Polymer Types and Structures Flashcards
1
Q
What is a linear molecule and monomer?
Give examples.
A
- Linear molecules: such as hydrocarbons are molecules that are straight when bonded together.
- Monomers: atoms or molecules that are bonded together to form a more complex structure, such as a polymer.
- Very long chains can build up, based on repeating monomer units.
- There is a C-C backbone that is covalently bonded for almost all polymers (at least all plastics we consider).
- Tetrahedral bonds around C-atoms. These monomers join up to make polymers.
2
Q
What are polymers?
A
- Made up of giant molecules
- Covalent bonding within molecules between the carbons making a strong bond
- Van der Waals forces holding long molecule chains together, e.g. Polyethylene
- Many other linear polymers incorporating O, F and Cl
3
Q
What do polymer chains look like? What causes it to look like that?
A
- The 109-degree C-C bond angle defines a possible cone in 3D
- Very long and tangled chains exist in 3D
- Essentially we will never have a “simple” crystal structure, but we may get regular chain packing => polymer crystallinity.
- High degree of rotation between C-C bonds. Bond can rotate around.
- The motion of one chain may affect another chain as they tangle.
- Random sort of pathways of polymer chains.
4
Q
What are the four types of polymer chains?
A
- Linear
- Branched
- Network
- Crosslinked
5
Q
How does chain length and molecular weight impact polymers?
A
- A distribution of chain lengths will exist after a certain polymerisation process
- Linear polymer molecules are termed thermoplastics
- Deformation (chains sliding past each other) is easier at higher temperatures
- Tensile strength increases with (molecular weight) Mw - longer chains are more entangled (anchored better). This is because molecular weight is essentially the number of chains. while length means they can entangle easily.
6
Q
What is the equation for:
- Number average molecular weight, Mn
- Weight-average molecular weight, Mw
A
- Mn = number average molecular weight: chains divided into a series of size ranges, then the number fraction of chains within each range is determined.
- Mw = weight-average molecular weight: the weight fraction of molecules within various size ranges.
- There is a difference as some molecules may get very big and skew the average. The higher molecular weights will improve the mechanics of the polymers.
7
Q
How does polymer crystallinity, cause the results below?
A
- In cooling from the liquid state, at Tm the polymer may become an Amorphous or crystalline solid: Crystallinity provides order and that ordering results in a reduction in specific volume.
- Crystallisation is easier for smaller regular molecule chains.
- Molecular movement is more difficult in crystalline solids.
- Below Tg for amorphous and semicrystalline polymers, molecular movement (chain sliding) is prevented.
- Note: these structures are found in thermoplastics.
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- Chain organisation can occur in polymers. We can see these differences based on how they contract. For amorphous structures, we get a contraction in dimensions between Tg and Tm. Past, Tg no chain slip can occur.
- For semicrystalline solids, we really need to understand that chains become more organised and aligned. Contraction in the volume occurs as a result.
- A very large shrinkage occurs for crystalline solids. For the same reason as semi-crystalline structures.
- When past the Tm stage, the chains become more and more organised and aligned, so the Van Der Waal forces become stronger between the chains, preventing chain slip. At Tg, bonding between chain structures becomes so strong that no chain sliding can occur and the polymer becomes more brittle, as plastic deformation can’t really occur.
8
Q
What is the glass transition temperature, Tg, and special about it?
A
9
Q
What are the two most common crystalline structures for polymers?
A
- Lamellar twisted ribbon structure is the most common
- It’s very unlikely that all the polymer chains (all of which are somewhat different lengths) can pack completely together.
->Partial cyrstallisation occurs. - Crystalline polymers are stiffer and stronger due to close chain packing.
- Semicrystalline, where you have some regions of high crystallinity, as chains line up with each other, but also amorphous regions. These aren’t the same chain, there are generally many different chains that align.
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- The most common polymer crystalline structure is the spherulite
- 3D structure consisting of spokes of lamellar chain folded crystallites, separated by amorphous polymer regions
- Cross polarised light => Maltese cross pattern in individual spherulites.
- The nucleation site is where they start to grow out of, the most organised section, where these organised regions and amorphous regions grow out of.
10
Q
What are elastomers and how are they structured and work?
A
- Rubbers
- These are lightly crosslinked polymer chains, such that chain sliding (i.e. deformation) is still possible.
- The relative position of the chains is lightly pinned so that when the applied stress is released, the elastomer returns to its original shape.
- The chains “want” to be coiled due to entropy, rubbers are effectively entropy springs.
- These chains have interactions that want to be in a specific structure and position. By stretching the chains, you are aligning the chains up, up until the crosslinks prohibit the motion, and they will want to spring themselves back due to these crosslinks and entropy.
11
Q
What are thermosetting plastics and how are they structured and work?
A
- 3D structure of covalent bonds
- Higher stiffness and strength than thermoplastics, chains can’t slide
- Good dimensional stability, i.e. no creep as above.
- Cannot be remelted - network destroyed at high enough temperatures
- Brittle with poor impact properties - no chain sliding to allow for deformation
- Examples: epoxies.
