TRANSITIONS IN POLYMERS

give some examples of natural polymers

• collagen

• silk

• DNA

• protein

• natural rubber

give examples of everyday polymers

• polystyrene

• polyamide

• low density polyethylene

give examples of aspects of dentistry that uses polymers

• prosthodontics: denture bases and teeth, soft liners, custom trays, temporary restoratives

• operative dentistry: dentine bonding agents, sealants, veneers

• orthodontics: brackets, spacers, bracket bonding resins/ cements

• endodontics: root canal sealants, rubber dams

• equipment: mixing bowls, spatulas, protective eyewear 

state advantages of dental resins

• mechanically strong

• physically stable 

• easily manipulated

• excellent aesthetic qualities

• chemically stable - in the mouth and in storage

• biocompatible

• reasonable cost

where are acrylic resins often used in dentistry

• denture base material

what is another name for acrylic resins

PMMA

when was PMMA introduced

1936

what type of polymer is PMMA

thermoplastic amorphous polymer

what are advantages of PMMA

• acceptable cost of material and processing method

• good mechanical properties - rigidity, strength, wear resistance

• biocompatible - tasteless, odourless, non-toxic, non-irritating, resistance to microbial colonisation

• suitable manipulation/ processing properties - easy to mix, shapable, simple to process and cure

• aesthetic properties - translucency and transparency

what are problems with PMMA

• colour stability

• shrinking problems

• adhesion problems

how can the shrinking problems of PMMA be resolved

adding MMA (methyl methacrylate)

when were room temperature polymerising methacrylates introduced

1940

when were self curing dimethacrylates reinforced by a ceramic particle ‘filler’

late 1950s

when were PMMA resins replaced by more durable monomers such as bisGMA

1940

define polymer

polymer: a large, chain-like molecule made up of monomers

what type of bonds are the monomers in a polymer joined by

covalent bonds

diagram showing polymer classification

what do the physical properties of polymers depend on

• how their molecules are arranged i.e. polymer structure

• the strength of the forces between these molecules i.e. intermolecular forces

in a polymer, where are intermolecular forces present

• weak intermolecular attract individual polymer chains together

when can covalent bonds join individual polymer chains together 

in cross linking, covalent bonds can join individual polymer chains together

what factors do the strength and flexibility of the polymer depend on

• chain length

• side groups

• branching

• cross-linking

outline the factors influencing strength and flexibility of the polymer

• chain length: generally, the longer the chains the stronger the polymer

• side groups: intramolecular forces e.g. hydrogen bonding give stronger attraction between polymer chains

• branching: straight, unbranched chains can pack together more closely than highly branched chains

  • » polymers with higher density, more crystalline and therefore stronger

• cross-linking: if polymer chains are linked together by covalent bonds, the polymer is harder and more difficult to melt

define crystallinity

crystallinity: the degree of structural order

diagram showing amorphous and crystalline structures

lines = chains of polymers

what do the highlighted areas in the crystalline diagram represent

• crystal-like ordered structure of randomly packed polymer chains

give examples of amorphous polymers

• acrylonitrile butadiene styrene (ABS)

• atactic polystyrene

• polycarbonate

• polyisoprene

• polybutadiene

give examples of crystalline polymers

• nylon

• crystalline PET (CPET)

• syndiotactic polystyrene

• Kevlar and Nomex

• polyketones

what polymer properties does crystallisation affect

• optical properties

• mechanical properties

• thermal properties

• chemical properties

outline thermoplastic polymers

• linear/ branches structures

• flow when heated and can be reshaped upon heating

• easily moulded and extruded into films

give an example of a thermoplastic polymer

polypropylene

outline thermosetting polymers

• crosslinked structures

• cannot be reshaped upon heating

• hard and durable

give an example of a thermosetting polymer

epoxy resin

outline elastomers

• rubbery polymers

• can be stretched easily

• return to their original dimensions when the applied stress is released

give an example of an elastomer

natural rubber

what is T_g

T_g: glass transition temperature

• the temperature at which the polymer chains begin to flow past each other

  • the polymer is soft, rubbery and easy to handle but NOT melted

graph showing T_g

is T_g higher or lower than the melting temperature

T_g is always lower than the melting temperature (T_m)

the ______ the secondary bonds the _____ the T_g

the weaker the secondary bonds the lower the T_g

define polymerisation

polymerisation: the chemical reaction in which monomers of a low molecular weight are converted into chains of polymers with a high molecular weight

• monomer molecules are bonded by covalent bonds

what are the types of polymerisation

• addition

• condensation

define addition polymerisation

addition polymerisation: occurs when a reaction between two molecules produces a larger molecule without the elimination of a smaller molecule

what are the steps of addition polymerisation

• activation

• initiation

• propagation

• termination

what are the common activators in the activation stage

• heat

• chemical compounds e.g. tertiary amines

• light

what initiates polymerisation

free radical (a molecule with only one free electron) created by the activator

what occurs in the propagation stage

reaction of an active chain with a monomer

what occurs in the termination stage

2 radicals join to make a big polymer

table showing common polymers prepared via addition polymerisation

define condensation polymerisation

condensation polymerisation: occurs when a reaction between two molecules produces a larger molecule with the elimination of a smaller molecule

diagram showing example of condensation polymerisation

what is a copolymer

copolymer: two different types of monomers from different polymers are joined chemically in the same polymer chain, creating a new molecule

• more noticeable in mechanical properties

what is a blend

• mixing polymers prior to moulding

• polymers generally miscible

• » achieve moderate changes in mechanical properties

• e.g. PLA + PCL

what is a plasticiser

• small molecule added to the polymer that reduces attraction forces between polymer chains

• » big effect in mechanical properties

  • lowers T_g and elastic modulus so good for brittle polymers

give an example of a composite

glass reinforced polymer

define resin-based composite

resin-based composite: highly crosslinked resin reinforced by a dispersion of amorphous silica or organic resin filler particles and/ or fibres bonded to the polymer matrix by a coupling agent