Calculating Polystyrene Mv Using GPC

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In summary, the conversation was about calculating Mv for a polystyrene polymer using GPC data. The IV and Mn were given, as well as the solvent and calibration method. The Mark-Houwink-Sakurada equation was used to calculate Mv for the standard and sample, but there was confusion about the result being much lower than the Mn and Mw. Eventually, it was discovered that the lab manual was worded confusingly and the correct procedure was to compare a and K values from the literature and use the GPC values in the Mark-Houwink-Sakurada equation, resulting in a more sensible Mv of 38419.
  • #1
NotJohnson
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Homework Statement


I am trying to calculate Mv as part of a lab using GPC data for a polystyrene polymer that I produced with a known polystyrene standard. for the produced PS I have a = .678 and K = .00018 and the standard has a = .725 and K = .011. intrinsic viscosity was measured as .2346
Mn for the polymer from GPC is 23097 g/mol (for reference)
The solvent is THF, I'm not sure if the GPC is conventionally or universally calibrated

Homework Equations


IV (eta) = K*Mv^a (Mark–Houwink-Sakurada equation)

ln (Mv) (of PS produced) = 1/(asa + 1) * ln (Kst/Ksa) + (ast + 1) / (asa + 1) * ln (Mvst)
(sa = produced sample, st = standard) (equation correcting for GPC universal calibration)

The Attempt at a Solution


I used the Mark–Houwink-Sakurada equation and got Mv = 68.08 for the standard
I then used the below equation to solve for my sample and got Mv = 880.62

I am confused as to why this number is so much lower than the Mn or Mw. Is this normal or am I using the wrong procedure or equation to calculate this value?
 
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  • #2


Never mind, apparently the lab manual worded this question very strangely. We were supposed to compare a and K from the literature to the ones from the GPC but only use the Mark–Houwink-Sakurada equation using the GPC values. Doing this gets an Mv of 38419 which is much more sensible (and ignore the effects of calibration method).
 
  • #3


I would first check to see if the values used in the calculations are correct. Make sure that the units are consistent and that the values for a and K are accurate. I would also double check the Mark-Houwink-Sakurada equation to ensure that it is appropriate for the specific polymer being analyzed. Additionally, it is important to confirm if the GPC is conventionally or universally calibrated as this can affect the calculations.

If all the values and equations are correct, it is possible that the calculated Mv may be lower than the Mn or Mw due to the limitations of GPC. GPC is a technique that separates polymers based on size, but it is not able to provide information on the actual molecular weight of the polymer. Therefore, the calculated Mv may not necessarily match the Mn or Mw values obtained from GPC. It is important to keep in mind that GPC is just one tool used to analyze polymers and it is important to use multiple techniques to fully characterize a polymer sample.

In conclusion, I would recommend double checking the values and equations used and also considering the limitations of GPC when interpreting the results. If there are still discrepancies, it may be helpful to consult with a colleague or supervisor for further guidance.
 

What is GPC and how is it used to calculate polystyrene Mv?

GPC stands for gel permeation chromatography and it is a technique used to determine the molecular weight of polymers. In the case of calculating polystyrene Mv, GPC is used to separate the polymer chains based on their size and then measure the distribution of molecular weights present in the sample. This information can then be used to calculate the average molecular weight of the polystyrene.

What equipment is needed to perform GPC for polystyrene Mv calculations?

The basic equipment needed for GPC includes a column, a pump, a detector, and a data collection system. For polystyrene Mv calculations, a refractive index detector is typically used to measure the concentration of the polymer in the eluent. A calibration curve of known molecular weight standards is also necessary to accurately determine the molecular weight of the sample.

How does GPC determine the molecular weight of polystyrene?

GPC separates the polymer chains based on their size, with larger molecules taking longer to elute from the column. The eluent containing the separated molecules is then passed through a detector, which measures the concentration of polymer at different points in time. This data is then used to create a chromatogram, which shows the distribution of molecular weights in the sample. The average molecular weight can be determined by comparing the chromatogram to the calibration curve of known standards.

What factors can affect the accuracy of GPC results for polystyrene Mv calculations?

Several factors can impact the accuracy of GPC results, including the type of column and detector used, the choice of eluent, and the calibration curve. Additionally, the sample preparation process, such as the use of solvents or additives, can also affect the accuracy of the results. It is important to carefully control and calibrate these variables in order to obtain reliable and accurate data.

What are the advantages of using GPC for calculating polystyrene Mv?

GPC is a widely used and accepted technique for determining the molecular weight of polymers such as polystyrene. It is relatively simple and quick to perform, provides high resolution and accuracy, and requires only a small amount of sample. Additionally, GPC can also provide information on the molecular weight distribution of the sample, which can be useful for understanding the properties and behavior of the polymer.

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