Polymers

Question 1

What characterizes thermoplastics?

Answer 1

• Long polymer chains (spaghetti)
• When applying heat above Tg - softens and melts
• Can be reheated and reshaped
• Is amorphhous or partly crystalline

Question 1

What sets polymers apart from conventional materials?

Answer 1

• Large time dependent deformations
• Mech. properties change radically with temperature
• Anisotropy, especially for polymer composites

Question 2

What characterizes thermosets?

Answer 2

• Cross-linked chains
• Cannot be reshaped
• When heated → softens and then burns and degrades
• Generally harder and stronger + better dimensional
  stability than thermoplastics.
• Amorphous

Question 3

What characterizes elastomer/rubbers?

Answer 3

• Lightly cross-linked(ca. one link every 100 monomer unit)
• non-linear elasticity to very large strains
• Tg is well below room temperature
• Amorphous, except at high strains

Question 4

How does covalent bonds work, and where in polymers are they present?

Answer 4

• Formed by sharing of electrons
• The backbone of the molecular structure is a long chain of covalently bonded atoms.
• Cross-links between chains are also covalent

Question 5

How does secondary bonds work, and where are they present?

Answer 5

• By coulumbic attraction betweet atomic or molecular dipoles
• Bonds together polymer molecules
• Has the lowest bonding energy

Question 6

Why does thermoplastics have lower Tg than thermosets?

Answer 6

Thermoplastics have weaker bonding (Secondary bonds). When the secondary bonding is diminished -> increasing molecular motion. Adjacent chains can slip.

Thermosets have crosslinking which is covalent. (stronger bond)

Question 7

What influences the machanical behaviour of polymers the most?

Answer 7

Mechanical behaviour is mainly determined by intermolecular secondary bonds between the chains, not by strong covalent bonds. As temperature is raised, these weaker secondary bonds are first gradually overcomed

Question 8

What characterizes a polymer before Tg?

Answer 8

• Hard, glassy state
• Small strains to failure (a few %) - brittle
• Higher density
• Low damping effect
• Polymer chains are frozen
• Higher E

Question 9

What characterizes a polymer as the Tg increases?

Answer 9

• Rubbery or viscous state
• Large strains to failure - tough
• Lower density
• Higher damping effect (before melting)
• Polymer chains gain more mobility (sliding, entanglements, cross-linking)
• lower E

Question 10

What happens with the molecular structure of polymers at Tm?

Answer 10

The polymer chains become more disordered, and the crystalline regions break down.

Question 11

What is viscoelasticity?

Answer 11

Viscoelasticity refers to the combination of viscous (flow-like) and elastic (spring-like) properties exhibited by certain materials. These materials display characteristics of both fluids and solids, and their behavior depends on the rate and duration of the applied stress or strain.

Question 12

Describe what the linear spring reperesent in a viscoelastic model, and how it behaves under stress/strain.

Answer 12

• Represents the elastic behaviour with the constant E. (σ=E*ɛ)
• Instant stress <---> Instant strain
• Constant stress <--> Constant strain

Question 13

Describe what the dashpot reperesent in a viscoelastic model, and how it behaves under stress/strain.

Answer 13

• Represents viscous behaviour wiht the constant μ (σ=μ*ε ̇ )
• Instant stress <--> Zero instant strain
• Constant stress <--> Ever increasing strain
• Constant strain <--> Zero stress

Question 14

When is a material viscoelastic?

Which temperature?

Answer 14

At temperatures around or above its glass transition temperature (Tg), and beow the melting temperature (Tm)

Question 15

What is stress relaxation, and why does it occur?

Answer 15

When a material is subjected to a constant deformation or strain, it experiences a decrease in internal stress over time, while maintaining a constant strain.
Occurs due to the movement of polymer chains or molecular segments within the material, which results in a decrease of internal stress.

Question 16

What are some factors that influence stress relaxation in polymers?

Answer 16

• Temperature: Elevated temperature tends to accelerate the relaxation process (above Tg –> more viscous)
• Time: The longer the material is under stress, the more prounonced the relaxation
• Molecular structure: Longer, more entangeled chains tend to exhibit slower relaxation. If crooss-linked –> slow realaxation

Question 17

What is the maxwell element?

Answer 17

A viscoelastic model used to represent the behavior of materials that exhibit both elastic and viscous characteristics. It is comprised of two key components in series: a spring (representing elasticity) and a dashpot (representing viscosity). This combination allows it to capture the time-dependent deformation response observed in viscoelastic materials.

Question 18

Constitutive equation for the Maxwell element?

Answer 18

\10/3

Question 19

What is the idealized relaxation response for thermoplastics and thermosets?

Answer 19

• Thermoplastic: The stress and the relaxation modulus approach zero value after a long time
• Thermoset: The stress, and relaxation modulus approach a constant non-zero value as t → ∞

Question 20

What is the Wiechert model?

Answer 20

A generalized Maxwell model with a free spring with the modulus 𝐸∞.