Key components in SEC analysis

1 The solvent. The solvent should be chosen to best dissolve the sample, to be compatible with the column packing and to permit detection. The most commonly used eluents in SEC are tetrahydrofuran (for polymers that dissolve at room temperature), 9-dichlorobenzene and trichlorobenzene at 130 C and 150°C (for crystalline polyolefins) and phenolic solvents at 100 C (for condensation polymers, such as polyamides and polyesters). For the more polar polymers, dimethylformamide and aqueous eluents may be employed, but care is required in avoiding interactions between the polymer (sample) and the gel (packing). Flow rates of 1 ml/min are typical for SEC analysis. 

Modern SEC columns are capable of achieving efficiencies up to 80 000 plates per metre and are available in 3, 5, 10 and 20 pm particle sizes. Analytical columns are typically 300 mm x 7.5 mm i.d. and preparative columns 300 or 600 mm x 25 mm i.d. The strategy of using multiple columns ultimately results in longer analysis times. An SEC separation normally takes 20-30 min. 

Another specific concentration detector is the infrared (IR) flow-through spectrometer. This detector monitors a specific absorbing group in the solute or polymer. Like the UV detector, the IR detector is relatively insensitive to fluctuations around a set temperature and is therefore particularly suitable for temperatures far above ambient conditions. Indeed, one of the uses for IR detection is in the molar-mass measurement of polyolefins at temperatures exceeding 140°C. Infrared detection is also useful in measuring polymer stoichiometry as a function of molar mass (by monitoring specific functional groups in a copolymer [4]). The detector, however, is solvent-limited in terms 

62232 Molecular-mass detectors. Size-exclusion molar-mass calibration is a complicated matter. Calibration curves differ for different polymer types, and for most commercial polymers, direct molar-mass calibration is not possible because of the lack of suitable known molecular-weight standards of sufficiently narrow mass distribution, and of the same chemical structure. The alternative is to determine the polymers molar mass in the SEC eluent, in situ by the use of an on-line molar-mass-sensitive detector. Two such detectors are now commercially available, the light-scattering detector and the recently introduced viscosity detector[5-8]. 

SEC with on-line light-scattering detection (SECILS) 

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