Polymer Fundamentals: Structure, Types, Polymerization, Additives, and Fabrication

Introduction

  • Naturally Occurring Polymers
    • Examples: Rubber, Cotton, Leather, Silk.
  • Polymers Inside Biological Bodies
    • Examples: Enzymes, Starches, Cellulose.
  • Scientifically Developed Polymers
    • Examples: Teflon, Biodegradable Polymers (e.g., bags), Synthetic rubbers.
  • Contextual note from slides: RUET (structure & processing of polymer) appears as a title/branding on slides.

About Polymers

  • Definition: A polymer is a large molecule consisting of repeated chemical units called ‘mers’ joined together.
  • Size of polymers:
    • Usually contain $5$ or more monomers.
    • Some polymer chains may contain $100$ or $1000$ monomers in each chain.
  • Molecular weight notation:
    • High molecular weight is often denoted as $M$; e.g., polyethylene (PE) has high $M$ values.
  • Monomer: A molecule that combines with others (identical or different) to form a polymer; examples include ethylene (as a monomer for polyethylene).
  • Key terms:
    • Polymerization: process of forming polymers from monomers.
    • Monomer units link to form long chains.

Hydrocarbon Molecules and Bonding

  • Most polymers are organic in origin and are hydrocarbons (composed of hydrogen and carbon).
  • Covalent bonds:
    • A single covalent bond exists when each bonding atom contributes one electron.
    • Double and triple bonds involve sharing two and three pairs of electrons, respectively.
  • Unsaturated vs Saturated:
    • Unsaturated: molecules with double or triple covalent bonds.
    • Saturated: all bonds are single.

About Polymers (Structural Backbone)

  • Polymers typically consist of long, flexible chains with a string of carbon (C) atoms as the backbone.
  • Side attachments:
    • C atoms are often bonded to H atoms or other radicals.
  • Double bonds:
    • Double bonds can occur in both the main chain and the side bonds.

About Polymers (Elements in Polymers)

  • While many polymers are primarily C–H, other elements can be present:
    • O, Cl, F, N, Si, P, S are also found in various polymers.
  • Examples:
    • Polyvinyl chloride (PVC) contains chlorine.
    • Nylon contains nitrogen.
    • Teflon contains fluorine.
  • Inorganic polymers (backbones): some polymers have silicon (Si) or phosphorus (P) backbones; these are classified as inorganic polymers (e.g., polysiloxanes and polyphosphazenes).

Types of Polymers

  • Homopolymer: all repeating units are the same.
  • Copolymer: more than one type of repeating unit.
  • Functionality: the number of bonds a given monomer can form.
    • Bifunctional example: Vinyl chloride (can form two bonds).
    • Trifunctional example: Phenol-formaldehyde (can form three bonds).

Molecular Structure

  • Physical characteristics depend on:
    • Molecular weight ($M$)
    • Shape
    • Structure of the molecular chains
  • Modern polymer synthesis allows control over structural possibilities.

Molecular Structure: Types (Morphologies)

  • (a) Linear Polymer
    • Monomers joined end-to-end in spaghetti-like chains by Van der Waals forces.
    • Examples: Polyethylene, PVC.
  • (b) Branched Polymer
    • Result from side reactions during synthesis.
    • Example: LDPE (low-density polyethylene).
  • (c) Crosslinked Polymer
    • Adjacent linear chains are joined by covalent bonds.
    • Example: Vulcanized rubber.
  • (d) Network Polymer
    • Highly cross-linked structure.
    • Example: Polyurethane.

Thermal Behavior of Polymers

  • Thermoplastics vs Thermosetting polymers:
    • Thermoplastics soften when heated and harden when cooled; the process is reversible and repeatable.
    • Thermosetting polymers are network polymers that form permanent hard structures during formation and do not soften upon heating.
  • Rationale:
    • Covalent crosslinks between adjacent chains resist vibrational and rotational motion at high temperature, limiting softening.

Copolymers

  • Classification of copolymers:
    • (a) Random copolymer
    • (b) Alternating copolymer
    • (c) Block copolymer
    • (d) Graft copolymer
  • Visual labels on slide indicate four categories: (a), (b), (c), (d).

Polymerization

  • Synthesis of large molecular weight polymers is termed polymerization.
  • Classifications by mechanism:
    • Addition polymerization (chain-growth polymerization): monomer units are added one at a time in a chain-like fashion to form a linear macromolecule.
    • Condensation polymerization (step-growth polymerization): involves intermolecular reactions between monomer species, often with the elimination of a small molecule (e.g., water).
  • For addition polymerization, the product composition is an exact multiple of the reactant monomer.
  • Three stages in addition polymerization:
    • Initiation
    • Propagation
    • Termination

Addition Polymerization

  • Initiation:
    • An active center is formed by a reaction between an initiator and the monomer unit.
  • Propagation:
    • Linear growth of the polymer chain by sequential addition of monomer units to the active growing chain.
  • Termination (end of growth):
    • Two propagating chain ends may link to form one molecule: ext{Two active ends join: } P^ullet + P^ullet
      ightarrow P-P
    • Or two growing molecules react to form two dead chains: P^ullet + Q^ullet
      ightarrow P-Q ext{ and three other ends terminate}

Condensation Polymerization

  • Formation of polymers by stepwise intermolecular chemical reactions that may involve more than one monomer species.
  • Byproduct: typically a small molecule such as water is eliminated during polymerization.

Polymer Additives

  • Additives are foreign substances intentionally added to enhance or modify properties.
  • Filler materials (fillers):
    • Improve tensile and compressive strengths, abrasion resistance, toughness, dimensional and thermal stability, and other properties.
    • Examples: wood flour, silica flour, sand, glass, limestone, and some synthetic fillers.
    • Particle sizes range from $10$ nm to macroscopic dimensions.
  • Plasticizers:
    • Increase flexibility, ductility, and toughness; often reduce hardness and stiffness.
    • Typically liquids with low vapor pressures and low molecular weights.
    • They insert between large polymer chains, increasing interchain distance and reducing secondary intermolecular bonding.
  • Stabilizers:
    • Counteract deterioration due to UV radiation or oxidation, protecting mechanical integrity.
  • Colorants:
    • Impart color via dyes (dissolve in polymer) or pigments (do not dissolve).
  • Flame retardants:
    • Many polymers are flammable; exceptions include polyvinyl chloride (PVC) and polytetrafluoroethylene (PTFE).
    • Flame retardants are additives to improve flammability resistance.

Polymer Forming

  • Fabrication of polymers depends on:
    • Whether material is thermoplastic or thermosetting.
    • For thermoplastics: the temperature at which it softens.
    • Atmospheric stability of the material being formed.
    • Geometry and size of the finished product.
  • Fabrication typically occurs at elevated temperatures and often under pressure.
  • Thermoplastics:
    • Formed above their melting temperatures.
    • A applied pressure is maintained during cooling to retain the shape.
  • Economic benefit: thermoplastics can be recycled by remelting scrap pieces into new shapes.

Fabrication of Polymers

  • Fabrication of thermosetting polymers occurs in two stages:
    • Stage 1: Preparation of a linear polymer (prepolymer) with low molecular weight in a liquid form.
    • Stage 2: Curing to form the final hard and stiff product, usually in a mold shaping the desired geometry.
  • Curing details:
    • May occur during heating or by adding catalysts, often under pressure.
    • During curing, crosslinking leads to a network structure.
  • Molding is a common forming method; several molding techniques are used:
    • Compression molding
    • Transfer molding
    • Injection molding
    • Extrusion molding
    • Blow molding
    • Casting

Compression Molding

  • Process:
    • Weigh polymer and additives, place in mold, close mold, apply heat and pressure.
    • The polymer becomes viscous and flows to conform to the mold shape.
  • Preform:
    • Raw materials may be mixed and cold-pressed into a disc called a preform.
  • Note:
    • Use of compression molding with thermoplastics can be more time-consuming and expensive than extrusion or injection molding.

Transfer Molding

  • Process variant of compression molding for thermosetting polymers and complex geometries:
    • Solid ingredients are melted in a heated transfer chamber.
    • The molten material is injected into the mold chamber, distributing pressure more uniformly.
    • Suitable for complex geometries.

Molding

  • Visual summary of common molding methods (as per slides):
    • Injection Molding: involves plastic granules or powder melted and injected into a mold via screw/ram mechanism.
    • Extrusion: plastic granules melted and forced through a die to form continuous profiles.
    • Blow Molding: forming hollow items by inflating a hot vial-shaped parison inside a mold.
    • Casting: pouring liquid polymer into a mold and allowing it to solidify.
  • Illustration references include terms like heating, molds, screws, dies, and rotating components (as depicted in slides).

Fabrication (Summary of Techniques)

  • Injection Molding:
    • Raw polymer is melted and injected into a mold under pressure.
  • Extrusion:
    • Material is melted and pushed through an orifice (die) to create continuous shapes.
  • Blow Molding:
    • A parison is inflated inside a mold to form hollow parts.
  • Casting:
    • Liquid polymer poured into a mold and cured.
  • Casting vs. other methods:
    • Casting is often used for polymers that may be difficult to mold with pressure-based methods.

End for Today

  • This summary recap aligns with classroom content on polymer structure, types, polymerization mechanisms, additives, and fabrication methods.
  • Key takeaways: understanding how structure (linear, branched, crosslinked, network) and thermal behavior (thermoplastic vs thermosetting) influence processing and applications; the role of additives in tuning properties; and the primary molding and fabrication techniques used to shape polymers into final products.