3. Polymer Structure

Question 1

Ways to classify polymer structures

Answer 1

• Composition
• Constitution
• Configuration
• Conformation

Question 2

Number average Mw

Answer 2

M^n = sum(n^i*M^i)/sum(n^i)

Question 3

Weight average molecular weight

Answer 3

M^w = sum(n^iM^i^2)/sum(n^iM^i)

Question 4

M^i, n^i

Answer 4

M^i = {molecular weight of a polymer chain}
n^i = {no. chains w/ a given Mw}

Question 5

Dispersity

Answer 5

D = M^w/M^n
A measure of how controlled a polymerisation is; influences properties of polymer

Question 6

How does structure of repeat unit influence properties?

Answer 6

• Nature and strength of interactions
• polymer backobone flexibility

Question 7

Types of polymer by constitution

Answer 7

• Linear (e.g. HDPE, UHMWPE)
  Very hard, high mp
• Branched (long, short, hyper-)
• Crosslinked network polymers

Question 8

Crosslinked Network polymers (description, thermal properties, example)

Answer 8

• Interconnectivity between all polymer segments
• All thermosets (can’t dissolve or form a liquid)
• Bakelite (hard), rubber (soft), polyisoprene

Question 9

Homo- vs Copolymers

Answer 9

• Homo: Single type of repeat unit e.g. PS, PMMA
• Co: 2+ distinct types of repeat unit e.g. PS-co-PMMA

Question 10

Types of copolymers

Answer 10

• Random
• Strictly alternating
• (linear) diblock
• (linear) tri…block
• Graft

-> all have different physical properties

Question 11

Tacticity

Answer 11

• Isotactic (same relative configuration)
  Tough w/ high crystallinity, Pm = 1
• Syndiotactic (alternating relative configuration)
  Softer with lower cyrstallinity, Pr = 1
  For polymerisation of alkenes with large side groups, sterics produce predominantly syndiotactic polymer
• Atactic (random relative configuration)
  Soft, amorphous, Pm/Pr = 0.5

Question 12

Configuration: Linkage

Answer 12

• Racemic linkage: change in stereochemistry between 2 stereocentres, Probability = Pr
• Meso linkage: no change in stereochemistry between 2 stereocentres, probability - Pm

Question 13

Explain the random coil

Answer 13

• The conformation changes continually (Gauche, anti etc.)
• Cooperative movement of chain segments
• Overall movement is slow
• 3^n possible conformations
• results in a ‘random coil’

Question 14

Random flight statistics

Answer 14

Rn = end-to-end distance,
l = step length
n = no. steps

mean (square(Rn)) = n*square(l)

Question 15

Random flight statistics in solution

Answer 15

• sqrt(mean(square(Rn)) = a * sqrt(n)*l
• a is alpha coefficient, measuring strength of interaction between solvent and polymer
• If alpha is less than 1, segment-segment interactions are favoured and the random coil collapses
• If alpha is greater than 1, segment-solvent interactions are favoured, random coil widens
• If alpha = 1, theta-state achieved; both interactions are of same strength and coil is as if in unperturbed state