Flashcards in Dental Materials Deck (293)
Describe light formation of FRs
Tertiary amine activator (dihydroxymethyl-p-toluidine) and light initiator (camphorquinone) added to monomer and react in presence of visible light source to form FRs
How does free radical formation being exothermic lead to porosity?
Boiling causes monomer to bubble creating airspaces which then set as material polymerises
Why can a molecule weight not be given for a polymer?
All polymers are not the same length
What 2 methods can be used to give a polymer a MWt?
1. Number average MWt (Mn)
2. Weight average MWt (Mw)
Define number average MWt
Total weight of sample divided by number of molecules in sample
Define weight average MWt
Sum of the number of molecules multiplied by their weight average
What are the 3 states polymers exist in? Give examples
1. Rubbers: silicone rubber
2. Hard resins: poly(methyl methacrylate)
3. Fibres: polyethylene
Rubber and hard resins: long chains exist in randomly coiled configuration and thermal motion
Fibres: long chains stiff and straightened
How are polymer chains held?
Weak 2ndary bonds: VDWs
Entanglement of chains
Describe the effect of stress on rubbers
Easy deformation due to low intermolecular forces
Reversible due to 1% crosslinks within polymer
Elastic modulus in MPa
Describe the effect of stress on hard resins
Difficult to deform due to high intermolecular forces
Elastic modulus in GPa
What are crosslinks?
Covalent bonds in polymers joining one polymer chain to another
Prevent polymers flowing irreversibly
Compare 1% and 30% crosslinks
30%: rigid material
Describe the effect of crosslinks on glassy polymers
Improves some properties such as impact strength (relevant for dentures)
Too many will make them brittle
Define thermosetting resin
Initially rubbers w/ low intermolecular forces, due to 30% crosslinks end up as rigid polymers
Compare thermoplastics and thermosetting plastics
Thermoplastics: once heated and formed into shape can be reheated and reshaped but red. quality of plastic
Thermosetting: once hearted and shaped can't be reheated and reshaped, difficult to recycle
Define glass transition temp
Explain with an example
Temp at which rigid material becomes soft and rubbery
Intermolecular forces are temp dependent
PMMA is glassy, rigid w/ high intermolecular forces
As temp inc., intermolecular forces dec. until material is soft and rubbery
Describe the Tg fro natural rubber
Elastomer w/ low intermolecular forces
As temp dec, forces inc. until reach Tg temp at which point material becomes glassy rigid
What is the significance of Tg to dentistry?
Denture bases require Tg for exceeding temps likely to be exposed to
Polishing be technician, cleaning by patient
What is a plasticiser?
Simple, organic liquid
What are plasticisers used for?
Added to monomer before polymerisation to red. Tg of material i.e. make naturally rigid, glassy material soft and rubbery at RT
How do plasticisers function?
Depress Tg by lubrication
Flow between chains of polymer red. high intermolecular forces
Rigid material becomes soft and rubbery w/ low intermolecular forces
What are the 2 disadvantages to plasticisers?
1. Material becomes hard again w/ time
2. Phthalates are carcinogens and leach out into patients mouth
What are crystalline polymers and what are their uses in dentistry?
Polymers w/ high degree of order formed by folding and stacking of polymer chains: long, stiff, straightened chains
1. Polypropylene: Instrument trays; withstand high pressure, temp
2. Polyethylene: reinforce acrylic dentures
What is a homopolymer?
Polymer formed when polymerised alone
What are the 3 stereoisomers that polymers can obtain?
1. Atactic: random arrangement of head - tail configurations along chain
2. Isotactic: all substituents on same side of chain
3. Syndiotactic: alternating head-tail linkages
What is a copolymer?
Product of polymerisation involving multiple types of monomer
What are the 3 types of copolymer?
1. Random: random arrangement of monomers along chain
2. Block: blocks of monomer groupings of 1 type along chain
3. Graft: main chain 1 monomer w/ branches of other monomers
Why impact can water uptake have on materials?
1. Red. strength
2. Extract potentially toxic materials
3. Can induce microorganism formation
What are 3 useful water soluble materials used in dentistry?
1. Alginates: impression
2. Poly(acrylic acid): cements, polymer is solid but dissolves readily and ionises, anion is polymeric
3. Hydroxyethyl methacrylate: resin modified GIC, can be used as hydrogel