做普适校正一定要用粘度检测器吗？

应助达人

在线粘度检测器可以得到特性粘度IV=K*M^a或者分子量密度的倒数，流体力学半径。
普氏校正本来的就相对的分子量，PS样品的a和K值是能查到的，但可能你的样品不一定有，一般就用0.7和14代入计算。粘度检测器或者现在流行的多检测器，只有PL和马尔文有。粘度检测器是可以直接得出特性粘度IV，流体半径，得到mark-houwink曲线的k和a值。

应助达人

Universal Calibration

The concept of Universal Calibration was introduced by Benoit, et. al. in 1967. Instead of plotting the log molecular weight of a series of narrow standards vs. retention, the log of the product of the intrinsic viscosity [η] and molecular weight M is plotted vs. retention. The [η]M product is related to the hydrodynamic volume. Benoit found that plotting a series of hydrodynamic volume values for a variety of narrow standards resulted in a singular calibration curve. In other words, all of the points fit the same curve. Once this "Universal" calibration has been established, any random coil polymer can be run in the appropriate solvent, and the molecular weight determined based on the Universal curve. Benoit used a glass capillary viscometer to measure the viscosities of the narrow standards and samples. After establishing the Universal curve, we can also plot the log of the intrinsic viscosity vs. the log of the molecular weight for the narrow standards. This plot is called the viscosity law plot, or, the Mark-Houwink plot. The slope of this plot is alpha, (sometimes called α), and the intercept is called log K. The resulting equation, known as the Mark-Houwink equation, is:


[η] = KMa





A Typical Universal Calibration curve and viscosity law plot for a series of polystyrene standards.


The Polymer Handbook contains many K and alpha values for a variety of different polymer/solvent combinations. One can input these empirical constants into many of the commercial GPC software packages available today, and obtain "absolute", or accurate molecular weights for many polymers. One must be sure that the values in the handbook are accurate for the polymer to be analyzed, or errors will occur.

Today, we can use an on-line viscometer detector, along with the differential refractive index (dRI) detector, to directly obtain the molecular weight of each slice. The dRI is the concentration (C) detector, and the viscometer detector gives us the product of intrinsic viscosity and concentration ([η]C). Dividing the viscometer signal by the dRI signal gives us the intrinsic viscosity [n i] of each slice across the polymer peak. We now know both the intrinsic viscosity and, of course, the retention time (or volume) of each slice, so we can go back to the Universal Calibration curve and obtain the molecular weight of each slice, Mi. This Universal Calibration concept has wide applicability, especially for random coil type polymers, which represents the majority of polymers being analyzed today. Other polymer conformations, such as rods, spheres, or globular shaped (such as proteins) may not behave the Universal concepts. There can be no interaction of the polymer and the eluent or column packing material for Universal Calibration to work.

Another advantage to using Universal Calibration and on-line viscometry/dRI detection is the ability to determine how branched a polymer is, relative to a known linear polymer standard. This technique is quite sensitive to long chain branching (as opposed to short chain branching), and is important to help predict how a certain polymer will process, or what the final physical properties will be, in comparison to the linear counterpart.

As an example, one can run a linear polyethylene broad polymer, (such as "NBS 1475", or any other known linear polyethylene), with the resulting Mark-Houwink values being determined from the experiment. The resulting Mark-Houwink plot (or viscosity law plot) will be linear, with a constant slope, (alpha will be constant across the molecular weight distribution). The K and alpha values can then be input into the software, and any subsequent unknown polyethylenes can be analyzed, with the viscosity law plot being compared to that of the known linear polyethylene.

If the unknown exhibits any long chain branching, the viscosity/molecular weight relationship is not linear; i.e. the viscosity will not increase linearly with molecular weight. The greater this deviation from linearity, the greater the level of long chain branching. An accurate alpha can be obtained for a branched polymer only at low molecular weights, where there is no long chain branching, and the slope is constant. Once the polymer is at a molecular weight where there is long chain branching, alpha is continuously changing, (may even approach zero), and becomes meaningless. A simple ratio of the viscosity law plot of the branched polymer to the linear polymer gives us the branching index, (g'), where: g' = [η ]br/[η]lin One can do further calculations to determine the branching frequency, what type of branch is present, etc. It is obvious that adding a viscometer detector on-line with a refractive index detector can provide much more information about your polymer, specifically:
?"Absolute" or accurate molecular weights for your polymer via Universal Calibration
?Calculation of the intrinsic viscosity of your polymer
?Determinationof branching