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Home > Application Notes
Copolymer Compositional Analysis Using GPC/SEC and Tetra Detection
Gel permeation chromatography, also known as Size Exclusion Chromatography, is a popular analytical tool for characterizing natural and synthetic polymers and proteins. Rapid developments in industrial, biomedical, pharmaceutical and food technologies, however, have provided a strong push to extend the capability of conventional calibration beyond relative molecular weight measurements. In response, many researchers have implemented the technique of Tetra Detection–the combination of selective concentration detectors with viscosity and light scattering detectors to obtain absolute Mw and information on structure and conformation.
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Figure 1. Tetra Detector chromatogram of PMMA-PMAS copolymer showing RI, UV, viscometer and light
scattering responses. | The development and synthesis of specialized copolymers, materials comprised of two or more distinct structural units, and their widespread use in industrial and pharmaceutical applications has introduced even more complexity into the characterization equation. Copolymers, as opposed to homopolymers, can present special analytical issues depending on their structural composition and its resultant impact on the dn/dc, or refractive index increment. Homogeneous copolymers with predictable monomer sequences can often be treated as homopolymers for characterization purposes. Inhomogeneous copolymers with varying dn/dc's cannot be properly treated as homopolymers and must be characterized using viscometric or hyphenated techniques, or by making assumptions regarding dn/dc–a process which can introduce significant error.
However, many copolymers are comprised of comonomers that display differential responses to selective concentration detectors. The most common examples are copolymers in which one of the comonomers contains a UV chromaphore, making it sensitive to UV detection. The presence of the UV chromaphore allows one to ratio RI and UV detectors to determine true concentration profiles. When the RI and UV are then combined with viscometric and light scattering detectors to create Tetra Detector systems, absolute molecular weight, molecular size, intrinsic viscosity, structure and compositional analysis can be obtained across the entire copolymer distribution.
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Figure 2. Overlay plot of weight fraction of PAMS versus retention volume as PMMA is added to the reaction. | Figure (1) displays a Tetra Detector chromatogram of a PMMA-PAMS copolymer in which the dn/dc has been determined from the UV/RI ratio, allowing a calculation of true concentration and % PAMS. The calculated concentration is then employed, along with the four capillary differential viscometer detector and a light scattering photometer, to determine absolute molecular weight and intrinsic viscosity. In Figure (2), a Tetra Detector featuring RI and UV detectors has been utilized to actually track the polymerization reaction as methyl-methacrylate is added to a reactor containing polyalpha-methyl-methacrylate, resulting in the formation of the PMMA-PAMS copolymer.
It's clear that copolymer characterization requires consideration of issues such as structure and composition. Homogenous copolymers can often be treated as homopolymers and characterized as such without introducing significant errors. More complex copolymers, however, require the use of differential, selective concentration detectors along with advanced viscometric and light scattering detectors for complete and accurate determination of molecular parameters.
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