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 Autor Paul Russo Titel Intrinsic Viscosity Jahr 2008 URL http://web.archive.org/web/20100702073803/http://macro.lsu.edu/HowTo/IntrinsicVisc.doc Literaturverz. no Fußnoten no Fragmente 3

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The Ubbelohde capillary viscometer

The most useful kind of viscometer for determining intrinsic viscosity is the "suspended level" or Ubbelohde viscometer, sketched below:

The Ubbelohde capillary viscometer

The most useful kind of viscometer for determining intrinsic viscosity is the "suspended level" or Ubbelohde viscometer, sketched below:

 Anmerkungen The source is not mentioned. To be continued on the next page. The sketches of the viscometer are not identical. A source for the sketch could be [1]. Sichter (Hindemith), SleepyHollow02

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The viscometer is called "suspended level" because the liquid initially drawn into the small upper bulb is not connected to the reservoir as it flows down the capillary during measurement. The capillary is suspended above the reservoir. In conjunction with the pressure-equalization tube, this ensures that the only pressure difference between the top of the bulb and the bottom of the capillary is that due to the hydrostatic pressure i.e., the weight of the liquid. Other designs, e.g., the Cannon-Fenske viscometer, do not provide for this, and will give erroneous results in an intrinsic viscosity determination. Such viscometers are useful in other experiments--e.g., checking the stability of some polymer solution, where one is only interested in measuring a change in the flow time.

Basic Relations for Capillary Viscometry

Here is presented the basic relation of capillary viscometry, which is known as Poiseulle's law [250].

Where:

· Q is the volumetric flow rate through the capillary in cm3/s;

· P is the pressure head forcing the liquid through the capillary (usually, just the hydrostatic pressure of the liquid itself);

· R is the radius of the capillary;

· l is the length of the capillary; and,

· η is the viscosity

The bulb volume in the Ubellohde viscometer is fixed. Thus, the flow rate, Q, is just inversely proportional to the time between marks. Since P is usually the hydrostatic pressure, which is proportional to the density of the fluid, we have:

η ∝ tρ

This simple relationship is the "ideal gas law" of capillary [sic]

[250] S.F. Sun, Physical chemistry of macromolecules: basic principles and issues, 2nd edition - Hoboken, NJ : Wiley, 2004

The viscometer is called "suspended level" because the liquid initially drawn into the small upper bulb is not connected to the reservoir as it flows down the capillary during measurement. The capillary is suspended above the reservoir. In conjunction with the pressure-equalization tube, this ensures that the only pressure difference between the top of the bulb and the bottom of the capillary is that due to the hydrostatic pressure--i.e., the weight of the liquid. Other designs, e.g., the Cannon-Fenske viscometer, do not provide for this, and will give erroneous results in an intrinsic viscosity determination. Such

[page 2]

viscometers are useful in other experiments--e.g., checking the stability of some polymer solution, where one is only interested in measuring a change in the flow time.

[page 9]

Appendix 1. Basic Relations for Capillary Viscometry

Here is presented the basic relation of capillary viscometry, which is known as Poiseulle's law. [...]

where:

• Q is the volumetric flow rate through the capillary in cm3/s;

• P is the pressure head forcing the liquid through the capillary (usually, just the hydrostatic pressure of the liquid itself);

• R is the radius of the capillary;

• l is the length of the capillary; and,

• η is the viscosity

[...]

The bulb volume in the Ubellohde viscometer is fixed. Thus, the flow rate, Q, is just inversely proportional to the time between marks. Since P is usually the hydrostatic pressure, which is proportional to the density of the fluid, we have:

η ∝ tρ

This simple relationship is the "ideal gas law" of capillary viscosity (i.e., you should remember it!).

 Anmerkungen The source is not given. The reference [250] does contain Poiseulle's law, but not the text parallels. Sichter (Hindemith) Singulus

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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

 Anmerkungen The source is not mentioned. Sichter (Hindemith), SleepyHollow02