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Trace Evidence > Lecture 8 - Polymers > Flashcards

Lecture 8 - Polymers Flashcards

1
Q

what is a polymer

A

a larger molecule made of lots of smaller repeating units called monomers that have been covalently bonded together

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2
Q

name some examples of polymers that may be useful in forensic science

A

fibres
adhesives
membranes
rope
textiles
additives
polystyrene
plastic

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3
Q

what are the two main categories of polymers

A

natural and synthetic
(can also get semi synthetic)

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4
Q

give three examples of a natural polymer

A

proteins
polysaccharides
resins

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5
Q

name 3 examples of synthetic polymers

A

plastics
elastomers (polymer that displays elastic properties)
rubber

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6
Q

what are the 4 main shape types polymers are found in

A

linear
branched
cross-linked
colloidal

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7
Q

what is a dimer

A

two monomers joined together

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8
Q

what is a trimer

A

three monomers joined together

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9
Q

what is a homopolymer

A

a polymer made up of all the same monomers

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10
Q

what is a co-polymer

A

a polymer made up of different monomers

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11
Q

give an example of a small monomer and a complex monomer

A

small = PVC
complex = nylon (an amine and a alcohol)

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12
Q

what feature do monomers need to be able to form polymer

A

complimentary reactivity - to react with themselves or other monomer types to allow chain growth

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13
Q

what does the shape of a polymer depend on

A

the functionality and reactivity of the monomers it is made up of

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14
Q

what is the purpose of an initiator

A

something that starts off the polymerisation reaction

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15
Q

what is the most complex polymer type we have considered and where is it often found

A

dendrimer - found in the coating of illicit substances

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16
Q

what allows the polymerisation reaction to stop and why are these useful in polymer analysis

A

the presence of an ending group

these have identifiable feature so are useful in forensic analysis

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17
Q

how can end groups be used in the analysis of polymers (3 ways)

A

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

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18
Q

what is something to be aware of when looking for an ending group in polymers

A

polymer ring structures do not have end groups

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19
Q

what do reactive end groups allow

A

further modification of the polymer and more control of its properties to tailor to your need

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20
Q

what properties of polymers allows them to be classified

A

their bulk properties

21
Q

name the three main polymer types when classifying them

A

thermoset
thermoplastic
elastomer

22
Q

what are the properties that allow a polymer to be classed as a thermoset (4)

A

burns when heated

irreversibly hardened when shaped

generally inflexible

curing is needed - a chemical process

e.g polyurethanes, epoxy resins (superglue) and silicones

23
Q

what happens when a polymer is cured

A

the polymer undergoes cross linking - normally requires heat - enhances the stability and durability of the polymer

24
Q

what are the properties that allow a polymer to be classed as a thermoplastic (4)

A

melts when heated

can be re- melted and reshaped

more flexible generally

no curing needed

e.g polystyrene, nylon and polycarbonate

25
Q

what are the properties that allow a polymer to be classed as an elastomer (3)

A

can also be thermoset or thermoplastic

is viscous (can flow)

has elasticity (returns to original shape)

e.g rubbers

26
Q

what are HDPE and LDPE polymers

A

high density polyethylene

low density polyethylene

27
Q

what are the properties of HDPE polymers compared to LDPE

A

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

28
Q

what properties allow us to tell HDPE and LDPE polymers apart

A

a comparison of their physical properties

29
Q

name some examples of where HDPE polymers may be found

A

containers
goof bottles
petrol tanks
pipes

30
Q

name some examples of where LDPE polymers may be found

A

films and sheet packaging
toys
squeezy bottles
plastic bags
wire coatings

31
Q

what largely determines how polymers behave macroscopically (to the naked eye)

A

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

32
Q

name two types of interactions that are often seen between adjacent chains in polymers

A

supramolecular interaction (hydrogen bonds, van der Waals)

covalent interactions (cross linking of chains)

33
Q

what does the crystallinity of a polymer refer to

A

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

34
Q

what physical properties does polymer crystallinity affect

A

impact resistance
stiffness
crease
thermal behaviour
transparency
tensile strength
young modulus (a measure of elasticity)

35
Q

how can we measure a polymers crystallinity

A

measuring tensile strength
or
measuring Youngs modulus and plotting stress vs strain

36
Q

what scale is used to assign a polymers crystallinity

A

0-1
0 = completely amorphous (disordered)
1 = completely crystalline (ordered)

37
Q

are synthetic polymers normally crystalline or amorphous

A

amorphous (very few are entirely crystalline)

38
Q

what can measuring the crystallinity of a polymer allow

A

identification and then comparisons between samples

39
Q

what determines if a destructive or non destructive method is used to measure a polymers crystallinity

A

the availability of the evidence and the processes that may follow my analysis

40
Q

what does the tensile strength of a polymer refer to

A

how much it can be stretched before it breaks

also called elongation stress

41
Q

in polymers what does tensile strength increase with

A

polymer chain length and crosslinking due to more interaction between chains

42
Q

why is measuring tensile strength a good method

A

cheap and easy

just stretch the polymer this is standardised by applying a force and look at the distance it is stretched

43
Q

what can be the problem with measure tensile strength

A

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

44
Q

when analysing solids and their tensile strength what terms do we use rather than stretch and force

A

instead of stretch we use strain
instead of force we use stress

45
Q

what are stress and strain independent of

A

the size of the sample

46
Q

what does Youngs modulus describe

A

how stiff a solid is (the higher the stiffer)

47
Q

in thermoplastics what are the two deformation regions classed as

A

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)

48
Q

for a crystalline material what will happen with an increase in temperature of the material

A

it will go from a crystalline to a thermoplastic to a liquid

Trace Evidence (16 decks)

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