Polymer Science Overview

Polymer Definition

• A polymer is a high molecular weight molecule composed of many repeating units (poly- many, mer- units).

Monomer Definition

• A monomer is the smallest repeating building unit of a polymer chain (mono- single).

Polymerization Reaction

• It's the reaction where monomers form a polymer chain.

Polymer Classification by Origin

• Natural polymers: Synthesized by living cells, examples include proteins, starch, and polysaccharides (agar & alginate).

  • Includes polypeptides, polynucleotides, and polysaccharides
  • There are issues with lack of pure material, difficult synthesis, and system flexibility, as well as branched structures for some types.

• Synthetic polymers: Synthesized by chemical reactions in the lab, an example is acrylic resin.

Polymer Classification by Spatial Configuration

• Linear polymer: The polymer molecule forms a thread-like structure without branches.

• Branched polymer: In the polymer molecule, shorter side chains are attached to the main backbone chain.

• Cross-linked polymer: The main polymer chains are bonded by covalent bonds.

Polymer Classification by Thermal Behavior

• Thermoplastic polymer: A polymer that softens when heated and hardens when cooled and can be molded repeatedly.

  • Has the same chemical composition after heating and cooling
  • Shows Glass Transition Temperature (Tg)

• Thermoset polymer: A polymer that decomposes when heated and cannot be reshaped.

  • The final product is chemically different than the original substances
  • Doesn't exhibit Glass Transition Temperature (Tg)

Polymer Classification by Mechanical Properties

• Elastomers (Elastic Polymers): Polymers with a wide range of elastic deformation and a low modulus of elasticity. Example: rubber base impression materials.

• Plastic polymers: Polymers with a moderate range of elastic deformation and a moderate modulus of elasticity. Example: Acrylic resin.

• Brittle polymers: Polymers with a small range of elastic deformation and a high modulus of elasticity.

Polymer Classification by Chemical Composition of Monomers

• Homopolymers: A polymer consisting of a single type of monomer.

• Copolymers: A polymer consisting of two or more different types of monomers.

  • Alternating copolymers: Different monomers are arranged regularly in an alternative pattern.
  • Random copolymers: Different monomers are arranged randomly.
  • Block copolymers: Each monomer forms a cluster and the clusters are arranged alternatively.
  • Graft copolymers: One polymer forms a backbone, and the other monomer is attached as side chains to the backbone.

Polymer Classification by Polymerization Reaction

• Addition Polymerization reaction: The monomers react together to form the polymer chains without the elimination of by-products (e.g., water, gas).

  • More accurate polymers than condensation polymers, due to no elimination of by-product.

• Condensation Polymerization reaction: The monomers react together to form the polymer chains with the elimination of a small molecule (e.g., water, gas) as a byproduct.

Stages of Polymerization Reaction

• Activation: The process of producing free radicals. Free radicals are the active forms of initiators.

• Initiation: The reaction of free radicals with the first monomer. The free radical breaks the double bond of the monomer and reacts with one carbon atom.

  • The other carbon atom in the monomer contains the unpaired electron, making it very reactive.
  • Reactivity is transferred to the monomer.

• Propagation: The process of adding monomers to the growing chains. Each activated monomer attacks the C=C of unreacted monomers, transferring the active part to it to extend the main chain.

• Termination: The termination process stops the growth chain.

  • Direct Coupling: Two growing chains react together to give one long stable chain.
  • Hydrogen atom transfer: Hydrogen atom transfers from the active chain to another active chain, forming two stable chains.

Polymer Classification by Chain Transfer

• Chain transfer is the transfer of reactivity from an active chain to an inactive chain.

Polymer Termination

• The termination process occurs under normal conditions.
  • Incomplete polymerization reaction results in residual monomers.

Ring Opening Polymerization Reaction

• The terminal group of the monomer is a ring.

Ring Opening Polymerization Advantages

• Less polymer shrinkage.
• Less heat evolution.

Inhibition & Retardation

• Inhibition: Stopping
• Retardation: Lowering the rate
• Hydroquinone: Added to monomers to inhibit premature polymerization during storage or accidental heat/light exposure.

Inhibition & Retardation Continued

• Eugenol: Inhibits the polymerization reaction of resin composite. Bases containing eugenol should not be used as a base under composite resin restorations.

• Oxygen: Inhibits the polymerization reaction of resin composite. The surfaces of restorations should be polished to remove the unreacted surface layer OR covered with a matrix band.

Factors Associated with Polymerization Reaction

• Heat evolution due to the breaking of monomers.
• Reduction in volume (polymerization shrinkage).
• Presence of residual monomers (not all participate in the reaction).

General Properties of Polymers

• Polymers are molecular solids.
• Polymers are amorphous solids (unless specific arrangements of monomers form crystalline regions).

Factors Affecting Polymer Properties

• Molecular weight (MW) and degree of polymerization (DP)
• Copolymerization
• Cross-linking
• Plasticizer
  • Internal plasticizers
  • External plasticizers
• Addition of inorganic fillers
• Rate of loading
• Degree of crystallinity
• Spatial configuration
• Temperature

Uses of Polymers in Dentistry

• Denture base materials
• Artificial teeth
• Teeth restorative materials
• Dental cements
• Temporary crowns
• Endodontic filling & sealers
• Maxillo-facial prosthesis
• Impression materials

This quiz covers the essential concepts of polymer science, including definitions of polymers and monomers, polymerization reactions, and classifications based on origin and spatial configuration. It aims to provide a foundational understanding of the types of polymers and their characteristics.