REPORT ON THE COOPERATIVE DETERMINATION O F MOLECULAR-WEIGHT AVERAGESOF POLYMERS BY SIZE-EXCLUSION CHROMATOGRAPHY .5. DEPENDENCE OF MOLECULAR-WEIGHT AVERAGES ON DETECTORS AND MOBILE PHASES

The effects of the differences in detectors and mobile phases on the c alculated molecular-weight averages are discussed based on the data ob tained from 1st and 2nd round-robin tests. Although the weight-average molecular weights (M(w)) of polystyrene (PS) samples remained unchang ed with the detectors, the Dumber-average molecular weights (M(n)) obt ained with an ultraviolet absorption detector (UV) were 74 similar to 88% of those with a differential refractometer (RI). The fractions at a lower molecular weight range of the differential molecular-weight di stributions, calculated using UV chromatograms, were higher than those based on RI chromatograms, which caused the M(n) obtained by a UV det ector to have smaller values than those by an RI detector. Peak broade ning due to the cell structure or the difference in the molecular-weig ht dependence of the response coefficients of UV and RI detectors were negative as the main reason. The most probable assumption was that th e end-structure of the PS samples affected the increase in the UV abso rption. When a comparison of the M(n) values between laboratories is r equired, the type of detector for size-exclusion chromatography must b e the same. The values of M(w) and M(w) for poly(methyl methacrylate), obtained with chloroform mobile phase, were 5 similar to 25% higher t han those with the tetrahydrofuran mobile phase. PS standards were use d for the calibration; in this case, the change in the values of M(n) and M(w) for other types of polymers than PS using different mobile ph ases had to be taken into account.