L. H. Sperling

Center for Polymer Science and Engineering & Polymer Interfaces Center, Materials Research Center, Department of Chemical Engineering and Materials Science and Engineering Department, Lehigh University, 5 E. Packer Ave., Bethlehem, PA 18015-3194

Before the advent of modern analytical methods, polymer scientists and engineers used osmometry and light-scattering to determine absolute molecular weights, and intrinsic viscosity to determine relative molecular weights. An absolute molecular weight is defined as one determined by theoretical first principles. A relative molecular weight depends on prior calibration. The value of intrinsic viscosity over the absolute methods was that it was fast and cheap. A newer term for molecular weight is molar mass.

Osmometry yields the number-average molecular weight, M_n, while light-scattering yields the weight-average molecular weight, M_w. Literally speaking, the number-average depends on a counting procedure, while the weight-average measures the weights(l, 2). Since synthetic polymers and many natural polymers as well, have a distribution of molecular weights, the ratio of these two averages, the polydispersity index (PDI), sometimes called the molecular weight distribution (MWD), of these two averages have classically defined the breadth of the distribution. For example, a sample of poly(vinyl chloride) might be listed as having M_n = 100,000 g/mol, with a PDI of 2.2. Intrinsic viscosity measures M_v, the viscosity-average molecular weight, which is usually intermediate between M_n and M_w. However, each of these methods yields a single number for the molecular weight, and the absolute methods tend to be slow and cumbersome.

A newer method involves gel permeation chromatography, GPC, a relative method that yields the complete molecular weight distribution in about half an hour per sample. This method is also known as size exclusion chromatography, SEC, or gel filtration chromatography. While GPC has its greatest value for measuring the molecular weight and polydispersity of synthetic polymers, a closely related method, high-performance liquid chromatography, HPLC, is more useful for polymers containing functional groups, such as proteins and pharmaceutical polymers containing special active groups.

In both GPC and HPLC, there is both a mobile phase and a stationary phase(3). The mobile phase, comprising a solvent and a portion of the polymer, moves past the stationary phase, which through physical or chemical means temporarily retains some portion of the polymer, thus providing a means of separation. Both of these methods depend on distribution coefficients, relating the selective distribution of an analyte between the mobile phase and the stationary phase, where the analyte is the component being analyzed. The GPC approach utilizes colunms containing finely divided, porous particles(4,5). Polymer molecules that are smaller than the pore sizes in the particles can enter the pores, and therefore have a longer path and longer transit time than larger molecules that cannot enter the pores. Motion in and out of the pores is statistical, being governed by Brownian motion. Thus, the larger molecules elute earlier in the chromatogram, while the smaller molecules elute later. This is largely an entropically governed phenomenon.

By contrast, HPLC utilizes interactions and weak bonding between the polymers and the surface of the particles composing the stationary phase(6). Two of the most important HPLC methods include reverse- phase chromatography and normal-phase chromatography, where the groups are either more polar or less polar than the stationary phase, respectively.

Important parts of GPC instrumentation include pumps for maintaining constant, pulseless rates of flow, column types for the molecular weight range of interest, and the detector system for quantifying the result. Detector systems are classified as either concentration sensitive or molar mass sensitive. The refractive index detector measures the change in refractive index as the concentration of polymer in the solution changes, and is usually operated on some type of differential refractive index method or Fresnel refraction. Another group of concentration methods involves the input of ultraviolet light, with the output being fluorescence or absorption by the polymer. Other methods include a density detector and the evaporative light-scattering method. In the latter method, the sample is nebulized (evaporated). Each droplet that contained non-volatile material forms an aerosol particle, which causes light to be scattered.

Molecular weight sensitive methods include light-scattering, viscometry, etc. If two angles are used in the light-scattering detector, the radius of gyration, a measure of size of the chain, can be determined.

If the analyte is anything but a simple homopolymer, multiple detectors will be required for accurate results. Cases include so-called statistical copolymers (nearly randomly placed multiple kinds of mers) which may actually contain systematic variation in composition as a consequence of the synthetic method, or solutions of two or more polymers. Examples might be a block copolymer(7) (a polymer containing two or more strings of different mers), or a polymer blend (a mixture of two or more polymers, either finely divided, or in solution). If the polymer is not linear, such as in hyperbranched(8) or dendritic materials (two types of polymers with multiple, short segments, each ending in a branch point), the latest methods must be employed.

Since GPC is a relative method, such instrumentation needs to be calibrated. Narrow PDI, anionically synthesized polystyrenes are used most often for this purpose.

1. H.-G. Elias, An Introduction to Polymer Science, VCH, Weinheim (1997).
2. L. H. Sperling, Introduction to Physical Polymer Science, 2nd ed., Wiley, New York (1992).
3. H. Pasch and B. Trathnigg, HPLC of Polymers, Springer-Verlag, Berlin (1998).
4. T. Provder (Ed.), Chromatography of Polymers: Characterization by SEC and FFF, American Chemical Society, Washington, DC, 1993.
5. M. Potschka and P. L. Dublin (Eds.), Strategies in Size Exclusion Chromatography, American Chemical Society, Washington, DC, 1996.
6. E. J. Swadesh, HPLC Practical and Industrial Applications, CRC Press, Boca Raton (1997).
7. C. S. Patrickios, A. B. Lowe, S. P. Armes, and N. C. Billingham, J. Polym. Sci., Part A, Polym. Chem., 36, 617 (1998).
8. J. F. Miravet and J. M. J. Fréchet, Macromolecules, 31, 3461 (1998).

First published: ACS Division of Polymeric Materials: Science and Engineering (PMSE), 81, 569 (1999).