transitions in polymers Flashcards
What are natural polymers?
Collagen Silk Natural rubber DNA Protein
What are synthetic polymers?
Polystyrene
Polyamide
Low density polyethene
How are polymers applied in dentistry?
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
What is an ideal resin?
Mechanically strong Physically stable Easily manipulated Aesthetic Chemically stable Biocompatible Reasonable cost
What is the history of dental resins?
1853- vulcanised rubber
1890- gutta percha
1936- PMMA
1940- room temp polymerising methacrylates (eg. bis-GMA)
1950- self curing dimethacrylates reinforced by filler
What is PMMA?
Polymethylmethacrylate Thermoplastic amorphous polymer ~bone cements ~contact and intraocular lens ~filler for bone cavities and skull defects ~screw fixation in bone
Why is PMMA used ?
Acceptable cost of material and processing method
Suitable manipulation/processing properties
Good mechanical properties
Biocompatible
Aesthetic properties
What are problems w PMMA?
Colour stability
Shrinking problems (reduced by adding MMA)
Adhesion problems
What is a polymer?
Large chain like molecule made up of monomers which are small molecules
Monomer repetition
Joined by covalent bonds
Natural/synthetic
How are polymers classified?
Structure- linear, cyclic, branches, cross linked
Molecular forces- thermoplastic, thermosetting, elastomers
Source- natural, synthetic, addition, condensation
How are the properties of polymers determined?
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
What are general properties of polymers?
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
What is crystallinity?
Degree of structural order
Amorphous- random eg. ABS
Crystalline- aligned parts eg. nylon
What can crystallisation affect?
Optical/mechanical/thermal/chemical properties
What are some highly crystalline polymers?
Crystalline PET
Nylon
Kevlar and nomex
What are some highly amorphous polymers?
Polycarbonate
ABS
Atactic polystyrene
What are thermoplastic polymers?
Linear, branched structures
Flow when heated, can be reshaped
Easily moulded and extruded into films
Eg. Polypropylene
What are thermosetting polymers?
Crosslinked structures
Can’t be reshaped when heated
Hard and durable
Eg. Epoxy resin
What are elastomers?
Rubbery polymers
Stretched easily
Reruns to original form when stress released
Eg. Natural rubber
What is glass transition temperature, Tg?
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
What is addition polymerisation?
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
What is polymerisation?
Chemical reaction in which monomers of a low molecular weight are converted into chains of polymers w a high molecular weight
How does addition polymerisation happen?
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
What is an acrylate and methacrylate?
Acrylate- [=CH2] important in addition polymerisation as radicals attack there due to high electron density
Methacrylate [acrylate and -CH3]
What is condensation polymerisation?
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)
What is a blend?
Mixing polymer prior to moulding Polymer- generally miscible Moderate changes to mechanical properties Eg. PLA+PCL to combine properties Not chemical
What is a copolymer?
Two different types of monomers are joined in the same polymer
More noticeable different mechanical properties
What is a plasticiser?
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
What is a composite?
Combination of materials w different mechanical properties in which the combination material offers superior properties
Eg. Glass reinforced polymer
What is a resin based composite?
Highly crosslinked resin reinforced by a dispersion of amorphous silica/organic resin filler particles+/fibers bonded to polymer matrix by a coupling agent
