By A. Ortín, B. Monrabal, J. Montesinos, P. Del Hierro, N. Mayo. Polymer Char, Valencia, Spain.
The analysis of the molecular weight distribution of polyethylene and polypropylene resins by Gel Permeation Chromatography (GPC), or Size Exclusion Chromatography (SEC), has always been considered a demanding task. Using multiple detection methods (with online viscometer and light scattering detectors) maximize the information on the polymer microstructure. A high-quality concentration signal is of paramount importance for the reliable application of those methods, in order to obtain accurate and precise information. Recent developments in online Infrared (IR) detection have meant a breakthrough in concentration detection for polyolefin applications, greatly improving the quality of data by those multiple detection methods. Highly sensitive and stable Polymer Char’s IR4 and IR5 MCT filter based IR detectors are now a convenient alternative to other concentration detection techniques, also providing superior performance.
In this document, data collected from IR detectors based on narrow band interference filters to select specific wavelengths is presented. The main advantages of the filter-based IR technique as concentration detection for GPC/SEC analysis of polyolefins are summarized: high sensitivity, superb stability, ability to work in staggered injection mode, low detection limit. In addition, this IR detection technology is also capable of detecting chemical composition, so short chain branching is quantified with high sensitivity across the molar mass distribution, complementing the microstructure information gathered in the GPC/SEC analysis.
Typical sample preparation:
Filter based Infrared detection is a very robust technique, not being affected by pressure changes, injection ghost peaks, temperature variations or other external variables. Very clean chromatograms on a highly stable baseline are collected when using this type of IR detector as seen in Figure 5, simplifying the task of setting the integration limits and performing baseline correction either manually or when using automated software algorithms. A high-quality concentration signal together with fully automated sample preparation results in enhanced reproducibility (Figure 2).
In polyolefin analysis, the IR detection provides high sensitivity not influenced by the rather low dn/dc values found in these polymers. This high sensitivity together with the low noise achieved in modern filter-based IR detectors results in superb signal to noise ratio. In GPC/SEC of polyolefins it is always desirable, and in some applications required, to reduce the polymer injected mass in order to obtain accurate molar mass distributions. This is the case when very high molar mass fractions are present in the sample. Filter-based IR detectors, such as Polymer Char’s IR5 MCT detector, can work successfully even when very low mass is injected, providing good quality chromatograms thanks to its low detection limit (Figures 3, 4a and 4b).
Additionally, the flow through cell allows direct connection of the detector after the GPC columns. Continuous absorbance chromatograms at selected IR bands are collected, without removal of the mobile phase. This results in very precise and accurate determination of the chromatograms, being a quantitative method in all the molecular weight range, from the low molecular weight fractions to very high molecular weight.
Triple detection (combination of concentration, viscosity and light scattering detectors) is the most comprehensive GPC/SEC technique, providing accurate molecular weight distributions as well as polymer architectural information. Determination of both absolute molar mass by light scattering and intrinsic viscosity by a capillary viscometer after the GPC/SEC separation, requires the measurement of polymer concentration at every retention volume slice. This means that the triple-detector GPC/SEC data accuracy and precision greatly benefit from an improved concentration signal. The low detection limit achievable by the filter-based IR detection also expands the log M range in which accurate, measured, not extrapolated, data is available for the Mark-Houwink plot. Absolute molar mass and intrinsic viscosity are accurately measured at the chromatogram tails beyond what other traditional detection techniques allow.
Multiple band Infrared detectors are sensitive to chemical composition in PE and PP copolymers. IR4 and IR5 MCT modern detectors based on narrow band optical filters are sensitive enough to be used in standard GPC/SEC analysis conditions or even at reduced injected mass. A single detector, using only one measurement cell, and in an extremely simple set-up, provides additional information on the compositional drifts across the molar mass distribution without additional hardware and, most important, without compromising the quality of the separation. Applications are found in pipe grade resins bimodal HDPE in which low amounts of comonomer are added to the high molar mass components (Figure 10), LLDPE based on different catalyst technologies (SSC data shown in Table 1) and Ethylene-Propylene Copolymers (Figure 11).
Table 1. Average value and standard deviation in the comonomer weight percent determination, calculated for 6 separate injections of 3 single site catalyst Polyethylene-co-Octene samples having different comonomer level.
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