Fast cooling of liquid polymer leads to restriction of motion(of chains) = crystallinity
4 of 25
What type of bonding are in 'crystalline' polymers?
Van der waals and hydrogen bonding,long chains are packed: usually two regions which are amorphous and crystalline respectfully = polycrystalline structure
5 of 25
What is 'degree of crystallinity' and what does it tells us?
Fraction polymer in crystalline region. if: <10%= amorphous , >10% = semicrystalline polymer, High crystallinity = "crystalline polymer"
6 of 25
How do spherulites form?
Successive lamellar layers of polymer chains are laid down with growth faster in one preferred direction. The other two axes grow at the same speed. The crystals grow in helical strands radiating from a nucleation point.
7 of 25
What determines the size of a spherulite?
Determined by impingement which is related to the nucleation rate. At low temperatures driving force for nucleation is large creating more spherulites = more impingement.
8 of 25
What are the effects of crystallinity on a polymer?
Crystalline regions are denser, they scatter light and increase the strength+stiffness of the polymer. They also alter the thermal properties of the polymer.
9 of 25
What affects crystallinity?
Strength of Intermolecular forces, architecture/shape of chains (branched/bulky side group), Rate of cooling from molten state.
10 of 25
Define 'glass transition temperature' (T_g)
Temperature at which, below: polymer is glassy and behaves in a brittle manner.(chains are in place). And above: polymer is rubbery,elastic (chains can move/slide over each other)
11 of 25
How is the processing temperature determined?
It needs to be above the T_g so that the polymer can be formed. T_m: T where crystalline regions in polymer melt ,T_m = 1.3-2T_g
12 of 25
Name the properties of a polymer between T_m and T_g
Useful stiffness and good toughness (amorphous region is reinforced by crystalline regions)
13 of 25
Name the properties of a polymer below T_g
Slightly stiffer than amorphous polymer. (Elastic deformation occurs)
14 of 25
Name the properties of a polymer above T_g
Permanent deformation can occur. Strength increases due to necking.
15 of 25
What occurs during necking?
Strength increases: chain alignment when neck forms (low cross-sectional area during necking --> less force required to deform)
16 of 25
State the general processing steps used form a polymer
It makes the polymer profile thinner. (enters haul - off rollers after the extruder 'die' is used to set the initial profile shape whilst the polymer is still molten.
19 of 25
What is the significance of orientation?
It improves tensile strength in the orientation direction.
20 of 25
How can uniaxial orientation be achieved?
Using 'draw-down'/haul-off.
21 of 25
What is biaxial orientation and why can it be desirable?
Orientation in 2 axes. It is important for films as it improves the strength in two directions.
22 of 25
Explain how a tenter frame works
Creates Biaxial orientation. Polymer enters the extruder die and the Haul-off rollers give stretch in one direction(oven heated above T_g). Tenter hooks give stretch in other direction.
23 of 25
Why is would it be useful to reheat a mould to just above the T_g?
To add more detail to the plastic (e.g bottles). as above T_g allows deformation but above T_m preform would melt
24 of 25
How does impingement affect the properties of a polymer?
There is a large effect on optical properties as large spherulites scatter light.
25 of 25
Other cards in this set
Card 2
Front
Name the types of polymers
Back
Thermoplastics:flow when heated;Thermosets: cannot flow,crosslinked;Elastomers:lightly crosslinked,
Card 3
Front
How are properties of polymers created
Back
Card 4
Front
How is crystallinity in polymers acheived?
Back
Card 5
Front
What type of bonding are in 'crystalline' polymers?
Comments
No comments have yet been made