Among the naturally-occurring polymers are cotton, silk, cellulose, proteins and DNA, natural rubber, and amber whereas those of synthetic polymers are nylon, polyethylene, polyester, Teflon, epoxy, synthetic rubber, silicone, polyvinyl chloride (PVC), and neoprene. As covalently bonded structures of macromolecules, polymers can be modified and be formed in chains that are linear, branched, cross-linked, or networked.
Like any other molecule or substance with certain characteristics, a polymer bears properties specific to its own composition and nature. Polymeric properties have been determined based on the identity of constituent monomers, the arrangement of these monomers along with repeating units into a ‘microstructure’ within a polymer, the phase behaviour, the polymer morphology, as well as the mechanical and chemical properties of a polymer.
Critical to the understanding of the morphology and phase behaviour of polymers is the temperature, for the degree of crystallinity of a polymer is a function of temperature so that through temperature variation, one can decide when a polymer becomes either crystalline or amorphous. A polymer transitions from a crystalline phase to an amorphous phase upon reaching its melting point. At high temperatures in which polymers behave as viscous liquid, thermoplastic polymers soften unstably while thermosetting polymers harden permanently where thermosets, such as epoxy and polyester, are found to be more brittle and dimensionally stable than thermoplasts, like polyetheretherketone, upon heating. During cooling period, however, a polymer transforms from a rubbery-viscous liquid phase to an amorphous solid phase, deforming elastically at low temperatures. Adjusting the temperature affects the modulus of elasticity of a polymer such that a range of polymeric material – whether viscous,