polymers and fibres Flashcards
what are the most common form of trace evidence and what are they made from
fibres; made from polymers
what are polymers
extremely large molecules composed of linked individual monomer units
name 3 examples of monomer units
vinyl chloride, styrene and glucose
what is process of monomer units linking together to form a polymer called
polymerisation
what are the 4 types of polymers and give examples of each
- Natural (biopolymers) - natural fibres, proteins and DNA e.g. hemp and cotton
- Semi-synthetic polymers - Rayon fibres and cellophane
- Synthetic polymers - PVC, polystyrene and nylon
- Co-polymers - rubber in tyres
what are co-polymers
polymers made using different monomer units
what are the 4 possible organisations of monomer units in co-polymers
- Random co-polymers A-B-AA-B-B-B
- Alternating co-polymers A-B-A-B-A-B
- Block co-polymer A-A-A-B-B-B
- Graft co-polymer A-A-A-A-A-A-A-A-A
B B
B B
what are the 4 interconnection/ interactions between polymer chains
- Branched
- Cross-linked
- Linear
- Dendrimer
explain a branched polymer and give an example
a main chain with side chains e.g. Polyethylene
explain cross-linked polymers and give an example
linear chains joined together by small vertical chains at random positions along the length of the main chain e.g. Poly(isoprene) Rubber - inner tubes of bicycle tyres
explain linear polymers and give an example
most common type of polymer as chains are straight with no branching e.g. PVC, polystyrene and nylon
explain a dendrimer polymer and give an example
specialised polymers which have specific uses - they have been engineered for that purpose. for example every single unit in a a chain being branched
name the 4 processes synthetic polymers are made from and give examples
chain growth polymerisation - most plastics
cationic polymerisation - bicycle tubes
anionic polymerisation - fingerprint superglue fuming
step growth polymerisation - nylon
name the 3 categories synthetic polymers can be classified into
thermoplastics
thermosetting
elastomer
describe thermoplastic synthetic polymers and give examples
something that has a high glass transition temperature (is hard at room temp like glass), yet is soft and viscous at higher temperatures so it can be shaped and moulded.
For example Polystyrene, polyethylene, acrylonitrile butadiene styrene, and most commonly Lego
describe thermosetting synthetic polymers and give 2 examples
when heated they produce an extensive 3D linkage molecule which is highly cross-linked, so that when it solidifies it forms a hard insoluble mass that cannot be reshaped or remoulded. For example Bakelite and automobile topcoat
name an issue surrounding thermosetting synthetic polymers
they are inflexible and they can become brittle and crack unless plasticisers are added
describe elastomer synthetic polymers and give an example
they have low glass transition temperatures, as well as the ability to stretch and return to their original shape - this is due to them being coiled chains that can be stretched horizontally due to van der waals forces
For example car tyres and rubber
define fibres
thin threads of molten polymer that are extruded through a spinneret by a high pressure, cooled down and drawn out along the axis of the fibre, to provide tension strength
why are fibres important forensically
they can be evidence in almost all crimes and they are easily transferable
what are the three classifications of fibres
natural
semi synthetic
synthetic
name the 11 natural fibres we look at in this module
Cellulose
Kapok
Flax
Hemp
Ramie
Jute
Coir
Sisal
Manilla
Cotton
Silk
out of the 11 natural fibres we look at which is the only animal fibre
silk
what are the monomer units in cellulose and what bond links them
glucose - hydrogen bonds
name a use of cellulose
provides strength to plants and wood
what is a dicot fibre
a fibre that comes from a plant that has 2 seed leaves
how does cellulose appear under microscope
transparent, colourless, curved, twisted fibres, with circular cross-sections and surface striations along the length of the fibre
what are Kapok fibres used for and why
they are used as a stuffing agent because they are brittle and inelastic meaning they cannot be spun into fabric
describe Kapok fibres
smooth, hollow, thin walled cylinders filled with air, that are undergoing sharp bending and twisting
why are Kapok fibres good in life jackets
as they can take on 30% of their weight in water - providing buoyancy
what is flax fibre more commonly known as
linen
why is Flax considered a skin/ bast fibre
its collected from the stem of the linen plant
What makes flax fibres so strong
they contain a very high concentration of cellulose
where do hemp fibres come from
the stem of the cannabis plant
describe hemp fibres being viewed under microscope
colourless, transparent cylinders that sometimes have swollen joints and tissues as well as surface irregularities
what are the two types of hemp fibres and how do they differ
- primary bast fibres - make up 70% of hemp fibre production as they are extremely strong
- secondary bast fibres - make up 30% of hemp fibre production as they have a high lignin content instead of cellulose, making them not as strong and therefore expensive
describe Ramie fibres being viewed under microscope
nodelike ridges, longitudinal striations down the length of the fibre, and smooth surfaces between the nodes
what is one advantage of Ramie fibres
they are on of the strongest natural fibres available and increase in strength when wet
describe jute fibres when viewed under microscope
straight, smooth cylinders that may have small protuberances, or bundles of individual fibres with adhesions
what is Jute used in and how is it altered for this purpose
used in cordage so its spun into a harsh fibre containing cellulose and lignin as its usually soft on its own
where does coir fibre come from
the husk of a coconut - its a fruit fibre
what is a monocot fibre
a fibre that comes from a plant with one seed leaf
describe what you see when Sisal is viewed under microscope
transparent, colourless to light yellowish cylinders which have broad strands in the middle
why is sisal fabric expensive
as to produce sisal fabric the fibres have to be beaten to a pulp
describe manilla fibres when viewed under a microscope
fibres that are polygonal to oval in shape, and they have rounded corners at the end
what are Manilla fibres used in and why
cordage, ropes and brushes as it is a hard fibre
as well as brown-paper envelopes if it is used in pulp
describe cotton fires when viewed under microscope
thin ribbons/ threads, with twists at regular intervals
describe silk fibres when viewed under a microscope
broad, continuous ribbons with longitudinal striations down the length of the fibre
why is silk a shiny fibre
due to its triangular prism like shape that can interact with light at different angles
name the one semi-synthetic we look at in this module
Rayon fibres
how are Rayon fibres made
○ Extract cellulose from wood, pulp or cotton
○ React that with sodium hydroxide (a strong base) and carbon disulphide
○ Results in cellulose Xanthate (viscose) - a fibre in its own right e.g. clothing
○ To produce Rayon fibres the cellulose Xanthate must be extruded through a spinneret into a bath of sulphuric acid and sulphate salts
○ This produces Rayon filaments that can be drawn out to form rayon fibres
describe rayon fibres
a chemically modified cotton that is strong and long
name the two synthetic fibres we look at in this module
nylon and kevlar
why can’t two synthetic fibres be told apart when viewed under microscopes
as they have no surface characteristics - regular and uniform in shape
what are bicomponent synthetic fibres
synthetic fibres made from 2 different polymers that are engineered for a specific purpose
what are the two subdivisions of bicomponent synthetic fibres
sheath and core design - allows for a message to be written down the fibre
side by side design - no message but patterns available
what kind of polymerisation creates nylon and kevlar
step-growth
what is Kevlar known as
the strongest synthetic fibre in the world
what are the monomer units of kevlar and what bond links them
aramids - hydrogen bonds
name the uses of nylon and kevlar
nylon; stockings, carpets and seatbelts
Kevlar; bullet-proof vests, armoured clothing, fireman’s suits
what are the 4 main steps in chain growth polymerisation
- Formation of free radicals
- Reaction of monomer with radicals
- Chain propagating
- Termination step - 2 options;
chain combination or impurity
what are the 3 main stages in cationic polymerisation
- An electrophilic initiator attacks a nucleophilic monomer
- Chain-propagating step
- Termination step - 2 options;
loos of proton or chain-transfer reaction with solvent
what are the 3 main steps in anionic polymerisation
- A nucleophilic initiator attacks an electrophilic group
- Chain-propagating step
- Termination step (reaction with water or carbon dioxide)
what are the 8 different ways to analyse fibres using a microscope
- general morphology
- cross section and diameter
- shape - modification ratio
- dichroism and pleochroism
- refractive index and birefringence
- sign of elongation of birefringent fibres
- isotropy and anisotropy
- interference colours
what is the difference between dichroic fibres and pleochroic fibres
dichroic = exhibits 2 different colours
pleochroic = exhibits more than 2 colours
how do we determine the RI of a fibre
- A fibre placed onto glass slide
- Immersed in a drop of oil of specific RI
- Cover slip and observe
- If the oil and fibre have the same RI = that RI
- If the oil and the fibre have different RI = test again
- Test using a series of oils of known refractive indices
what does birefringence indicate
the absolute difference in the RI’s of the two rays
what are other forensic techniques used for analysis of fibres
Raman
FTIR
visible micro spectroscopy
