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GPC - Gel Permeation Chromatography

GPC Introduction

Why is GPC important?

How GPC works

GPC systems

Gel permeation chromatography (GPC) is one of the most powerful and versatile analytical techniques available

for understanding and predicting polymer performance. It is the most convenient technique for characterizing

the complete molecular weight distribution of a polymer.

Waters commercially pioneered GPC in 1963. Since then, Waters has continued to develop and explore new

GPC applications and improve the instrumentation that makes GPC so powerful.

Why is GPC important?

GPC can determine several important parameters. These include number average molecular weight, weight

average molecular weight, Z weight average molecular weight, and the most fundamental characteristic of a

polymer its molecular weight distribution.

These values are important, since they affect many of the characteristic physical properties of a polymer. Subtle

batch-to-batch differences in these measurable values can cause significant differences in the end-use

properties of a polymer. Some of these properties include:

  Tensile strength Adhesive strenth

  Elastomer relaxation time Cure time

  Brittleness Elastic modules

  Flex life Melt viscosity

  Impact strength Hardness

  Toughness Softening temperature

  Drawability Tear Strength

  Adhesive tack Stress-crack resistance

  Coefficient of friction

Materials characterization

Understanding the makeup of a polymer is particularly important due to the variety of resins available for the

same purpose, the high cost of specialty resins or compounds, and the value added to the polymer duringmanufacturing. For example, the cost of a resin used in a printed circuit board is very low, but the cost of the

finished board is very high. Poor quality resin can result in an unacceptable finished circuit board.

Where a polymer's end-use application requires precision performance or endurance under harsh conditions,

the need for polymer characterization is particularly acute. Because GPC fulfills these needs better than any

other single technique, it has become an extremely valuable tool for materials characterization in the polymer

industry.

Telling good from bad

Two samples of the same polymer resin can have identical tensile strengths and melt viscosities, and yet differ

markedly in their ability to be fabricated into usable, durable products. These differences can be attributed to

subtle, yet significant variations in the molecular weight distributions of the two resin samples. Such

differences, if undetected, can cause serious product defects.

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Improved peak

resolution, especially for

low MW polymers and

oligomers; up to 20x

faster than GPC.

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Though they are subtle, differences such as those shown in the molecular-weight distributions to the left, could cause

marked variations in the performance of the polymer.

In addition to providing the molecular weight distribution, GPC also separates a complex polymeric compound into its

component parts - polymer, oligomer, monomer, and additives.

How GPC works

GPC separates molecules in solution by their "effective size in solution." To prepare a sample for GPC analysis

the resin is first dissolved in an appropriate solvent.

Inside the gel permeation chromatograph, the dissolved resin is injected into a continually flowing stream of 

solvent (mobile phase). The mobile phase flows through millions of highly porous, rigid particles (stationary

phase) tightly packed together in a column. The pore sizes of these particles are controlled and available in a

range of sizes.

Cross sectional view of porous particle

The width of the individual peaks reflects the distribution of the size of molecules for a given resin and its

components. The distribution curve is also known as the molecular weight distribution (MWD) curve. Taken

together the peaks reflect the MWD of a sample. The broader the MWD, the broader the peaks become and vice

versa. The higher the average molecular weight, the further along the molecular weight axis the curve shifts

and vice versa.

You can see then how easily the MWD profiles of two resins can be compared. If the MWD profile of an incoming

resin doesn't match that of the control resin (i.e. one that is known to process well) closely enough, the

incoming resin can be modified or process conditions can be changed to make sure the resin processes

properly. If the differences between the control resin and the incoming resin are too severe, the incoming resincan be returned to the supplier as unacceptable.

The Size Separation Mechanism

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Molecules of various sizes elute from the column at different rates. The column retains low molecular weight material

(small black dots) longer than the high molecular weight material (large black dots). The time it takes for a specific

fraction to elute is called its "retention time".

GPC Systems

In designing instrumentation for GPC, a variety of requirements must be satisfied. Injectors are needed to

introduce the polymer solution into the flowing system. Pumps deliver the sample and solvent through the

columns and system. Detectors monitor and record the separation. Data acquisition accessories control the test

automatically, record the results, and calculate the molecular weight averages. The gel permeation

chromatograph contains a number of different components that work together to provide optimum system

performance with minimum effort. Schematic of a basic gel permeation chromatograph.

Schematic of a basic gel permeation chromatograph

This diagram illustrates how the sample is injected into the mobile phase and the path the sample takes to the detector.

1. Pump

Pumps the polymer in solution through the system.

Different polymers produce solutions of different viscosities. To compare data from one analysis to the next, the

pump must deliver the same flow rates independent of viscosity differences. In addition, some detectors are

very sensitive to the solvent flow rate precision. Such constant flow must be a critical feature of the instrument.

2. Injector 

Introduces the polymer solution into the mobile phase.

The injector must be capable of small volume injections (for molecular weight determinations) and large

volume injections (if fraction collecting is desirable). The injector should not disturb the continuous mobile

phase flow. It should also be capable of automatic multiple sample injection when the sample volume is large.

3. Column Set

Efficiently separates sample components from one another.

High efficiency columns give maximum separating capability and rapid analyses. Every column must provide

reproducible information over extended periods for both analytical and fraction collecting purposes.

4. Detector 

Monitors the separation and responds to components as they elute from the column.

Detectors must be nondestructive to eluting components if they are to be collected for further analysis.

In addition, the detectors must be sensitive and have a wide linear range in order to respond to both trace

amounts and large quantities of material if necessary.

Since all compounds refract light, the differential refractometer (RI) is referred to as a "universal" detector. As a

result it is the most widely used detector to monitor molecular weight distribution. The refractive index of 

polymers is constant above approximately 1000 MW. Therefore, the detector response is directly proportional to

concentration.

Beside information about molecular weight averages and distribution obtained with RI, the use of UV

absorbance detectors may provide information about composition, while on-line light scattering detectors and

viscometers provide information about polymer structure.

5. Automatic data processing equipment

Automatically calculates, records, and report numerical values for Mz, Mw, Mv, Mn, and MWD.

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Data systems can also provide complete control of GPC systems so that large numbers of samples can be run

unattended and raw data can be automatically processed. Today's GPC software offerings need to be able to

provide special calculations for multi-detection processing, band broadening correction, special calibration

routines and polymer branching determination, just to name a few.

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PC Basic Chemistry

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Gel Permeation Chromatography - Basic Chemistry

alibration of the GPC System

(http://www.waters.com/waters/en_ID/Calibration-of-the-GPC-System/nav.htm?

locale=en_ID&cid=10167839)

In order to assign a molecular weight to each retention time slice for the eluted polymer, we mustcalibrate our system, or more specifically, the column set.

requently Asked GPC/SEC Questions

(http://www.waters.com/waters/en_ID/Frequently-Asked-GPC-SEC-Questions/nav.htm?

locale=en_ID&cid=10167847)

Frequently Asked GPC/SEC Questions

pplications for Room Temperature GPC

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locale=en_ID&cid=10167876)

Applications for Room Temperature GPC

eferences

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GPC Primer references