polymers and fibres Flashcards
polymers and fibres introduction
fibres are the most common trace evidence found at a crime scene. fibres are made from polymers.
polymers are extremely large molecules made up of individual monomer units linked together. the process of monomer units linking together to form a polymer is called polymerisation.
types of polymers:
- natural (biopolymers)- natural, proteins, DNA.
- semi synthetic polymers- rayon fibres and cellophane.
- synthetic polymers- PVC, polystyrene, PTFE and nylon. kevlar is the strongest synthetic fibre in the world.
co polymers- have different individual monomer units that are all linked together within the polymer.
possible organisation of monomer units in co polymers:
random: ABAABABBBAAAB
alternating: ABABABABABAB
block: AAAAABBBBB
graft: AAAAAAAAAAAA
B B B
B B B
interaction between polymer chains:
branched- polyethylene (LDPE)(HDPE).
cross linked- poly(isoprene) rubber, epoxy resin (glue).
most common interaction is linear- PVC, polystyrene, nylon, dendrimer.
synthesis and classification of synthetic polymers:
chain growth, cationic, anionic and step growth polymerisation.
classification of synthetic polymers: 1. thermoplastics
high glass transition temperature, hard like glass at room temperature. when heated it becomes soft and viscous, can be shaped and moulded. polystyrene, polyethylene, acrylonitrile butadiene styrene (lego) and PET.
classification of synthetic polymers: 2. thermosetting
when heated becomes highly cross linked and solidifies. can’t be reshaped or remoulded. bakelite and automobile top coats.
classification of synthetic polymers: 3. elastomers
low glass transition temperatue, have the ability to stretch and return to their original shape because they are random coiled chains that can be stretched horizontally. horizontal stretch cant be maintained because the forces involved with the stretch are wan der waals which are very weak, so it returns to its original shape. rubber and tyres.
fibres:
thin threads of molten fibres that are extruded through a spinneret. the molten polymer is then cooled and drawn out along the axis of the fibre, to give the fibre its tension strength.
fibres are the most important transfer and trace evidence found at a crime scene (largest category of polymer that will be analysed on a regular basis). important in homicide, assault, sexual offences.
natural fibres: cellulose
extremely large natural polymer which is made from glucose monomer units. the glucose monomer units are held together by hydrogen bonds which makes cellulose very strong. cotton fibres are made up of 90% cellulose. cellulose is a dicot fibre as it comes from a plant with 2 seed leaves. under a microscope they are transparent, colourless, curved and twisted fibres with a circular cross section and surface striations.
natural fibres: kapok
java cotton. sharp, bending and twisting tubes. silky substance, tiny cellulose tubes with air sealed inside. removed by hand, dried, separated and exported. brittle and inelastic which means it cant be spun into a fabric so is used as a stuffing agent. can take 30% of its weight in water. its non toxic, non allergenic, used for life jackets, mattresses, quilts and pillows.
natural fibres: flax (linen)
dicot fibre. skin or bast fibre as its collected from the stem of the plant. high concentration of cellulose, more than kapok so its much stronger. under microscope it has slight thickenings of transverse nodes in the shape of I, X, V, Y.
natural fibres: hemp
dicot fibre. comes from cannabis plant. under microscope its colourless and transparent cylinders with surface striations. primary bast fibres (70%) of hemp fibre production. one of the strongest natural fibres as its long and its got a very high concentration of cellulose. secondary bast fibres (30%) of hemp fibre production. its got a high lignin content which makes it not as strong as primary (very sensitive fibres) .
natural fibres: ramie
dicot fibre. nodelike ridges, longitudinal striations, smooth surfaces, raised nodes. long, glossy, naturally white and silky in appearance. composed of cellulose, extremely porous. strongest natural fibres and greater strength when wet.
natural fibres: jute
second most affordable fibre (after cotton). under microscope its straight smooth cylinders, bundles of individual fibres. long, soft and shiny fibres but spun into harsh fibre containing cellulose and lignin.
natural fibres: coir
comes from a coconut (fruit fibre). composed of fibre bundles. floor mats and ropes.
natural fibres: manilla
monocot fibre. under microscope is oval in shape with rounded corners. ropes, brushes. used in pulp for paper.
natural fibres: silk
animal fibre, larvae of silkworm. pale to yellow fibres, broad continuous ribbons and longitudinal striations. shiny and one of the strongest fibres but decreases when wet. clothing, bedding and parachutes.
natural fibres: cotton
seed fibre. thin ribbons or thin threads with a ribbon shape. grow in the seed pods. when the pods open, they reveal cotton fibres which are very weak. have to spin them into fabric to give them strength. can count the number of fibres in fabric. microscope is an 8 stage process.
natural fibres: sisal
monocot fibre (comes from a plant with 1 seed leaf). hard fibres. transparent colourless to yellowish tan cylinders. used in ropes, floor mats and brushes. can be pulled into pulp, beaten to produce sisal which is worn in hot countries but extremely expensive.
semi synthetic fibres:
chemically modified forms of cellulose. the hydroxyl groups are extremely active.
semi synthetic polymers- rayon fibres
extract cellulose from wood pulp or cotton, react that with strong base carbon disulphate. this results in cellulose xanthate (viscose). spinneret into a bath of acid. rayon filaments drawn out to form rayon fibres. long, smooth, highly reflective and shiny fibre. chemically modified by cotton.
synthetic fibres:
bicomponent fibres: 2 different fibres present. sheath and core and side by side design. no surface characteristics, regular and uniform in shape. cant identify them under a microscope but can differentiate a synthetic vs natural fibre
synthetic fibres: nylon
step growth polymer. nylon stockings, carpets and seatbelts
