POLYMER

What is polymer? A polymer is composed of many simple molecules that are repeating structural units called monomers. A single polymer molecule may consist of hundreds to a million monomers and may have a linear, branched, or network structure. 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. .

Polymer structure Hydrocarbon molecules - Most polymers are organic in origin. - Many organic materials are hydrocarbon.

Polymer molecules For carbon chain polymers, the backbone of each chain is string of carbon atoms. Many times each carbon atom singly bonds of two adjacent carbon atoms on either sides, represented schematically in 2D. These long molecules are composed of structural entities called repeated unit.

Some of the list of repeat unit of polymeric materials:

- A weight-average molecular weight: - The number-average molecular weight: 𝑀 𝑛 = 𝑥 𝑖 𝑀 𝑖 mean (middle) molecular weight of size range I - A weight-average molecular weight: 𝑀 𝑤 = 𝑤 𝑖 𝑀 𝑖 Degree of polimerization: DP = 𝑀 𝑛 𝑚 Fraction of the total number of chains Weight fraction of molecules Repeat unit molecular weight

Molecular shape Schematic representations of how polymer chain shape is influenced by the positioning of backbone carbon atoms (grey circle). (a) The rightmost atom may lie anywhere an the dashed circle and still subtend a 109° angle with the bond between the other two atoms. Straight and twisted chain segments are generated when the backbone atoms are situated as in (b) and (c).

Molecular structure Linear polymer Branched polymer The repeat units are joined together end to end in single chains. Branched polymer Which is side-branch chains are connected to the main ones.

Cross-linked polymer The adjacent linear chains are joined one to another at various positions by covalent bonds. - Network polymer Multifuntional monomers forming three or more active covalent bonds make three-dimensional networks.

Molecular configurations For polymers having more than one side atom or group of atoms bonded to the main chain, the regularity and symmetry of the side group arrangement can significantly influence the properties. Consider the repeat unit R represents an atom or side group other than hydrogen. Head -to –tail configuration Head – to – head configuration

Stereoisomerism Situation in which atoms are linked together in the same order(heat-to-tail) but differ in their spatial arrangement. Asotactic configuration Syndiotactic configuration Atactic configuration

Geometrical Isomerism Repeat units having a double bond between chain carbon atoms. Example of cis structure: Example of trans structure: Cis-isoprene Trans-isoprene

Thermoplastic and Thermosetting polymers With regard to behavior at elevated temperature, polymer are classified as either thermoplastic or thermosetting. Thermoplastic polymer: Linear and branched structure Soften when heat and harden when cooled. Thermosetting: Once they have hardened, will not soften upon heating, their structures are cross-linked and network.

Copolymers random alternating Graft copolymers block

Repeat unit that employed in copolymer rubber materials:

Crystallinity When the molecular chains are alligned and packed in an ordered atomic arrangement, the condition of crystallinity is said to exist. Amorphous polymers are also possible wherein the chains are misaligned and disordered. In addiction to being entirely amorphous, polymers may degrees of cryatallinity ; crystalline regions are interdispersed within amorphous areas. Crystallinity is facilitated for polymers that are chemically simple and that have regular and symmetrical chain structure. The percent depends on density:

Polymer crystal - Crystalline region are plate-shaped and have a chain-folded structure: - Many semicrystalline polymers form spherulites;each spherilite consist of a collection of ribbonlike chain-folded lamellar crysrallites that radiate outward from its center. Chains within the platelet are aligned and fold back and forth on themselves, with folds occuring at the faces.

Diffusion in polymeric materials Defects in polymers -chain ends -dangling and loose chains -dislocations Diffusion in polymeric materials Small molecules of foreign substances diffuse between molecular chains by an interstitial-type mechanism from one void to an adjacent one. Diffusion of gaseous species is often characterized in terms of the permeability coefficient, which is the product of the diffusion coefficient and solubility and the polymer: Diffusion flow rates are expressed using a modified form of Fick’s first law:

MECHANICAL PROPERTIES STRESS – STRAIN BEHAVIOUR - polymers fall within 3 classifications : Brittle polymers ( it fractures while deforming elastically ) Plastic materials ( metallic materials ) Elastics (elastromers- large recoverable strains produced at low stress levels )

- Polymers neither as strong nor as stiff as metals - Polymers neither as strong nor as stiff as metals. - It high flexibilities, low densities, and resistance to corrosion make them the materials of choice for many applications - mechanicals properties of polymers are sensitive to changes in temperature and strain rate. - if rising or decreasing strain rate, modulus of elasticity diminishes, tensile strength decreases, and ductility increases

2) VISCOELASTIC DEFORMATION -Viscoelastic mechanical behavior, being intermediate between totally elastics and totally viscous, is displayed by a number of polymeric materials. -This behavior is characterized by the relaxation modulus, time-independent modulus of elasticity. -Magnitude of the relaxation modulus is sensitive to temperature.

3) FRACTURE OF POLYMERS - Fracture strengths of polymeric materials are low relative to metals and ceramics. -Both brittle and ductile fracture are possible -Some thermoplastic materials experience a ductile to brittle transition with a lowering a temperature, an increase in strain rate, and/or an alteration of specimen thickness or geometry -In some thermoplastics, the crack-formation process my be preceded by crazing -Crazing can lead to an increase in ductility and toughness of the material

4) DEFORMATION OF SEMICRYSTALLINE POLYMERS -Having a spherulitic structure that is stressed in tension, molecules in amorphous regions elongate in the stress direction -The tensile plastic deformation of spherulitic polymers occur in several stages as both amorphous tie chains and chain-folded block segments become oriented with the tensile axis -During deformation the shapes of spherulites are altered; relatively large degrees of deformation lead to a complete destruction to form highly aligned structures

FACTORS THAT INFLUENCE THE MECHANICAL PROPERTIES OF SEMICRYSTALLINE POLYMERS MOLECULAR WEIGHT DEGREE OF CRYSTALLINITY PREDEFORMATION BY DRAWING HEAT -TREATING INCREASING THE TEMPERATURE AND DIMINISHING THE STRAIN RATE INFLUENCED BY BOTH IN SERVICE AND STRUCTURAL FACTORS

5) DEFORMATION OF ELASTOMERS - Large elastic extensions are responsible for elastomeric materials that are amorphous and lightly crosslinked. - Deformation corresponds to the unkinking and uncoiling of chains in response to an applied tensile stress. - crosslinking is often achieved during a vulcanization process; increased crosslinking enhances modulus of elasticityand tensile strenght of the elastomer - many of the elastomer are copolymers,whereas the silicone elastomers are really inorganic materials

6) CRSTALLIZATION - Randomly oriented molecules in the liquid phase transform into chain-folded crystallites that have ordered and aligned molecular structures. 7) MELTING - The melting of crystalline regions of a polymer corresponds to the transformation of a solid materials, having an ordered structure of aligned molecular chains, to viscous liquid in which the structure is highly random.

8) THE GLASS TRANSITION - The glass transition occurs in amorphous regions of polymers. - Upon cooling, this phenomenon corresponds to the gradual transformation from the liquid to a rubbery materials, and finally to a rigid solid. - With the increasing temperature there is a reduction in the motion of large segments of molecular chains

9) FACTORS THAT INFLUENCE MELTING AND GLASS TRANSITION TEMPERATURES - The magnitudes of T(melting) and T(glass transition) with increasing chain stiffness - At low molecular weight T (melting) and T(glass transition) increase with increasing M.

STRESS-STRAIN BEHAVIOR MECHANICAL BEHAVIOR STRESS-STRAIN BEHAVIOR DEFORMATION MECHANISMS (SEMICRYSTALLINE POLYMERS) FACTORS THAT INFLUENCE THE MECHANICAL PROPERTIES MOLECULAR WEIGHT DEGREE OF CRYSTALLINITY PREDEFORMATION BY DRAWING HEAT-TREATING MACROSCOPIC DEFORMATION (NECKING PHENOMENON)

APPLICATIONS OF POLYMER Elastomers Rubber is the most important of all elastomers. Natural rubber is a polymer whose repeating unit is isoprene. Obtained from the bark of the rubber tree. 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 The two main types of plastics are thermoplastics and thermosets. Among the most important and versatile of the hundreds of commercial plastics is polyethylene. This polymer is characterized by a large degree of branching, forcing the molecules to be packed rather loosely forming a low density material. LDPE is soft and pliable and has applications ranging from plastic bags, containers, textiles, and electrical insulation, to coatings for packaging materials.

Fibers 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. fibers are at least 100 times longer than they are wide. Typical natural and artificial fibers can have axial ratios (ratio of length to diameter) of 3000 or more. 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. These polymers are then formed into fibers with various characteristics.