Polymers:
At what temperature does benzyl peroxide begin to dissociate and form free radicals?
>60 degrees
What is a polymer?
A molecule that is made of many parts
How do you do chemically activated polymerisation, what must you have?
A tertiary amine, or activator that is incorporated in the monomer. Such as Benzyol peroxide
What are the polymer requirements for aesthetic properties in dental appliances?
Able to match tooth colour and translucence
What is crazing of acylics?
Acrylics are brittle materials, that are sensitive to size of defects such as cracks and pores, when stressed they develop crazes of crack like features, perpendicular to the tensile stress
How is the free radical usually formed?
Activated by thermal decompositon (heat causes molecule to dissociate), photo dissociatio or redox reactions
What is propagation?
Activated monomers attack the double bonds of additional available monomers, resulting in the rapid addition of monomer molecules to the free radical -Condensation will have the by-product
How can you inhibit early termination?
Adding an inhibitor such as hydroquinone
What are the two types of polymerization?
Addition Condensation
What are the polymer requirements for Physical, chemical, and mechanical properties in dental appliances?
Adequate strength and resilience to resist occlusal forces Should not be degraded or work in the presence of oral fluid The shape should not change by time or temperature
What happens below Tg?
Amorphous (glass) structure Rigid solid
What is the initiator in composite resin?
An organic perioxide: Benzoyl peroxide -Free radicals are made by the presence of blue curing light
What is the initiator for denture base acrylic resins and is it in powder of liquid?
An organic peroxide such a: Benzoyl Peroxide
Why is the development of porosity, less likely in cold cured acrylic exotherm?
Because the temperature is not likely to go over 60 degrees therefore there will not be vapourising
What are common methods to reduce shrinkage- fiiller, what is the effects??
Binding the monomer to an inert filler such as a glass powder. This not only decreases shrinkage but also reduces thermal expansion Increases mechanical properties Helps to deal with dimension change
How do you achieve cross-linking?
By adding glyco dimethylacrylate, a molecule that has two double bonds, so joins two chains. This is added to the monomer
How can you avoid cracks in acrylics?
By developing high impact resistant acrylics, that is developed from a copolymer that contains PMMA as well as a rubber particle
How is acrylic resins polymerised?
By heat curing Reaction of the initiator, generally benzyl peroxide,-were it has a free radical.
How do you decrease the liklihood of vapourising acrylics?
By pressurising the flask, it increases the boiling point temperature of the monomer, and therefore decreases the vapour release and chance of bubbles
How can you have cross-linking
Can have a cross-linking agent involved, that vastly increases the strength by joining chains together to make a more stable structure in the end
What are the different chemically activated polymerisation?
Cold curing Self curing Autopolymerisation
What is a terpolymer?
Contains three monomers
What is a copolymer?
Contains two monomer
What are the bonds between polymeric chains?
Covalent bonds
What are the effects of crazes
Crazes are a void like feature that develops at sites of stress concentration and weaken dentures. A crack may eventually form from this
What is cross-linking?
Cross-linked molecules are a network structure that may result in one giant molecule
What do plasticisers do on the relationship between acrylics mechanical behaviour and temperature?
Decrease the temperature for onset of ductility
How does the fillers affect properties of polymers
E.g. glass paritcles or fibres Increase Tg and modulus of elasticity.
What are the polymer requirements for handling properties in dental appliances?
Easy to use and form
What is the cross-linking agent?
Ethyleneglycoldmethycrylate
How does the chemical composition affect properties of polymers
Example: polymer backbones which contain phenyl groups are more rigid than those that contain only C-C
How does the plasticizers affect properties of polymers
Facilitate in the movement of polymers chains therefore decreases Tg and modulus of elasticity Make things more like dough, so easier to deal with
If the chain is longer, what is the property of it more likely to be?
Flexible, so a soft lining material
For acrylic polymerization reaction what happens in the inititation stage?
Free radicals react with monomer, breaking the double bond and attaching to monomer
What are the thermal properties of polymers?
Heating the polymer--> increase distance between the atoms --> softening the polymer
What is a typical stabliser or inhibitor?
Hydroquinone, it works because it has a higher probability of reacting with free radicals, so BPO free radicals are consumed firstly by hydroquinone, prior to the polymerisation
What is the inhibitor?
Hydroquione
How does porosities occur in acrylics?
If the temperature of the denture exceeds 100 degrees during polymerisation, the monomer can boil > vapour > bubbles, which move very slowly through the viscous partially polymerised resin
What is the effects of changing chain length in polymers?
If you change chain length, you change the function of the polymer, therefore composition change. Length gives it a certain property
What is causes termination of polymerization?
Impurities in the monomer: can react with free radicals, and therefore lower the amount free, and lower the degree of polymerisatin Presence of O2
Why should Tg be considered?
In dental polymersdue to wide temperature fluctuations in the mouth
How do you prevent shrinkage?
Incorporate pre-cured PMMA granules to reduce the change in volume from 21% -> 7%
What are the effects of cross-linking?
Increase the strength of the polymer Decreases flow effects due to temperature Makes structure more rigid/stable
How does the number of branches affect properties of polymers
Increases degree of chain branches lower the glass Tg because chain branches prevent adjacent chains from moving close enough to form inter chain-bonding
What does cross-linking do on the relationship between acrylics mechanical behaviour and temperature?
Increases the temperature where ductility commences
How does the degree of cross linking affect properties of polymers
Increasing the number of cross-linking increases Tg and modulus of elasticity, much more rigid
What are the three stages for acrylic resin?
Initiation Propagation Termination
What are the three stages of polymerisation?
Initiation Propagation Termination
How are acrylic resins cured?
Light cured Heat cured Cold cured
What is a acrylic resin?
Like a denture base
What is the spatial structure of polymers?
Linear Branched- back bone chain, then have side chains coming off
What is condensation polymerisation?
Low-molecular weight by product such as water or alcohol is formed
What is the monomer?
Methylymethacrylate
What are the physcial properties of the polymers?
Molecular weight of polymer (composed of varying chain legnths) Degree of polymerisation High degree of polymerisation--> better mechanical properties --> better clincal performance
What is the composition of the liquid part of the denture base acrylic resins?
Monomer Cross-linking agent Inhibitor Activator
How is acrylic resins denture bases manufactured?
Monomer (powder) is added to polymethylmethacrylate (PMMA/ Liquid), which also contains the intiator benzyol peroxide, it is mixed to make a dough. PMMA dissolves. No polymerisation, the monomer must be dispersed in water bath and heated in a controlled manner
What happens if temp goes above 100 degrees in acrylics?
Monomer boils Porosity is a result Lowering the strength is the outcome
What are the phase changes during polymerisation?
Most monomers are liquid at room temp During polymerisation average molecular weight increases --> increase in viscosity --> very viscous (amorphous solid structure)--> final product with some degree of rigidity.
What is the activator?
N N`-Dimethyl-P-toluidine Only used in self curing materials
What is addition polymerization
No by-product is formed, just two monomers joined together
What are the polymer requirements for biollogical properties in dental appliances?
Non-toxic and non-carcinogenic Does not cause allergic or inflammatory reactions Odourless and tasteless
What is the powder composition of denture base acrylic resins?
Polymer Initiator Pigments
What is the shrinkage of polymerisation?
Polymers undergo considerable shrinkage upon curing, because the free volume of monomers is > Polymer. From monomer--> polymer 21% volume shrinkage
What polymer of denture base acrylic resins?
Polymethylmethacrylate
What is the consequences of residual monomer?
Presence of residual monomer in dentures is a serious problem, as it decreases the strength and toughness of the denture. Also the monomer may be released to the patient over time- toxic
What is initiation?
Production of free radicals, which intiate the growth of polymer chains.
What are the temperature changes that occur during polymerisation?
Reactions are exothermic Produce heat -Want to avoid getting so high that they cause vaporisation of unconverted monomer--> porosity
What do the rubber particels do in the copolymer for high impact resistant acrylics?
Rubber particles bridge and stop crack developing through brittle acrylic
What are the pigments that could be in the powder composition of denture base acrylic resins?
Salts of cadmium or iron or organic dyes
What are the dimensional changes in polyemerisation?
Shrinkage occurs during polymerisation because polymeric chains occupy less space than monomer 21%, when methylmethacrylate --> polymethymethacrylate.
What is the relationship between acrylics mechanical behaviour and temperature?
Temperature has a dramatic effect on the stress-strain behaviour of PMMA At 40 degrees and above you observe some ductility will commence
What is the accelerator of composite resin?
Tertiary amine -Aromatic amine Helps speed up the propagation
How does the degree of polymerisation affect properties of polymers
The higher the average molecular weight, the higher the glass transition and modulus of elasticity, so the stiffer the material
What is polymerisation?
The process by which monomer units become chemically linked together to form high molecular weight molecules
What is the glass transition temperature?
The temperature at which the polymer ceases to be glass-like (brittle) and becomes rubber-like
Why is polymerisation likely to be less complete in thinner denture sections?
The thinner it is the greater surface area there is in proportion to the volume of O2
For acrylic polymerization reaction what happens in the propagation stage?
There is chain elongation Free radical of monomer are able to react with another monomer unit
What polymers dont soften by heating?
Thermosets, because they are solidified during polymerisation, with covalent cross-linking.
What materials might it be impossible to find a Tg in/
Thermosetting materials Since the material will often decompose before softening
What are the bonds between adjacent chains?
These could be secondary bonds such as hydrogen or van der Waals forces
What are inhibitors added for?
They are needed to be incorporated to prevent spontaneous polymerisation due to UV radiation, O2
For acrylic polymerization reaction what happens in the termination stage?
This can occur at any stage, irrespective of chain length. It can depend on the denisty of free radicals
What is the effect of cross linking in acrylic resin?
To increase (harden) the mechanical properties as well as reducing the solubility of water.
What are common methods to reduce shrinkage is the effects??
Use pre-polymerisaed beads of monomer, where some are already cured, therefore pre-shrunk Use larger monomers
What happens above Tg?
Viscous liquid, flexible material
What is cold cured acrylic exotherm?
Where the polymerisation occurs at room temperature, but there is still an exothermic reaction, resulting in a temperature rise
Is the polymerisation of PMMA a exothermic reaction, and what may be the consequences of this?
Yes it is, it releases approx 80kJ/mol, as the C=C bonds --> C-C. Acrylic is a poor thermal conductor, therefore it doesnt draw away the heat, this can result in temperatures exceeding the boiling point of the monomer, which means that it may generate vapour, vapour= porosity.
Is polymethylmethacrylate pre-polymerised monomers Y/N?
yes
