Toward Absolute Chemical Composition Distribution
Measurement of Polyolefins by High-Temperature Liquid Chromatography
Hyphenated with Infrared Absorbance and Light Scattering Detectors
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Abstract
Chemical composition distribution
(CCD) is a fundamental metric
for representing molecular structures of copolymers in addition to
molecular weight distribution (MWD). Solvent gradient interaction
chromatography (SGIC) is commonly used to separate copolymers by chemical
composition in order to obtain CCD. The separation of polymer in SGIC
is, however, not only affected by chemical composition but also by
molecular weight and architecture. The ability to measure composition
and MW simultaneously after separation would be beneficial for understanding
the impact of different factors and deriving true CCD. In this study,
comprehensive two-dimensional chromatography (2D) was coupled with
infrared absorbance (IR5) and light scattering (LS) detectors for
characterization of ethylene–propylene copolymers. Polymers
were first separated by SGIC as the first dimension chromatography
(D1). The separated fractions were then characterized by the second
dimension (D2) size exclusion chromatography (SEC) with IR5 and LS
detectors. The concentrations and compositions of the separated fractions
were measured online using the IR5 detector. The MWs of the fractions
were measured by the ratio of LS to IR5 signals. A metric was derived
from online concentration and composition data to represent CCD breadth.
The metric was shown to be independent of separation gradients for
an “absolute” measurement of CCD breadth. By combining
online composition and MW data, the relationship of MW as a function
of chemical composition was obtained. This relationship was qualitatively
consistent with the results by SEC coupled to IR5, which measures
chemical composition as a function of logMW. The simultaneous measurements
of composition and MW give the opportunity to study the SGIC separation
mechanism and derive chain architectural characteristics of polymer
chains