Comparing multi-detector GPC and APC for polyalphaolefin analysis

Introduction

Polyalfaolefins (PAO) are a range of low molecular weight hydrocarbons, used in a number of lubricant applications.  In particular, they are widely used in the industrial and automotive industries where they are used as base oils.  The wide functional temperature ranges, high oxidative stability and high viscosity indices all contribute to their desirable qualities.

As with so many polymers, the specific physical properties of PAO are strongly determined by their molecular weights.  As a polymer of decene, a PAO sample will likely be a mixture of different decene oligomers and polymers, thus the final molecular weight distribution of a PAO sample will depend on polymerization parameters.  As a lubricant, the primary tool for characterizing PAO samples will be a viscosity or rheology measurement, however, gel-permeation/size-exclusion chromatography (GPC/SEC) can be used to make measurements of PAO molecular weight distribution, which may offer insight into small differences between samples.

Historically, GPC has been a slow technique with measurements taking 30-60 minutes to separate the sample.  Additionally, the molecular weight result is determined by comparison to polymer standards of different structure and chemistry, making the result only relative.  Despite this, the technique is a powerful tool for comparing samples and has become the gold-standard of molecular weight measurement in the polymer industry.

The addition of advanced detectors such as light scattering and a viscometer can help overcome some of the limitations of GPC.  Light scattering allows for the direct measurement of the polymer’s molecular weight, independent of its structure or chemistry providing what is often referred to as absolute molecular weight.  The use of a viscometer allows for the measurement of some structural aspects including branching.  In the last few years, further developments in GPC column chemistry have resulted in the development of Advanced Polymer Chromatography (APC) systems.  These use smaller particle sizes to achieve superior resolution but generate higher backpressure.  This also offers the benefits of significantly reduced run-time and solvent use.  Previously, the narrow peaks generated by APC were incompatible with multi-detection because of the effects of band-broadening (also called dispersion) caused by entry and exit of the detector cells. This has now been overcome and it is possible to combine multi-detection with APC with multi-detection to perform absolute characterization of polymers at the timescales and resolutions of APC. As low molecular weight molecules which are mixtures of oligomers, PAO, are an ideal example of this. 

In this application note, the separation of PAO by multi-detector GPC and APC are compared and the benefits of multi-detector APC are discussed.

Materials and methods

GPC conditions

Three samples of PAO (PAO6, PAO40, PAO100) of different viscosities and molecular weights were dissolved in THF to concentrations of approximately 18-20 mg/ml (analytical GPC), and 9-13 mg/ml (multi-detector APC). 

For analytical GPC, a Malvern Panalytical OMNISEC system including refractive index (RI), UV-Vis absorbance (UV), light scattering (including right-angle, and low-angle light scattering (RALS/LALS)), and viscometer (IV) detectors was used.  100 µl injections were separated through a column set containing Viscotek columns T2500, T2000, and T1000.  The mobile phase was THF and the runs were 45 minutes at 1 ml/minute.

For the multi-detector APC, a Waters ACQUITY APC system was combined with a Malvern Panalytical OMNISEC REVEAL detector including RI, UV, RALS/LALS, and IV detectors was used.  20-27 µl injections were separated through a column set containing Waters ACQUITY APC columns (4.6 mm x 300 mm), with pore sizes of 45, 125, and 450 Å.  The mobile phase was THF and the runs were 20 minutes at 0.8 ml/minute.

Results & Discussion

Figure 1 shows overlaid RI detector responses for the PAO samples.  Analytical GPC and multi-detector APC are compared.  As can be seen, multi-detector APC offers some clear advantages.  Firstly, the run length is significantly reduced from 45 minutes to 20 minutes.  This equated to a solvent use reduction from 45 ml to 16 ml of THF, which when considering typical costs of solvent and disposal can reduce the cost of a single injection from $1 to around $0.35.  Over the course of a year, this could add up $1500 or more in solvent savings alone (see appendix). 

Additionally, the APC columns afford significantly more resolution than the analytical GPC columns.  The polydispersity of the PAO40 and PAO100 samples is more apparent, while the PAO6 sample has been separated into three distinct populations.

Figure 1: RI chromatograms of duplicate injections of PAO6, PAO40, and PAO100, comparing analytical-scale GPC (top) and multi-detector APC (bottom).

Multi-detector chromatograms for the three samples are shown in figure 2 where the detector responses for each can be seen.

Figure 2: Multi-detector APC chromatograms for PAO6 (A), PAO40 (B), and PAO100 (C), showing RI (red), RALS (green), and viscometer (blue) responses.  Measured molecular weights are overlaid in gold.

Table 1 shows the molecular data for the three samples.  The three peaks in the PAO6 sample have been analyzed individually.  With this information, and knowing that decene has a molecular weight of 120 g/mol, it can be seen that this sample is primarily comprised of a decene tetramer (47%) with 30% trimer and then 23% pentamer/hexamer.  This knowledge could be critical in understanding small performance differences between products.

The PAO40 and PAO60 products are more polydisperse with molecular weights around 2000 g/mol and 2900 g/mol, respectively.  The intrinsic viscosities and hydrodynamic radii for the samples as well.  Since the PAO samples are linear polymers, these properties can be seen to increase in proportion to their molecular weights.

Table 1: Measured molecular data for the PAO6, PAO40, and PAO100 samples measured using multi-detector APC.

Conclusions

While traditional analytical GPC measurements are relatively slow and only convey limited information in terms of a relative molecular weight, the use of multi-detector APC offers direct measurements of a range of molecular properties including molecular weight, size, and intrinsic viscosity in shorter times and at higher resolution.  The addition of advanced detectors brings direct measurements of these properties without the need to refer to standards and giving results independent of the material’s chemistry or structure.  The use of APC allows these measurements to be made in much less time and with significant solvent savings.

Here, we have shown that while these measurements can be made using multi-detector GPC, the use of multi-detector APC offers significant improvements in value including solvent and time savings, and in the additional information that comes from improved resolution for these three PAO samples.

Appendix

THF use cost

• 16 L = $225
  
• 63c per run by GPC of THF
  
• 22c per run by APC of THF
  

THF disposal cost

• 55 gallons = 208198 ml = $1688
  
• 45/208198 x 1688 = 36c per GPC run of THF
  
• 16/208198 x 1688 = 13c per APC run of THF
  

Total THF cost/saving

It costs $1 per 45 min GPC run and 35c per 20 min APC run.

That’s a 65c saving per run.  At 10 runs a day for 250 days that’s 2500 runs = $1,600