Lecture 8 - Polymers

Question 1

what is a polymer

Answer 1

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

Question 2

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

Answer 2

fibres
adhesives
membranes
rope
textiles
additives
polystyrene
plastic

Question 3

what are the two main categories of polymers

Answer 3

natural and synthetic
(can also get semi synthetic)

Question 4

give three examples of a natural polymer

Answer 4

proteins
polysaccharides
resins

Question 5

name 3 examples of synthetic polymers

Answer 5

plastics
elastomers (polymer that displays elastic properties)
rubber

Question 6

what are the 4 main shape types polymers are found in

Answer 6

linear
branched
cross-linked
colloidal

Question 7

what is a dimer

Answer 7

two monomers joined together

Question 8

what is a trimer

Answer 8

three monomers joined together

Question 9

what is a homopolymer

Answer 9

a polymer made up of all the same monomers

Question 10

what is a co-polymer

Answer 10

a polymer made up of different monomers

Question 11

give an example of a small monomer and a complex monomer

Answer 11

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

Question 12

what feature do monomers need to be able to form polymer

Answer 12

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

Question 13

what does the shape of a polymer depend on

Answer 13

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

Question 14

what is the purpose of an initiator

Answer 14

something that starts off the polymerisation reaction

Question 15

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

Answer 15

dendrimer - found in the coating of illicit substances

Question 16

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

Answer 16

the presence of an ending group

these have identifiable feature so are useful in forensic analysis

Question 17

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

Answer 17

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

Question 18

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

Answer 18

polymer ring structures do not have end groups

Question 19

what do reactive end groups allow

Answer 19

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

Question 20

what properties of polymers allows them to be classified

Answer 20

their bulk properties

Question 21

name the three main polymer types when classifying them

Answer 21

thermoset
thermoplastic
elastomer

Question 22

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

Answer 22

burns when heated

irreversibly hardened when shaped

generally inflexible

curing is needed - a chemical process

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

Question 23

what happens when a polymer is cured

Answer 23

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

Question 24

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

Answer 24

melts when heated

can be re- melted and reshaped

more flexible generally

no curing needed

e.g polystyrene, nylon and polycarbonate

Question 25

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

Answer 25

can also be thermoset or thermoplastic is viscous (can flow) has elasticity (returns to original shape) e.g rubbers

Question 26

what are HDPE and LDPE polymers

Answer 26

high density polyethylene low density polyethylene

Question 27

what are the properties of HDPE polymers compared to LDPE

Answer 27

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

Question 28

what properties allow us to tell HDPE and LDPE polymers apart

Answer 28

a comparison of their physical properties

Question 29

name some examples of where HDPE polymers may be found

Answer 29

containers goof bottles petrol tanks pipes

Question 30

name some examples of where LDPE polymers may be found

Answer 30

films and sheet packaging toys squeezy bottles plastic bags wire coatings

Question 31

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

Answer 31

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

Question 32

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

Answer 32

supramolecular interaction (hydrogen bonds, van der Waals) covalent interactions (cross linking of chains)

Question 33

what does the crystallinity of a polymer refer to

Answer 33

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

Question 34

what physical properties does polymer crystallinity affect

Answer 34

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

Question 35

how can we measure a polymers crystallinity

Answer 35

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

Question 36

what scale is used to assign a polymers crystallinity

Answer 36

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

Question 37

are synthetic polymers normally crystalline or amorphous

Answer 37

amorphous (very few are entirely crystalline)

Question 38

what can measuring the crystallinity of a polymer allow

Answer 38

identification and then comparisons between samples

Question 39

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

Answer 39

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

Question 40

what does the tensile strength of a polymer refer to

Answer 40

how much it can be stretched before it breaks also called elongation stress

Question 41

in polymers what does tensile strength increase with

Answer 41

polymer chain length and crosslinking due to more interaction between chains

Question 42

why is measuring tensile strength a good method

Answer 42

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

Question 43

what can be the problem with measure tensile strength

Answer 43

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

Question 44

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

Answer 44

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

Question 45

what are stress and strain independent of

Answer 45

the size of the sample

Question 46

what does Youngs modulus describe

Answer 46

how stiff a solid is (the higher the stiffer)

Question 47

in thermoplastics what are the two deformation regions classed as

Answer 47

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)

Question 48

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

Answer 48

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