DMT.6

Question 1

What is the minium number of functional groups required for crosslinking to occur?

Answer 1

3 or more reactive groups capable of reaction (possible only condensation- unsure)

Question 2

addition or polymerisation reaction:

acetic acid + ethyl alcohol = ethyl acetate + water

Answer 2

condensation (it is an esterification)

Question 3

Why do we need to check by products of condensation reactions?

Answer 3

may be toxic, so dangerous to use chair side

Question 4

why may air bubbles occur in denture?

Answer 4

1. contraction of the polymer , as polymers are more dense than monomers
2. reactions are exothermic and may result in vaporisation of monomer

Question 5

What is a solution to shrinkage of fillings?

Answer 5

one method of reducing shrinkage is to use larger monomers e.g. Methyl methacrylate (blue background molecule above).

Question 6

Why is shrinkage a problem in fillings?

Answer 6

1. staining occur between filling material and tooth. Aesthetically unpleasing
2. provides entry way for pathogens.

Question 7

What is the function of Cross-linking ?

Answer 7

: makes it harder to deform as binds chains together thus increase the glass transition temperature, it can make a material that is ductile, brittle

Question 8

What effect does plasticiser have on the glass transition temperature?
2) why?

Answer 8

1. reduces it

2. allows polymer chains to flow over one another more easily

Question 9

1. What do dentists typically do with liquid polymers and use them for?

Answer 9

1. no real uses, normally at cross-linkers

Question 10

describe the structure of the following:

1. crystalline polymers
2. glass polymers
3. amorphous solids
4. super-cooled liquids

Answer 10

1. structures with a regular long ranged arrangement e.g. nylon
2. same as 1?
3. no long range order, its random (the chains look like they do in liquid state but it is a solid)
4. has the same structure as an amophous solid

Question 11

If something that has a crystalline structure what happens on heating? (MP)

Answer 11

regular structure vibrates, and has a melting point

Question 12

Do all crystalline polymers melt?

Answer 12

some will burn instead or melting , as they undergoes oxidation

Question 13

What is a thermosetting polymer?

2. what is a thermoplastic ?

Answer 13

1. harden on setting and cannot be softened on reheating

2. softens on reheating so has a MPt and Tg, hardens on cooling

Question 14

Give an example of a thermoplastic?

Answer 14

boil and bite mouthguards

Question 15

do amophous solids have a glass transition temperature?

2. Does it have a melting point?

Answer 15

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

Question 16

which one can be recycled:
thermosets or thermoplastics
2. Which ones burn and char on heating?
3. which one has lots of cross links?

Answer 16

thermoplastics

2. thermosets
3. thermosets

Question 17

If the pendant group is larger in the polymer , mentioning the structure, how will that effect the glass transition temperature?

Answer 17

acts as barriers between the chains, thus larger= further apart and thus the more easily polymers move past each other.

Question 18

What could you do to a polymer to make it softer?

Answer 18

lower the glass transition temperature

1. add a plasticiser
2. use a polymer with less than 3 reactive groups= no cross-linking
3. use monomers with a large pendant group
4. if more residual monomer is left over as reaction as not gone to completion (never will): they act as another plasticisor

Question 19

structurally what changes when we lower glass transition temperature overall?

Answer 19

the polymer chains flow over each other more easily

Question 20

How are dentures moulded?

Answer 20

dough moulding

-look at practical

Question 21

How are composite fillings moulded?

Answer 21

paste moulding

-look at practical

Question 22

Why do we use injection and compression moulding in dentistry?

Answer 22

it is not practical

Question 23

why is the glass transition temperature effected if the manufacturers guide on setting isn’t followed?

Answer 23

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