Polymers in Dentistry

-Macromolecule consisting of repeating structure units joined together by covalent chemical bonds

• Silicone impression materials • Soft lining materials • Composites • Acrylic denture bases • Alginate impression materials

• Light curing a composite • Preparing a denture base by polymerising the monomer • Repairing a denture by polymerising chairside

Addition Condensation Cationic Anionic

-Initiation Producing the free radicals -Propagation -Termination Mopping up of free radicals eg. Composites and denture bases are cured via a free radical addition polymerisation reaction

-Method in which free radicals are produced for the initiation reaction • Chemical • Thermal • Photochemical • Microwave • Combining heat and light • must be safe for the patient • eg. thermal initiation must not be so hot that it can burn the patient

• Initiator is the thing that will make the free radical • Activator is the method in which you use to make it For example, when repairing denture bases: • Tertiary amine is the activator • Dibenzoyl peroxide is the initiator

• Dibenzoyl peroxide is the initiator that can produce the free radicals • In the presence of a tertiary amine (N,N dimethyl p toluidine) which is the activator • Forms an initiator-activator complex • Denture bases are repaired by this process • Dibenzoyl peroxide is NOT a catalyst, but rather an initiator • Dibenzoyl peroxide can also decompose into free radicles with thermal activation if heated about 65 degrees C which is used in production of polymers but this obviously cannot be done inside the mouth so chairside chemical activation is used instead

• Dibenzoyl peroxide can undergo demposition when subjected to heat • Free radicals are produced as a result -Heat cured denture bases are cured through this process

• Light initiated • Visible light usually 480nm • Camphorquinone (a diketone) • Tertiary amine used as an activator (dimethylaminoethyl methacrylate) • Dental composites cured by this method

Incremental curing is required due to: • Limited cure depth • Reduce the consequences of shrinkage stress (C factor) • Depth of cure refers to the thickness of composite that is adequately cured

Limited by light absorption and scatter within the material, which depends on: • Amount, size and type of fillers • Shade • Photoinitiator type and concentration • Refractive index mismatch • Light irradiation source and irradiation duration

• Generally used for composite inlays • Very high degrees of curing are required • Indirect restorations often cured by light first and then heat

• Step Growth polymerisation • Reaction between two molecules such as an organic alcohol or amine with an organic acid in which a small molecule is eliminated • To synthesise linear chains, bifunctional molecules are required

• Polyesters, polyamides and polycarbonates are all produced through condensation polymerisation reactions of bifunctional molecules • Condensation cured silicones are used for taking impressions

• Typically a reaction between epimine and a benzene sulphonate ester to supply the cations • Example is the polymerisation of polyether impression materials eg Impregum

• Initiated by an anion, for example OH- - Polymerisation cyanoacrylated eg superglue

• Through the method of polymerisation eg. step growth, free radical etc • Through the structure of the polymer eg. elastomere (uncross linked or lightly cross linked systems) or glasses (amorphous systems) -Through the thermal behaviour eg. thermoset- cross linked systems that decompose irreversibly (do not soften) thermoplastic- melt when heated and resolidify when cooled

-Homopolymer: Repeating unit is the same eg. Linear Polymer Branched Polymer Cross-Linked Polymer (won’t melt because of the branching so will decompose instead) -Copolymer: Polymer made up of 2 or more monomers eg. Random copolymer -A-B-B-B-A-A-B-A-A-A-A-A-B easy to make but properties shit Block copolymer- A-A-A-A-A-B-B-B some of the material will have properties of A and others B Alternating copolymer- A-B-A-B-A-B best properties but difficult to make Graft copolymer

-Particular type of architecture of the polymer chains controls the properties of the material -Linear polymers (long continuous chains of carbon-carbon bonds with the two valence bonds attached primarily to hydrogen or another relatively small hydrocarbon moeity) Crystalline as in order Require more energy to melt as you have to overcome the forces of attraction between the chains of polymers which is regular -Branched chain polymer -Cross-linked polymers Do not undergo melting But do decompose

• Length of polymer chain (generally, the greater the chain length and higher molecular weight, the better the mechanical properties) • Intermolecular forces • Flexibility of the backbone • Stereoregularity (how arranged in space)

• Polymers have a characteristic temperature below which is behaves as a hard and rigid solid more like a glass • At temperatures greater than the Tg the polymer exhibits significant elastic behaviour • Long range segmental motion of polymer does not occur below Tg • Chains are able to wiggle about, turning to and fro like a corkscrew past the glass transition temperature, known as segmental motion

-As you heat up the polymer, Intermolecular forces get disrupted as you add more energy to the systme Polymer chains begin to wiggle about Go from a very ordered structure to wiggly state The temperature at which the properties of the material changes from a hard solid structure to a soft wiggly one is known as the glass transition temperature -You want a material with a Tg 20 degrees higher than the highest temperature the mouth will ever be exposed to Otherwise after a hot drink it can degrade quickly Don’t want it to ever soften

-Property of polymers that allow it to be pliable well above its glass transition temperature