| Abstract: | Data are presented which show that when a polymer contains an appreciable amount of low molecular weight species below the diffusion limit of the osmometer membrane, the osmotic molecular weight, M?n, is generally higher than the M?n calculated from gel-permeation chromatography (GPC). Experiments were performed on samples of poly(vinyl chloride) (PVC) and high-cis polybutadiene polymers. Osmotic data were obtained in the usual manner, while GPC data were obtained using the universal calibration approach. It was found that when all polymer species below approximately 10,000 molecular weight were excluded from the calculation of M?n by GPC, agreement in M?n was obtained between membrane osmometry and GPC. The data obtained suggest that the choice of M?n as measured by membrane osmometry in the calibration of the GPC should not be done casually, as the measured M?n may not reflect the “true” value of that sample, especially when the polymer sample contains an appreciable amount of low molecular weight material. |
