Introduction to Polymers

• Polymers are plastics and rubbers.
• Polymers are long chain molecules with repeating units.
• The smallest repeating unit in a polymer is a monomer.
• Polymerization is the chemical reaction that creates polymers from monomers, like ethylene.

Classifications of Polymers

• Polymers can be classified based on origin, thermal properties, spatial arrangement, and polymerization reaction.

Classification by Origin

• Synthetic polymers are produced in labs through chemical reactions using acrylic resin, nylon, and polystyrene.
• Synthetic polymers serve as substitutes for natural materials like rubber and silk, and are used in dental applications like fillings.
• Natural polymers include rubber, gutta-percha, and polysaccharides.

Thermal Classification

• Thermoplastics are linear polymers with secondary bonds and entanglement of chains, they undergo reversible reactions.
• Thermosets are cross-linked polymers, with chains bonded by primary bonds, they undergo irreversible reactions.

Thermoplastic Resin

• Thermoplastic resins can be shaped by heat, and maintain their shape after cooling in a reversible reaction.
• Bonds between thermoplastic polymer chains are considered secondary.
• Thermoplastics are relatively soft, and their mechanical properties are heat-sensitive.

Thermoset Polymers

• Thermoset polymers harden during fabrication through an irreversible chemical reaction.
• Examples of thermoset polymers are cross-linked polymethylmethacrylate and silicone.

Classification by Spatial Configuration

• Polymers can be classified as linear, branched, or cross-linked.
• Linear polymers can be simple, with one type of monomer (homopolymer), or copolymer.
• Branched polymers can be homo- or copolymer.

Polymerization Reactions

• Polymerization reactions are chemical reactions that prepare polymers from monomers.
• Polymerization is classified into condensation and addition.

Condensation Polymerization

• Condensation polymerization involves a reaction between two molecules to form a larger molecule by eliminating a smaller molecule, usually water.
• Polysulphide rubber impression material are examples of Condensation polymerization

Addition Polymerization

• Addition polymerization is a reaction between molecules to form a larger molecule without eliminating smaller molecules.
• The monomer's structure is repeated in the polymer without composition changes, forming poly(ethylene), poly(acrylic acid) and polymethacrylic acid.

Stages of Addition Polymerization

• Addition polymerization involves initiation, propagation, and termination stages.

Activation and Initiation Stage

• A free radical, with an unpaired electron, breaks double bonds of the monomers.
• Benzoyl peroxide generates free radicals before initiation.
• Activation results from light, heat, or a chemical compound like dimethyl paratoluidine, causing the peroxide to decompose and release free radicals.

Initiation Process

• Benzoyl peroxide breaks down into free radicals and carbon dioxide.
• Free radicals combine with monomers, activating them.

Propagation Stage

• Chain reactions continue with heat until all monomer turns into polymer.
• Polymer free radicals combine with monomers to grow the chain.

Termination Stage

• Polymerization terminates by direct coupling, where two free radicals react to form a stable molecule.
• Polymerization terminates by exchange of a hydrogen atom.

Chain Transfer

• Hydrogen atoms are transferred during chain reactions.

Factors Associated with Polymerization

• Reactions are exothermic, breaking bonds while releasing heat.
• Polymerization results in a volume reduction (shrinkage).

Inhibition of Polymerization

• Impurities in the monomer reacting with free radicals can inhibit or retard the reaction, increasing the polymer's shelf life.

Factors Affecting Polymer Properties

• Polymer properties are affected by many factors.
• Chemical composition of the chain.
• Polymerization degree.
• Number of branches.
• Cross-links between polymer chains.

General Properties of Polymers

• Longer chains and higher molecular weight polymers show increased strength, hardness, brittleness, and stiffness.
• Exhibits increased resistance to creep.
• Composite resins with a cross-linked matrix featuring strong covalent linkages create a rigid, high molecular weight material displaying great strength, stiffness, and resistance to occlusal stresses.

Glass Transition Temperature

• The glass transition temperature (Tg) is where a polymer changes from glassy to rubbery.
• Molecular weight affects Tg; increased molecular weight increases Tg.

Co-Polymerization

• Two or more types of chemically different monomers are used to modify properties.
• A copolymer process enables chemists to tailor make molecules of predicted properties for special applications
• Adding octyl-methacrylate creates a copolymer that is soft and flexible at mouth temperature, used as a soft liner.
• Hydroxyethyl methacrylate enhances water absorption and the copolymer's wettability by saliva, softening and relaxing the material when exposed to water or saliva; used as a soft liner for dentures.

Cross-Linking

• Cross-linking involves a chemical bond between polymer chains.
• Compounds with two double bonds can act as cross-linking agents
• Ethylene glycol dimethacrylate exhibits superior properties.
• It limits polymer chain movement when stressed.
• Deformation is elastic rather than plastic.
• Polymers have high glass transition temperatures.
• The polymer is harder and more resistant to solvents

Plasticizers

• Liquids separate polymer chains reducing intermolecular forces.
• These liquids are called plasticizers.

Characteristics of Plasticizers

• Plasticizers soften the material.
• They enhance flexibility by reducing the glass transition temperature.
• When added in sufficient quantities they produce soft lining materials in mouth.

Applications of Polymers in Dentistry

• Polymers are used in denture base materials.
• Polymers are used in artificial teeth.
• Polymers are used in tooth restorative materials.
• Polymers are used in cements.
• Polymers are used in crown and bridge facings.
• Polymers are used in impression materials.
• Polymers are used in dies.
• Polymers are used in endodontic fillings.
• Polymers are used in mouth protectors.
• Polymers are used in maxillofacial prostheses.
• Polymers are used in varnishes.
• Polymers are used in pit and fissure sealants.