Polymers Flashcards
What is a polymer? And the 2 broad categories?
- Consist of large molecules
- Long chains not atoms
- Repeating sub-units
- Rigif plastics and elastic rubbers
- Most are organic
- 2 broad categories:
- Natural
- Synthetic
What are the general properties of polymers?
- Low density
- Low strength
- Low stiffness
- Ductile and pliable
- Chemically inert and resistant to degradation
- Low electrical conductivity
- Low thermal conductivity
- Soften/decompose at relatively low temperatures
What are some polymer forms?
- Elastic (rubbers)
- Films/membranes
- Foams/expanded foams
- ‘Rigid’ plastics
- Paints
- Fibres/monofilament/ropes
- Textiles
- Adhesives/fillers/sealants
- Compposties
What are the characteristics of thermoplastics?
- Relatively soft and flexible
- Soften/melt when heated -> can be reshaped
- Excessive heat leads to degradation
- Can be supplied in pellet form for melting/moulding
- Recyclable
What are the characteristics of thermosetting plastics?
- Harder, stronger and more rigid than thermoplastics
- Do not soften or melt when heated
- Excessive heat leafs to degradation
- Often supplied as pre polymer resin
- Cured using heat/pressure/catalysts
- More difficult to recycle
What are the characteristics of elastomers?
- Highly elastic
- Very flexible and soft
- Mostly thermosets
- Vulcanisation creates cross links
- Exhibits hysteresis
The production of polymers involving fractional distillation?
- Crude oil is heated
- Oil changes to gases
- Separation (fractioning)
- ‘Cracking’ process is used to separate Naptha into smaller molecules called monomers
- Smaller molecules synthesised into polymers during polymerisation
What are some additives that can be added during the production of polymers?
- Plasticisers: Improve ‘flow’ properties such as flexibility and ductility
- Catalysts/Accelerators: Chemical compounds that speed up reactions
- Fillers: Powdered solids (e.g. woodflour, clay) reduce costs and improve properties
- Colorants: Dyes or pigments
- Stabilisers: Enhance resistance to degradation
- Flame retardants: Chemical compounds that inhibit/retard the ignition/burning
What are hydrocarbons?
Synthetic polymers derived from crude oil
What are the 4 different structures of molecular chains?
- Linear
- Branched
- Crosslinked
- Network (3D)
Properties of linear molecular chains?
- Relatively high density, tensile strength and melting points (compared to short branched)
Properties of Branched molecular chains
- Lower chain packing efficiency
- Long branches increase TS / melting points
Properties of Crosslinked molecular chains
- Crosslinks make Covalent bonds
- Formed during synthesis or additional reaction
Properties of network molecular chains
- Monomers form _>3 covalent bonds
- 3D network formed
- High crosslink density
What is a crystalline structure?
When atoms are arranged in a repeating 3D structure
What is an amorphous structure?
No regular structure to atoms
What crystalline structure are polymers?
Polymers are often semi-crystalline.
What are the factors that affect crystallinity?
- Molecular chemistry
- Molecular structure (simpler the structure the higher the crystallinity)
- Cooling rate (rapid cooling gives insufficient time or polymer chains to arrange into regular pattern)
What affect does increased crystallinity have on the properties of a polymer?
- Higher density
- Enhances strength
- Stiffer
- Harder
- Increased creep resistance
- More brittle
- Improved heat resistance
- Clear melting point
- less transparent
When it comes to tensile properties of polymers, what are the 3 main categories of polymers?
- Brittle Polymers
- Plastic Polymers
- Elastomeric Polymers
What is viscoelastic behaviour?
Ability of a material to display both fluid and elastic properties whilst in same environment
What is ‘Creep’
The time dependent deformation of a polymer that is subject to a constant load below yield.
It can occur in all materials but is relatively rapid in polymers at room temperature.
Accelerated by temperature increase
What are the stages of ‘Creep’?
- Primary Creep: rapid initial deformation that slows down as the polymer adjusts.
- Secondary Creep: steady-state deformation with a nearly constant rate.
- Tertiary Creep: accelerated deformation leading to failure.
- Maybe Creep Recovery: process by which a polymer attempts to return to its original shape
What is stress relaxation
- When a polymer specimen is deformed up to a predetermined strain, this will induce a stress in the specimen.
- Over time the stress will decrease due to molecular relaxation within the polymer.
- The relaxation modulus will decrease with temperature.
Difference between the melting temperatures (Tm) of amorphous and crystalline materials?
- Crystalline materials have a clear melting temperature (Tm)
- Amorphous materials melt gradually
- The higher the degree of crystallinity the more clearly defined melting point there is
- E.G semi crystallines exhibit combination of both melting characteristics
What is Glass transition temperature (Tg)
- Temperature at which an amorphous polymer (or the amorphous regions of a semi-crystalline polymer) transitions from a hard and brittle “glassy” state to a soft and rubbery state.
- Helps define useful working temperature
Factors influencing Tm and Tg?
- More complex molecular structures have higher Tm and Tg (generally)
- Higher values caused by:
- Greater polymer chains stiffness
- Increased molecular weight
- Polar groups
- Cross linking
- Large amount of long branching
- Lower values caused by:
- Short branches
What is bond rupture?
- When molecular chains break apart and the molecular weight reduces
What are the 3 main causes of bond rupture?
- Radiation (e.g. X-ray, UV)
- Chemical (e.g. O2, O3)
- Thermal
What is swelling and dissolution?
- Swelling is when the liquid solute is absorbed by the polymer
- Dissolution is when the polymer is completely dissolved by a solvent
Both happen when:- Small liquid molecules fit into gaps
- Polymer chains forced apart
- Secondary forces weaken
- Becomes softer and more ductile
- Tg is lowered
- Chemical similarity can increase likelihood
- More resistant to acid/alkaline solutions
What are some potential positives of degradation in polymers?
- Medicines (capsules)
- Disposable waste
- Sealing applications
- Spill absorption
