5. Polymers + composites in dentistry Flashcards
AIMS
What is a polymer?
- A polymer is a large molecule (macromolecule) composed of repeating structural units typically connected by covalent chemical bonds
Made of monomer residues
Why do we need polymers in dentistry?
Polymers are used in dentistry in a wide range of applications:
- Silicone impression material
- Soft lining material ( in dentures )
- Alginate impression material
- Acrylic denture base
- Composites ( dental resin composites - widely used restorative material )
Polymer synthesis
Why do dentists need to understand polymer synthesis ?
Dentists need to understand polymer synthesis because :
- You conduct a polymerisation when you use a light to cure a dental composite
- You prepare a denture base by polymerising a monomer
- You repair a denture by polymerising at the chairside
Polymer synthesis
What are the methods of polymer synthesis / how do we make polymers?
What are the main methods of polymer synthesis ?
- ADDITION
- CONDENSATION
- CATIONIC
- ANIONIC
- Main methods of polymer synthesis - ADDITION AND CONDENSATION
Addition polymerisation
Free radical addition polymerisation
What dental materials are cured using this?
Composites & denture bases are cured via a free radical addition polymerisation
Addition polymerisation - Free radical addition polymerisation
What is an addition reaction?
Addition reaction - joining of two molecules to form a third larger molecule
Addiction polymerisation involves addicting of reactive species with a monomer to form larger reactive species
This reactive species then adds onto another monomer unit continuing until get a long chain
Reactive species involved can be IONIC or FREE RADICAL
If reactive species is free radical reaction is - free radical addition polymeration
What are the 3 steps in free radical addition polymerisation
( Activation first )
What is the initiator in free radical addition polymerisation?
Initiator is Free radical
Free radical addition polymerisation
Initiation and activation?
What does step 2 in free radical addition polymerisation include?
What is step 3 of addition polymerisation and what happens?
What are the different activation methods (addition polymerisation) ?
Activation - chemical ?
What’s used?
Activation - thermal or heat initiation ?
Activation - photo initiation?
What is the activator?
What is it commonly used for?
Resin based composite restorations
Usually cured by?
Disadvantage?
Activation - combination of heat and light?
What are they used for ?
Why is combination of heat and light used?
- Generally used for ‘composite inlays’ as very high degrees of curing are required.
- Often used to improve degree of polymerisation
Higher the extent of polymerisation the better the mechanical properties - Composite inlays first cured using light and then subjected to heat cure cycle to improve degree of curing and thereby improving mechanical properties
Condensation polymerisation
What is condensation polymerisation?
What type of molecules are needed for condensation polymerisation?
Why?
Examples of products formed by condensation polymerisation ?
Examples of materials in dentistry that undergo condensation polymerisation?
Need BIFUNCTIONAL molecules (2 functional groups )
Examples are polyesters, polyamides, polycarbonates
In dentistry: Condensation cured silicones are used for taking impressions
Cationic polymerisation
Occurs between?
Used for?
Anionic polymerisation
What is it initiated by?
Used in?
Classification of polymers
How can we classify polymers?
Polymerisation method
Structure
Thermal method
Classification of polymers
Polymerisation method?
Classification of polymers
Structure?
Classification of polymers
Thermal behaviour
What does thermal behaviour involve?
Thermal behaviour can be either?
Thermal behaviour - how polymers respond when subjected to heat
Thermal behaviour can be :
Thermoset or Thermoplastic
Classification of polymers
Polymers can be :
Examples of polymers
What do these polymers look like:
random copolymer
Linear polymer
Branched polymer
Block copolymer
Graft copolymer
Crosslinked copolymer?
Which type of polymer would be thermoplastic and which would be thermosetting?
If a polymer has the same molecular weight for example, but different type of polymer would it’s properties be the same?
no
Linear polymers
Molecular architecture
What does molecular architecture control
types of architecture?
Properties of polymers
What do the properties of polymers depend on?
Glass transition temp tg
Tg graph
Difference between glassy state and rubbery state?
Can polymer properties be modified ?
What are plasticizers and why might they be used?
Yes
Polymers are versatile materials and can change their properties in different ways . Before seen how copolymers affect properties and how can change backbone so different properties. Another way to change properties is by making them soft or rigid
Cross linking
What is it and what does it do?
Polymer backbones
Which backbone is flexible?
Which backbone is used in composites?
Geometric isomers
Examples of ones that are relevant in dentistry?
Stereoregularity
What is tacticity? (Not from lectures just for understanding )
What are the three types of tacticity of polymers?
Tacticity - stereochemical arrangement of the units in the main chain of polymer (google def)
Viscoelastocity
What type of characteristics do viscoelastic materials exhibit?
Mechanical response is highly sensitive to ?
Viscoelastic materials exhibit both vicious and elastic characteristics when undergoing deformation
Stress has been used to define elastic range of material beyond which material becomes permanently distorted
When materials undergo elastic recovery after removal of load returns to original shape and size however if material coni tubes to recover slowly over time or a certain amount of permanent deformation remains, the material is said to exhibit viscoelasticc behaviour
Stress and strain relationship is time dependant in viscoelastic polymers
When stresses are removed from polymeric material before it fractures, the strain recovery path is not necessarily identical to that of the loading part of the deformation cycle,
Energy is dissipated during the deformation of materials which is indication of deviation from elastic behaviour
Degree of viscoelastcity is dependant on :
(All the bps)
Viscoelasticity
When is a polymer called elastic?
When is a polymer called viscoelastic?
When the polymer is able to undergo full elastic recovery after removal of applied load it is called ‘elastic’
If the recovery is slow and there is a degree of permanent deformation it is called viscoelastic
Viscoelasticity
Maxwell model?
Voigt model?
Viscoelastic nature of solids can be explained using a spring-dash pot model
Spring represents elastic component
Once load applied - it extends
When release the load - recovers instantly reflecting no time dependancy and complete recovery occurs to original shape and size
Modulus of elasticity indicates amount of load a material can withstand
The dash pot is used to describe the viscous behaviour
On application of load, it open dashpot slowly with strain being a function of time
Time dependancy is similar in the way in which the flow is of viscous material that is controlled by its viscosity
What are two important properties exhibited by polymers?
Stress relaxation
Creep
What is stress relaxation?
Graph of strain against time?
Graph of stress against time?
Creep
When does creep occur?
When is creep more severe?
Creep can occur as result of long term exposure to high levels of stress that are still below the yield strength of material
Creep - more severe in materials that are subjected to heat for long periods and generally increases as their near their mp
What does creep involve
Creep involves the gradual increase in strain under a constant applied load. Creep is relatively small deformations produced by a relatively large stress over a period of time.
creep curve
Summary
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