poly-
mer and to a significant extent also of the process whereby the object was
fabricated.
Δ
S
m
when these molecules crystallize. Since
Δ
4.4
The Glass Transition
The mechanical properties of amorphous polymers change profoundly (three
orders of magnitude) as the temperature is decreased through the glass transition
region. The corresponding changes in the behavior of semicrystalline polymers
are less pronounced in general, although they are also evident. At present, we do
not have a complete theoretical understanding of glass transition, particularly the
