5. Block Copolymer Morphology

Question 1

‘Mixing’ enthalpy vs entropy; what does it depend on, what are its results?

Answer 1

• Enthalpy
  Depends on relative strength of intra- and intermolecular chain segment interactions (A<->A, B<->B, A<->B) Results in segregation or uniting
• Entropy
  Depends on DP, N = N^A + N^B
  Applied, results in Flory-Huggins parameter (allows determination of N^A and N^B)

Question 2

Flory-Huggins Interaction parameter (denoted ‘Chi’)

Answer 2

Describes enthalpic contributions
X = 0 -> v. similar int.
X > 0-> A-B are unfavourable
X < 0-> A-B are favourable
Temperature dependent: X~1/T

Question 3

Weak segregation limit

Answer 3

XN > 10
Minimal value for which microphase segregation can occur
Means that copolymer will assemble to form ordered phases w/ bulk periodicity
(strong seg. limit: XN > 50)

Question 4

Volume Fraction (f^A)

Answer 4

f^A = approx. N^A/(N^A + N^B)
f^A + f^B = 1

Question 5

min-max principle

Answer 5

• As little curvature as possible, as much chain entropy as possible
• i.e. the chains stretch out to some degree but above a certain limit of block imbalance interfacial curvature will occur

Question 6

Ordered phase morphologies

Answer 6

• Lamellar phase:
  (f^a = approx. 0.5), alternating layers of A, B
• Cylindrical hexagonal phase:
  (f^A = approx. 0.33), cylinders of minority block A in a matrix of B
• Body centred cubic phase:
  (f^A<0.25), Spherical domains of A in a matrix of B
• Gyroid (intermediate)

Question 7

Synthesised block copolymers

Answer 7

• Choice of microphase separating monomers allows design of specific nanopatterned soft materials
• Contrasting properties in domains…
  Compositional
  Conformational
  Morphological
  Functional

Question 8

Polymers that obey the weak segregation limit

Answer 8

• Microphase separate and self-organise into ordered phases with bulk periodicity
• domain a is 2xN^A wide

Question 9

Production of precise nano-patterns

Answer 9

• Nanolithography
• Photovoltaics