Polymers and Composites in Dentistry Flashcards
What is a polymer?
- What is a polymer?A large molecule (macromolecule) composed of repeating structural units (monomers) typically connected by covalent chemical bonds.
Give 5 examples of polymer use in dentistry.
- Silicone impression material
- Soft lining material
- Acrylic denture base
- Composites
- Alginate impression material
Give 4 methods polymers are synthesised by.
- Addition
- Condensation
- Cationic
- Anionic
What are the 3 steps in free radical addition polymerisation?
- Initiation: Formation of free radicals.
- Propagation: Long chains are formed through the addition of reactive species to a monomer, which becomes reactive (a new active site is made) and in return longer chains are formed.
- Termination: 2 free radicals form neutral species.
In order for initiation and activation to occur what is required?
- A reactive species.
- This can be free radicals or ionic.
What are free radicals?
Molecules that contain 1 weak bond that can breakdown to form 2 reactive species with an unpaired electron each.
Give an example of a free radical.
Dibenzoyl peroxide.
Give 5 activation methods for polymerisation.
- Chemical
- Thermal
- Photochemical
- Microwave
- Combined heat and light
Give an example of chemical activation and apply this to dentistry.
- Dibenzoyl peroxide (initiator) can produce free radicals.
- The chemical can be a tertiary amine such as N,N dimethyl p toluidine. This acts as an activator.
- Both the dibenzoyl peroxide and dimethyl p toluidine come together to form an initiator-activator complex.
- This produced free radicals.
- This process can be used to repair denture bases.
Give an example of thermal activation and apply this to dentistry.
- Dibenzoyl peroxide can undergo decomposition when subjected to heat.
- Free radicals produced as a result.
- Heat cured denture bases are cured by this process
Give an example of photoinitiation and apply this to dentistry.
- Visible light is applied to a photoinitiator.
- This is typically camphorquinone and this is combined with a tertiary amine as an activator (e.g. dimethylaminoethyl methacrylate)
- This is how dental composites are cured (composites have camphorquinone within its structure)
Why is incremental curing required in resin based composite restorations?
- Due to the limited cure depth.
- To reduce the consequences of shrinkage stress.
- The depth of cure refers to the thickness of the composite that is adequately cured.
What does the light absorption and scatter within the material depend on in the curing of resin based composite restorations?
- The amount, size and type of fillers.
- Shade
- Photoinitiator type and concentration
- Refractive index mismatch
- Light irradiation source and irradiation duration
In what dental procedure is a combination of heat and light used, and why is this?
- Composite inlays
- This improves the degree of polymerisation and very high degrees are required for this procedure.
What is condensation polymerisation and what is it also known as?
Reaction between 2 molecule such as an organic alcohol or amine with an organic acid in which a small molecule is eliminated
Step Growth polymerisation
In condensation polymerisation, what type of molecules are required to synthesise linear chains?
Bifunctional molecules (habve 2 different functional groups):
Give 3 examples of bifunctional molecules.
- Polyesters
- Polyamides
- Polycarbonates
Give an example of a condensation polymerisation reaction used in dentistry.
- Condensation cured silicones
- Used for taking impressions
What is cationic polymerisation?
A reaction between an epimine and a benzene sulphonate ester to supply the cations.
The polymerisation of polyether impression materials e.g. impregum
The polymerisation of polyether impression materials e.g. impregum
What is anionic polymerisation?
A reaction initiated by an anion such as OH-.
ive an example of anionic polymerisation.
The polymerisation of cyanoacrylates (e.g. superglue)
Used as a surface dressing after oral surgical procedures.
What are the 2 thermal behaviours polymers can have?
- Thermoset
They do not melt as they are cross linked systems that decompose irreversibly
- Thermoplastic
Melt when heated and resolidify when cooled.
What is a homopolymer and what is a copolymer?
- Made up of the same monomer.
- Made up of 2 or more different monomers.
What are the 5 different types of copolymers?
- Random
- Block
- Branched
- Cross-linked
- Graft
What 4 variables affect the properties of a polymer?
- The flexibility of the backbone
- The length of the polymer chain
- Intermolecular forces
- Stereoregularity
What is the glass transtional temperature Tg?
- The temperature at which if exceeded, the polymer exhibits significant elastic behaviour.
- Acts ‘glassy’ as opposed to hard and rigid, when Tg is exceeded.
What is different between amorphous and elastomer polymers when it comes to Tg?
- Amorphous polymers coming up to Tg exhibit a glassy state, before melting to a liquid. Elastomers exhibit a crystalline state
- Amorphous polymers beyond Tg exhibit a rubbery state, before melting to a liquid. Elastomers do not have a rubbery state.
What do plasticizers do to polymers when added to them?
- They get in between polymer chains.
- Space them out from each other.
- Increasing free volume.
What are the advantages of adding plasticizers to polymers in dentistry?
- The polymer chains slide past each other mroe easily and move around at lower temperatures.
- Therefore the Tg of the polymer cna be lowered, making the polymer mroe pliable and easier to work with.
- They can be added to PMMA denture bases to soften them for a temporary soft lining, aiding patients with ulcers.
What is a disadvantage of adding plasticizers to polymers in dentistry?
Plasticizers are prone to leaching out of polymers.
What does cross linking do to the properties of a polymer?
- Changes from thermoplastic to thermoset
- As a result the Tg of the polymer does not exist anymore
What polymer backbone is used in dental composites and why?
Bis GMA
Very rigid, has a high Tg
What type of polymer backbone is used in gutta percha for root canal?
Geometric isomer- namely the trans-somer of polyisoprene:
What are the 3 types of stereoregular polymers?
- Isotactic
An arrangement where all substituents are on the same side of the polymer chain. - Syndiotactic
Composed of alternating groups. - Atactic
Random arrangement
What is the difference between an elastic polymer and viscoelastic polymer?
- Elastic polymer
When a polymer is able to undergo full elastic recovery after removal of applied load it is called elastic.
- Viscoelastic polymer
If recovery is slow and there is a degree of permanent deformation is it called viscoelactic polymer
What is the difference between stress relaxation and creep?
- Stress relaxation
A decrease in stress under constant strain.
- Creep
An increase in plastic strain under constant stress.
In a viscoelastic material this eventually fails (initial curve ending). When this stress is released the strain decreases gradually with time (2nd depressed curve)
