transitions in polymers

Question 1

What are natural polymers?

Answer 1

Collagen
Silk
Natural rubber
DNA
Protein

Question 2

What are synthetic polymers?

Answer 2

Polystyrene
Polyamide
Low density polyethene

Question 3

How are polymers applied in dentistry?

Answer 3

Denture based/teeth, soft liners, custom trays, impression materials, temps, cements, prostheses

Dentine bonds, fillings, cements, sealants, veneers

Brackets, bonding resins/cements, spacers

RCT sealants, rubber dams

Mixing bowls, spatulas, eyewear

Question 4

What is an ideal resin?

Answer 4

Mechanically strong
Physically stable
Easily manipulated
Aesthetic 
Chemically stable
Biocompatible 
Reasonable cost

Question 5

What is the history of dental resins?

Answer 5

1853- vulcanised rubber
1890- gutta percha
1936- PMMA
1940- room temp polymerising methacrylates (eg. bis-GMA)
1950- self curing dimethacrylates reinforced by filler

Question 6

What is PMMA?

Answer 6

Polymethylmethacrylate
Thermoplastic amorphous polymer
~bone cements
~contact and intraocular lens
~filler for bone cavities and skull defects
~screw fixation in bone

Question 7

Why is PMMA used ?

Answer 7

Acceptable cost of material and processing method
Suitable manipulation/processing properties
Good mechanical properties
Biocompatible
Aesthetic properties

Question 8

What are problems w PMMA?

Answer 8

Colour stability
Shrinking problems (reduced by adding MMA)
Adhesion problems

Question 9

What is a polymer?

Answer 9

Large chain like molecule made up of monomers which are small molecules
Monomer repetition
Joined by covalent bonds
Natural/synthetic

Question 10

How are polymers classified?

Answer 10

Structure- linear, cyclic, branches, cross linked

Molecular forces- thermoplastic, thermosetting, elastomers

Source- natural, synthetic, addition, condensation

Question 11

How are the properties of polymers determined?

Answer 11

Structure related to properties

Physical properties depend on-
~How molecules are arranged
~Strength of forces between molecules

Covalent bonds- strong- join atoms to eachother

Intermolecular forces- weak- attract molecules to eachother

Question 12

What are general properties of polymers?

Answer 12

Strength and flexibility depend on-

CHAIN LENGTH- longer is stronger

SIDE GROUPS- intra molecular forces give stronger attraction between chains- therefore stronger

BRANCHING- straight, unbranched can pack together more so more dense, more crystalline, more strong

CROSS LINKING- more links via covalent bonds= harder and more difficult to melt

Question 13

What is crystallinity?

Answer 13

Degree of structural order

Amorphous- random eg. ABS
Crystalline- aligned parts eg. nylon

Question 14

What can crystallisation affect?

Answer 14

Optical/mechanical/thermal/chemical properties

Question 15

What are some highly crystalline polymers?

Answer 15

Crystalline PET
Nylon
Kevlar and nomex

Question 16

What are some highly amorphous polymers?

Answer 16

Polycarbonate
ABS
Atactic polystyrene

Question 17

What are thermoplastic polymers?

Answer 17

Linear, branched structures
Flow when heated, can be reshaped
Easily moulded and extruded into films
Eg. Polypropylene

Question 18

What are thermosetting polymers?

Answer 18

Crosslinked structures
Can’t be reshaped when heated
Hard and durable
Eg. Epoxy resin

Question 19

What are elastomers?

Answer 19

Rubbery polymers
Stretched easily
Reruns to original form when stress released
Eg. Natural rubber

Question 20

What is glass transition temperature, Tg?

Answer 20

Temp at which the polymer chains begin to flow past eachother

Below Tg- solid
above Tg- flows

Tg is always lower than the melting temperature

The weaker the secondary bonds, the lower the Tg

Question 21

What is addition polymerisation?

Answer 21

When a reaction between two molecules produces a larger molecule w/o elimination of smaller molecule

(Activation, initiation, propagation, termination)

Creates radical (molecule that has one free electron)

Activators- heat, chemical compounds (tertiary amines), light

Initiation- free radical initiated polymerisation

Question 22

What is polymerisation?

Answer 22

Chemical reaction in which monomers of a low molecular weight are converted into chains of polymers w a high molecular weight

Question 23

How does addition polymerisation happen?

Answer 23

Initiation- free radical is created and looks for a double bond (high density of electrons- many in acrylates)
Creates another bigger radical

Propagation- creates a bigger radical and another bigger etc

Termination- 2 big radicals join to create the polymer

Question 24

What is an acrylate and methacrylate?

Answer 24

Acrylate- [=CH2] important in addition polymerisation as radicals attack there due to high electron density
Methacrylate [acrylate and -CH3]

Question 25

What is condensation polymerisation?

Answer 25

When a reaction between two molecules produce a larger molecule with the elimination of a smaller molecule

Eg. Setting reaction of silicone impression materials, Si(OCH3)4 joins 4x to OH to create polymer and give off 4CH3OH (methanol)

Question 26

What is a blend?

Answer 26

Mixing polymer prior to moulding
Polymer- generally miscible
Moderate changes to mechanical properties
Eg. PLA+PCL to combine properties
Not chemical

Question 27

What is a copolymer?

Answer 27

Two different types of monomers are joined in the same polymer
More noticeable different mechanical properties

Question 28

What is a plasticiser?

Answer 28

Small molecule added to polymer
Reduced attraction forced between polymer chains
Big impact on mechanical properties
Eg. Lowers Tg and elastic modulus
Eg. Good for brittle polymers

Question 29

What is a composite?

Answer 29

Combination of materials w different mechanical properties in which the combination material offers superior properties
Eg. Glass reinforced polymer

Question 30

What is a resin based composite?

Answer 30

Highly crosslinked resin reinforced by a dispersion of amorphous silica/organic resin filler particles+/fibers bonded to polymer matrix by a coupling agent