Polymers and Polymerization Methods

Polymers

• Polymers are large molecules made up of repeating smaller units called monomers.
• Polymers are formed by the process of polymerization.
• Examples of polymers we encounter daily include sandwich bags, carpets, nylon stockings, milk cartons, and more.
• Different polymers have different properties due to variations in their chemical compositions, structures, and fabrication processes.
• LDPE (Low-Density Polyethylene) is more transparent, flexible, and waxy compared to HDPE (High-Density Polyethylene), which is more opaque, rigid, and non-waxy.

Step-Growth vs Chain-Growth Polymerization

• Step-Growth Polymerization: Growth occurs throughout the matrix, the monomer is rapidly lost early in the reaction, and average molecular weight increases slowly.
• Chain-Growth Polymerization: Growth happens by adding monomers at one end of the chain, some monomers remain even after long reaction times, different mechanisms operate at different stages, and the molar mass of the backbone chain increases rapidly at the beginning.

Glass Transition Temperature (Tg)

• At low temperatures, amorphous polymers are stiff and glassy.
• Upon warming, polymers soften in a specific temperature range known as the glass-rubber transition region.
• Tg is the temperature when the amorphous phase of the polymer transitions between rubbery and glassy states, considered a key mechanical property.
• Below Tg, an amorphous polymer is brittle, hard, and glassy. Above Tg, it becomes flexible, soft, and rubbery.

Bakelite

• Bakelite is a thermosetting polymer formed by condensation of phenol (P) and formaldehyde (F).
• Novolacs are prepolymers with a molar ratio (P/F) greater than one. They are soluble and fusible and do not undergo cross-linking on their own.
• Resoles are formed with a molar ratio (P/F) less than one and a base catalyst. They contain alcoholic groups and are soluble and fusible, undergoing cross-linking upon curing.
• Bakelite is rigid, hard, scratch-resistant, infusible, water-resistant, insoluble, and resistant to non-oxidizing acids, salts, and many organic solvents.
• Uses: Electric insulators, molded articles, impregnating fabrics, woods, and paper, adhesives, paints and varnishes, and in water softening.

Elastomers

• Elastomers are long-chain polymers that can be stretched several times under stress and regain their original shape when the stress is released.

Natural Rubber

• Natural rubber is obtained from the latex of rubber trees.
• Latex is a milky colloidal solution containing 25-45% rubber dispersed in water, with small amounts of protein and other materials.
• Natural rubber is a polymer of isoprene (2-methyl-1,3-butadiene) with an average degree of polymerization (DP) around 5000.

Drawbacks of Raw Rubber

• Plasticity: Soft at high temperatures and brittle at low temperatures, limiting its use to 10-60℃.
• Weakness: Low tensile strength of 200 kg/cm2.
• Water Absorption: High water absorption capacity.
• Solvent Resistance: Non-resistant to non-polar solvents like vegetable and mineral oils.
• Oxidizing Agent: Easily attacked by oxidizing agents.
• Swelling and Degradation: Swells in organic solvents and degrades over time.
• Durability: Poor durability.

Vulcanization

• Vulcanization is a process that increases rubber's elasticity and reduces its plasticity by forming a cross-linked molecular network.
• Vulcanization involves heating rubber with sulfur or other vulcanizing agents under pressure.
• The amount of sulfur used influences the properties of the final rubber product.

Compounding of Rubber

• Compounding involves mixing raw rubber with other substances to impart specific properties for specific applications.

Explore the fascinating world of polymers, focusing on their formation through polymerization processes. Understand the differences between step-growth and chain-growth polymerization, along with examples of daily-use polymers. Dive into the properties of various types of polyethylene and their applications.