|Composites Design and Manufacture (Plymouth University teaching support materials)
Molecular shapes and morphology of polymer molecules.
Most polymers are based on the element carbon. This is the sixth element in the Periodic Table.
A number of on-line tutorials/textbooks on polymers are available:Case Western Reserve University
The Polymers and Liquid Crystals website at Case Western Reserve University has a useful series of pages:
Polymers are chemicals usually based on long carbon chains or 3-D networks of small repeating units.
Important concepts are electron orbitals, chirality, geometric isomers and the nomenclature.
The study of polymer science begins with understanding the methods in which these materials are synthesised.
Polymer synthesis is a complex procedure and can take place in a variety of ways.
The terms configuration and conformation are used to describe the geometric structure of a polymer and are often confused. Configuration refers to the order that is determined by chemical bonds. The configuration of a polymer cannot be altered unless chemical bonds are broken and reformed. Stereoregularity is the term used to describe the configuration (tacticity) of polymer chains. Conformation refers to order that arises from the rotation of molecules about the single bonds. These two structures are studied below.
Both crystalline and amorphous phases coexist in polymers.
Consider a comparison between glass, an amorphous material, and ice which is crystalline.
Despite their common appearance as hard, clear material, capable of being melted,
a difference is apparent when viewed between crossed polarizers.
The following site gives thermal transition data:
Glass transition temperature (Tg)
Most polymers can be defined by one or more thermal transitions.
The segmental motion of short sections of polymer chains can be "frozen out" by cooling.
This transition, known as the glass transition temperature (Tg), has a characteristic temperature value for each polymer type.
Below Tg the polymer usually acts as a brittle elastic solid.
Above Tg the polymer is normally tougher and has a viscoelastic response to stress.
Polymers are susceptible to creep above Tg.
Crystalline melting temperature range (Tm)
Polymer chains with a regular repeating unit can align to form crystalline
regions within an amorphous matrix.
It is rare for 100% crystallinity to be achieved, especially with commodity polymers.
The breakdown (melting) of the crystal structure occurs in a specific temperature range at a value characteristic of the polymer.
Note that amorphous (i.e. no crystallinity) polymers do not have a melting point.
As a broad rule-of-thumb Tm = Tg + ~200° C.
Note that a thermoplastic component to be used under stress at ambient temperature (~25° C)
will thus need to be processed at 250° C or higher to avoid creep in-service.
Processing temperature (Tp)
To manufacture thermoplastic polymer components, the most common procedure is
to heat the material above Tm,
form it to shape, then cool to below Tm to retain the form of the component.
The viscosity of a polymer normally decreases as temperature increases and hence forming becomes easier at higher temperatures.
Decomposition temperature (Td)
The backbone of a polymer chain will degrade at high temperatures.
This limits the processing and service temperatures of these materials.
Polymers with good performance at high-temperature normally have aromatic segments (notably the benzene ring)
in the backbone often with delocalisation of electrons over adjacent groups.
RA Pethrick, Polymer Structure Characterization: from nano to macro organization, RSC Publishing, 2007. ISBN: 978-0-85404-466-5.