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News USA:
Check out the latest USA News involving Viscotek.
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HTGPC
The Viscotek High Temperature GPC (HT-GPC) System is a revolutionary advanced detector system specifically designed for the characterization of polyolefins, natural and synthetic polymers, nanoparticles and other large molecules.
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Product Portfolio
Browse our product portfolio to discover which Viscotek products can help with your macromolecular characterization.
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Complete DSV Systems
For Dilute Solution Viscosity, Viscotek takes pride in being the only manufacturer capable of supplying you with a comprehensive system solution, regardless of your viscosity testing requirements. |
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GPC Theory: Universal Calibration |
In Conventional Calibration we discovered an inherent problem occurs when we analyze
a sample which is chemically different than the standards used to calibrate the
column. Unfortunately this is frequently the case, so to improve on Conventional Calibration we use an additional detector, the
Four-capillary Differential Viscometer Detector, and a technique called Universal Calibration.
As previously mentioned in Conventional Calibration and Separation,
the principle behind GPC is that macromolecules are separated based on their Hydrodynamic Radius
or Volume. If this is the case, to accurately
calibrate a column we need to form a relationship between the Hydrodynamic Volume and
the Retention Volume (Universal
Calibration), as opposed to the Molecular Weight and Retention Volume (Conventional
Calibration). The Viscometer Detector makes this possible because it directly measures
Intrinsic Viscosity (IV), which is inversely proportional to the molecular density
of the polymer coil.
The product of molecular mass and Intrinsic Viscosity provides the Hydrodynamic
Volume:
MW • IV = 5/2 • NA • Vh
Because a Viscometer Detector coupled with a concentration detector (RI or UV/VIS) can give
us a direct measurement of Intrinsic Viscosity (IV), we can now create a calibration
curve of Log(Vh) vs. Retention Volume (RV).
The procedure is as follows:
- Inject a series of Narrow Standards of known Molecular Weight.
- Measure the Retention Volume of the resulting peak apex.
- Calculate the IV of the peak.
- Construct a calibration curve of Log(MW • IV) vs. Retention Volume.
Now when we analyze our unknown sample, at each data point at RVi, we
can look up on the calibration curve to find Y = Log(MWi • IVi).
Since
we calculate the IVi from the viscometer we can deduce that MWi =
10Y/ IVi.
We can now obtain MWi by simple re-arrangement. From here, we can use the equations described in Conventional Calibration to determine the Mn, Mw and Mz averages.
 In addition to being able to determine true molecular weight distributions (Mn,
Mw and Mz), molecular size (Rh and Vh) and Intrinsic Viscosity (IV), the Viscometer
Detector also gives us important
structural information on conformation, branching
and aggregation.
As already stated, the Intrinsic Viscosity of polymer
samples is measured by means of GPC viscosity detectors. Intrinsic viscosity, which is the
inverse density of the polymer coil in solution, represents a direct and sensitive structural parameter and, therefore, a traditional
parameter in the polymer industry.
The well-known Mark-Houwink plot:
IV = K • MWa
can be obtained by means of the double-logarithmic plot
of intrinsic viscosity against MW. The Mark-Houwink plot is the
central plot used for the analysis of polymer structure.
It reflects structural
changes in the polymer, such as
branching and chain rigidity. The
slope, described by the Mark-Houwink exponent “a” can vary between 0 for
solid spheres and 2 for rod-shaped structures.
Changes in slope in the Mark-Houwink plot are used to identify when branching occurs.
This is described in more detail in Polymer Structure Theory.
Almost all the benefits drawn from the technique of Universal Calibration can also
be obtained by the method of triple or advanced detection
which bestows one additional unique benefit: expediency; no lengthy column calibration
procedure is required. |
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