Unit II

Introduction to Polymers

• Polymers are high molecular weight compounds formed by linking together many low molecular weight molecules (monomers).

• The reaction through which monomers combine to form polymers is called polymerization.

• Degree of Polymerization (Dp): Number of repeating units in the polymer chain.

• Molecular Weight Measurement:

  • Number Average Molecular Weight (Mn): Total mass of all molecules divided by total number of molecules.

  • Weight Average Molecular Weight (Mw): Takes into account the weight of each molecular weight in the total mass.

• Glass Transition Temperature (Tg): The temperature at which a polymer transitions from a rubbery to a rigid state.

Classification of Polymers

• Polymerization Mechanism:

  • Step Polymers: Formed through step-growth polymerization where any two molecular species react.

  • Chain Polymers: Formed through chain-growth (addition) polymerization.

• Types of Polymerization Reactions:

  • Addition Polymerization: Involves the simple addition of monomers without the elimination of small molecules.

  • Condensation Polymerization: Involves the elimination of small molecules such as water upon polymer formation.

• Thermal Behavior:

  • Thermoplastics: Can soften when heated and harden when cooled, allowing for reprocessing.

  • Thermosetting Polymers: Set permanently upon heating and cannot be reshaped.

Types of Monomers

• Types:

  • Linear Polymers: Continuous chains of monomers.

  • Branched Polymers: Chains with branching structures.

  • Cross-linked Polymers: Networks of interconnected polymer chains.

  • Homo Polymers: Consist of identical monomer units.

  • Copolymers: Contain two or more different monomer units.

Commercial Polymers

Synthesis, Properties, and Applications:

1. Polyethylene (PE): Produced through the polymerization of ethylene. Used in packaging, containers, and films.

2. Polypropylene (PP): Made from propylene polymers; famous for durability in containers and automotive parts.

3. Polyvinyl Chloride (PVC): Produced from vinyl chloride; widely used in pipes, electrical cables, and construction material.

4. Polystyrene (PS): Used in disposable containers, insulation, and as a model material.

5. Phenol Formaldehyde (PF): Known as Bakelite; used in electrical insulators and kitchenware.

6. Epoxy Resins: Synthetic polymers known for their high chemical resistance and are used as adhesives and coatings.

Specialty Polymers

• Conductive Polymers: Allow electrical conductivity; used in electronic devices.

• Biodegradable Polymers: Designed to break down under environmental conditions, reducing plastic waste.

• Recycling of Polymers: Important for sustainability and reducing plastic pollution.

Composites

• Classification:

  • Fiber Reinforced Composites: Combine fibers with a matrix for enhanced strength (e.g., carbon fibers).

  • Particle Reinforced Composites: Incorporate particulates for strength and functional enhancements.

• Commercial Applications: Used in aerospace, automotive, and construction industries for their lightweight and strong properties.

Natural and Synthetic Polymers

• Natural Polymers: Include cellulose (in wood), DNA, and rubber.

• Synthetic Polymers: Include polyethylene, polypropylene, nylon, and PVC.

  • Early synthetic polymer: Bakelite (1906), modern plastics began in the 1920s.

Molecular Weight of Polymers

• Directly influences properties like viscosity, thermal stability, and mechanical strength.

• Polydispersity Index: Describes the distribution of molecular weight in a given polymer sample. Mw/Mn ratio indicates the breadth of the molecular weight distribution.

Types of Polymerization

• Addition Polymerization: Involves the addition of monomers without byproducts. Examples include PVC and polystyrene.

• Condensation Polymerization: Involves bifunctional or multifunctional monomers forming larger structures with byproduct elimination. Examples include polyesters and nylon 6,6.

Thermal Behavior of Polymers

Thermoplastics vs Thermosetting Polymers

• Thermoplastics:

  • Can be melted and re-molded. Examples: polyethylene, PVC.

• Thermosetting Polymers:

  • Set permanently upon initial application of heat. Examples: Bakelite, epoxy resins.

Glass Transition Temperature (Tg)

• Transition temperature defining the change from a rubbery state to a rigid state; crucial for polymer applications. Factors affecting Tg include chain stiffness, side groups, and molecular structure.

Applications and Uses

• Polyethylene: Used extensively in packaging and household items.

• Polypropylene: Utilized in automotive components and household goods.

• PVC: Major material in piping and construction.

• Polystyrene: Common in disposable products and insulating materials.

• Phenol Formaldehyde and Epoxy Resins: Used in advanced materials for electrical and structural applications.