Transitions in polymers

Question 1

Polymer definition

Answer 1

A polymer is a large, chain-like molecule made up of monomers, which are small molecules
We can find ‘natural’ and ‘synthetic’ polymers

Question 2

Natural polymers

Answer 2

Collagen
Silk
Natural rubber
DNA
Protein

Question 3

Synthetic polymers

Answer 3

Everywhere
Polystyrene
Polyamide
Low-density polymer

Question 4

Applications in dentistry

Answer 4

Dentures (bases, liners, artificial teeth)
Cavity filling materials (composites)
Sealants
Impression materials
Cements (resin based)
Orthodontics (elastics)
Equipment (mixing bowl)

Question 5

Denture base acrylic

Answer 5

Denture-base material –> pouring –> typical acrylic denture

Question 6

Denture resins

Answer 6

1853 Vulcanised rubber introduced
1890 Gutta-percha temp crowns and cavity fillings, root canal filling materials
1936 PMMA introduced
1940 Room-T PMs e.g. bis-GMA (self-curing prosthetic and restorative resins)
1950 Self-curing dimethacrylates reinforced by dispersed phase ceramic particle ‘filler’ in late 50s

Question 7

PMMA uses

Answer 7

Bone cements
Contact and intraocular lens
Screw fixation in bone
Filler for bone cavities and skull defects
Vertebrae stabilisation in osteoporotic patients

Question 8

PMMA advantages

Answer 8

Good mechanical properties (rigidity, strength, wear resistance)
Suitable manipulation/ processing properties (easy to mix, shapeable simple to process and cure)
Acceptable cost of both material and processing method
Biocompatible (tasteless, odourless, non-toxic, non-irritating, resistance to microbial colonisation)
Aesthetic properties translucency and transparency

Question 9

PMMA problems

Answer 9

Colour stability
Shrinking problems
Adhesion

Question 10

How can shrinkage of PMMA be reduced

Answer 10

Adding MMA

Question 11

Self-curing dimethacrylate with dispersed phase ceramic particle ‘filler’ advantages

Answer 11

Self-curable
UV-photocurable
Blue-light photopolymerisable resins

Question 12

What do the physical properties of polymers depend on?

Answer 12

How their molecules are arranged (polymer structure) - covalent bonds
The strength of forces between these molecules (intermolecular forces)

Question 13

Strength and flexibility of polymer depends on:

Answer 13

Chain length (longer chain, stronger polymer)
Side groups (intramolecular forces e.g. H-bonds give stronger attraction between polymer chains, making polymer stronger)
Branching (straight and unbranched more dense, therefore crystalline and stronger)
Cross-linking (if polymer chains linked together by covalent bonds, polymer is harder and more difficult to melt)

Question 14

Crystallinity of polymers

Answer 14

Aligned parts 'crystal-like' ordered structure
Crystallisation affects
-optical
-mechanical
-thermal
-chemical properties

Question 15

Some highly crystalline polymers

Answer 15

Crystalline PET (CPET)
Nylon

Question 16

Some highly amorphous (random structure) polymers

Answer 16

Acrylonitrile buadeine styrene (ABS)

Polycarbonate

Question 17

Thermoplastic polymers

Answer 17

Linear/ branched structures
Flow when heated and can be reshaped
Easily moulded and extruded into films
e.g. polypropylene

Question 18

Thermosetting polymers

Answer 18

Crosslinked structures
Cannot be reshaped upon heating
Hard and durable
e.g. epxoy resin

Question 19

Elastomers

Answer 19

Rubbery polymers
Can be stretched easily
Return to original dimensions when applied stress released

Question 20

Polymer properties: Tg (see graph on slides)

Answer 20

Tg is glass transition T: the T at which polymer chains begin to flow past each other
Below Tg polymer is solid (hard and glassy)
Above Tg the polymer can flow (soft and rubbery)
Tg is always lower than melting T (Tm)
The weaker the secondary bonds the lower the Tg

Question 21

Polymerisation: mechanisms

Answer 21

Chemical reaction in which monomers of low molecular weight are converted into chains of polymers with high molecular weight
Monomer molecules bonded by covalent bonds

Question 22

Types of polymerisation

Answer 22

Addition polymerisation

Condensation polymerisation

Question 23

Addition polymerisation

Answer 23

Addition polymerisation occurs when reaction between 2 molecules produces larger molecule without elimination of smaller molecule

Question 24

Addition polymerisation steps

Answer 24

1. Activation (by heat/ chemical compounds e.g. tertiary amines/ light)
2. Initiation (free radical initiates polymerisation e.g. benzoyloxy radical from benzoyl peroxide)
3. Propagation (reactive end-groups of a polymer chain react with a new monomer molecule transferring the reactive group to the last unit)
4. Termination (ceases the formation of reactive intermediates in a chain propagation)

Question 25

Common polymers formed via addition polymerisation

Answer 25

Polyethylene (low and high density)
Polystyrene
PTFE, Teflon
PMMA

Question 26

Condensation polymerisation

Answer 26

Occurs when reaction between 2 molecules produces a larger molecule with the elimination of a smaller moecule
e.g. silicone impression material setting reaction, producing water

Question 27

Blend

Answer 27

Mixing polymer prior to moulding
Polymers generally miscible
Moderate changes in mechanical properties
e.g. PLA + PCL to combine properties

Question 28

Copolymer

Answer 28

2 different types of monomers joined in same polymer chain

More noticeable changes in mechanical properties

Question 29

Plasticizer

Answer 29

Small molecule added to polymer
Reduces attraction forces between polymer chains
Big effect in mechanical properties: lowers Tg and elastic modulus (e.g. good for brittle polymers)

Question 30

Composite

Answer 30

Combination of materials with different mechanical properties in which the combination material offers superior properties comparing them to the individual components

e. g. matrix and filler
e. g. glass reinforced polymer