Intrinsic Viscosity
Intrinsic Viscosity
Introduction
One of the most precise measurements in polymer science is also the simplest and cheapest. Intrinsic viscosity, which is measured from the flow time of a solution through a simple glass capillary, has considerable historical importance for establishing the very existence of polymer molecules. It also provides considerable physical insight. In this lab, each group will study the intrinsic viscosity of hydroxypropylcellulose, a common polymer derived from cellulose--hopefully at a different temperature.
Viscosity in general
For a discussion of viscosity, refer to the "HowTo" document on using the Wells-Brookfield cone and plate viscometer. The viscosity measured in a capillary viscometer is not obtained at a defined shear rate. Of several fixes to this problem, the simplest is simply to ignore it. This amounts to assuming that the fluid is Newtonian over the entire range of shear rates encountered by the fluid as it passes down the capillary.
The Ubbelohde capillary viscometer
The most useful kind of viscometer for determining intrinsic viscosity is the "suspended level" or Ubbelohde viscometer, sketched below:
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.
Use of the
Introduction
One of the most precise measurements in polymer science is also the simplest and cheapest. Intrinsic viscosity, which is measured from the flow time of a solution through a simple glass capillary, has considerable historical importance for establishing the very existence of polymer molecules. It also provides considerable physical insight. In this lab, each group will study the intrinsic viscosity of hydroxypropylcellulose, a common polymer derived from cellulose--hopefully at a different temperature.
Viscosity in general
For a discussion of viscosity, refer to the "HowTo" document on using the Wells-Brookfield cone and plate viscometer. The viscosity measured in a capillary viscometer is not obtained at a defined shear rate. Of several fixes to this problem, the simplest is simply to ignore it. This amounts to assuming that the fluid is Newtonian over the entire range of shear rates encountered by the fluid as it passes down the capillary.
The Ubbelohde capillary viscometer
The most useful kind of viscometer for determining intrinsic viscosity is the "suspended level" or Ubbelohde viscometer, sketched below:
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.
Use of the