Polymers Flashcards
What are thermoplastics? (4)
● Considered linear, can be branched – chains not cross-linked by covalent bonds – only by Van der Waals bonds. ● Soften when heated, harden when cooled ● Secondary bonds break easily with heat ● Have range of Tm , due to: -- Range of molecular weights -- Varying “packing” states (part crystalline/ part amorphous)
What are 5 types of thermoplastics?
PE, PP, PS, PTFE, PVC
What are thermosets? (2)
● Heavily cross-linked or networked polymers
- Lateral covalent bonds join long polymer chains.
- Consequently application of heat does not break lateral bonds - no “softening”.
● Produced by chemical reaction on mixing two components (resin + hardener).
Why are thermoplastics linear? (2)
– chains not cross-linked by covalent bonds
– only by Van der Waals bonds.
Why do thermoplastics have a range of temperatures? (2)
– Range of molecular weights
– Varying “packing” states (part crystalline/
part amorphous)
Why do thermosets have heavily cross-links or networked polymers? (3)
- Lateral covalent bonds join long polymer chains.
- Consequently application of heat does not break lateral bonds
- no “softening”.
What are elastomers?
Linear polymers with occasional cross‐links, hence elastic “memory”, and amorphous
What are 8 factors affecting tensile properties?
- Chemical compositions
- Molecular weight/length
- Structure
- Tacticity
- Polymer additives, such fillers, plasticisers…
- Temperature
- Strain rate
- Environments
How does molecular weight/length affect tensile properties? (2)
Increase DP (degree of polymerisation)
– Increases strength
– Increases viscosity
How does structure affect the tensile properties? (crosslinks) (3)
Cross Linked (Thermosets) – Primary lateral bonds between chains. – Greater degree of cross linking – leads to ‘stiffer’ more ‘stable’ structure. – More ‘brittle’ at ambient temperature.
How does structure affect the tensile properties? (interpenetrating polymer network (IPN)) (2)
– During manufacture catalytic additions can result
in intertwining network formation.
– Restricts polymer chain movements, therefore
strengthening.
How does structure affect the tensile properties? (Linearity) (2)
– Linear polymers have large degree of flexibility due to independent movement of chains (van de waals & hydrogen bonding).
– Branched polymers are considered linear.
• Branches restricts polymer chain movement, therefore strengthening
• Low packing efficiency therefore low density
How does structure affect the tensile properties? (branches) (2)
– Branched polymers are considered linear.
• Branches restricts polymer chain movement, therefore strengthening
• Low packing efficiency therefore low density
What is tacticity?
Stereoregularity or Spatial Arrangement of R Units
Along Chain
What are three types of tacticity?
Isotactic – all R groups on same side of chain Syndiotactic – R groups
alternate sides
Atactic – R groups randomly positioned
What happens if you increase crystallinity in polymers?
Increasing crystallinity in polymers can lead to improved strength, modulus, stiffness/brittle, lowering toughness.
What are liquid crystal polymers?
crystal formation promoted in liquid state (“self‐reinforced” plastics), high strength at high temperature.
What are semicrystalline polymers? (2)
- Some semicrystalline polymers form spherulite structures
* Alternating chain‐ folded crystallites and amorphous regions
What are two ways to increase polymer crystallinity? (2)
Heat treating
The molecular chemistry and chain configuration
How does heat treating increase polymer crystallinity?
– Slow cooling rate during solidification causes crystalline
regions to grow and % crystallinity to increase.
How does molecular chemistry and chain configuration increase polymer crystallinity? (4)
– an ordered structure is needed. The more irregular & random the repeated unit arrangement, the greater tendency for the development of non‐crystallinity
– Simple linear polymer form crystalline region even at high cooling rate
– Complex structure, and excessive branching, prevent crystallization.
– Networked and crosslinked polymers are almost totally amorphous.
How does drawing affect a polymer?(3)
– increases the elastic modulus (E) in the
stretching direction
– increases the tensile strength (TS) in the
stretching direction
– decreases ductility (%EL)
What is drawing a polymer? (2)
- stretches the polymer prior to use
- - aligns chains in the stretching direction
What happens if you do annealing after drawing? (2)
– decreases chain alignment
– reverses effects of drawing (reduces E and
TS, enhances %EL)
What happens if you do annealing to undrawn materials? (2)
- increased TS and E
- - Reduced %EL
What is fracturing in thermosets like?
brittle fracture
What is fracturing in thermoplastics like? (4)
transition between ductile and brittle, if
- reducing temperature, or increasing strain rate, the presence of sharp notch, or anything reducing Tg.
- Temperature falls below a specific level (0.75TG) polymers can behave like brittle solids (eg glasses and ceramics)
- Low ductility, high viscosity, sensitive to flaws. – Can lead to “crazing” – sometimes encouraged.
What does crazing do to a polmer?
Craze formation prior to cracking
– regions of localized plastic deformation
– formation of microvoids and fibrillar bridges
– molecular chains oriented (Spherulites), bridges elongated and break, causing growth and coalescence of micro‐voids
What are some benefits to polymers? (8)
– Low friction coefficients.
– Low modulus allows high degree of deflection (elastic) in contact
– Light
– Design favourable for flow, less joints..
– Easy and low cost in manufacture/process – Large varieties
– Additives
– Anti corrosion
