Gel Permeation Chromatography and End group Analysis - Polymer chemistry

The determination of the molecular weight of the polymer is calculated by Gel permeation chromatography(GPC) and End group analysis method.

1. By JEEVANANTHAM P II- MSc., Chemistry GEL PERMEATION CHROMATOGRAPHY (GPC)

2. GEL PERMEATION CHROMATOGRAPHY ( GPC) It was first developed in 1955 by Lathe and Ruthven GPC is an analytical technique that separates molecules in polymers by size. It provides the molecular weight distribution of a material. GPC is also sometimes known as size exclusion chromatography (SEC ).

3. Outline of GPC How GPC Works? Dissolve the sample with suitable solvent. inject your sample into continuously flowing mobile phase of the same solvent. Your dissolved sample and the mobile phase is then pumped through a column. finally leads to a detector and the data system. 

4. Components In GPC GEL : Gels are used as stationary phase for GPC.  Other desirable properties of the gel forming agent are the absence of ionizing groups and, in a given solvent, low affinity for the substances to be separated.  Commercial gels like PLgel & Styragel (cross-linked polystyrene- divinylbenzene ), LH-20 ( hydroxypropylated   Sephadex ), Bio-Gel (cross-linked polyacrylamide), HW-20 ( hydroxylated   methacrylic polymer),  agarose gel and are often used based on different separation requirements.

5. SOLVENT : Choosing an appropriate solvent for your sample is very important. There are a wide range of solvents used in GPC, from non-polar to aqueous. The solvent must also be compatible with the column being used. A common non-polar solvent is tetra hydrofuran (THF), while even water can be used as an aqueous solvent. INJECTOR : Most GPCs have an auto sampler with an injector. Using an auto sampler , you can set up a very large run and let it run overnight

6. COLOUMN : A GPC column (Figure 3) is made up of the mobile phase and a stationary phase of porous particles all packed together into a column . You can select your needed pore size and a wide range exists for GPC. GPC columns separate material by size. higher molecular weight material passes through the column first. While lower molecular weight material enters the pores and takes longer to elute out of the column. they are expensive- often around $3,000 .

7. PUMP : The pump pushes the mobile phase and your samples through the column. Pump pressure and flow rate are two very important variables in GPC . A flow rate that is too slow makes the test unnecessarily slow, while a flow rate that is too fast can provide inaccurate measurements. DETECTOR : Numerous types of detectors also exist for GPC, including ultraviolet, refractive index, infrared, density, etc . Normally two detectors are used in conjunction with each other, so that molecular weight can be directly calculated.

8. The two peaks would translate to different molecular weights . The peak on the left would have a higher molecular weight , while the peak on the right would have a lower molecular weight GPC can provide number average molecular weight ( M n ), weight average molecular weight (M w ), z average molecular weight ( M z ), molecular weight distribution (MWD), and the polydispersity index (PDI). Example :

9. samples often have overlapping molecular weights. you might not be able to get an accurate molecular weight of a certain component of your sample. Also , some studied samples are more finicky with GPC than others when it comes to solvent and column selection. Limitation :

10. REFERENCES https://bitesizebio.com/28296/how-to-measure-molecular-weight-with-gel-permeation-chromatography-gpc / https:// en.wikipedia.org/wiki/Gel_permeation_chromatography https:// www.slideshare.net/shradhabasu1/gel-permeation-chromatography https://slideplayer.com/slide/777116/

11. Molecular Weight Determination by END GROUP ANALYSIS

12. End Group Analysis End group analysis is a chemical method used for calculating the molecular weight of the polymer whose molecule has a reactive funtional group. End groups  are an important aspect of  polymer  synthesis and characterization . In this method the polymer weight limited upto 50,000

13. End Group Analysis end-groups are commonly used and can be analyzed for example by nuclear magnetic resonance ( NMR ) to determine the average length of the polymer . Other methods for characterization of polymers where end-groups are used are  mass spectrometry  and vibrational spectrometry, like  infrared  and  Raman  spectrometry . Mass Spectrometry is helpful for the  determination  of the  molecular weight  of the polymer, structure of the polymer etc.

14. 1) Calculation, integral per proton: Locate the end-group proton signals (ca. 5.8, 6.2 & 6.4 ppm) integral per proton = sum of vinyl proton integrals / # of protons in the two vinyl end groups (10.00 + 9.66 + 10.17)/6 =  4.97 per proton 2) Calculation, number of repeating monomer units, n: Locate the OCH2CH2 proton signals (ca. 3.6, 3.7 & 4.3 ppm) n = ((sum of CH2 proton integrals)/# of CH2 protons)/{integral per proton value} ((20.79 + 151.87)/4)/4.97=  8.69 repeating units, n; 3) Calculation, Mn : Mn = (FW end groups) + (FW repeating unit)(n) = (55.06 + 71.60) + (44.05)(8.69) =509; Therefore, the Mn of this polymer is approx. 509

15. Reference https:// en.wikipedia.org/wiki/End-group https://www.sigmaaldrich.com/technical-documents/articles/material-matters/polymer-analysis-by.html