Characterization of Polymers by Gradient Polymer Elution Chromatography
S.J. Kok1, 2
and P.J. Schoenmakers2
click here for PDF-file.
Voeding, Analytical Sciences Division, Utrechtseweg 48,
3704 HE Zeist, The Netherlands
of Amsterdam, Faculty of Chemistry, Nieuwe Achtergracht
166, 1018 WV Amsterdam, The Netherlands
|Due to the rapid development of new types of
polymers, e.g. random, block, graft, branched, gradient (co)polymers,
telechelics, etc., liquid chromatography in the size exclusion mode on its
own is inadequate in the characterization of such complex polymers. Size
exclusion chromatography (SEC) is not able to separate polymers if they
consist of the same size (hydrodynamic volume) but with different chemical
composition. Therefore, in recent years a gradient liquid chromatography
technique called gradient polymer elution chromatography has been developed
for the separation and characterization of these types of polymers.
Principle of gradient polymer elution
chromatography [1, 2]
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
To sum up, gradient polymer elution
chromatography can be used:
for the characterization of (complex)
for impurity profiling
for prediction of correlation between
physical characteristics with composition
for detection / identification of minor
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
Detection methods in gradient polymer elution chromatography
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.
Interfacing LC- infrared
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 .
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 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.
Research will be focussed on studying solubility
effects and improvement of the reproducibility (precipitation mechanism,
effect of cristallinity). Furthermore, hyphenation of spectroscopic (infrared)
and spectrometric (MALDI-TOF-MS) techniques will be investigated. A self-learning
data processing method will be developed based on multi-variate data analysis
to interpret 3-dimensional LC-IR chromatograms.
- Philipsen, H.J.A.; Ph.D. thesis, Eindhoven
University of Technology, Eindhoven, The Netherlands, 1998.
- Cools, P.J.C.H.; Ph.D. thesis, Eindhoven University
of Technology, Eindhoven, The Netherlands, 1999.
- Glockner, G.; Gradient HPLC of Copolymers
and Chromatographic Cross-fractionation, Springer Verlag, Berlin
Heidelberg, New York, 1991.