CHARACTERIZATION OF POLYOLEFINS BY SIZE EXCLUSION CHROMATOGRAPHY WITH LOW-ANGLE LIGHT-SCATTERING AND CONTINUOUS VISCOMETER DETECTORS

Molecular-weight distributions (MWDs) of polyolefins are currently measured by high-temperature size exclusion chromatography. The raw data are converted to MWDs using universal calibration with a differential refractive index (d.r.i.) concentration detector or by direct molecular-weight measurements with a continuous viscometer (c.v.) or a low-angle laser light scattering (l.a.l.l.s.) detector. None of these techniques provides a picture of the true distribution. Simple d.r.i.- universal calibration may fail to sense the high-molecular-weight tail of the distribution, which is important in some applications. The l.a.l.l.s. technique is most sensitive to high-molecular-weight polymers but is also least precise. The c.v. method is presently the most convenient single technique. The most information is obtained, however, when all these detectors are employed. Both the l.a.l.l.s. and c.v., for example, should be used to verify that the polymer sample is actually dissolved, since dissolution of semicrystalline polyolefins is not a trivial exercise.