Polymers : are compounds comprising many modern synthetic materials and a large percentage of biological components. The Structure of Polymers : They are molecules made up of a large number of small molecular units referred to as monomers. A typical polymer molecule may contain several thousand monomer units that may be the same or they may be varied. Polymer molecules can be either a single long chain,like a strand of spaghetti.

The length of the polymer chain, or its branches, depends heavily upon the conditions under which the polymer is synthesized. The length of individual polymer molecules in a given sample will usually vary somewhat about a mean value, although certain polymers, such as many proteins, will have well defined sizes. (A polymer molecule built as a single linear chain) (A polymer molecule with branching)

Covalent bonds hold the atoms in the polymer molecules together and secondary bonds then hold groups of polymer chains together to form the polymeric material. Copolymers are polymers composed of two or more different types of monomers. The three dimensional structure of these polymer molecules also varies considerably : - At one extreme, the polymer chains can be intertwined in a spaghetti like mass. - At the other, the polymer will be tightly coiled as a single molecule dissolved in an appropriate solvent.

The exact three dimensional structure of the polymer molecule is determined by the chemical makeup of the polymer's monomer units and the chemical environment, typically the solvent, in which the polymer finds itself. (A tightly coiled, unbranched polymer molecule) Polymers Natural polymers Synthetic polymers

The nature of the polymerization reaction used to form the polymer such : 1) Addition Polymerization occurs because two monomers are added to each other, with elimination of a double bond in the monomer. n CH 2 =CH 2 -CH 2 CH 2 -CH 2 CH 2 -CH 2 CH 2 -CH 2 CH 2 - 2) Condensation Polymerization involves condensing two monomer units together, with concomitant elimination of a small molecule once the monomers are linked.

Polymer Chains (Thermoplastics and Thermosets) A polymer is an organic material and the backbone of every organic material is a chain of carbon atoms. The carbon atom has four electrons in the outer shell. Each of these valence electrons can form a covalent bond to another carbon atom or to a foreign atom. The key to the polymer structure is that two carbon atoms can have up to three common bonds and still bond with other atoms. The elements found most frequently in polymers and their valence numbers are: H, F, Cl, Bf, and I with 1 valence electron; O and S with 2 valence electrons; n with 3 valence electrons and C and Si with 4 valence electrons.

The length of the polymer chain is very important. As the number of carbon atoms in the chain is increased to beyond several hundred, the material will pass through the liquid state and become a waxy solid. It should also be noted that the molecules are not generally straight but are a tangled mass. The binding forces are the result of van der Waals forces between molecules and mechanical entanglement between the chains. When thermoplastics are heated, there is more molecular movement and the bonds between molecules can be easily broken. This is why thermoplastic materials can be remelted.

Polymer Crystallinity : Most polymers, unlike metals, do not every fully crystallize; some are fully amorphous (no crystallinity at all). Polymers that partially crystallize upon cooling from the melt state are called semicrystalline polymers. Chain disorder or misalignment, which is common, leads to amorphous material since twisting, kinking and coiling prevent strict ordering required in the crystalline state. Crystalline polymers are denser than amorphous polymers, so the degree of crystallinity can be obtained from the measurement of density.

Mechanical properties StrengthToughnessModulusElongation.

Strength 1.Tensile – A polymer has tensile strength if it is strong when one pulls on. – Important for a material that is going to be stretched or under tension. – Example:fiber 2.Compressional – A polymer sample has compressional strength if it is strong when one tries to compress it. – Example : Concrete 3.Flexural – A polymer sample has flexural strength if it is strong when one tries to bend it.

Elongation A type of deformation Deformation is simply a change in shape that anything undergoes under stress. The sample deforms by stretching, becoming longer. Two type of elongation: Ultimate elongation Elastic elongation

Modulus If we want to know how well a material resists deformation, we measure something called modulus. Modulus is measured by calculating stress and dividing by elongation, and would be measured in units of stress divided by units of elongation. In general, fibers have the highest tensile moduli, and elastomers have the lowest, and plastics have tensile moduli somewhere in between fibers and elastomers.

Toughness A measure of the energy a sample can absorb before it breaks.

Toughness different from strength: – Strength tells how much force is needed to break a sample, and toughness tells how much energy is needed to break a sample. – Just because a material is strong, it isn't necessarily going to be tough as well.

Application of polymers. Elastomers Rubber is the most important of all elastomers. Natural rubber is a polymer whose repeating unit is isoprene. 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.

Plastics Two main types of plastics: – Thermoplastics – Thermosets The most important and versatile of the hundreds of commercial plastics is polyethylene. Polyethylene is used in a wide variety of applications because, based on its structure, it can be produced in many different forms. LDPE is soft and pliable and has applications ranging from plastic bags, containers, textiles, and electrical insulation, to coatings for packaging materials.

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 have been used by humans for many centuries. In 1885, artificial silk was patented and launched the modern fiber industry. Man-made fibers include materials such as nylon, polyester, rayon, and acrylic.

Synthetic polymers have been developed that posess desirable characteristics, such as a high softening point to allow for ironing, high tensile strength, adequate stiffness, and desirable fabric qualities. Nylon used for parachutes in World War II. This synthetic fiber, known for its strength, elasticity, toughness, and resistance to abrasion, has commercial applications including clothing and carpeting.