Friday, May 11, 2007

A much better QC test

So as you can tell, I don’t like MFI much as a QC test. Another less common test is IV – which in this case means inherent viscosity. Polyesters are commonly spec’ed on this basis. Here again, running the test at two different conditions can provide a huge increase in the data and what you can conclude from it.

Background: This test is also run with the polymer flowing through a capillary, but with big differences. First off, the capillary is glass. (A variety of different glasses are available each with different advantages and disadvantages.) Also, instead of being a melt (100% polymer), the polymer is dissolved in a solvent and the concentration is so low that each polymer molecule does not interact with any other one because the distance between them is greater than the size of the molecules. The polymer solutions flows down under the pull of gravity and a stopwatch is used to measure the time that it takes for a certain volume of fluid to drain. The same measurement is also made for the pure solvent. It’s a very easy test to run – I’ve had high school students get fantastic results – and even automated systems exist. End of Background

The ratio of the drain time for the polymer solution divided by the drain time for pure solvent is the relative viscosity. Take the natural log of this, divide it by the concentration and you have the IV. There also another “IV” which is called the intrinsic viscosity (Inherent/intrinsic viscosity – very confusing. I didn’t name these things, I just live with them.) The intrinsic viscosity is found by rerunning the inherent viscosity at a different concentration and then extrapolating the inherent viscosity to zero concentration. The neat thing is that this can be self correcting by doing a second set of independent calculations. Going back to the relative viscosity, subtracting “1” from it gives you the specific viscosity. Dividing this by the concentration gives you the reduced viscosity. Extrapolating the reduced viscosity to zero concentration also gives you the intrinsic viscosity. (The IV line has a negative slope while the reduced viscosity line has a positive slope. If they don’t cross at the y axis, something is wrong with the test.)

Like the MFI, measuing IV at just one point is of dubious value, as there are a large number of lines that can be drawn through that point. I know that it is an infinite number of lines, but for practical purposes, it is much less than this. First we know that the lines have to have a negative slope. Also, intrinsic viscosity is determined to two decimal places, so lines that lead to changes in the intrinsic viscosity of less than 0.01 are of grouped together. Nonetheless, there still is a large number of lines that can be drawn though a single IV measurement. Having a second IV measurement lets you calculate the slope AND the intrinsic viscosity AND also lets you calculate the reduced viscosity line too. All that from a second measurement. It's a heckofa deal.!

The intrinsic viscosity is magical. Despite being a measurement made at infinite dilution (commonly called “a single molecule in a sea of solvent”), you can actually learn much about the melt properties of the polymer. But that will have to wait for the next post.


Unknown said...

nice article!
i just have a question relating to the lab we did: can intrinsic viscosity be negative and what is the reason for it?

John said...

It certainly can be negative. This link is to the first page of research paper on the subject.

I'm not real familiar with this area. I would argue that you really are going outside the theoretical basis with these results and that trying to fit them to the theory is not going to be helpful. Physically, I woudl expect that the polymer is reducing the viscosity of the solvent, probably by breaking up hydrogen bonds or other polar-polar interactions between the solvent molecules.

Whatever the explanation is, it does raise this question on the other side: is there a similar effect that "artifically" increases the viscosity of the solvent but it is not "detected" since it is expected and "explained" that all the viscosity increases are due to the hydrodynamic radius of the polymer

Anonymous said...

I don't know what I did wrong, but my inherent viscosity has a positive slope, what did I do wrong?