# Mrs/Fragment 073 01

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Use of the Ubbelohde viscometer

Capillary viscometry is conceptually simple: the time it takes a volume of polymer solution to flow through a thin capillary is compared to the time for a solvent flow. It turns out that the flow time for either is proportional to the viscosity, and inversely proportional to the density

The relative viscosity is defined to be the ratio ηsol'nsolvent . For most polymer solutions at the concentrations of interest, ρsol'n / ρsolvent ࣈ 1 . Thus, to a very good approximation, the relative viscosity is a simple time ratio:

"specific viscosity" is also defined to be the fractional change in viscosity upon addition of polymer:

Both ηrel and ηsp depend on the polymer concentration, so to extract the "intrinsic" properties of the polymer chain itself, one must extrapolate to zero concentration. Measuring at zero concentration (c=0) would be useless, but this concept of extrapolating to c=0 is very important in polymer characterization and in thermodynamics generally. The two quantities that are commonly plotted versus concentration and extrapolated to c=0 are ηsp and c-1ln (ηrel). A typical plot is Figure 5.5.

Use of the Ubbelohde viscometer

[...] Capillary viscometry is conceptually simple: the time it takes a volume of polymer solution to flow through a thin capillary is compared to the time for a solvent flow. It turns out that the flow time for either is proportional to the viscosity, and inversely proportional to the density.

We define the relative viscosity to be the ratio ηsol'nsolvent. For most polymer solutions at the concentrations of interest, ρsol'n / ρsolvent ࣈ 1. Thus, to a very good approximation, the relative viscosity is a simple time ratio:

We also define a "specific viscosity" to be the fractional change in viscosity upon addition of polymer:

Both ηrel and ηsp depend on the polymer concentration, so to extract the "intrinsic" properties of the polymer chain itself, one must extrapolate to zero concentration. Measuring at zero concentration (c=0) would be useless, but this concept of extrapolating to c=0 is very important in polymer characterization and in thermodynamics generally. The two quantities that are commonly plotted vs. concentration and extrapolated to c=0 are ηsp and c-1ln (ηrel). A typical plot is shown below

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