Presentation on theme: "Lecture # 9 Polymer structure Characteristic,Applications and processing of polymers Learning objectives: 1- Describe a typical polymer molecule in terms."— Presentation transcript:
Lecture # 9 Polymer structure Characteristic,Applications and processing of polymers Learning objectives: 1- Describe a typical polymer molecule in terms of its structure and how the molecule may be generated from repeating unit. 2- Cite the differences in behavior for thermoplastic and thermosetting polymers. 3- Describe the mechanical properties of polymers. 4-State the applications and processing of polymers.
2 2 What are Polymers? Greek word Poly = many; Mer = unit Polymer = many units The term polymer denotes a molecule made up by the repetition of smaller molecules, the monomer. The repeating structure is mostly based on a carbon backbone.
3 Polymers Polymer = Poly + mer Polymerization MersPolymer
THERMOPLASTIC AND THERMOSETTING POLYMERS Thermoplastics: Thermoplastics soften when heated (and eventually liquefy) and harden when cooled—processes that are totally reversible and may be repeated. Thermosetting polymers are network polymers.They become permanently hard during their formation, and do not soften upon heating. THERMOSETTING POLYMERS
POLYMER CRYSTALLINITY The crystalline state may exist in polymeric materials. However, since it involves molecules instead of just atoms or ions, as with metals and ceramics, the atomic arrangements will be more complex for polymers. We think of polymer crystallinity as the packing of molecular chains to produce an ordered atomic array. Crystal structures may be specified in terms of unit cells, which are often quite complex.
DEFECTS IN POLYMERS
Mechanical Behavior of Polymers STRESS–STRAIN BEHAVIOR Three typically different types of stress–strain behavior are found for polymeric materials, as represented in Figure Curve A illustrates the stress–strain character for a brittle polymer, inasmuch as it fractures while deforming elastically. The behavior for a plastic material, curve B, is similar to that for many metallic materials; the initial deformation is elastic, which is followed by yielding and a region of plastic deformation. Finally, the deformation displayed by curve C is totally elastic; this rubber-like elasticity (large recoverable strains produced at low stress levels) is displayed by a class of polymers termed the elastomers
Polymer Synthesis and Processing Polymerization Polymer Additives The final sections of this lecture treated synthesis and fabrication techniques for polymeric materials. Synthesis of large molecular weight polymers is attained by polymerization, of which there are two types: addition and condensation. The properties of polymers may be further modified by using additives; these include fillers, plasticizers, stabilizers, colorants, and flame retardants. Forming Techniques for Plastics Fabrication of plastic polymers is usually accomplished by shaping the material in molten form at an elevated temperature, using at least one of several different molding techniques—compression, transfer, injection, and blow. Extrusion and casting are also possible.
Fabrication of Fibers and Films Some fibers are spun from a viscous melt, after which they are plastically elongated during a drawing operation, which improves the mechanical strength. Films are formed by extrusion and blowing..