POLYMER STRUCTURE, MECHANICAL PROPERTIES AND APPLICATION

What is Polymer ? Most polymeric materials are composed oof very large molecular chains with side groups of various atom or organic material such as methyl, ethyl or phenyl groups. These macromolecules are composed of repeat units, smaller structural entities, which are repeated along the chain repeat unit repeat unit repeat unit C H Polyethylene (PE) Cl C H Polyvinyl chloride (PVC) H Polypropylene (PP) C CH3

Repeat units for some of the chemically simple polymers A homopolymer is one of which all of the repeat units are the same type The chains of copolymer are composed of two or more kind of repeat units

MOLECULAR WEIGHT • Molecular weight, Mi: Mass of a mole of chains. Lower M higher M Simple for small molecules All the same size Number of grams/mole Polymers – distribution of chain sizes Mw is more sensitive to higher molecular weights

Degree of Polymerization, n n = number of repeat units per chain ni = 6 Chain fraction mol. wt of repeat unit i

The terms configuration and conformation are used to describe the geometric structure of a polymer. Configuration refers to the order that is determined by chemical bonds. The configuration of a polymer cannot be altered unless chemical bonds are broken and reformed.  Conformation refers to order that arises from the rotation of molecules about the single bonds. These two structures are studied below.

Configuration The two types of polymer configurations are cis and trans. These structures can not be changed by physical means (e.g. rotation). The cis configuration arises when substituent groups are on the same side of a carbon-carbon double bond.  Trans refers to the substituents on opposite sides of the double bond.

Configuration of polymer chains  Three distinct structures can be obtained.  Isotactic is an arrangement where all substituents are on the same side of the polymer chain. Asyndiotactic polymer chain is composed of alternating groups and atactic is a random combination of the groups. Isotactic   Syndiotactic

Conformation If two atoms are joined by a single bond then rotation about that bond is possible since, unlike a double bond, it does not require breaking the bond.

Thermoplastic and Thermosetting Polymers Thermoplastic polymers have linear and branched structure where they soften when heated and harden when cooled Thermosetting polymers , once they have hardened will not soften when heating and their structure are crosslinked and network

COPOLYMER When a polymer is made by linking only one type of small molecule, or monomer, together, it is called a homopolymer. When two different types of monomers are joined in the same polymer chain, the polymer is called a copolymer When the two monomers are arranged in an alternating fashion, the polymer is called, of course, an alternating copolymer: In a random copolymer, the two monomers may follow in any order:

In a block copolymer, all of one type of monomer are grouped together, and all of the other are grouped together. A block copolymer can be thought of as two homopolymers joined together at the ends: A block copolymer that you know very well, that is, if you wear shoes, is SBS rubber. It's used for the soles of shoes and for tire treads, too. When chains of a polymer made of monomer B are grafted onto a polymer chain of monomer A we have a graft copolymer:

Polymer Crystallinity The condition of crystallinity is said to exist when the molecular chains are aligned and packed in ordered atomic arrangement Amorphous polymers are also possible when the chains are misaligned and disordered Crystalline region are interdispensed within amorphous areas bacause polymer may also exhibit varying degrees of crystallinity Crystallinity is facilitated for polymers that are chemically simple and have regular and symmetrical chain stucture

Polymer Crystals Crystalline region ( or crystallites) are plated-shape and have chain-folded structure Many semicrystalline polymers form spherulites

MECHANICAL PROPERTIES

Stress – Strain Behavior (I) A: Brittle Polymer B: Plastic Polymer C: Elastomer Stress-strain behavior can be brittle (A), plastic (B), and highly elastic (C) Curve C is totally elastic (rubber-like elasticity). This class of polymers - elastomers

Mechanical properties of polymers change dramatically with temperature, going from glass-like brittle behavior at low temperatures to a rubber-like behavior at high temperatures. Polymers are also very sensitive to the rate of deformation (strain rate). Decreasing rate of deformation has the same effect as increasing T.

Factors that Influence Mechanical properties Temperature and strain rate Chain entanglement, strong intermolecular bonding (van der Waals)increase strength Tensile strength increases with molecular weight – effect of entanglement Higher degree of crystallinity – stronger secondary bonding - stronger and more brittle

APPLICATION OF POLYMER

Elastomers Rubber is the most important of all elastomers. Natural rubber is a polymer whose repeating unit is isoprene. This material, obtained from the bark of the rubber tree, has been used by humans for many centuries. It was not until 1823, however, that rubber became the valuable material we know today. In that year, Charles Goodyear succeeded in "vulcanizing" natural rubber by heating it with sulfur. In this process, sulfur chain fragments attack the polymer chains and lead to cross-linking. The term vulcanization is often used now to describe the cross-linking of all elastomers.

Plastics The two main types of plastics are thermoplastics and thermosets. Thermoplastics soften on heating and harden on cooling Thermosets, on heating, flow and cross-link to form rigid material which does not soften on future heating. Thermoplastics account for the majority of commercial usage.

Fibers Fibers represent a very important application of polymeric materials, including many examples from the categories of plastics and elastomers. Natural fibers such as cotton, wool, and silk Man-made fibers include materials such as nylon, polyester, rayon, and acrylic. The combination of strength, weight, and durability have made these materials very important in modern industry.