Questions from class - Lecture 5 Flashcards
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.
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/(6πeta*R),
and solved for the hydrodynamic radius R.
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.
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.
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.
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
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.
