Understanding Polymer Materials

What is a Polymer?

• A high molecular weight material.
• (10³ - 10⁶ Daltons or more).
• Formed by the combination of a large number of small molecular units.
• These small units are called "monomers".

Polymer Applications

• Aerospace
• Engineering
• Aromatic high-strength fibers
• Non-flammable polymers
• Degradable polymers
• Medical applications
• Electrical conductivity comparable to metals
• Insoluble support for catalysis or synthesis of proteins and nucleic acids
• Electronics

Polymer Classification

Organic Polymers

Natural Organic Polymers

• Polysaccharides (cellulose, starch, cotton)
• Proteins (Biopolymers, wool)
• Natural rubber (cis-1,4-polyisoprene)

Synthetic Organic Polymers

• Rubber
• Plastics
• Fibers
• Coatings
• Adhesives

Inorganic Polymers

Natural Inorganic Polymers

• Clay
• Sand

Synthetic Inorganic Polymers

• Fibers (fiber optics)
• Rubber (silicone)

Classification by Synthesis Method

• Carothers (1929): Condensation and Addition
• Flory (1950): Step-growth and Chain-growth reactions

Addition Polymers

Formed by the addition of one monomer molecule to another.

Condensation Polymers

Formed by joining two different types of functional groups with the elimination of a small, stable molecule.

Polymerization Mechanisms

• Chain-growth polymerization
• Step-growth polymerization

Classification by Behavior

Thermoplastics vs. Thermosets

Thermoplastics: When heated sufficiently, they melt. This distinguishes them from crosslinked materials that do not melt.

Thermosets: Crosslinked materials that do not melt when heated.

Plastics vs. Elastomers vs. Fibers

Why do we call a material a plastic and not a rubber or elastomer? The distinction lies in its ease of deformation and recovery.

You can stretch an elastomer, and it returns to its original shape. However, if you stretch a plastic, it tends to undergo permanent deformation.

Plastics resist deformation better than elastomers. You can stretch plastic, but it will retain the shape it took after you stop stretching. In contrast, elastomers regain their original form.

Fibers deform very little when stretched.

Hard and Soft Plastics

Your keyboard is stiff, but the plastic covering computer cables is soft. This difference is due to their glass transition temperature (Tg) and the addition of plasticizers, which increase flexibility.

Polyethylene (PE)

Common applications include grocery bags, shampoo bottles, children's toys, and even bulletproof vests.

Long chains of polyethylene with branches are called branched polyethylene, or LDPE (Low-Density Polyethylene). When there are no branches, it is called linear polyethylene, or HDPE (High-Density Polyethylene).

Ultra-high molecular weight polyethylene (UHMWPE) fiber can replace materials like Kevlar.

Polypropylene (PP)

Polypropylene serves a dual purpose as both a plastic and a fiber. Applications include dishwasher-safe food containers and indoor/outdoor rugs (e.g., for poolsides).

Like polyethylene, polypropylene does not absorb water, unlike nylon.

Polystyrene (PS)