Polymers & Solid State

Question 1

Describe the definition of a polymer at molecular level and point out the most distinct feature of a polymer in contrast to metals and ceramics from a molecular perspective.

Answer 1

• Many units of monomers covalently bonded together to form a long chain.
• Distinct feature is its long chainlike molecules, which is different than the basic metallic and ionic atoms in metals and ceramics respectively.

Question 2

Describe the difference between thermoplastic and thermosetting polymers in terms of behaviour, solid material structure, and reversibility of their formation.

Answer 2

• Thermoplastic polymers can turn into viscous liquid by heating and can then restore to a solid after cooling. This process is reversible and used repeatedly to shape it.
• Thermosetting polymers start as a liquid of monomers and can turn into hard solid by heating. This process is irreversible.
• Thermoplastic polymers are typically made up of linear or branched molecules without crosslinking.
• Thermosetting polymers contain cross-linked molecules.

Question 3

Which type of polymers will have a chance to crystallise; thermoplastics or thermosets?

Answer 3

• Thermoplastic polymers may partially crystallise if their molecules have ordered molecular structure and regular spatial arrangement.
• Thermosetting polymers cannot crystallise due to cross-linked structure.

Question 4

What is the number average molar mass formula and what is its units?

Answer 4

Mn = n*M/n or Mn = W/m given in g/mol

Mn = number average molar mass
n = moles of polymer
M =  molar mass

Where n = W/M

Question 5

What is the degree of polymerisation formula?

Answer 5

Dp = Mn of fraction / M molar mass of monomer

Question 6

If you were tasked to improve the toughness/strength of a polymer by changing weighting of the fractions, what would you suggest and why?

Answer 6

Increase the fraction of high molar mass (M) as it gives better mechanical properties (e.g. strength and toughness) due to increased chain interaction and entanglement

and/or decrease the fraction of low molar mass

Question 7

If you were tasked to improve the stiffness of the polymer, what would you suggest and why?

Answer 7

1. Increase average molar mass (increased chain interaction and entanglement).
2. Choose a polymer that can crystallise (isotatic or syndiotatic polystyrene).
3. Incorporate reinforcing materials such as glass/carbon fibres as fibres are stiffer than the matrix.

Question 8

Describe how the Young’s modulus changes in relation to their crystallinity and why?

Answer 8

The modulus will increase with crystallinity, there are stronger interchains, which make them more resistant to deformation.

Question 9

What is the relationships for density using the rules of mixtures? State the volume fraction crystallinity formula as well as the amorphous and crystalline density.

Answer 9

Row = Row[c]*Xv + Row[a] *(1-Xv)

Xv = volume fraction crystallinity = V[c] / V

Row[c] = 1 / V[c]

Row [a] = 1 / V[a]