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Molecular weight
of a polymer can be measured by several techniques and are separated into 2
categories: absolute and relative. Absolute methods, such as osmometry and
light scattering, are based on theoretical ovservations for molecular weight
calculations. Relative methods, which include intrinsic viscosity and size
exclusion chromatography, depend on calibration standards. Relative methods are
usually used as it is easy and have a low cost of operation.

 

Viscosity
measurements are used to determine the molecular weight of a polymer by
relating the polymer size to the viscosity. Due to frictional forces at the
walls of the capillary, a pure solvent experiences shear rate under laminar
flow. Depending on the size of a polymer particle in solution, it will
experience different shear rates as it flows through the capillary. This
increases the drag and viscosity, thus viscosity is related to the polymer
size. The relative viscosity, ?rel, of the polymer solution is given
by the ratio of the solution viscosity, ?, to the solvent viscosity, ?o.
Intrinsic viscosities can be calculated experimentally by using the Huggins (Equation
1) and Kraemer (Equation 2) relationships.

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   Equation 1: Huggins                                       Equation 2: Kraemer

 

Using an Ubbelohde
capillary viscometer, the viscosity is measured by the time it takes for the
solution to pass through the viscometer. The specific viscosity is measured
using equation 3. After obtaining the intrinsic viscosity, the viscosity
average molecular weight, Mv, can be calualated using the
Mark-Houwink equation (equation 4).

 

                                                      

                                               

Equation 3: Specific viscosity           Equation 4: Mark-Houwink equation

 

In this experiment, the
intrinsic viscosity was obtained by plotting both the Huggins and Kraemer
equations and the viscosity molecular weight of the sample was then calculated.

 

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