Polymers and Composites

Question 1

What is a polymer?

Answer 1

A large molecule composed of repeating units typically connected by covalent chemical bonds.

Question 2

What do we use polymers for in dentistry?

Answer 2

• Denture base
• Impression material (silicone based and alginate)
• Composites

Question 3

What are the types of polymerisation?

Answer 3

• Addition
• Condensation
• Cationic
• Anionic

Question 4

What are the 3 steps in free radical addition polymerisation?

Answer 4

1) Initiation
- Forming free radicals

2) Propagation
- Long chains react with free radicals which then become reactive

3) Termination
- Two free radicals react together to form a neutral species

Question 5

How is initiation occuring?

Answer 5

• Source of free radicals required

- Reactive species may be a free radical or ionic

Question 6

What is disporportionation?

Answer 6

Another way that a free radical polymerisation is terminated.
- Breakdown of molecule generates one unsaturated molecule and one saturated molecule

Question 7

What are the main methods of activation a polymerisation reaction?

Answer 7

• Light
• Chemical
• Thermal (high temp cant be used in dentistry)
• Microwave (cant be used in dentistry)
• Combined heat and light

Question 8

How is polymerisation activated chemically?

Answer 8

• Peroxide molecules are used (such as dibenzoyl peroxide)
• This has a COO-OOC link that can be broken down with a weak bond
• Free radicals can be produced
• Activators can be used (tertiary amines) to break down and form free radicals also

Question 9

How does thermal activation work?

Answer 9

Peroxide molecules can undergo decomposition when subjected to heat and this forms free radicals.

Question 10

How does light activation work?

Answer 10

Commonly used polymerisation method.
The light used is 470nm wavelength.
Camphorquinone is the light activator used with a tertiary amine activator.

Question 11

What is the drawback of using light activation with resin?

Answer 11

• Incremental (1-2mm) placement

due to fillers type, size and distribution affecting light movement through material

Question 12

Why do we want a high extent of polymerisation?

Answer 12

To give better mechanical properties.

Question 13

What is condensation polymerisation?

Answer 13

A reaction between 2 molecules that results in a small molecule being eliminated.

Question 14

What type of molecule is required for a condensation reaction to occur?

Answer 14

• Bifunctional molecules

Question 15

What is cationic polymerisation?

Answer 15

Cations (+) generated to drive the polymerization reaction.

Used in polyether impression materials.

Question 16

What is anionic polymerisation?

Answer 16

Anions can also drive the polymerisation reaction.

Question 17

What are the 3 methods of classifying polymers?

Answer 17

1) Polymerisation method (addition, condensation, cationic, anionic)
2) Structure - elastomers or glasses
3) Thermal behaviour - thermosetting (dont melt under heat) or thermoplastic (do melt under heat)

Question 18

What is a homopolymer and a copolymer?

Answer 18

Homopolymer = repeating unit is the same

Copolymer = polymer made up of different monomers

Question 19

What are the 3 types of copolymer?

Answer 19

1) Random
2) Alternating
3) Block

Question 20

What types of molecular architecture can a polymer have?

Answer 20

• Linear
• Branched
• Cross-linked

Question 21

What does branching do to the polymer?

Answer 21

Stops the ingress of solvents into the polymer (stops polymer dissolving).

Question 22

What do the properties of a polymer depend on?

Answer 22

• The flexibility of the backbone
• The length of the polymer chain
• Intermolecular forces
• Stereoregularity

Question 23

What do we need to be aware of with the glass transition temperature?

Answer 23

• Below the glass transition temperature, there is insufficient thermal energy so the motion of the polymer is resisted
• As the thermal energy is provided, the long motions between possible above the glass transition temperature

Question 24

What does adding plasticisers to the polymer do?

Answer 24

They get between the polymer chains and increase the FREE VOLUME.
This makes the polymer softer.

Question 25

When we cross-link a polymer, what does this do to the properties?

Answer 25

Th polymer will not undergo any melting as it has changed from a thermoplastic to a thermosetting polymer.

Question 26

How can the backbone of the polymer be changed when still using the same atoms?

Answer 26

ISOMERS!

Geoisomers (cis and trans isomer).

Stereoregularity

• Isotactic (all the substituents are on the same side of the chain)
• Syndiotactic (the substituents are on the opposite sides of the chain)
• Atactic (random arrangement)

Question 27

What are the properties of a viscoelastic material?

Answer 27

• Exhibit both viscous and elastic properties when undergoing deformation (intermediate between the two)
• The strain recovery path is not identical to the loading part
• Mechanical response is highly sensitive to: temperature, rate of deformation, magnitude of deformation

Question 28

What is the model for viscoelasticity?

Answer 28

SPRING - DASH pot model

• The spring represents the elastic component (load is applied and it extends but it recovers immediately)
• The dashpot is used to describe the viscous behaviour (on application, the dash pot opens slowly, time dependant with strain)

If the spring and dashpot are in series, the load allows the spring to extend immediately which is then followed by opening of the dash pot slowly.

When in parallel, the load opens the spring slowly due to damping affect of dashpot.
Both recover slowly when the load is removed.

Question 29

What is stress relaxation?

Answer 29

The reduction in stress in a material subjected to a constant strain whilst creep is the increase in a material under constant stress.

When subjected to a constant deformation, a viscoelastic materials internal resistance to this deformation relaxes with time.

Question 30

What is creep?

Answer 30

Creep can occur due to long term exposure of high stress.

The rate of deformation is a function of the materials properties, then exposure temperature and exposure time, and the applied structural load.