Presented at the 5th International Conference on Polyolefin Characterization (ICPC), 2014. Valencia, Spain.
Huang Honghong, Guo Meifang, Wei Dong. Beijing Research Institute of Chemical Industry, SINOPEC, China.
Baudilio Coto. Rey Juan Carlos University, Spain.
Esther López, Alberto Ortín. Polymer Char, Spain.
Wallace W. Yau. Polyolefin Characterization Consultant, USA.
It is true that Differential Refractive Index (DRI) detector is well recognized as one of the concentration detectors most commonly used for Gel Permeation Chromatography (GPC), but for the particular use in the high-temperature GPC (HT-GPC) analysis of polyolefins, an online filter-based Infrared (IR) detector specific to the C-H absorption band is a more preferred detector, such as the IR5-MCT detector developed by Polymer Char.
The advantages of filter-based IR detectors for HT-GPC of polyolefins are: (1) capability for the copolymer compositional analysis with the multi-wavelength feature, and (2) improvements in detector signal to noise ratio (S/N) and GPC baseline stability, as the result of IR being much less affected by temperature and flow-rate variations. Item (2) is the focus of this current investigation.
In this study, we coupled an IR5-MCT detector to a GPC system already equipped with its built-in DRI detector. This is done so that we can make a direct and objective comparison for the performance of these two detectors. The DRI and the IR5-MCT detection data on several test samples were collected simultaneously and processed in the same way to compare the MMD results calculated from data of either the DRI or the IR5-MCT detector. The attempt here is to show the link between the improved baselines of IR over DRI, to the quality of the processed MMD results between the two detectors.
The study result showed that the IR data led to a more reproducible Mn value of the samples. This is because, as expected, the IR5 baseline shows fewer impurity peaks than the DRI baseline at the low molar mass tail of the GPC curve at the long elution time. Although not being widely publicized, the same precision problems of DRI detectors exist at the high molar mass side of the GPC curve that affect the reproducibility of the Mw and Mz values, and the quality of the Mark-Houwink plot derived from the triple-detector GPC, using data from additional online viscometer and light scattering detectors. A novel approach to construct MH plots not using concentration detector is also presented and compared.
Another relevant difference in the behavior of both detector types is the fact that the DRI response changes at low molar masses, due to the dependence of dn/dc with M, while the IR response factor is quite uniform with M. A correction for the effect of M in DRI response is here outlined.