What characterizes thermoplastics?
- Long polymer chains (spaghetti)
- When applying heat above Tg - softens and melts
- Can be reheated and reshaped
- Is amorphhous or partly crystalline
What sets polymers apart from conventional materials?
- Large time dependent deformations
- Mech. properties change radically with temperature
- Anisotropy, especially for polymer composites
What characterizes thermosets?
- Cross-linked chains
- Cannot be reshaped
- When heated → softens and then burns and degrades
- Generally harder and stronger + better dimensional
stability than thermoplastics. - Amorphous
What characterizes elastomer/rubbers?
- 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
How does covalent bonds work, and where in polymers are they present?
- 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
How does secondary bonds work, and where are they present?
- By coulumbic attraction betweet atomic or molecular dipoles
- Bonds together polymer molecules
- Has the lowest bonding energy
Why does thermoplastics have lower Tg than thermosets?
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)
What influences the machanical behaviour of polymers the most?
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
What characterizes a polymer before Tg?
- Hard, glassy state
- Small strains to failure (a few %) - brittle
- Higher density
- Low damping effect
- Polymer chains are frozen
- Higher E
What characterizes a polymer as the Tg increases?
- 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
What happens with the molecular structure of polymers at Tm?
The polymer chains become more disordered, and the crystalline regions break down.
What is viscoelasticity?
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.
Describe what the linear spring reperesent in a viscoelastic model, and how it behaves under stress/strain.
- Represents the elastic behaviour with the constant E. (σ=E*ɛ)
- Instant stress
<--->Instant strain - Constant stress
<-->Constant strain
Describe what the dashpot reperesent in a viscoelastic model, and how it behaves under stress/strain.
- Represents viscous behaviour wiht the constant μ (σ=μ*ε ̇ )
- Instant stress
<-->Zero instant strain - Constant stress
<-->Ever increasing strain - Constant strain
<-->Zero stress
When is a material viscoelastic?
Which temperature?
At temperatures around or above its glass transition temperature (Tg), and beow the melting temperature (Tm)
What is stress relaxation, and why does it occur?
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.
What are some factors that influence stress relaxation in polymers?
- 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
What is the maxwell element?
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.
Constitutive equation for the Maxwell element?
\10/3
What is the idealized relaxation response for thermoplastics and thermosets?
- 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 → ∞
What is the Wiechert model?
A generalized Maxwell model with a free spring with the modulus 𝐸∞.
