Use of continuous viscometer and light scattering detectors in characterization of polyolefins: Comparisons of data from individual and combined detectors

Abstract

SYNOPSISMolecular weights of polyethylenes have been characterized using differential refractometer (DRI) , continuous viscometer (CV) , and low-angle laser light (LALLS) detection. In normal operation with the latter two detectors, the DRI is also employed as a concentration detector. The intrinsic viscosity of the whole polymer can be derived from the CV without use of a DRI concentration detector. If one calibrates the size exclusion chromatography (SEC) columns using the CV detector, it is possible to use this universal calibration relation and the CV detector to calculate number average molecular weight ( M , ) of the polymer. Weight average molecular weight (M,) of the sample can be calculated using data from the LALLS alone, without reference to the DRI. These variations of the analysis were tested and the advantages and limitations of the different detectors were compared using standard reference polyethylene samples in solution in 1,2,4-trichlorobenzene at 145°C.

High molecular weight tail and long-chain branching in SRM 1476 polyethylene

“…The detection of a second peak at small elution volumes is consistent with earlier reports. 6,10,18,20 This secondary peak was not present in the response of the DRI and VD detectors, which is consistent with earlier on-line viscometer and SEC studies. [3][4][5][6][7]9,10 The difference in response between the LSD and the VD detectors is not surprising because it reflects the difference in sensitivity.…”

“…Over most of the range the distributions are quite similar. The higher sensitivity of the LSD, results in a small tail with M in excess of 10 6 Da. Clearly, in order to calculate the molecular weight averages one needs to rely on an extrapolation.…”

“…Particularly striking, however, are the values of z-average molecular weight (M z ) obtained by SEC-LSD, which can be up to an order of magnitude higher than the value obtained by SEC-VD. 12,17,18 It was these differences in molecular weight averages that first led some authors 6,7,10,18,19 to suggest the presence of a high molecular weight tail in the polymer.…”

“…This can be rationalized in terms of a particular combination of LCB concentration effects and molecular architecture. The population M Ͼ 10 6 is different in kind from that at M Ͻ 10 6 . These findings explain the discrepancies observed in the molecular weight averages of SRM 1476 reported in the literature, in particular M z .…”

Correction for interdetector volume in size exclusion chromatography (SEC)

“…It is expected, then, that the three different detectors will produce A?, and higher averages that rank in the order DRI < viscometer < LALLS. 24 This is what is observed with the differences between the three detectors greater for than for M,. (In the case of an, the LALLS is often too noisy and insensitive to low molecular weight species to provide good values.…”