Polymers: Structure, Types, and Applications

Polymers

Introduction to Polymers

Polymers are large macromolecules composed of repeating structural units called monomers. The term "polymer" is derived from the Greek words poly (many) and meros (part). Polymers can have molecular weights in the thousands and are essential in both natural and synthetic materials.

• Monomer: The small, repeating unit that joins to form a polymer.

• Macromolecule: A very large molecule, such as a polymer, with a high molecular weight.

Natural Polymers

Many substances found in nature are polymers. These include:

• Starch

• Cotton (cellulose)

• Wood (cellulose)

• Proteins

Early Synthetic Polymers

Celluloid, produced by treating natural cellulose with nitric acid, was one of the earliest synthetic polymers. It was used for billiard balls, men’s collars, and movie film. The first completely synthetic polymers were phenol–formaldehyde resins.

Types of Synthetic Polymers

Polyethylene

Polyethylene is one of the most common synthetic polymers. It exists in two main forms:

• High-density polyethylene (HDPE): Mostly linear molecules that pack closely together, used for milk jugs, bottle caps, and toys.

• Low-density polyethylene (LDPE): Highly branched molecules, used for plastic bags, films, and wire insulation.

Thermoplastic and Thermosetting Polymers

Polymers can be classified based on their response to heat:

• Thermoplastic polymers: Soften upon heating and can be remolded (e.g., polyethylene).

• Thermosetting polymers: Harden permanently when formed and do not soften upon reheating; they decompose instead.

Addition Polymers

Mechanism and Examples

Addition polymerization occurs when monomers add together without the loss of any atoms, forming polymers that contain all the atoms of the monomers. Many common plastics are addition polymers.

• Polyethylene: Formed from ethylene monomers.

• Polypropylene: Formed from propylene monomers.

• Polystyrene: Formed from styrene monomers.

• Polyvinyl chloride (PVC): Formed from vinyl chloride monomers.

• Polytetrafluoroethylene (PTFE or Teflon): Formed from tetrafluoroethylene monomers.

Table: Common Addition Polymers and Their Uses

Monomer	Polymer Name	Some Uses
Ethylene (CH2=CH2)	Polyethylene	Bags, bottles, toys, electrical insulation
Propylene (CH2=CHCH3)	Polypropylene	Carpeting, bottles, luggage
Styrene (CH2=CH–C6H5)	Polystyrene	Insulation, cups, toys, packing materials
Vinyl chloride (CH2=CHCl)	Polyvinyl chloride (PVC)	Food wrap, plumbing, garden hoses, floor tile
Tetrafluoroethylene (CF2=CF2)	Polytetrafluoroethylene (Teflon)	Nonstick coatings, electrical insulation

Conducting Polymers

Some polymers, such as polyacetylene, contain conjugated double bonds and can conduct electricity.

Molding and Processing of Plastics

Common Molding Techniques

• Compression molding: Heat and pressure are applied directly to polymer powder in a mold.

• Injection molding: Plastic is melted and forced into cold molds to set.

• Extrusion molding: Molten polymer is extruded through a die and cut or coiled.

• Blow molding: A bubble of molten polymer is blown inside a mold to form hollow objects like bottles.

Elastomers and Rubber

Natural Rubber and Vulcanization

Elastomers are polymers that stretch under force and return to their original shape. Natural rubber is an elastomer made from isoprene units. Vulcanization, discovered by Charles Goodyear, involves cross-linking rubber with sulfur to make it harder and more durable.

Synthetic Rubber

Synthetic rubbers are designed to mimic or improve upon natural rubber. Examples include:

• Polybutadiene

• Neoprene: Contains chlorine, making it more resistant to oils and solvents.

• Styrene–butadiene rubber (SBR): A copolymer of styrene and butadiene, used in tires and other products.

Elastomers in Paints

Elastomeric binders in paints increase resistance to cracking. Latex paints use water as a solvent, making them more environmentally friendly than oil-based paints.

Condensation Polymers

Mechanism and Examples

Condensation polymerization involves the joining of monomers with the release of small molecules such as water, alcohols, ammonia, or HCl. Common condensation polymers include:

• Nylon (polyamide): Used mainly as fibers.

• Polyesters: Made from molecules containing alcohol and carboxylic acid groups; the linkage is an ester.

• Phenol–formaldehyde resin (Bakelite): The first synthetic polymer.

• Urea–formaldehyde and melamine–formaldehyde resins: Used in adhesives and plastics.

• Polycarbonates: Tough, clear polymers used in safety equipment.

• Polyurethanes: Used in foam rubber, skate wheels, and furniture finishes.

• Epoxy resins: Durable adhesives and coatings, often sold as two-part liquids.

Composite Materials

Composite materials consist of high-strength fibers (such as fiberglass, graphite, or ceramics) embedded in a polymeric matrix. They are used in boat hulls, tennis rackets, automobile panels, and fishing rods.

Silicones

Structure and Properties

Silicones are polymers with silicon-oxygen backbones instead of carbon. They can be oils, gels, or elastomers, depending on their structure and degree of cross-linking.

Properties of Polymers

Molecular Weight and Structure

Polymers have high molecular weights, resulting in strong intermolecular forces and the ability to form strong fibers and viscous solutions. The arrangement of polymer chains affects their properties:

• Crystalline polymers: Chains line up in neat rows, making the material rigid and strong.

• Amorphous polymers: Chains are randomly tangled, making the material soft and rubbery.

Glass Transition Temperature (Tg)

The glass transition temperature is the point above which a polymer is tough and rubbery, and below which it is hard, brittle, and glass-like.

Polymer Fibers

Many synthetic polymers can be spun into fibers with properties superior to natural fibers. Most fibers and fabrics in the United States are synthetic.

Plastics and the Environment

Disposal Methods

• Landfills: Plastics make up a significant portion of solid waste by mass and volume. Landfill space is limited.

• Incineration: Plastics can be burned for energy, but may release toxic gases (e.g., HCl from PVC).

• Degradable Plastics: Biodegradable and photodegradable plastics are being developed, especially for packaging.

• Recycling: Plastics can be separated, chopped, melted, and remolded or spun into fibers.

Fire Hazards

Some plastics are flammable and may release toxic gases when burned. Regulations require flame-retardant materials for certain uses, such as children's sleepwear.

Plasticizers and Pollution

Plasticizers are added to make plastics more flexible. Early plasticizers, such as polychlorinated biphenyls (PCBs), were toxic. Modern plasticizers, such as phthalate esters, are less toxic but still a concern for environmental pollution.