Polymers and Composites in Dentistry

Question 1

What is a polymer?

Answer 1

• What is a polymer?A large molecule (macromolecule) composed of repeating structural units (monomers) typically connected by covalent chemical bonds.

Question 2

Give 5 examples of polymer use in dentistry.

Answer 2

1. Silicone impression material
2. Soft lining material
3. Acrylic denture base
4. Composites
5. Alginate impression material

Question 3

Give 4 methods polymers are synthesised by.

Answer 3

1. Addition
2. Condensation
3. Cationic
4. Anionic

Question 4

What are the 3 steps in free radical addition polymerisation?

Answer 4

1. Initiation: Formation of free radicals.
2. 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.
3. Termination: 2 free radicals form neutral species.

Question 5

In order for initiation and activation to occur what is required?

Answer 5

1. A reactive species.
2. This can be free radicals or ionic.

Question 6

What are free radicals?

Answer 6

Molecules that contain 1 weak bond that can breakdown to form 2 reactive species with an unpaired electron each.

Question 7

Give an example of a free radical.

Answer 7

Dibenzoyl peroxide.

Question 8

Give 5 activation methods for polymerisation.

Answer 8

1. Chemical
   
   2. Thermal
   3. Photochemical
   4. Microwave
   5. Combined heat and light

Question 9

Give an example of chemical activation and apply this to dentistry.

Answer 9

1. Dibenzoyl peroxide (initiator) can produce free radicals.
2. The chemical can be a tertiary amine such as N,N dimethyl p toluidine. This acts as an activator.
3. Both the dibenzoyl peroxide and dimethyl p toluidine come together to form an initiator-activator complex.
4. This produced free radicals.
5. This process can be used to repair denture bases.

Question 10

Give an example of thermal activation and apply this to dentistry.

Answer 10

1. Dibenzoyl peroxide can undergo decomposition when subjected to heat.
2. Free radicals produced as a result.
3. Heat cured denture bases are cured by this process

Question 11

Give an example of photoinitiation and apply this to dentistry.

Answer 11

1. Visible light is applied to a photoinitiator.
2. This is typically camphorquinone and this is combined with a tertiary amine as an activator (e.g. dimethylaminoethyl methacrylate)
3. This is how dental composites are cured (composites have camphorquinone within its structure)

Question 12

Why is incremental curing required in resin based composite restorations?

Answer 12

1. Due to the limited cure depth.
2. To reduce the consequences of shrinkage stress.
3. The depth of cure refers to the thickness of the composite that is adequately cured.

Question 13

What does the light absorption and scatter within the material depend on in the curing of resin based composite restorations?

Answer 13

1. The amount, size and type of fillers.
2. Shade
3. Photoinitiator type and concentration
4. Refractive index mismatch
5. Light irradiation source and irradiation duration

Question 14

In what dental procedure is a combination of heat and light used, and why is this?

Answer 14

1. Composite inlays
2. This improves the degree of polymerisation and very high degrees are required for this procedure.

Question 15

What is condensation polymerisation and what is it also known as?

Answer 15

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

Question 16

In condensation polymerisation, what type of molecules are required to synthesise linear chains?

Answer 16

Bifunctional molecules (habve 2 different functional groups):

Question 17

Give 3 examples of bifunctional molecules.

Answer 17

1. Polyesters
2. Polyamides
3. Polycarbonates

Question 18

Give an example of a condensation polymerisation reaction used in dentistry.

Answer 18

1. Condensation cured silicones
2. Used for taking impressions

Question 19

What is cationic polymerisation?

Answer 19

A reaction between an epimine and a benzene sulphonate ester to supply the cations.

Question 20

example of cationic polymerisation in dentistry?

Answer 20

The polymerisation of polyether impression materials e.g. impregum

Question 21

What is anionic polymerisation?

Answer 21

A reaction initiated by an anion such as OH-.

Question 22

ive an example of anionic polymerisation.

Answer 22

The polymerisation of cyanoacrylates (e.g. superglue)

Used as a surface dressing after oral surgical procedures.

Question 23

What are the 2 thermal behaviours polymers can have?

Answer 23

1. Thermoset

They do not melt as they are cross linked systems that decompose irreversibly

2. Thermoplastic

Melt when heated and resolidify when cooled.

Question 24

What is a homopolymer and what is a copolymer?

Answer 24

1. Made up of the same monomer.
2. Made up of 2 or more different monomers.

Question 25

What are the 5 different types of copolymers?

Answer 25

1. Random 2. Block 3. Branched 4. Cross-linked 5. Graft

Question 26

What 4 variables affect the properties of a polymer?

Answer 26

1. The flexibility of the backbone 2. The length of the polymer chain 3. Intermolecular forces 4. Stereoregularity

Question 27

What is the glass transtional temperature Tg?

Answer 27

1. The temperature at which if exceeded, the polymer exhibits significant elastic behaviour. 2. Acts ‘glassy’ as opposed to hard and rigid, when Tg is exceeded.

Question 28

What is different between amorphous and elastomer polymers when it comes to Tg?

Answer 28

1. Elastic polymers coming up to Tg exhibit a glassy state, before melting to a liquid. Elastomers exhibit a crystalline state 2. Amorphous polymers beyond Tg exhibit a rubbery state, before melting to a liquid. Elastomers do not have a rubbery state.

Question 29

What do plasticizers do to polymers when added to them?

Answer 29

1. They get in between polymer chains. 2. Space them out from each other. 3. Increasing free volume.

Question 30

What are the advantages of adding plasticizers to polymers in dentistry?

Answer 30

1. The polymer chains slide past each other mroe easily and move around at lower temperatures. 2. Therefore the Tg of the polymer cna be lowered, making the polymer mroe pliable and easier to work with. 3. They can be added to PMMA denture bases to soften them for a temporary soft lining, aiding patients with ulcers.

Question 31

What is a disadvantage of adding plasticizers to polymers in dentistry?

Answer 31

Plasticizers are prone to leaching out of polymers.

Question 32

What does cross linking do to the properties of a polymer?

Answer 32

1. Changes from thermoplastic to thermoset 2. As a result the Tg of the polymer does not exist anymore

Question 33

What polymer backbone is used in dental composites and why?

Answer 33

Bis GMA Very rigid, has a high Tg

Question 34

What type of polymer backbone is used in gutta percha for root canal?

Answer 34

Geometric isomer- namely the trans-somer of **polyisoprene**:

Question 35

What are the 3 types of stereoregular polymers?

Answer 35

1. Isotactic An arrangement where all substituents are on the same side of the polymer chain. 2. Syndiotactic Composed of alternating groups. 1. Atactic Random arrangement

Question 36

What is the difference between an elastic polymer and viscoelastic polymer?

Answer 36

1. Elastic polymer When a polymer is able to undergo full elastic recovery after removal of applied load it is called elastic. 1. Viscoelastic polymer If recovery is slow and there is a degree of permanent deformation is it called viscoelactic polymer

Question 37

What is the difference between stress relaxation and creep?

Answer 37

1. Stress relaxation A decrease in stress under constant strain. 2. 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)