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.