Polymers

• natural (proteins, polysaccharides, resins/gums) • semi-synthetic • synthetic (plastics, elastomers, rubber)

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

dimer - bonded to one of self (O-O) trimer - bonded to two of self (O-O-O)

homopolymers - all the same monomer (doesn’t have to be linear) co-polymers - made up of different monomers

small (PVC) complex (nylon)

• linear or branched structures • depends on monomer functionality • sometimes they’ll self assemble but most of the time they need to be initiated

• monomers may have complimentary reactivity (react with self to form dimer etc) or react with another different monomer to perpetuate chain growth

• used in drug delivery • if can identify polymer then can find out where drug was manufactured

• 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

• allows for measuring of polymer length • nature of end group is unique • this allows the method of synthesis to be identified

• reactive end groups allow further modification to control properties or add additional functionality

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)

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 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)

• 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)

• 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)

• 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

• 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

• E = strain/stress • describes how stiff a solid is • so large they are usually reported in GPa

for a spring, F = -kx where k = force/spring constant

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

• 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

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

• 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