1. Polymers:
Molecular Structures and Properties
• The properties of polymers are
directly affected by their atomic and
molecular structures.
• In this lecture, we will explore how
polymer molecules are made up, and
how factors affect their
– strength,
– stiffness, and
– other properties.
3. Crystallinity in Polymers
• Most polymers, unlike metals, do not every
fully crystallize; some are fully amorphous
(no crystallinity at all)
– Polymers that partially crystallize upon
cooling from the melt state are called
semicrystalline polymers
– Factors affecting crystallinity
• Linearity of the polymer chain
• Presence of more than one monomer type
• Arrangement of side groups on the
backbone
• processing conditions: how the materials is
shaped into products.
– The following slide shows a polymer
material in both molded (solid) and fiber
forms…
5. Factors Affected by
Crystallinity
• The degree of crystallization (volume fraction of
crystalline regions in a polymer) affects the optical,
thermal, and mechanical properties of a polymer
– Optical properties
• Semicrystalline polymers can be recognized easily
because they are opaque, light is not transmitted
through the polymer; rather, it is reflected or scattered
by the crystalline planes
• amorphous polymers are clear: visible light can pass
through them
– Thermal properties – Semicrystalline polymers will
have a definite melting point range at which the
crystalline volume is destroyed; amorphous polymers
do not have a defined melting range because they are
already without any crystallinity
– Mechanical properties – as a polymer’s crystallinity
increases, so also will its level of secondary bonding.
This in turn increases the stiffness (elastic modulus)
and strength of the polymer. On the other hand,
ductility declines as crystallinity increases.
6. Processing Conditions Affecting
Crystallinity
• Deformation from applied stress
– Processes such as extrusion of narrow
sections, fiber pulling (pultrusion), and
molding all affect the degree of
crystallinity in a polymer
– Deformation from directional stress has the
effect of orienting polymer chains in the
direction of applied force; in extrusion and
pultrusion chains are oriented along the
direction of extrusion and along the fiber
axis. This is called uniaxial orientation
– If stress is applied in more than one
direction, such as in blowmolding, polymer
chains are directed in a biaxial manner,
typically in a plane
7. Disadvantages of Crystallinity
• While crystallinity is a powerful tool
for improving strength and stiffness,
such strengthening is almost always
directional – that is, the properties
are not the same in all directions.
• A crystalline polymer will be much
stronger in the direction of molecular
alignment, and much weaker at right
angles to that alignment.
• Such materials, whose properties are
not the same in all directions, are
called anisotropic.
