Confirmation of Polymer Structure Using GPC

Confirmation of Polymer Structure Using GPC

Introduction

   Gel Permeation Chromatography (GPC) is a method developed to measure the molecular weight of polymers. GPC is a classification technique that can measure not only the average molecular weight but also the entire distribution. However, traditional GPC (using a single concentration detector) can only determine relative molecular weights.

   To meet the increasing demand for analysis of increasingly complex polymer characteristics, one can use detectors sensitive to molecular weight, such as light scattering detectors and viscometers, together with GPC to overcome these limitations. Using these detectors together allows obtaining not only the distribution of molecular weight and viscosity but also the absolute molecular weight distribution. Based on this, one can construct a Mark-Houwink plot, which provides additional structural information such as the degree of branching of the molecule.

Conventional GPC

   In GPC, molecules are separated based on hydrodynamic volume. The molecular weight (MW) and molecular weight distribution are calculated from the retention volume (RV) measured by a calibration curve (MW vs. RV log curve) which needs to be constructed with the help of many standards of known molecular weights.

   However, since the relationship between molecular weight and molecular size varies depending on the type of polymer, the calibration curve also changes accordingly, and it is only possible to obtain the actual molecular weight when the calibration standards and the samples are the same type of polymer.

   In other cases, the results are merely relative values. This is especially compounded in branched samples where the molecular density is substantially higher than linear chain polymers, causing a significant deviation from the actual molecular weight(1, 2). In conventional GPC, the detectors commonly used were either refractive index (RI) detectors or ultraviolet (UV) detectors. The signal from these detectors only varies with concentration and does not change with molecular weight or molecular size.

Detectors Sensitive to Molecular Weight

   Today, to overcome the limitations of traditional GPC, viscometer and/or light scattering detectors are very frequently used. These mass-sensitive detectors provide complementary information.

     Static Light Scattering Detector

   The signal from a light scattering detector is directly proportional to the molecular weight, concentration, and the square of dn/dc of the polymer.

   Due to the squared dependence on the refractive index increment (dn/dc), if this value is inaccurate, it can result in significant deviations in molecular weight.

   The ability to use a light scattering detector is critically dependent on the refractive index increment of the polymer solvent compound. For large values, the usable signal range is up to a molecular weight of about 1000 g/mol.

   For other polymers like polylactide in THF (dn/dc = 0.049), the signal magnitude is only 7% of that of polystyrene with the same concentration and molecular weight in THF, which can make evaluations at small MW unreliable.

   The advantage of using light scattering in GPC is that once calibrated, the signal-to-noise ratio is sufficient for calculating molecular weight right away without a calibration curve.

     Viscometer

   For a viscometer, the signal is proportional to the intrinsic viscosity (IV or [η]) and concentration of the polymer.

   For low molecular weight, the sensitivity of the viscometer exceeds that of the light scattering detector even at high dn/dc (see Figure 1). This means that regardless of whether the signal-to-noise ratio of the light scattering detector is sufficient, one can obtain the actual molecular weight through universal calibration.

   Using the double log plot of intrinsic viscosity versus MW, one can obtain the well-known Mark-Houwink plot. The Mark-Houwink plot is central to polymer structure analysis as it reflects structural changes such as branching and chain rigidity.

   The slope expressed as the Mark-Houwink exponent can vary between 0, which represents a spherical form, and 2, which represents a rod-shaped structure (3).

   Combining the advantages of the two detectors yields the effect of triple detection (RI/viscosity/LS). You can obtain structural information through intrinsic viscosity and calculate molecular weight using light scattering. This combination also makes it possible to measure and differentiate aggregates and micro-gels. To analyze polymers with low MW and/or low dn/dc, you can use universal calibration without needing to change the GPC system.

Summary

   If the signal is sufficiently strong, there is the advantage of using light scattering to calculate molecular weight. Triple detection allows these detection features to be combined into a single system, enabling the calculation of molecular weight and the determination of structure without constraints.

References

– W.W. Yau, J.J. Kirkland and D.D. Bly, Modern Size Exclusion Liquid Chromatography, (Wiley and Sons, New York, USA, 1979).

– S. Mori and H.G. Barth, Size Exclusion Chromatography, (Springer-Verlag Berlin, Heidelberg, Germany, 1999).

– H.-G. Elias, Makromolekule, 5th Edition,(Huthig & Wepf, Basel, Switzerland, 1984).