Introduction Gradient Polymer Elution Chromatography

S.J. Kok^{1, 2} and P.J. Schoenmakers^2
1 TNO Voeding, Analytical Sciences Division, Utrechtseweg 48, 3704 HE Zeist, The Netherlands
² University of Amsterdam, Faculty of Chemistry, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands

Initial chromatography conditions in GPEC are poor with respect to solubility for the injected polymer. The polymer is dissolved in a solvent which solves the polymer completely and makes reproducible sample injection possible. This solution is injected in a chromatographic system with thermodynamically poor conditions and complete retention of the polymer occurs on top of the LC column. During gradient elution, the thermodynamic quality of the mobile phase increases, which leads to the gradual redissolution of the polymer. When the chromatographic strength of the eluent is large enough, the polymer starts migrating and the sample is separated into monomers, additives, oligomers and polymers.

Retention is influenced by three parameters: solubility effects, size exclusion and interaction. Solubility and interaction depend on the molecular mass and chemical composition. At low molecular masses, where oligomers will elute, information on molar mass and chemical composition, e.g. copolymers, can be directly obtained from the chromatogram. Major application areas are determination of chemical composition distributions (CCD’s) of copolymers. Gradient polymer elution chromatography can be used in the normal phase as well as in reversed phase mode.

To sum up, gradient polymer elution chromatography can be used:

• for the characterization of (complex) polymer mixtures

• for impurity profiling

• for prediction of correlation between physical characteristics with composition

• for detection / identification of minor differences

• where size exclusion chromatography (SEC) is incompetent in separating polymer blends due to similar hydrodynamic volume (molecular weight)

• characterization of polymer blends, (co)polymers, paints, glues, laminates, packaging materials

Untill now, evaporative light scattering and UV/Vis detectors are widely used for detection of polymers in the eluent stream (see Figure 1). One of the major drawbacks of these detectors is their characterization possibilities of the separated polymers, or calculate CCD’s from copolymers.

When coupling advanced detection methods, like infrared (IR) spectroscopy and matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS) or electrospray ionization (ESI-MS), one can obtain structure information and calculate CCD’s.

Figure 1: RP-HPLC-ELSD chromatogram of 7 µg polymer mixture. Peak assignment: 1, PMA; 2, PMMA 7k8; 3, PMMA 64k, 107k, 1420k; 4, PS 7k; 5, PS 21k; 6, PS 72k; 7, PS 1460k. Mobile phase: water / acetonitril / tetrahydrofuran. Column: Novapak C18.

In the early days of LC-IR coupling, IR detectors can only be used when eluent mixtures are available which have no absorption in a wavenumber region where the (co)polymer can be detected [3]. Usually, one is limited to normal phase separations, because water (used in reversed phase systems) will absorb IR radiation over a wide range, limiting the applicability of IR as an on-line detection technique.

Therefore, a commercial spraying interface is developed which evaporates the effluent and deposits the (almost) dry polymer sample, including additives, oligomers and polymers, on a rotating IR transparent germanium disc. After deposition the disc is transferred to the IR spectrometer and spectra from the entire chromatogram are collected. Spectra are evaluated, used for, e.g. structure prediction (Figure 2) or determination of the CCD by extracting functional group chromatograms (Figure 3).

Figure 2: RP-GPEC-IR spectra at increasing retention times from Figure 1 collected after deposition: black trace, PMA tr=25.5 min; red trace, PMMA tr=35.6 min; blue trace, PS tr=59.6 min).

Figure 3: Functional group RP-HPLC-IR-chromatogram of a 35 mg polymer mixture: black trace, IR of wavenumber region 1744 - 1724 cm-1, characteristic for C-O (MA); blue trace, IR of wavenumber region 688 - 708 cm-1, characteristic for phenyl (S). Peak assignment: 1, PMA; 2, PMMA 7k8; 3, PMMA 64k, 107k, 1420k; 4, PS 7k; 5, PS 21k; 6, PS 72k,, PS 1460k. Conditions: see Figure 1.

1. Philipsen, H.J.A.; Ph.D. thesis, Eindhoven University of Technology, Eindhoven, The Netherlands, 1998.
2. Cools, P.J.C.H.; Ph.D. thesis, Eindhoven University of Technology, Eindhoven, The Netherlands, 1999.
3. Glockner, G.; Gradient HPLC of Copolymers and Chromatographic Cross-fractionation, Springer Verlag, Berlin Heidelberg, New York, 1991.