Lecture # 9Polymer 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.

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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.

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Polymers

Polymer = Poly + mer

Polymerization

Mers Polymer

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Poly(ethylene) (PE)

Poly(tetrafluoroethylene)

Poly(vinyl chloride) (PVC)

Poly(propylene) (PP)CH3

(PTFE Teflon)

Cl

F

FF

F

H

HH

H

Other Examples:

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Chain Microstructure

Polymer structure chain:

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 BEHAVIORThree typically different types of stress–strain behavior are found for polymeric materials, as represented in Figure 15.1. 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

PolymerizationPolymer Additives•The final sections of this lecture treated synthesis and fabrication techniques forpolymeric materials. Synthesis of large molecular weight polymers is attained bypolymerization, of which there are two types: addition and condensation. The propertiesof 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..