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Recent Advancements in Polymer Characterization

Thu, 06/06/2013 - 12:17pm
Ed Bouvier, Manager, Advanced Materials Research Group, Mia Summers, Principal Applications Chemist, and Michael O’Leary, Principal Scientist, Chemical Materials Business Operations, Waters Corp., Milford, Mass.

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A collaborative effort brings forth a new era in chromatography.

The ACQUITY UPLC APC system. Image: Waters Corp.Innovation in liquid chromatography instrument design and column technology over the last decade has led to substantial improvements in chromatographic throughput and resolution. This has been achieved by enabling the system to achieve pressures up to 15,000 psi, reducing the system contributions to peak broadening, and utilizing well-packed columns containing sub-2-micron particles. These advances now allow chromatographers to routinely perform faster separations with improved resolution compared to those previously achieved.

Until now, these benefits had not been realized for one important application area: characterization of synthetic polymers by size-exclusion chromatography (SEC). Polymer SEC technology has seen minimal technological developments since its inception fifty years ago by J. C. Moore of The Dow Chemical Corp., Torrance, Calif.

At that time, 7.8-by-3,650-mm-packed beds of porous crosslinked styrene-divinylbenzene particles were used to separate polymers in 2.5 hours. To this day, similar styrene-divinylbenzene particles are still employed as the primary chromatographic media for polymer separations, albeit with a reduction in particle size and column length. However, the physical properties of these materials limits further improvements in chromatographic performance.

The low tensile strength of porous styrene-divinylbenzene prevents their use at pressures greater than 2,000 psi. The low chromatographic efficiencies and low flow rates needed typically require banks of columns, and can result in run times greater than 30 minutes. Additionally, these polymeric columns are unforgiving if one wants to optimize the method by varying the mobile phase. The resin volume changes that occur when switching solvents can lead to rapid column failure. Typically, manufacturers provide columns packed in several different solvents, and the user purchases the appropriate column pre-filled with the mobile phase to be used for the intended application.

Because there is no adsorption occurring in SEC, analytes elute within one column volume; resulting peak widths can be substantially narrower compared to adsorption chromatography. However, traditional high-performance liquid chromatography (HPLC) instrumentation can deleteriously impact the observed chromatographic efficiency. This is because the injector, tubing, connections, and detector all contribute to band broadening. Typically, SEC columns are 8 mm in diameter, significantly larger than column dimensions used in other chromatographic modes. By increasing the column diameter, the system contribution to band broadening is reduced. However, one tradeoff is the need to use larger quantities of mobile phase for a given separation.

Recently, researchers at Waters Corp., Milford, Mass., have developed the ACQUITY UPLC advanced polymer chromatography (APC) system to address some of these challenges and limitations of existing SEC technology for polymer characterization. The materials used in the construction of the system were chosen to enable robust usage of a wide range of compatible organic mobile phases, while still enabling operation at pressures up to 15,000 psi. A new low-dispersion ACQUITY refractive index detector complements the existing ACQUITY tunable UV detector, photodiode array detector, and evaporative light scattering detectors. The low system dispersion enables the use of smaller 4.6-mm-diameter columns without any tradeoffs in chromatographic performance. This reduces solvent usage, which is particularly important when using expensive mobile phases that are often used for polymer characterization.

Additionally, ACQUITY APC columns are comprised of a new type of high-strength packing media, which was developed to meet the needs of SEC separations under UPLC conditions. SEC is a separation technique that requires no adsorption between analyte and stationary phase surface. Separation occurs as analytes are physically prevented from probing certain regions of the pore volume due to their differences in hydrodynamic size. Separation thus requires that the pore size distribution within the pores be sufficient to distinguish between the different size analytes.

The chromatographic media used in ACQUITY APC columns is comprised of inert bridged-ethyl hybrid (BEH) particles, similar in composition to the standard ACQUITY BEH columns used today. By modifying the surface of the particles with a trimethylsilane reagent, residual surface silanols are masked, and thus provide an inert and non-retentive chromatographic surface for use with organic mobile phases. In the case where aqueous mobile phases are employed, unbonded materials provide the best chromatographic media due to the inherent low surface acidity of BEH particles. To achieve the desired selectivity, the pore volume of the media was increased by almost two-fold compared to the traditional ACQUITY UPLC media. Despite the significantly greater pore volume, particles still maintain the mechanical stability necessary for operation under UPLC conditions.

Furthermore, the ACQUITY APC columns comprise well-packed beds of sub-3-micron, high-pore-volume particles, with mean pore sizes of 45 Å, 125 Å, 200 Å, and 450 Å. This range of pore sizes enables separation of a broad range of molecular weights. Combining the column/particle technology with the ability to operate at higher flow rates, run times can be up to 10 times shorter than traditional SEC, without any sacrifice in chromatographic resolution.

A challenging application is the separation of low molecular weight oligomers. Synthetic oligomers are used for numerous applications: lubricants, plasticizers, coatings, and as intermediate prepolymers. The goal in oligomeric SEC is to resolve the individual components from each other, typically to quantitate the total oligomeric species. The quality of the oligomer separation can be impacted by both the bandspread of the chromatographic system as well as the efficiency of the liquid chromatography (LC) column.

One practical limitation of traditional SEC columns is their sensitivity to changes in mobile phase composition. The volume of styrene-divinylbenzene particles change when exposed to different solvents. If one desires to use a different mobile phase for an application, a long, tedious equilibration process is required to ensure the chromatographic bed remains intact. Often the bed will collapse and render the column useless despite the best attempts to prevent this from happening. BEH particles do not undergo a similar volume change when exposed to different solvents. This means that solvent switching can be easily performed, and thus a single chromatographic system can readily be used to characterize a diversity of polymers with different mobile phases. Additionally, the same column can be readily used for method development to identify the most appropriate choice of mobile phase for a given separation.

The ACQUITY APC system provides improvements over conventional SEC systems for polymer characterization, due to the improved system components and columns with BEH particles.

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