Manufacturing Technology Tutorials
The Conformation and Configuration of Polymer Molecules
The conformation (morphology) and configuration of polymer molecules are important because these factors affect the properties and behavior of the polymer in manufacturing processes. Both conformation and configuration are related to the organic chemistry principles on which polymer science is based.
The conformation of a molecule describes the preferential positions of atoms on a molecule.
Polymer plastic molecules all contain carbon. Carbon is tetravalent, meaning that it can bind with four other substituents (atoms or other molecules), forming a symmetric tetrahedral geometry.
Tetrahedral is another intimidating word. A tetrahedral molecule has four lateral planes. The concept is best demonstrated by looking at an actual tetrahedral molecule (called a tetrahedron). Below is a schematic drawing of methane (CH4), which consists of one carbon atom bonded to four hydrogen atoms. The tetrahedral geometry of methane sets each bond angle at 109.5 degrees.
Configuration describes the spatial layout of a molecule. Polymers are built on carbon backbones. Side groups (such as a styrene [C8H8]) are then attached to this carbon-carbon chain. The order in which these groups are arranged is the atacticity of the polymer.
If the side groups are arranged randomly, then the polymer is described as atactic. A polymer that has all its side groups on one side is isotactic. And if the side groups alternate between one side and the other, the polymer is syndiotactic.
Also be aware that some polymers contain double carbon-carbon bonds in their backbones. Double bonds restrict the rotation of the carbon atoms on the axis polymer’s backbone.
Thermoplastic polymers are grouped into three broad categories based on their conformation:
Amorphous thermoplastics: These are polymers that have a random molecular structure. When heated, their molecular chains become disentangled. When cooled, these polymers return to a rigid state. Examples of amorphous plastics include polystyrene and ABS (acrylonitrile butadiene styrene).
Semi-crystalline thermoplastics: In chemistry, crystal is a generic term for a regularly ordered, repeating molecular pattern.
The polymer molecules of semi-crystalline thermoplastics form more orderly arrangements than those of the amorphous thermoplastics. The crystal structures of these molecules remain intact when heated, until the melt temperature of the material is reached, at which point the molecular structure becomes random and amorphous.
When the polymer cools, some of the material returns to the crystalline state, and some remains amorphous. The exact mixture of crystallinity and amorphousness in the cooled polymer depends on the chemical composition of the material, as well as the processing conditions.
Examples of semi-crystalline thermoplastics include polyethylene and polypropylene.
Liquid crystalline thermoplastics: These polymers retain their ordered molecular structure even when melted. They are extremely rigid, and offer a high degree of chemical resistance. Examples of liquid crystalline thermoplastics include Kevlar (DuPont trade name) and Ultrax (BASF trade name)
