Lecture 4: Polymers and fibres Flashcards
Introduction to polymers
- Large molecules
- Linked molecule units
Some examples of monomer units: vinyl chloride, styrene and glucose
- Linked molecule units
Interconnection/interaction between chain polymers
- Branched
- Polyethylene (LDPE and HDPE)
- Cross linked
- Poly(isoprene) rubber
- Epoxy resin (glue)
- Linear
- PVC, polystyrene, and nylon
- Dendrimer
- Specialised polymers
Types of polymers
- Natural (biopolymers)
-Natural fibres, proteins
and DNA - Semi-synthetic polymers
-Rayon fibres and
cellophane - Synthetic polymers
-PVC, polystyrene, PTFE,
and nylon - Co-polymers
-different linker monomers
-styrene (25%) and 1,3-
butadiene
Synthesis and classification of synthetic polymers
- Chain growth polymerisation- thermoplastic
- Cationic polymerisation- Thermosetting
- Anionic polymerisation- Thermosetting
Step growth polymerisation- Elastomer
Classification of synthetic polymers
- Thermoplastics
- Hard at room temperature
- Soft and viscpus at higher
temperature- shaped and
moulded - Polystrene, polyethylene,
acrylonitrile butadiene
styrene (lego) and PET
- Thermosetting polymers
- On heating: become highly
cross-linked and solidify - Cannot be reshaped or
remoulded - Bakelite and automobile
and
- On heating: become highly
- Elastomers
- Ability to stretch and
return to their original
shape - Rubber Tyres
- Ability to stretch and
What are fibers?
- Thin threads
- Spinneret
- Cooled and drawn
out - Tension strength
Why are fibers important?
-Transfer and trace evidence
-Largest category of polymer evidence analysed
-Homicide, assault, sexual offences and hit and run
Classification of fibers
-Natural
* Animal source
* Plant Source
* Mineral
-Semi-Synthetic
-Synthetic
Cellulose
-Cellulose Fibres
-Cotton Fibres
-Other Natural fibres
Cellulose- Dicot fibers
-Transparent, colourless, curved twisted fibres
-Circular Cross Section and Surface Striations
Kapok- Seed fiber
- Smooth, hollow, thin walled cylinders
- 2-3 cm/10-35 µm, Twisting and sharp bending and surface irregularities
- Silky cotton like substance
- Tiny cellulose tubes air sealed inside
- Removed by hand, dried, separated and exported
- Brittle and inelastic
Buoyancy aids
Flax (linen)- Dicot fiber
-Skin Fibre
-Vegetable Fibre
-Cellulose
-Stronger than cotton
-Transverse Nodes shapes I, X, V and Y
Hemp- Dicot fibers
- Colourless, transparent cylinders
- Surface irregularities
Polygonal in cross section - Cannabis plant
- Primary Bast Fibres (70%)
-Long, high cellulose, low lignin
-One of the strongest natural fibre - Secondary Bast Fibres (30%)
-Medium, high lignin, low cellulose
Ramie- Dicot
- Nodelike ridges
- Longitudinal striations
- Smooth Surface
- Raised Nodes
- Long, Glossy, Naturally white and silky appearance
- Composed of Cellulose
- Extremely porous
- Strongest natural fibres
Greater strength when wet
Jute
- Straight, smooth cylinders
- Bundles of individual fibres
- 1-4 m long, soft and shiny fibres
- White to brown
- Spun
Cellulose and Lignin
Coir- fruit fibre
-Cordage and brushes
-Fibre Bundles
-Floor Mats and Ropes
Sisal- monocot fibre
-Transparent colourless to light yellowish tan
-cylinders.
Manilla- monocot fibers
-Bundles of individual fibres
-High magnification 0.2-0.4 μm protuberances
Silk- animal fibres
-Pale yellow to brown/yellow fibres
-Broad Continuous Ribbons
-Longitudinal Striations
Silk
-Protein fibre
-Silk worm
-Shiny fibre
-Strongest Fibres
-Decreases when wet
Cellulose- cotton fibres
-90% Cellulose
-Seed fibre
-Seed Pods “boll”
-Cotton Fibres: yellow-white colour length around 2 inches
-Microscopic analysis
Semi synthetic fibres: Rayon fibres
- Extraction from wood pulp or cotton
- Reaction with a strong base and carbon disulphide
- Cellulose Xanthate (viscose)
- Spinneret into a bath of acid
- Rayon filaments drawn out to form rayon fibres
- Long, Smooth, highly reflective and shiny fibre
Synthetic fibres: characteristics
No surface Characteristics, Regular and Uniform in shape
Synthetic fibres: nylon
- Step-Growth Polymer
- 6-aminohexanoic acid
- Uses:
-Nylon Stockings
-Carpets
-Seat Belts
Microscopic analysis of fibres
1) General Morphology
2) Cross Section and Diameter
3) Shape – Modification Ratio (MR)
MR = R
r
4) Dichroism and Pleochroism
-Dichroic fibre: Exhibits 2 different colours
-Example: Nylon 66
-Pleochroic fibre: Exhibits more than 2 colours
-Example: Asbestos Fibres
-PLM
5) Refractive Index (RI) and Birefringence
6) Sign of elongation of Birefringent Fibres
-Compare Parallel RI against Perpendicular RI
-Sign of elongation (+) =
-Sign of elongation (-) =
7) Isotropy and Anisotropy
-Isotropic Substances/Fibres:
-Anisotropic Substances/Fibres:
-An anisotropic substance produces interference colours
8) Interference Colours
-Colours produced by Double Refraction
-Different degrees of retardation = different Interference colours
Refractive index
‘When a ray of light strikes the interface between a medium (air) and a denser medium (fibre) Part of the light is reflected back off the interface and the other part is passed (refracted) into the fibre. The ray that passes into the fibre is bent due to refraction (polarised) and will travel at a specific velocity through the fibre i.e. a characteristic refractive index (RI)’
Isotropic Substances/Fibres
-Amorphous
-The rays of light travel with the same velocity in all directions
-The rays of light have the same RI
Anisotropic Substances/Fibres
-Have a crystalline or pseudocrystalline nature
-Light transmitted at different velocities in different directions due to different refractive indices.
Double refraction
As light passes into an anisotropic substance the single light ray is broken into two polarised rays, which travel at different velocities i.e. different refractive indices
Birefringence
-Symbolised by: Parallel RI – Perpendicular RI
-Indicates the absolute difference in the refractive indices of the 2 rays
Determination of a RI of a fiber
1) A fibre placed onto glass slide
2) Immersed in a drop of oil of specific RI
3) Cover slip and observe
4) If the oil and fiber have the same RI =
5) If the oil and the fiber have different RI =
6) Test using a series of oils of known Refractive indices
Determination of the Birefringence of a Fiber
- Same technique as RI
- Except observe RI in two orientations:
1) Parallel RI
2) Perpendicular RI
What are polymers?
Large molecules linked by smaller molecule units
Examples of monomer units include vinyl chloride, styrene, and glucose.
What are the types of polymers?
- Natural (biopolymers)
- Semi-synthetic polymers
- Synthetic polymers
- Co-polymers
Co-polymers consist of different linker monomers.
What are examples of natural polymers?
Natural fibres, proteins, and DNA
What are examples of semi-synthetic polymers?
Rayon fibres and cellophane
What are examples of synthetic polymers?
PVC, polystyrene, PTFE, and nylon
What are the types of interconnection/interaction between chain polymers?
- Branched
- Cross linked
- Linear
- Dendrimer
Examples include polyethylene (LDPE and HDPE), poly(isoprene) rubber, and epoxy resin.
What is chain growth polymerisation?
A method of synthesizing thermoplastic polymers
What is cationic polymerisation?
A method of synthesizing thermosetting polymers
What is anionic polymerisation?
A method of synthesizing thermosetting polymers
What is step growth polymerisation?
A method of synthesizing elastomers
What are thermoplastics?
Polymers that are hard at room temperature and soft and viscous at higher temperatures
Examples include polystyrene, polyethylene, and PET.
What are thermosetting polymers?
Polymers that become highly cross-linked and solidify upon heating, cannot be reshaped
Examples include Bakelite.
What are elastomers?
Polymers that can stretch and return to their original shape
An example is rubber.
What are fibres?
Thin threads that are cooled and drawn out with tension strength
Why are fibres important in forensic analysis?
They are the largest category of polymer evidence analyzed in cases such as homicide and assault.
What are the classifications of fibres?
- Natural
- Semi-Synthetic
- Synthetic
What are cellulose fibres?
Fibres derived from cellulose, such as cotton fibres and other natural fibres
What characterizes Kapok seed fibres?
Smooth, hollow, thin-walled cylinders with air-sealed tiny cellulose tubes
What are the characteristics of flax (linen) fibres?
Skin fibre, stronger than cotton, with transverse node shapes I, X, V and Y
What are the primary bast fibres from hemp?
Long, high cellulose, low lignin fibres; one of the strongest natural fibres
What are the secondary bast fibres from hemp?
Medium, high lignin, low cellulose fibres
What are the characteristics of ramie fibres?
Nodelike ridges, longitudinal striations, smooth surface, and extremely porous
What are jute fibres characterized by?
Straight, smooth cylinders, soft and shiny, bundles of individual fibres
What is coir?
Fruit fibre used for cordage and brushes, made from fibre bundles
What are the characteristics of sisal fibres?
Transparent colourless to light yellowish tan cylinders
What are the features of silk fibres?
Pale yellow to brown/yellow fibres, broad continuous ribbons, strong and shiny
What is the composition of cellulose in cotton fibres?
90% cellulose
What are cotton fibres derived from?
Seed pods known as ‘boll’
Fill in the blank: Cotton fibres have a length of around _______.
2 inches
What are the key components of microscopic analysis of fibres?
- General Morphology
- Cross Section and Diameter
- Shape – Modification Ratio (MR)
- Dichroism and Pleochroism
- Refractive Index (RI) and Birefringence
- Sign of elongation of Birefringent Fibres
- Isotropy and Anisotropy
- Interference Colours
Each component plays a crucial role in understanding the properties of fibres under microscopic analysis.
What is the formula for Modification Ratio (MR)?
MR = R / r
R is the major axis and r is the minor axis of the fibre.
What is a dichroic fibre?
A fibre that exhibits 2 different colours
Example: Nylon 66.
What is a pleochroic fibre?
A fibre that exhibits more than 2 colours
Example: Asbestos Fibres.
What does Birefringence indicate?
The absolute difference in the refractive indices of the 2 rays
Birefringence is symbolised by: Parallel RI – Perpendicular RI.
What happens when light strikes the interface between air and a denser medium?
Part of the light is reflected back, and part is refracted into the fibre
The ray that passes into the fibre is bent due to refraction.
What characterizes isotropic substances/fibres?
- Amorphous
- Light travels with the same velocity in all directions
- Light has the same refractive index
Isotropic materials do not show different optical properties when viewed from different angles.
What characterizes anisotropic substances/fibres?
- Crystalline or pseudocrystalline nature
- Light transmitted at different velocities in different directions
Anisotropic materials have varying optical properties based on the direction of observation.
What is double refraction?
The phenomenon where a single light ray is broken into two polarised rays in an anisotropic substance
These rays travel at different velocities, resulting in different refractive indices.
How is the refractive index (RI) of a fibre determined?
- Place the fibre on a glass slide
- Immerse in a drop of oil of specific RI
- Cover slip and observe
- Compare RI of oil and fibre
If they match, it indicates the RI of the fibre.
What is the sign of elongation in birefringent fibres?
Compare Parallel RI against Perpendicular RI
A positive sign indicates one orientation, while a negative sign indicates the opposite.
What is the process for determining the birefringence of a fibre?
Use the same technique as RI determination, observing RI in two orientations: Parallel RI and Perpendicular RI
This allows for the assessment of optical properties in different directions.
Fill in the blank: An anisotropic substance produces _______.
interference colours
This occurs due to the different velocities of light in various directions.
What causes interference colours in fibres?
Different degrees of retardation due to double refraction
These colours are visible when observing anisotropic materials under polarized light.
