Questions from class - Lecture 5 Flashcards Preview

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Flashcards in Questions from class - Lecture 5 Deck (7):
1

What is the signal measured in dynamic light scattering?

The signal measured in dynamic light scattering is the fluctuations of the scattered intensity as a function of time.

2

How is the hydrodynamic radius of a single polymer determined?

The average relaxation time of the scattering intensity is related to the diffusion constant by

tau = 1/2(q^2*D).

This can be plotted into the Stokes-Einstein relation,

D = kT/(6*π*eta*R),

and solved for the hydrodynamic radius R.

3

What is a real chain?

A real chain is a chain in which the interactions between the monomers are also included. This means that the monomers can't occupy the same volume.

4

How does the potential between two monomers look like? Which possibilities are there?

The potential between two monomers is equal to 0 at ∞ distance. At r -> 0, there is a hard-core repulsion. For r-values larger than this there are three possibilities:

i) The monomers like the solvent more than each other, thus giving an additional repulsive term.
ii) The monomers is indifferent to the monomer and solvent, and the potential is 0 as the hard-core repulsion dissipates.
iii) The monomers like each other more than the solvent, thus giving an additional attractive term.

5

How is the excluded volume calculated from the potential?

The excluded volume is defined as the negative of the intergral of the Mayer f-function.

The Mayer f-function is equal to the relative probability of finding two monomers a distance r apart minus 1.

6

Which solvent qualities are distinguished? What is the excluded volume for the different cases?

Athermal solvent - v = b^3
Good solvent - 0 < v < b^3
Theta solvent - v = 0 (@ theta-point)
Poor solvent - -b^3 < v < 0
Non-solvent - v ≈ -b^3

7

How does the end-to-end distance scale with the degree of polymerization for a chain in a good solvent?

In a real chain in a good solvent, the end-to-end distance scales with N^3/5.