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.

Some Organic Monomer Types

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…

Solid & Crystalline Forms

Amorphous – Polydiacetylene – (100%) Crystalline

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.

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

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.