Polymers

• Naturally occurring polymers, such as wood, rubber, cotton, wool, leather, and silk, have been used for many centuries.
• Other natural polymers, including proteins, enzymes, starches, and cellulose, are important in biological and physiological processes in plants and animals.
• Most polymers are organic in origin, and a lot of organic materials are hydrocarbons, composed of hydrogen and carbon, with covalent intramolecular bonds.
• Molecules that have double and triple covalent bonds are termed unsaturated, and for a saturated hydrocarbon, all bonds are single ones.
• The molecules in polymers are gigantic in comparison to the hydrocarbon molecules already discussed, often referred to as macromolecules.
• These long molecules are composed of structural entities called repeat units.
• The term monomer refers to the small molecule from which a polymer is synthesized.
• A repeat unit is also sometimes called a mer, originating from the Greek word polymer meros, which means part, and the term polymer was coined to mean “many mers”.
• Inorganic polymers include cement, glass, sand, clays.
• Organic polymers can be synthetic or natural, including adhesives, fibers, coatings, rubbers, polysaccharides, proteins, DNA, and polyisoprene rubber.
• If the ethylene gas is reacted under appropriate conditions, it will transform to polyethylene, a solid polymeric material.
• The physical characteristics of a polymer depend on its molecular weight and shape, as well as differences in the structure of the molecular chains.
• Several molecular structures including linear, branched, crosslinked, and network, in addition to various isomeric configurations, can be found in polymers.
• Linear Polymers are those in which the repeat units are joined together end to end in single chains, making them flexible, and each circle represents a repeat unit.
• For linear polymers, there may be extensive van der Waals and hydrogen bonding between the chains.
• A finished piece having a desired shape must be fashioned during a forming operation.
• Fillers are materials added to polymers to improve tensile and compressive strengths, abrasion resistance, toughness, dimensional and thermal stability, and other properties.
• Materials used as particulate fillers include wood flour, silica flour, sand, glass, clay, talc, limestone, and even some synthetic polymers.
• Fillers are often inexpensive materials that replace some volume of the more expensive polymer, reducing the cost of the final product.
• Plasticizers are additives that improve the flexibility, ductility, and toughness of polymers, and produce reductions in hardness and stiffness.
• Plasticizers are generally liquids having low vapor pressures and low molecular weights.
• The small plasticizer molecules occupy positions between the large polymer chains, effectively increasing the interchain distance with a reduction in the secondary intermolecular bonding.
• Plasticizers are commonly used in polymers that are intrinsically brittle at room temperature, such as poly(vinyl chloride) and some of the acetate copolymers.
• The plasticizer lowers the glass transition temperature, so that at ambient conditions the polymers may be used in applications requiring some degree of pliability and ductility.
• These applications include thin sheets or films, tubing, raincoats, and curtains.
• Stabilizers are additives that counteract deteriorative processes in polymeric materials, often under normal environmental conditions.
• Colorants impart a specific color to a polymer; they may be added in the form of dyes or pigments.
• The molecules in a dye actually dissolve in the polymer.
• Pigments are filler materials that do not dissolve, but remain as a separate phase; normally they have a small particle size and a refractive index near to that of the parent polymer.
• Others may impart opacity as well as color to the polymer.
• Flame retardants are additives that enhance the flammability resistance of polymeric materials, often by interfering with the combustion process through the gas phase, or by initiating a different combustion reaction that generates less heat, thereby reducing the temperature; this causes a slowing or cessation of burning.
• Some of the common polymers that form with linear structures are polyethylene, poly(vinyl chloride), polystyrene, poly(methyl methacrylate), nylon, and the fluorocarbons.
• Impact strength is the degree of resistance of a polymeric material to impact loading, which may be of concern in some applications.
• Polymers may exhibit ductile or brittle fracture under impact loading conditions, depending on the temperature, specimen size, strain rate, and mode of loading.
• Fatigue is a phenomenon where polymers may experience failure under conditions of cyclic loading, occurring at stress levels that are low relative to the yield strength.
• Fatigue testing in polymers has not been nearly as extensive as with metals.
• Tear strength, the mechanical parameter that is measured, is the energy required to tear apart a cut specimen that has a standard geometry.
• The magnitude of tensile and tear strengths are related.
• Hardness represents a material’s resistance to scratching, penetration, marring, and so on.
• Plastics are materials that have some structural rigidity under load, and are used in general-purpose applications.