What is the minium number of functional groups required for crosslinking to occur?
3 or more reactive groups capable of reaction (possible only condensation- unsure)
addition or polymerisation reaction:
acetic acid + ethyl alcohol = ethyl acetate + water
condensation (it is an esterification)
Why do we need to check by products of condensation reactions?
may be toxic, so dangerous to use chair side
why may air bubbles occur in denture?
- contraction of the polymer , as polymers are more dense than monomers
- reactions are exothermic and may result in vaporisation of monomer
What is a solution to shrinkage of fillings?
one method of reducing shrinkage is to use larger monomers e.g. Methyl methacrylate (blue background molecule above).
Why is shrinkage a problem in fillings?
- staining occur between filling material and tooth. Aesthetically unpleasing
- provides entry way for pathogens.
What is the function of Cross-linking ?
: makes it harder to deform as binds chains together thus increase the glass transition temperature, it can make a material that is ductile, brittle
What effect does plasticiser have on the glass transition temperature?
2) why?
- reduces it
2. allows polymer chains to flow over one another more easily
- What do dentists typically do with liquid polymers and use them for?
- no real uses, normally at cross-linkers
describe the structure of the following:
- crystalline polymers
- glass polymers
- amorphous solids
- super-cooled liquids
- structures with a regular long ranged arrangement e.g. nylon
- same as 1?
- no long range order, its random (the chains look like they do in liquid state but it is a solid)
- has the same structure as an amophous solid
If something that has a crystalline structure what happens on heating? (MP)
regular structure vibrates, and has a melting point
Do all crystalline polymers melt?
some will burn instead or melting , as they undergoes oxidation
What is a thermosetting polymer?
2. what is a thermoplastic ?
- harden on setting and cannot be softened on reheating
2. softens on reheating so has a MPt and Tg, hardens on cooling
Give an example of a thermoplastic?
boil and bite mouthguards
do amophous solids have a glass transition temperature?
2. Does it have a melting point?
yes
2. not a definite melting point; instead, it melts gradually over a range of temperatures, because the bonds do not break all at once
which one can be recycled:
thermosets or thermoplastics
2. Which ones burn and char on heating?
3. which one has lots of cross links?
thermoplastics
- thermosets
- thermosets
If the pendant group is larger in the polymer , mentioning the structure, how will that effect the glass transition temperature?
acts as barriers between the chains, thus larger= further apart and thus the more easily polymers move past each other.
What could you do to a polymer to make it softer?
lower the glass transition temperature
- add a plasticiser
- use a polymer with less than 3 reactive groups= no cross-linking
- use monomers with a large pendant group
- if more residual monomer is left over as reaction as not gone to completion (never will): they act as another plasticisor
structurally what changes when we lower glass transition temperature overall?
the polymer chains flow over each other more easily
How are dentures moulded?
dough moulding
-look at practical
How are composite fillings moulded?
paste moulding
-look at practical
Why do we use injection and compression moulding in dentistry?
it is not practical
why is the glass transition temperature effected if the manufacturers guide on setting isn’t followed?
as the reaction will stop, not go to completion sooner, thus there will be a lots of residual monomer left over, acting as a plasticiser, reducing the glass transition temperature
