Polymers are long chain molecules with properties dominated by their chain behaviour and the nature of their chemical make-up or constitution. The distinction between thermoplastics and thermosets has become rather blurred with the development of new materials for more demanding environments than previously. They include high performance polymers which are more resistant to high temperatures, possess greater moduli or strengths, and can be combined with additives to enhance their intrinsic properties yet further. An understanding of the atomic and molecular construction of polymers provides an insight into how improved materials can be developed, in the subject of molecular engineering. It includes an understanding of both molecular configuration and conformation.
The manufacture of polymers from oil and gas feedstocks is dominated by a handful of processes operated in large scale petrochemical complexes. Intermediates and monomers are often interconvertible so that supply and demand can be matched. However, in the thermal cracking of naphtha the production of olefins varies within relatively narrow margins, so that excess quantities of propylene, for example, may be produced at medium and high cracking severity. Ethylene is the major petrochemical building block for vinyl chain growth polymers, and benzene and para-xylene are the main sources of aromatic structures within polymer repeat groups. Polymer consumption continues to grow and the range of both general-purpose materials and speciality polymers has widened considerably over the past two decades. Within each polymer type the range of grades available to the design engineer has also continued to widen, owing to the close control over molecular mass and structure that is available with the latest polymerization catalysts. Copolymerization, for example, is a major way in which structure can be tailored at a molecular level to produce the desired balance of physical properties in the end product. It allows both Tg and Tm to be modified to match the temperature scale of exposure of the end product, as well as affecting other properties like toughness and stiffness. A compromise between conflicting property demands often needs to be reached in the selection of the appropriate grade for a particular product. But there is an extra level of complexity in the effect of processing – it can affect molecular orientation and crystallisation and hence end properties. In addition, the constraints of processing can severely restrict the grades which can be processed effectively. So some compromise between design for function and design for manufacture must be planned in order to maximise and exploit the extra design freedom that polymers offer.