Polymer Mechanics

Question 1

How do thermoplastics deform?

Answer 1

• Covalent intra-chain bonds are strong
• Van der Waals forces are weak electrostatic forces which increase at low temperatures - thermally activated
• Linear polymers flow under load (chains slip past each other) particularly at high temperatures e.g. PVC
• Time-dependent molecular flow, more time for chains to rearrange and slip => easier plastic flow.

Question 2

How does temperature of a polymer (for semi-crystalline or amorphuos structure) affect it’s physical behaviour?

i.e. brittle, ductile, etc. Think in terms of Tg and Tm.

Answer 2

Question 3

Describe the deformation chain behaviour for an amorphous polymer.

Answer 3

• 1->2: Elastic deformation
• 3: Necking
  -Stable neck formation occurs when tangled amorphous polymer chains start to straighten out and pack closer together in the neck region.
  -This transmits stress more effectively to the unnecked portion as closer-packed chains are stiffer
• 4->7: Plastic flow
  -Alignment and flow of linear molecules
• ================================================
• Initially, we start to stretch these bonds up until this yield point.
• Then we start to form the neck. In the neck region, the chains start to line up, and the van der Waals forces between the chains start to become stronger. We get a higher level of bonding and the strength in the neck region will stop deforming. So the areas around the neck start to deform.
• This is why the neck continues to grow until all are aligned and a fracture occurs.
• Also, in the region of the neck, you start to form voids, which is why plastics might turn white or change opacity.

Question 4

Describe the deformation chain behaviour for a semi-crystalline polymer?

Answer 4

• Original structure, with amorphous regions and crystalline regions.
• The amorphous region will start to stretch originally.
• Then close to yield, you start to initiate shear in the crystalline regions. Causing some deformation
• Once past yield, the crystalline regions will start to break up.
• Then these broken bits will start to stretch out more, growing the neck further.

Question 5

Describe the stress-strain graph for themoset plastics, thermoplastics and elastomers. Noting their promanent feature.

Basically, ductility, brittleness, etc.

Answer 5

• Can see that the thermosets are brittle materials, no chain sliding, 3d structure
• Thermoplastic: shows the characteristic curve for thermoplastic in the Tg and Tm region
• Elastomer: will have low stress and high strain, until hits the point where it can no longer stretch and will snap.

Question 6

What is Viscoelastic deformation and what are the two main types of viscoelastic deformation?

Answer 6

• Viscoelastic deformation: is when a material exhibitis both viscous and elastic characteristics when deforming. This results in things like time dependencies for strain and stress when deforming.
• ==================================================
• Creep (not diffusion-based, but chain sliding based):
  -Increasing strain at constant load/stress
  -The stress on the polymer will allow chain sliding to occur, to form a gradual stretch of the material.
  -This is shown on the graph on right.
• =================================================
• Stress Relaxation:
  -Reduced stress at constant strain
  -Once an initial deformation is done and is held there, the stress peaks, but then chain sliding will occur and reduce the stress on the material.
  -The chains will restructure to reduce the stress on the material.
  -We use the Kelvin model to help explain this effect.
• ===============================================
• Extra clarifiers:
  -Creep will be a stress put on the material to cause the deformation in a fixed manner.
  -Stress relaxation is a deformation that results in stress which then causes a reduction in stress over time exponentially (as stress is the driving force of chain sliding and that is affecting stress).

Question 7

What is the relaxation modulus equation?

Answer 7

• Stress relaxation can be defined also in terms of a relaxation modulus: Er(t)
  -(see equation below)
• Temperature has an effect along with the time as time effects the stress level and therefore stress modulus and temperature effects the polymer structure and behaviour.

Question 8

How is the relaxation modulus affected by temperature? Give and describe the different characteristics and their behaviours.

Try to mention how amorphous and cystalline structures differ

Answer 8

Different regions based on temp and relaxation modulus for amorphous strucuture.
* Glassy -< Tg no molecular sliding
* leathery - Viscoeleastic (time-dependent) deformation.
* Rubbery - like an elastomer but no crosslinks only entanglements
* Rubbery flow: elastic and viscous components are present. Van der Waals forces starting to weaken.
* Viscous flow - independent chain motion, rotation and vibration

Crystalline polymers are stiffer and stronger.

-Red line = amorphous
-Green = crystalline structure.

Question 9

How can we dampen polymers? Why do we do it? How do different polymers do at dampining?

Answer 9

• There is a time-dependent “anelastic” deformation: non-linear stress-strain behaviour for some polymers, like thermoplastics.
  -Can see this for the viscoelastic response there will likely be a lag time in providing the energy back from the system once deformed, unlike in elastomers.
• A lag between stress and strain provides the damping capacity.
• With different responses, we get a lag between stress and strain due to the chain sliding.
• Thermosetting plastics have low damping capacity due to 3D network of bonds - chains can’t slide
• Thermoplastic plastics have a high damping capacity for Tg<T<Tm where molecular chains are mobile
• Temperature range for high internal damping may be increased by:
  -Mixing amorphous polymers with different Tg
  -Copolymerisation of suitable monomers.