Lecture 8 - Polymers Flashcards
what is a polymer
a larger molecule made of lots of smaller repeating units called monomers that have been covalently bonded together
name some examples of polymers that may be useful in forensic science
fibres
adhesives
membranes
rope
textiles
additives
polystyrene
plastic
what are the two main categories of polymers
natural and synthetic
(can also get semi synthetic)
give three examples of a natural polymer
proteins
polysaccharides
resins
name 3 examples of synthetic polymers
plastics
elastomers (polymer that displays elastic properties)
rubber
what are the 4 main shape types polymers are found in
linear
branched
cross-linked
colloidal
what is a dimer
two monomers joined together
what is a trimer
three monomers joined together
what is a homopolymer
a polymer made up of all the same monomers
what is a co-polymer
a polymer made up of different monomers
give an example of a small monomer and a complex monomer
small = PVC
complex = nylon (an amine and a alcohol)
what feature do monomers need to be able to form polymer
complimentary reactivity - to react with themselves or other monomer types to allow chain growth
what does the shape of a polymer depend on
the functionality and reactivity of the monomers it is made up of
what is the purpose of an initiator
something that starts off the polymerisation reaction
what is the most complex polymer type we have considered and where is it often found
dendrimer - found in the coating of illicit substances
what allows the polymerisation reaction to stop and why are these useful in polymer analysis
the presence of an ending group
these have identifiable feature so are useful in forensic analysis
how can end groups be used in the analysis of polymers (3 ways)
can be used to identify and compare materials
the ratio of the end group to the in chain groups can be used to measure polymer length
the nature of the end group can suggest the method of synthesis of the polymer
what is something to be aware of when looking for an ending group in polymers
polymer ring structures do not have end groups
what do reactive end groups allow
further modification of the polymer and more control of its properties to tailor to your need
what properties of polymers allows them to be classified
their bulk properties
name the three main polymer types when classifying them
thermoset
thermoplastic
elastomer
what are the properties that allow a polymer to be classed as a thermoset (4)
burns when heated
irreversibly hardened when shaped
generally inflexible
curing is needed - a chemical process
e.g polyurethanes, epoxy resins (superglue) and silicones
what happens when a polymer is cured
the polymer undergoes cross linking - normally requires heat - enhances the stability and durability of the polymer
what are the properties that allow a polymer to be classed as a thermoplastic (4)
melts when heated
can be re- melted and reshaped
more flexible generally
no curing needed
e.g polystyrene, nylon and polycarbonate
what are the properties that allow a polymer to be classed as an elastomer (3)
can also be thermoset or thermoplastic
is viscous (can flow)
has elasticity (returns to original shape)
e.g rubbers
what are HDPE and LDPE polymers
high density polyethylene
low density polyethylene
what are the properties of HDPE polymers compared to LDPE
HDPE compared to LDPE
- higher crystalline content (70-90%)
- less transparent
- harder and stiffer
- less gas permeable
- very little or no branching
- higher density
LDPE form good films
what properties allow us to tell HDPE and LDPE polymers apart
a comparison of their physical properties
name some examples of where HDPE polymers may be found
containers
goof bottles
petrol tanks
pipes
name some examples of where LDPE polymers may be found
films and sheet packaging
toys
squeezy bottles
plastic bags
wire coatings
what largely determines how polymers behave macroscopically (to the naked eye)
how the adjacent chains interact and link - this is controlled by the chemistry of the monomers that make up the polymer and how they are arranged
name two types of interactions that are often seen between adjacent chains in polymers
supramolecular interaction (hydrogen bonds, van der Waals)
covalent interactions (cross linking of chains)
what does the crystallinity of a polymer refer to
how ordered and structured the molecules are within the polymer structure
high crystallinity = well structures
low crystallinity = disorder and amorphous structure
amorphous = lacks a defined structure
what physical properties does polymer crystallinity affect
impact resistance
stiffness
crease
thermal behaviour
transparency
tensile strength
young modulus (a measure of elasticity)
how can we measure a polymers crystallinity
measuring tensile strength
or
measuring Youngs modulus and plotting stress vs strain
what scale is used to assign a polymers crystallinity
0-1
0 = completely amorphous (disordered)
1 = completely crystalline (ordered)
are synthetic polymers normally crystalline or amorphous
amorphous (very few are entirely crystalline)
what can measuring the crystallinity of a polymer allow
identification and then comparisons between samples
what determines if a destructive or non destructive method is used to measure a polymers crystallinity
the availability of the evidence and the processes that may follow my analysis
what does the tensile strength of a polymer refer to
how much it can be stretched before it breaks
also called elongation stress
in polymers what does tensile strength increase with
polymer chain length and crosslinking due to more interaction between chains
why is measuring tensile strength a good method
cheap and easy
just stretch the polymer this is standardised by applying a force and look at the distance it is stretched
what can be the problem with measure tensile strength
the size of the material being stretched matters - it is easier to stretch smaller things than bigger things
more matter = greater forces needed to move
with evidence you are in control over how much you receive
when analysing solids and their tensile strength what terms do we use rather than stretch and force
instead of stretch we use strain
instead of force we use stress
what are stress and strain independent of
the size of the sample
what does Youngs modulus describe
how stiff a solid is (the higher the stiffer)
in thermoplastics what are the two deformation regions classed as
plastic and elastic
plastic = higher end of stretch vs strain - material is unlikely to return to its original form if within this limit
elastic = likely to return to its original form (lower end of strain vs stress)
for a crystalline material what will happen with an increase in temperature of the material
it will go from a crystalline to a thermoplastic to a liquid
