Materialteknik 2017 Flashcards
Vad är molekylvikten ett mått på hos polymerer?
Polymerkedjans längd
Vad är det för skillnad mellan en sampolymer och en polymerblandning?
Sampolymerer är polymerer med två eller fler monomerer i samma kedja (polymeriserade) medan polymerblandningar innehåller två eller fler olika typer av polymerer som är blandade (ej polymeriserade)
Hur påverkar temperaturen viskositeten hos polymerer?
En ökning i temperatur orsakar en minskning av viskositeten.
What are polymeric materials?
Large molecules, repetitive units
What molecular forces hold a polymer component?
- Strong forces C-C (covalent bonds; backbone).
2. Weak forces (van-der-Waals; intermolecular). These dominate the strength of a polymer component
How do we quantify the size of polymer molecules?
- The molecular weight
- The molecular number
- Weight averaged
- The polydispersity index
How are polymers defined in terms of structure/conformation/configuration/arrangement?
rotation around the carbon bonds ->
- Flexibility
- Tacticity
- Branching/Topology
What are the main types of polymers based on their molecular arrangement and their characteristics?
Thermoplastics, can be reshaped and remelted:
- Amorphous, random orientation of chains
- Semi-crystalline, defined by degree of crystallinity which dictates how organized the polymer i
Cross-linked polymers, obtained via crosslinking/curing and cannot be reshaped or remelted:
- Thermosets
- Elastomers
What is polymerization?
The process by which monomers are added to each other through a chemical reaction to become a
polymer
What can polymers contain?
- Copolymers
- Blend
- Thermal stabilizers
- Pigments
- Additives
What are the thermal characteristics of polymers and what do thermal properties tell us about the microstructure of polymers?
As a polymer is heated the distance between the molecules increases as the vibration amplitude increases. At certain temperatures, the material flows at macroscopic level.
What transition temperatures are there?
- Tg: Glass transition temperature.
- Melting temperature.
For amorphous: usefulness below Tg.
For semi-crystalline: usefulness between Tg & Tm.
For thermosets: usefulness below Tg
What can you say about the thermal conductivity of polymers?
Polymers are excellent thermal insulators, best: foams.
What are the fundamental concepts of polymer melt flow?
- Both amorphous and semi-crystalline polymers have similar structure in the melt state
- polymers are viscoelastic when deformed in the melt state (viscous part: molecular friction between molecules)
> small/slow deformations -> no change in orientation; large/fast deformations: molecular orientation in the flow direction
What are the flow characteristics of polymers in the melt state?
- Thermoplastic polymers: viscosity versus shear rate: polymer materials exhibit shear-thinning behavior; the viscosity function has two regions.
- Thermosets: generally start from a viscous liquid (viscosity is independent of the shear rate), during curing they become viscoelastic (with shear thinning behavior) until they become solid.
How do we measure rheological properties?
Melt flow indexer and the melt flow index
What are the main processing methods for polymers and how do they work?
- Extrusion, parts: https://i.imgur.com/uRvEW3e.png
2. Injection modling, parts: injection unit (think extruder), mold, clamp
Explain the solidification of thermoplastics & p-V-T diagrams
As temperature is lowered, polymers gradually loose their ability of move freely. Influencing parameters: Pressure & Temperature.
Higher cooling rate results in ‘freeze-in’
free volume between molecules
p-V-T: https://imgur.com/a/EN8H2
What post-solidification problems can occur in polymers?
- Thermoplastics: residual stresses, warpage, shrinkage, anyisotropy
- Thermosets: non-uniform
curing, voids, cracks, imperfections inside the part.
What is the Boltzmann superposition principle?
The deformation of a polymer component is the sum/ superposition of all strains that result from various loads acting on the part at different times.
Explain creep & stress relaxation
Stress relaxation: A material held at a constant strain will require less stress to hold it there with time.
Creep: A material held at a constant stress will strain over time.
https://i.imgur.com/FjUwuCa.png
What is tacticity in polymers? Name two tactic configurations that can be achieved and represent them.
Tacticity - side groups can be arranged in different directions.
Ex of configurations:
Isotactic - All arranged on one side
Atactic - Arranged randomly
Syndiotactic - Alternatingly arranged
Briefly explain the working principle of film casting as secondary shaping operation for extrusion. Use a sketch and brief description
thereof.
https://i.imgur.com/g0xPXPv.png
Briefly explain the working principle of film casting as secondary shaping operation for extrusion. Use a sketch and brief description
thereof.
https://i.imgur.com/g0xPXPv.png
What is ‘necking’ in polymers and in what type(s) of polymer(s) does it occur?
‘necking’ - localized reduction in
cross-sectional area.
Occurs in semi-cristalline polymers.
Consider the morphology of semi-crystalline thermoplastic polymers. Name the possible state of the crystals
and how can they be obtained.
- Single crystals - formed in solution
- Spherulites - form at the solidification of semi-crystalline materials (no deformation)
- Deformed crystals (ordered lamellae) - if a semi-crystalline material is deformed during crystallization
.4 Shish-kebab (plate and ribbon structures) - shear deformation during solidification
Describe 4 different types of copolymers
- Alternating
- Block
- Random
- Graft
What is the effect of strain rate on the occurrence of crazes in thermoplastic amorphous polymers at room temperature?
- At high def speeds: c. are small, appear shortly before failure, or do not appear at all!
- At low def speeds: c. are large and appear earlier during loading
Förklara lite om isokront och isometrisk krypning.
Isokront: x-axel= töjning, y-axel= spänning. linje= tid
Isometrisk: x-axel= tid, y-axel= spänning. linje= töjning
Vad är den eutektiska & eutektoida punkten?
Eutektisk punkt: Den lägre gränsen för det likvida enfasområdet, utgörs av en punkt där likviduslinjerna möts. Detta är den eutektiska punkten.
Eutektoid punkt: Den lägre gränsen för ett fasta enfasområde, utgörs av en punkt där fasgränslinjerna sammanfaller i ett V. Detta är den eutektoidiska punkten.
