Lecture 4 : Gel Permeation or

Size Exclusion Chromatography

Polymer Fractionation

Sedimentation Centrifugation Evaporation of the solvent Gel permeation chromatography

Gel Permeation Chromatography (GPC)

I. GPC is the mos twidely used and convienent method to measure

the molecular weight distribution of polymers

II. Discovered in the 1950’s , and it became a well established

technique in the early 1970’s.

III. Uses size exclusion for separation . The stationary phase contains

solvent filled pores of certain sizes that allow smaller molecules to

enter while excluding larger molecules.

IV. Thus GPC is best used as a relative technique requiring molecular

weight standards for calibration

Effect of MW and MWD on polymer properties

Gel Permeation Chromatography (GPC)

GPC Seperation Mechanism

Elution Profiles

Separation by Size Exclusion

ioe VKVV secVo= volume of solvent in the system outside the porous beads Vi= volume of solvent inside the beads Kse= fraction of the internal pore volume penetrated by te particular polymer molecules For small molecules that can penetrate all of the internal volume ;

For very large polymer molecules that are unable to penetrate the pores;

Clearly for polymer moelcules of intermediate size Kse and Ve lie between these limits

Ve is independent of flow rate. Separation process takes place under equilibrium conditions , Kse can be

considered as as the equilibrium constant defined by Kse=ci/co Wgen the polymer molecules penetrate the pores to the same extent as

the solvent ci=co and Kse=1

ie VVVK 0sec and 1

0sec and 0 VVK e

• The relation between Kse and Gibss free enery ( )change is given by ;

• For seperation exclusively by size-exclusion , the enthalpy must change associated with the transfer of the solute species into the pores must be zero. is controlled by the entropy change ( ) and is given by .

Separation by Size Exclusion 0

pG

0

pG0

pS00

pp STG

sec

0 ln KRTGp

)/exp(sec RSKo

p

Calibration and Evaluation of Molar Masses

A range of mixed gel columns designated A and E: Eluent:THF, flow rate; 1mL/min, calibrants: polystyrene standarts

Eluent Selection The primary consideration in eluent selection is that the polymer must be fully dissolved for SEC analysis. a) Organic – most common: THF (Tetrahydrofuran), Toluene, CHCl3, CH3Cl b) Polar organic or organic/aqueous mixtures – DMF (Dimethylformamide),

DMAc(Dimethylacetamide) c) DMSO (dimethylsulfoxide) d) Aggressive solvents/temperatures – TCB (1,2,3,-trichlorobenzene), ODCB (Ortho

Dichlorobenzene), NMP (N-Methyl Pyrrolidone) e) Aqueous – water, water/buffer, some small %organic

The physical characteristics of the organic solvent selected must be taken into consideration with respect to detection, for instance, the refractive index of the solvent if DRI is to be used, or the UV or IR spectrum of the solvent if spectrophotometric detection at a given wavelength is to be employed. Furthermore, the viscosity of the eluent is important; if it is relatively high, then the SEC separation may need to be carried out at elevated temperature in order to reduce the eluent viscosity, thus decreasing column operating pressure and improving mass transfer. In aqueous SEC, many water-soluble polymers contain charged species or relatively hydrophobic groups which are likely to give rise to sample-column packing interactions in the SEC separation. Water containing salts and buffers successfully inhibit ionic interactions and the addition of a minor volume of organic solvent (eg methanol) is used to suppress hydrophobic interactions between the polymer and the column packing material.

GPC Column Packings

With organic solvents; rigid porous beads of either crosslinked PS or surface treated silica

For aqueous GPC separations, porous beads of water swellable crosslinked polymers (crosslinked polycrylamide gels), glass or silica gel.

An appropriate range of gel porosities is required to obtain the necessary resolution a) connected series of short columns each or which is packed with a gel of different porosity b) one long column packed with mixed gels.

Resolution increased approximately with l1/2 where l is the total column length and also with (1/d2) where d is the bead diameter.

The early gels had d of about 50 m and large column lengths are required for separations. Nowadays, most GPC gels have d of either 5 m or 10 m, and much shorter column lengths can be used. GPC performed using gels of small bead diameter is termed as high performance GPC (HPGPC)

GPC aparatus

Common detectors used Differential Refractive Index Detector (DRI) UV Detector (UV) Evaporative Mass Detector (ELSD) Viscometry Light scattering

Concentration of polymer in the eluate

Molar mass of the polymer

Concentration detectors

Molecular weight sensitive detectors

Universal Calibration Curve

ssxx MM loglog

sssxxx MMKMMKsx loglog

Mark–Houwink equation

KM

s

x

s

x

s

s

x MK

KM log

1

1log

1

1log

Lo

g [

]

Log M

Data Treatment

Separation of narrow polymer standards of low polydispersity, and subsequent SEC calibration plot.