Questions from class - Lecture 8

Question 1

What is UCST/LCST behaviour?

Answer 1

The LCST (Lower Critical Solution Temperature) is the temperature below which the components of a mixture are miscible for all compositions. The LCTS polymers become unsolvable at temperatures above the critical temperature.

This is opposite to the normal behaviour (UCTS), at which the components of the mixture are miscible for all compositions at high temperatures and phase separate at lower temperatures.

Question 2

How does the scattering of a polymer blend look like and which quantities can be determined from its shape?

Answer 2

Composition fluctuations in blend make us able to use scattering as the way of determining different parameters. By plotting the scattering as a function of q one can determine the Flory-Huggins parameter from the low-wavevector limit, S(0). At the high wavevector limit one can obtain the Ornstein-Zernike scattering function and the correlation length. The parameter ξ (correlation length) gives an estimate of the average distance between entanglements.

Question 3

Which mechanisms of phase separation are distinguished?

Answer 3

1. Nucleation and growth

2. Spinodal decomposition

Question 4

What is the Rouse time and how does it depend on the degree of polymerisation?

Answer 4

The Rouse time is the “average” time needed for a polymer to diffuse a distance equal to its own diameter in the Rouse model.

The Rouse time is proportional to the degree of polymerisation.

Question 5

Which situation is described by the Zimm model and what is the Zimm time?

Answer 5

The Zimm model best describes dilute polymer solutions, and states that the movement of one monomer causes a force on the solvent as well as a force on other monomers.

The Zimm time is the “average” time needed for a polymer to diffuse a distance equal to its own diameter in the Zimm model.