Lecture 4 : Gel Permeation or Size Exclusion Chromatography

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 V e V o K sec V o = volume of solvent in the system outside the porous beads V i = volume of solvent inside the beads K se = fraction of the internal pore volume penetrated by te particular polymer molecules For small molecules that can penetrate all of the internal volume ; K sec 1and Ve V0 V i For very large polymer molecules that are unable to penetrate the pores; K sec 0 and V V Clearly for polymer moelcules of intermediate size K se and V e lie between these limits V e is independent of flow rate. Separation process takes place under equilibrium conditions, K se can be considered as as the equilibrium constant defined by K se =c i /c o Wgen the polymer molecules penetrate the pores to the same extent as the solvent c i =c o and K se =1 e 0 V i

Separation by Size Exclusion The relation between K se and Gibss free enery ( 0 G p )change is given by ; 0 G p RT ln Ksec For seperation exclusively by size-exclusion, the enthalpy must change associated 0 with the transfer of the solute species into the pores must be zero. G p is controlled 0 0 0 by the entropy change ( S p ) and is given by G. p TS p K sec exp( S o p / R)

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, CHCl 3, CH 3 Cl 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 l 1/2 where l is the total column length and also with (1/d 2 ) 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) Concentration of polymer in the eluate Viscometry Light scattering Molar mass of the polymer

Concentration detectors

Molecular weight sensitive detectors

Universal Calibration Curve s s x x M M log log s s s x x x M M K M M K s x log log Mark Houwink equation KM s x s x s s x M K K M log 1 1 log 1 1 log

Log [] Log M

Data Treatment

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