Boosting the productivity of GPC/SEC experiments

Boosting the informational productivity of GPC/SEC experiments: Using a multi-detector array to quantify polymer branching. By Paul Clarke.

Gel permeation or size exclusion chromatography (GPC/SEC), is the technique of choice for measuring the molecular weight (MW) of polymers, proteins and macromolecules. As the application of these macromolecules diversifies, there is an increasing demand on GPC/SEC systems to deliver more accurate MW and more detailed insight into polymer structure. This article explores how multi-detector GPC/SEC meets this demand, focusing on its application for quantifying branching in polyolefins. 

Seeing the full picture
The functionality of a polymer is defined by features such as MW, molecular weight distribution and structural characteristics such as branching. By varying these properties developers are able to tailor polymeric materials to meet specific applications. Detailed, accurate polymer characterisation provides the information required to drive product and process development, and for effective QC.

GPC/SEC is a two-stage process. The first stage involves separation of the dissolved polymer, using a microporous packing material, on the basis of hydrodynamic volume. The second is analysis of the eluting size fractionated sample. Conventional, single detector GPC/SEC relies on concentration measurement alone, typically via a refractive index (RI) detector. External calibration with an appropriate reference standard enables the conversion of this data into a relative (to the standard) MW distribution. 

With this single detector system, the reported MW is only accurate if the MW/hydrodynamic size ratio of the standard is closely similar to that of the sample. Differences in molecular density, between the reference and the sample, or between different samples, are undetectable. Equally importantly, single detection provides no insight into polymer structure.

Multi-detector GPC/SEC directly addresses these limitations and delivers a far more complete view of polymer features. Figure 1 shows the information generated by a multi-detector array incorporating RI, light scattering, and viscometer detectors (“triple detection”). Light scattering is particularly valuable as it provides absolute MW data without calibration. In combination with intrinsic viscosity (IV) data, MW information can be used to quantify structural characteristics, via the construction of a Mark-Houwink (M-H) plot. 

Branching out with multi-detector GPC/SEC
An M-H plot (see Figure 2) is a double logarithmic plot of IV against MW. As IV is a measure of molecular density, this plot shows how molecular structure changes with MW thereby revealing characteristics such as branching and cross-linking. The degree of branching can also be quantified numerically from light scattering and IV data. Branching ratio and frequency for larger polymers can be calculated from radius of gyration (Rg) data, or more practically and reliably, for polymers of all sizes, by directly comparing IV measurements (see equation 1).

This capacity to quantify branching is particularly useful for polyolefins where the induction and control of branching is essential to develop and manufacture products with different characteristics. However, the application of multi-detector GPC/SEC in polyolefin analysis has traditionally been restricted because of the difficult application requirements of working at high temperature. The emergence of integrated multi-detector GPC/SEC systems that provide controlled measurements at temperatures up to 160oC has changed this situation. These systems enable the accurate and efficient measurement of polyolefins, and of filled materials containing, for example, carbon black. 

Employing high temperature GPC/SEC to assess the extent of branching in different polyethylene samples

Three reportedly similarly branched polyethylene samples were analysed, together with a linear polyethylene standard material at 140oC using a high temperature triple detection GPC/SEC (Viscotek HT-GPC Malvern Instruments). The solvent used was 1,2,4-trichlorobenzene (TCB) with 500 ppm BHT. An M-H plot was generated to qualitatively compare the structural characteristics of the polymers (Figure 2).

The plot shows, by the difference from the linear reference, that all three samples are branched but they are clearly not identical. The outstanding sensitivity of the M-H plot reveals that the samples have significantly different levels of branching which would cause a difference in the material performance.

These results demonstrate the informational productivity of triple detector GPC/SEC, relative to conventional single detector analysis, and the robustness and value of HT-GPC systems developed specifically to extend these capabilities to polyolefins. These capabilities support the fast and effective development of new products and processes, and provide the sensitive differentiation required to meet the very highest standards in QC.

Paul Clarke is industrial portfolio manager with Malvern Instruments.

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