Molecular architecture and classification of polymers

The main feature which sets polymers apart from other materials is that polymer samples are made up of long molecules. A typical sample of polyethylene may have molecules which contain an average 50 000 atoms and would be 25 OOOA long. The presence of these macromolecules has a dominant effect upon the properties of the polymeric material. There can be a considerable variation in the architecture of the individual molecules in different polymer samples. Although by definition polymer molecules are long they can be linear, branched or even in the form of a three-dimensional network as shown in Fig. 1.1. The particular type of architecture the molecules possess controls the properties of the material. 

Any attempts to classify polymers into different qategories tend to be somewhat arbitrary. One useful way is to piit them ihto groups displaying similar properties which also has the advantage reflecting the underlying molecular structure. This classification is outlined ip Fig. 1.2 where they are separated into three groups thermoplastics, rubbers and thermosets. In addition thermoplastics are separated into those whiqh are either crystalline or non-crystalline (amorphous). 

Thermoplastics, which are often referred to just as plastics are linear or branched polymers which can be melted upon the application of heat. They can be moulded and remoulded using conventional techniques and now make up the largest bulk of polymers used. Thermoplastics can be sub-divided into those which crystallize on cooling and those which do not and are normally used as polymer glasses. The ability of the polymers to crystallize depends upon many factors such as the degree of branching and the regularity of the molecules. However, crystalline thermoplastics are invariably only partly (semi-) crystalline and do not crystallize completely when cooled from the melt. 

Rubbers are materials which display elastomeric properties, i.e. they can be stretched easily to high extensions and will spring back rapidly when the stress is released. This extremely important and useful property is a reflection of the molecular structure of the polymer which consists of a lightly cross-linked macromolecular network. The molecules slide past each other on deformation, but the cross-links prevent permanent flow and the molecules spring back to their original position on removal of the 

Thermosets are heavily cross-linked polymers which are normally rigid and intractable. They consist of a dense three-dimensional molecular network and, like rubbers, degrade rather than melt on the application of heat. Common thermosetting polymers include phenol-formaldehyde or urea-formaldehyde resins and high-performance adhesives such as epoxy resins. 

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