Polymers Flashcards
where do polymers come from in terms of raw material?
crude oil
what are polymers?
polymers are materials with large macro-molecules, of which plastics is just one group - long chains of hydrocarbons with sometimes other atoms like oxygen and silicon
what are some general properties of polymers?
- good thermal/electrical insulation
- low density
- easy to manufacture and low cost
- useful as adhesives (including composites)
- good transparency
- durable
what is the main difference between thermoplastics and thermosets?
thermoplastics - can be re-heated
thermosets - cannot be re-heated
what is an example of an elastomer?
rubber (can undergo higher strain)
what are polysiloxanes?
alternating oxygen and silicon atoms
what is the bonding in polymers?
atoms are bounded by covalent bonds to form long and flexible chains, secondary bonds between chains (VdW)
what is the structure of polymers?
amorphous or partially crystalline materials
what is teflon?
a polymer made from a hydrocarbon chain with fluorine - used in frying pans
what are the repeated units called in a polymer?
mers
what is polymerization?
the chemical process that causes a large no. of monomers to combine to form the polymer
why do polymers have low stiffness and low melting points (generally)?
because although they have a strong covalent backbone, chains will often bond by weaker secondary attraction
what determines the mechanical properties of the material?
molecular structure and molecular weight
what are linear polymers?
- single chains
- flexible
- ‘mass of spaghetti’
- van der walls between molecules
what are branched polymers?
- side branches
- packing is less efficient
- lower density
- less crystalline than linear polymers
what are cross-linked polymers?
- adjacent chains joined at various positions by covalent bonds, e.g rubbers
- cross-links are often non reversible due to chemical reactions
- when heavily cross linked, may be referred to as a network
order the molecular structures of polymer from lowest to highest strength?
- network
- cross-linked
- branched
- linear
how can we tailor the properties of polymers?
using different ‘mer’ units
what is condensation polymerisation?
- elimination of small molecule like H2O or HCl etc.
- reaction between an hydroxyl (-OH) and carboxylic acid (-COOH) to make an ester and water
what is free radical polymerisation?
- produces an addition reaction
- the 3 steps are: initiation, propagation, termination
what are the three main classes of polymers?
- thermoplastics
- thermosets
- elastomers
what are some properties of thermoplastics?
- they soften on heating
- linear/branches chains (minimal cross linking)
- weak intermolecular bonding
- greater ability to form (semi-) crystalline regions
- ductile: long period of plastic deformation
- e.g PP, PE, PS
what are some properties of thermosets?
- only degrade on heating
- react and harden (e.g epoxy resin)
- heavily cross-linked (10-50% of mers)
- do not soften on heating once formed, difficult to recycle
- brittle: minimal period of plastic deformation
- e.g vulcanised rubber, epoxies, urea formaldehyde, polyester resin, polyesters
what are some properties of elastomers?
- linear with some cross-linking, which provides shape ‘memory’ / retention
- useful for heat shrink, to insulate electrical cables and structural bearing pads in construction
what is the molecular weight Mw in terms of polymers?
the molecular weight Mw, is the mass of a mole (fixed number) of chains
what is linked to molecular weight?
- stiffness
- tensile strength
- viscosity
what does a large chain length in a polymer result in?
increased tensile strength (longer chains are more likely to entangle and can ‘anchor’ better)
what can occur when solidifying a polymer (thermoplastic)?
- you can form amorphous regions where they have solidified randomly which is encouraged by random copolymerisation, bulky side groups and chain branching
- some semi crystalline regions where the chains are lined up (caused by annealing), these crystalline regions are resistant to solvent
why do amorphous/non crystalline polymers look like in their purest state?
transparent
what is the nucleation rate?
rate at which nucleation occurs (crystals forming)
how do you work out the overall crystallisation rate of a polymer?
nucleation rate x growth rate
about halfway between Tg and Tm
out of crystalline and amorphous polymers, which takes up more volume?
amorphous = more volume crystalline = less volume
what is a semi-crystalline polymer?
a polymer with amorphous and crystalline regions (we can never have a polymer with purely crystalline regions)
what is the equation for the total volume of the polymer?
V = Vc + Va
where Vc and Va are the volumes of the crystalline and amorphous regions respectively
what is the equation for the total mass of the polymer?
W = Wc + Wa
where Wc and Wa are the masses of the crystalline and amorphous regions respectively
what is the mass fraction equation of crystalline regions and how else can it be rewritten?
Xc = Wc / W = ρc x Vc / pV
OR Xc = ρc (ρ - ρa) / ρ (ρc - ρa)
why do samples have considerably different crystallinities?
different crystallinities are observed because branches disrupt chain folding
what is crazing?
(like when you bend a ruler and it goes white)
crazing creates lots of little voids/cracks that scatter light which is why you cannot see through it - similar process when the polymer comes into contact with a solvent
what is a technique used to improve yield strength of polymers?
cold drawing
what is cold drawing?
the process of which polymers are stretched out:
- molecules are drawn out and aligned
- material much stronger in draw direction than before
- increasing elastic modulus and tensile strength
- reduces ductility
- annealing possible
what is the glass transition temp Tg?
the temp. at which you go from brittle to plastic behaviour
what factors effect Tm and Tg?
chain stiffness (additional side groups)
- branching (additional bulky side groups) increases the melting point
- harder for chains to melt and move and slide past each other
how else can side groups affect Tm?
very polar, electronegative side groups leads to significant intermolecular bonding forces so Tm increases
if you increase the temperature of a polymer what happens?
it goes from behaving in a brittle way to a plastic way, ductility increases
what is the equation for glass transition temp?
Tg = Tg∞ - (ρyNaθ / αfMn) where Tg = glass transition temp. Tg∞ = glass transition temp. at infinite molecular weight ρ = density y = total number of ends per chain Na = avogadro number θ = contribution of chain end to free volume αf = thermal expansion coefficient Mn = number average molecular mass
what are some advantages/disadvantages to injection mounding?
- can make complex shapes
- rapid manufacturing, used to make thousands and thousands of the same component
- very cheap once infrastructure in place
- expensive moulds
- pigments in pellet form can be added to colour the polymer
how does extrusion work in terms of processing polymers?
- similar to injection moulding
- polymer is heated to melt into liquid
- force through a die or required cross sectional shape
- cool extrusion
- used to make linear sections
- for high volume manufacture
- e.g UVPC window frames
can both thermoplastics and thermosets be injection moulded? what is the main difference?
yes - the mould is cooled for thermoplastics with 2 hoppers and heated for thermosets with only 1 hopper
what is the process of thermoforming?
- includes: vacuum forming, pressure forming and mechanical forming
- using a sheet of polymer
- heat is applied
- formed against a one sided mould
- for thermoplastics
- cheap and easy to use
- uses air pressure
- e.g trays
what is blow moulding?
- used to make plastic bottles (PET)
- parison extrusion
- high temperature and stretching in the desired direction causes the molecules to polarize, line up and essentially crystallise to produce a bottle of superior strength
what is compression moulding?
- used for thermosets
- a preheated preform is placed in the open mould
- as the mould closes, the preform is compresses and starts to flow
- the mould is almost closed and the resin is now a hot flowing fluid
- the heated mould is held shut until the resin cures
- then the mould is opened and the finished part is ejected
what is an antimicrobial? (polymer additive)
to control the build up of bacteria, fungi and algae on the surface of the polymer (prevent mould) - used in sealants for bathrooms
what is an antioxidant? (polymer additive)
for higher temp. stability
what is an antistatic? (polymer additive)
to prevent the build up of static electric charge
what are fibres? (polymer additive)
fibres are added to increase strength and stiffness
what are plasticizers? (polymer additive)
added to increase flexibility
what are lubricants? (polymer additive)
for easier moulding or for increasing adhesion and viscosity of the moulded parts
what are UV stabilizers? (polymer additive)
for resistance to sunlight
what are fillers? (polymer additive)
for economical reasons, to bulk it out
what are flame retardants/smoke suppressants? (polymer additive)
it’s in the name
what are conductive fibres or flakes? (polymer additive)
for special applications
what are polysiloxanes? and what are their applications?
- backbones consist of Si-O-Si units polymeric organosilicon compound commonly referred to as silicones
- alternating oxygen and silicon atoms
- most widely used silicon-based organic polymer
- used in consumer applications, such as silicone caulk and adhesives, sealants in kitchens, showers and bathrooms
what are some properties of silicone?
- good electrical insulator
- can be formulated to be electrically conductive
- good flexibility
- thermal stability over a wide range of (-100 to 250 degrees C)
- ability to repel water and form watertight seals
- resistance to oxygen, ozone and UV light
- does not stick after cure (non stick, so easy to clean)
- low chemical reactivity
- low toxicity
- does not support microbiological growth
what are some applications of silicone in construction?
- sealants and caulks
- coatings
- fire protection
- glazing seals
- in plumbing, silicon grease is typically applied to O-rings in taps and valves
are the properties of plastic temperature dependent?
yes
what must you remember to do if you want to calculate the number of branches from y?
minus 2 from y to get NUMBER OF POLYMER BRANCHES
