Chapter 3 - Methods for characterization of polymers Flashcards
What is size-exclusion chromatography? How does it work, and what calculations are done to obtain useful information?
In size-exclusion chromatography, a solution is poured through a column with swollen polymer beads with pores of different sizes. The larger particles will explore the large pores only, while the smaller particles will explore the small ones also. That means that the smaller particles will use more time to pass through the column. The time for different size polymers to come through is measured, and the time is converted to elution volume by means of a siphon. A calibration curve is used to match elution volume to molar mass.
What challenges exists with size-exclusion chromatography?
Choice of solvent, choice of column material (don’t want adhesion of polymer to walls).
What information is gained by size-exclusion chromatography?
The molar mass distribution, and then the number average molar mass and weight average molar mass from weighted integration of the peak.
Describe how osmometry works.
Two compartments with polymer solution and solvent respectively, separated by a semi-permeable membrane that is only permeable to the solvent. Due to differences in chemical potential between the polymer solution and the solvent, solvent will flow into the other chamber, increasing the pressure. This can be measured by having columns sticking up from both compartments, and measuring the difference in height of the polymer solution and solvent.
Measured at low concentrations, and extrapolated to 0 (to fulfill the van’t Hoff Law).
What is the van’t Hoff Law?
The van’t Hoff Law described the limit of the ratio between osmotic pressure and molar mass in the limit where the concentration goes to 0.
lim π/c = RT/M = RT/M_n (for polydisperse solutions).
c->0
What information is retrieved from an osmometry experiment?
The number average molar mass, as the osmotic pressure is a thermodynamic colligative property, that is only sensitive to number of polymers.
What information can we derive from a dynamic light scattering experiment?
We can look at the average relaxation time of the intensity fluctuations, which is determined by the diffusion coefficient (tau = 1/[2q^2D]). This can be related to the hydrodynamic radius through the Stokes-Einstein relation.
What is the typical ratio between the hydrodynamic radius and the radius of gyration in a theta or good solvent?
R_g/R_h ≈ 1.24-1.56.
