Polymers and Composites Flashcards
What is a polymer?
A large molecule composed of repeating units typically connected by covalent chemical bonds.
What do we use polymers for in dentistry?
- Denture base
- Impression material (silicone based and alginate)
- Composites
What are the types of polymerisation?
- Addition
- Condensation
- Cationic
- Anionic
What are the 3 steps in free radical addition polymerisation?
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
How is initiation occuring?
- Source of free radicals required
- Reactive species may be a free radical or ionic
What is disporportionation?
Another way that a free radical polymerisation is terminated.
- Breakdown of molecule generates one unsaturated molecule and one saturated molecule
What are the main methods of activation a polymerisation reaction?
- Light
- Chemical
- Thermal (high temp cant be used in dentistry)
- Microwave (cant be used in dentistry)
- Combined heat and light
How is polymerisation activated chemically?
- 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
How does thermal activation work?
Peroxide molecules can undergo decomposition when subjected to heat and this forms free radicals.
How does light activation work?
Commonly used polymerisation method.
The light used is 470nm wavelength.
Camphorquinone is the light activator used with a tertiary amine activator.
What is the drawback of using light activation with resin?
- Incremental (1-2mm) placement
due to fillers type, size and distribution affecting light movement through material
Why do we want a high extent of polymerisation?
To give better mechanical properties.
What is condensation polymerisation?
A reaction between 2 molecules that results in a small molecule being eliminated.
What type of molecule is required for a condensation reaction to occur?
- Bifunctional molecules
What is cationic polymerisation?
Cations (+) generated to drive the polymerization reaction.
Used in polyether impression materials.
What is anionic polymerisation?
Anions can also drive the polymerisation reaction.
What are the 3 methods of classifying polymers?
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)
What is a homopolymer and a copolymer?
Homopolymer = repeating unit is the same
Copolymer = polymer made up of different monomers
What are the 3 types of copolymer?
1) Random
2) Alternating
3) Block
What types of molecular architecture can a polymer have?
- Linear
- Branched
- Cross-linked
What does branching do to the polymer?
Stops the ingress of solvents into the polymer (stops polymer dissolving).
What do the properties of a polymer depend on?
- The flexibility of the backbone
- The length of the polymer chain
- Intermolecular forces
- Stereoregularity
What do we need to be aware of with the glass transition temperature?
- 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
What does adding plasticisers to the polymer do?
They get between the polymer chains and increase the FREE VOLUME.
This makes the polymer softer.
When we cross-link a polymer, what does this do to the properties?
Th polymer will not undergo any melting as it has changed from a thermoplastic to a thermosetting polymer.
How can the backbone of the polymer be changed when still using the same atoms?
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)
What are the properties of a viscoelastic material?
- 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
What is the model for viscoelasticity?
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.
What is stress relaxation?
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.
What is creep?
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.
