Polymers in Dentistry Flashcards
Definition of a polymer
-Macromolecule consisting of repeating structure units joined together by covalent chemical bonds
Examples of polymers used in dentistry
- Silicone impression materials
- Soft lining materials
- Composites
- Acrylic denture bases
- Alginate impression materials
When would the polymerisation process be occuring in the dental setting
- Light curing a composite
- Preparing a denture base by polymerising the monomer
- Repairing a denture by polymerising chairside
Main chemical methods in which polymer synthesis can occur
Addition
Condensation
Cationic
Anionic
Free Radical Addition Polymerisation Reaction explanation and dental relevance
-Initiation
Producing the free radicals
-Propagation
-Termination
Mopping up of free radicals
eg.
Composites and denture bases are cured via a free radical addition polymerisation reaction
Activation definition and methods of activation
Requirement of this method
-Method in which free radicals are produced for the initiation reaction
- Chemical
- Thermal
- Photochemical
- Microwave
- Combining heat and light
- must be safe for the patient
- eg. thermal initiation must not be so hot that it can burn the patient
Difference between an activator and an initiator
- Initiator is the thing that will make the free radical
- Activator is the method in which you use to make it
For example, when repairing denture bases:
- Tertiary amine is the activator
- Dibenzoyl peroxide is the initiator
Chemical activation example in free radical addition polymerisation
- Dibenzoyl peroxide is the initiator that can produce the free radicals
- In the presence of a tertiary amine (N,N dimethyl p toluidine) which is the activator
- Forms an initiator-activator complex
- Denture bases are repaired by this process
- Dibenzoyl peroxide is NOT a catalyst, but rather an initiator
- Dibenzoyl peroxide can also decompose into free radicles with thermal activation if heated about 65 degrees C which is used in production of polymers but this obviously cannot be done inside the mouth so chairside chemical activation is used instead
Thermal or Heat Initiation example in free radical addition polymerisation
- Dibenzoyl peroxide can undergo demposition when subjected to heat
- Free radicals are produced as a result
-Heat cured denture bases are cured through this process
Photoinitiation example in free radical addition polymerisation
- Light initiated
- Visible light usually 480nm
- Camphorquinone (a diketone)
- Tertiary amine used as an activator (dimethylaminoethyl methacrylate)
- Dental composites cured by this method
Principle of incremental curing
Incremental curing is required due to:
- Limited cure depth
- Reduce the consequences of shrinkage stress (C factor)
- Depth of cure refers to the thickness of composite that is adequately cured
Properties that affect the depth of cure
Limited by light absorption and scatter within the material, which depends on:
- Amount, size and type of fillers
- Shade
- Photoinitiator type and concentration
- Refractive index mismatch
- Light irradiation source and irradiation duration
Heat and Light Combo for Initiation in free radical addition polymerisation
- Generally used for composite inlays
- Very high degrees of curing are required
- Indirect restorations often cured by light first and then heat
Condensation Polymerisation alternative name and definition
- Step Growth polymerisation
- Reaction between two molecules such as an organic alcohol or amine with an organic acid in which a small molecule is eliminated
- To synthesise linear chains, bifunctional molecules are required
Examples of condensation polymers in general and 1 use in dentistry
- Polyesters, polyamides and polycarbonates are all produced through condensation polymerisation reactions of bifunctional molecules
- Condensation cured silicones are used for taking impressions
Cationic Polymerisation and example in dentistry
- Typically a reaction between epimine and a benzene sulphonate ester to supply the cations
- Example is the polymerisation of polyether impression materials eg Impregum
Anionic Polymerisation and example
- Initiated by an anion, for example OH-
- Polymerisation cyanoacrylated eg superglue
Ways to classify polymers
- Through the method of polymerisation
eg. step growth, free radical etc - Through the structure of the polymer
eg. elastomere (uncross linked or lightly cross linked systems) or glasses (amorphous systems)
-Through the thermal behaviour
eg. thermoset- cross linked systems that decompose irreversibly (do not soften)
thermoplastic- melt when heated and resolidify when cooled
Types of polymers in term of repeating unit and monomers
Advantages of certain types over others
-Homopolymer: Repeating unit is the same
eg.
Linear Polymer
Branched Polymer
Cross-Linked Polymer (won’t melt because of the branching so will decompose instead)
-Copolymer: Polymer made up of 2 or more monomers
eg.
Random copolymer -A-B-B-B-A-A-B-A-A-A-A-A-B easy to make but properties shit
Block copolymer- A-A-A-A-A-B-B-B some of the material will have properties of A and others B
Alternating copolymer- A-B-A-B-A-B best properties but difficult to make
Graft copolymer
Different types of molecular architecture in polymers
-Particular type of architecture of the polymer chains controls the properties of the material
-Linear polymers (long continuous chains of carbon-carbon bonds with the two valence bonds attached primarily to hydrogen or another relatively small hydrocarbon moeity)
Crystalline as in order
Require more energy to melt as you have to overcome the forces of attraction between the chains of polymers which is regular
-Branched chain polymer
-Cross-linked polymers
Do not undergo melting
But do decompose
Properties depend on…
- Length of polymer chain (generally, the greater the chain length and higher molecular weight, the better the mechanical properties)
- Intermolecular forces
- Flexibility of the backbone
- Stereoregularity (how arranged in space)
Glass transition temperature of polymers definition. What is the term used to describe the motion of chains when you heat polymers
- Polymers have a characteristic temperature below which is behaves as a hard and rigid solid more like a glass
- At temperatures greater than the Tg the polymer exhibits significant elastic behaviour
- Long range segmental motion of polymer does not occur below Tg
- Chains are able to wiggle about, turning to and fro like a corkscrew past the glass transition temperature, known as segmental motion
What happens when you heat up a polymer and significance of Tg in dentistry
-As you heat up the polymer,
Intermolecular forces get disrupted as you add more energy to the systme
Polymer chains begin to wiggle about
Go from a very ordered structure to wiggly state
The temperature at which the properties of the material changes from a hard solid structure to a soft wiggly one is known as the glass transition temperature
-You want a material with a Tg 20 degrees higher than the highest temperature the mouth will ever be exposed to
Otherwise after a hot drink it can degrade quickly
Don’t want it to ever soften
Significance of segmental motion
-Property of polymers that allow it to be pliable well above its glass transition temperature
Difference between glassy and crystalline polymers on temperature volume graph
Look on slides
Additions to polymers and significance of this. Issues and dental example
-Plasticizers
Small molecules which when added to polymer get in between the polymer chains
Space out from each other
Increase the free volume
- Polymer chains slide past each other more easily and move around at lower temperatures than they would without the plasticizer
- Tg of the polymer is lowered, making a polymer more pliable and easy to work with
- However, plasticizers prone to leaching out of the polymer
- Added to PMMA denture bases to soften them for temporary soft lining
Affect of cross linking in a polymer
- Chemically joins the polymer chains together
- Tends to enhance physical properties and Tg
- In a heavily cross linked system, polymer changes from a thermoplastic polymer that has a Tg to a thermoset one which does not and decomposes instead
Affect of flexibility and example in dentistry
- The higher the flexibility, the lower the Tg
- Polydimethysiloxane is so flexible that it is a liquid at RT
- BisGMA used in dental composites has a very rigid backbone with a high Tg
Stereoregularity types and example in dentistry
-Cis and trans isomers
- Isotactic is an arrangement where all substituents are on the same side of the polymer chain
- Syndiotactic polymer chain is composed of alternating groups
- Atactic is random arrangement
- Natural rubber is a cis isomer
- Gutta percha is a transisomer
Excluded volume definition
-Idea that one part of the long chain molecule can not occupy space that is already occupied y another part of the same molecule
Viscoelasticity definition
-Real materials exhibit properties intermediate between a viscous liquid and an elastic solid
-If a polymer is able to undergo full elastic recovery it is known as elastic
-If recovery is slow and there is a degree of permanent deformation it is known as viscoelastic
-Mechanical response is dependent on
Temperature
Rate of deformation
Deformation history
Magnitude of deformation
Stress Relaxation definition
- Reduction in stress in a material when a material is subjected to a constant strain
- When subjected to constant deformation, a viscoelastic material’s internal resistance to this deformation relaxes with time
Creep definition
- Involves the gradual increase in strain under a constant applied load
- Relatively small deformations produced by a relatively large stress over a period of time
- Increase in strain in a material under constant stress
- Look at graph on slides for creep curves
