What are the three categories of polymers?
- natural (proteins, polysaccharides, resins/gums)
- semi-synthetic
- synthetic (plastics, elastomers, rubber)
Define polymer chemically
What are four potential structures?
a long chain molecule (MW of several thousand/million atomic units) constructed from many monomers (like structured molecules) covalently bonded together in any conceivable pattern
linear
branched
cross-linked
colloidal
Define dimer and trimer
dimer - bonded to one of self (O-O)
trimer - bonded to two of self (O-O-O)
What are the two polymer types in terms of diversity?
homopolymers - all the same monomer (doesn’t have to be linear)
co-polymers - made up of different monomers
What two things can monomers be?
Give examples of these
small (PVC)
complex (nylon)
What are the different types of chemical linkages?
What does this depend on?
What does this usually require?
- linear or branched structures
- depends on monomer functionality
- sometimes they’ll self assemble but most of the time they need to be initiated
How can monomers perpetuate chain growth?
- monomers may have complimentary reactivity (react with self to form dimer etc) or react with another different monomer to perpetuate chain growth
Where are dendrimers used?
How are these helpful?
- used in drug delivery
- if can identify polymer then can find out where drug was manufactured
How do polymers form?
How do polymers end?
What is an exception to this?
- by chains reacting and forming bonds
- the end cannot have dangling half bond - must be something there
- polymers end by something called end groups (important for polymer analysis)
- no end groups in rings
- very rare as most industrial processes will have rings
What does ratio of end groups to in-chain groups allow?
What does the uniqueness of end group mean?
- allows for measuring of polymer length
- nature of end group is unique
- this allows the method of synthesis to be identified
What type of end groups allow for further modification and of what?
- reactive end groups allow further modification to control properties or add additional functionality
What are the three main types of polymer when classed by bulk properties?
Describe the first two in terms of:
- what happens when heated
- what happens after being heated?
- flexibility (why?)
- what is required for them to be produced
- examples of them
Describe the third one in terms of:
- what type it can be
- its two properties
- examples
thermoset:
- burns when heated
- irreversibly hardened once shaped
- generally inflexible (crosslinking in curing process means chains linked together inhibit molecular motion)
- requires curing (chemical process) in order to produce them (also requires curing agent)
- e.g. polyurethanes, epoxy resin (superglue), silicones
thermoplastics
- melts when heated
- can be remelted and reshaped
- generally more flexible (disordered regions facilitate molecular motion)
- no chemical curing required
- e.g. polystyrene, nylon, polycarbonate
elastomer
- can be thermoset or thermoplastic
- viscoelastic so has viscosity (can flow) and elasticity (returns to original shape)
- e.g. rubbers (natural, butyl, silicone)
Describe the two types of polyethylene polymers in terms of density:
- examples
- density
- branching
- crystalline content
- transparency
- hardness
- gas permeability
high density polyethylene (HDPE)
- containers/lids, food bottles, petrol tank, motor oil bottles, crates, pipers
- higher density than LDPE
- very low or no branching
- high crystalline content - 70-90 % (very well aligned)
- less transparent than LDPE
- stiffer and harder than LDPE
- less gas permeable than LDPE
low density polyethylene (LDPE)
- film/sheet packaging, toys, squeeze bottles, plastic bags, wire and cable coatings
- lower density than HDPE
- highly branched (around 60 branch points per 1000 carbon atoms)
- low crystalline content - 40-60 % (far less aligned)
- more transparent than HDPE
- not as stiff and hard as HDPE - forms good films
- more gas permeable than HDPE
How are physical properties of polymer determined?
How are they controlled?
What does measuring properties and understanding of chemistry allow for?
- how adjacent chains interact and are linked
- supramolecular interactions (H bonds, van der Waals interactions etc)
- covalent interactions (cross-linking of chains etc.)
- controlled by underlying chemistry
- allows us to identify and compare polymeric trace evidence types (compare known and unknown)
Define crystallinity?
what is the scale used to measure crystallinity?
what 7 physical properties does crystallinity affect?
what does measuring crystallinity allow for? in what ways is this done?
- the regions of atomic ordering where intramolecular folding/stacking of adjacent chains occur
- degree of crystallinity:
- 0 = completely amorphous (not aligned)
- 1 = completely crystalline (aligned)
- impact resistance
- Young’s Modulus
- tensile strength
- stiffness
- crease
- thermal behaviour
- transparency
- allows for identification of polymer and comparison between samples
- directly or indirectly
- destructively (cheaper and easier) - depending on availability of evidence type as it is useful if can and do not have to delve into more complex non-destructive testing
- non-destructive testing (optical/spectroscopic methods)
define tensile strength
what is the correlation between tensile strength, polymer chain length and crosslinking (why)
- how much something will stretch before it breaks (elongation stress)
- for polymers, tensile strength increases with polymer chain length and crosslinking (more interaction between chains)
How is tensile strength measured?
How is this method normalised/standardised?
- take a sample and stretch (this destructive method is cheaper and easier than non-destructive method)
- need to normalise this so we can standardise for sample size
- it is easier to stretch smaller things than large things (more matter to move = more force required)
- take into account sample length and sample cross section
What are stretch and force termed in solids?
How are these calculated?
How are these equations standardised/normalised?
Positive/negative sign? and why?
- stretch is called strain
- force is called stress
- strain = I/L
- strain = distance stretched/length
- this is a fractional change so is independent of length
- stress = F/A
- stress = force/area
- independent of sample size
- minus sign as force in opposite direction to applied force - if you in one direction (x) then the spring pulls back in the opposite way
How is young modulus calculated?
What does it describe?
What unit is it usually given in?
- E = strain/stress
- describes how stiff a solid is
- so large they are usually reported in GPa
Define Hooke’s Law
for a spring, F = -kx
where k = force/spring constant
Describe the behaviour of a thermoplastic polymer being stretched?
ELASTIC deformation region:
- necking - where bits on end are quite bulky but things in middle start to stretch out
- this is shown as turning point on plot
PLASTIC deformation region:
- keep stretching beyond that - we get drawing
- much less disordered now (more ordered)
- when fully drawn = polymer breaks = fracture
How can young’s moduli be useful in trace evidence?
What does this look like for three types of polymers
- measuring young’s moduli and plotting stress (y) vs strain (x) plots allows for identification of polymer and comparison between polymeric samples
thermoset - almost immediately upright until fractures
thermoplastic - up then down then across
elastomer - almost immediately directly proportional
What are the phase transitions for amorphous and Pseudocrystalline materials as increase temperature?
What is glass transition temp higher for?
amorphous:
- glass
- reach glass transition temp
- rubber
- gum
- liquid
Pseudocrystalline
- crystalline
- reach glass transition temp
- flexible thermoplastic
- reach melting temp
- liquid
- glass transition temp higher for Pseduocrystalline
define glass transition temperature
how is this useful in forensics?
- not the melting point but a temp at which the tension in the polymer backbone lessens sufficiently to impart flexibility, but not flow
- it is unique to material and therefore identifiable region of a material that allows us to identify it and see more information
