Molar mass distribution of poly(methyl methacrylate) by sedimentation analysis in a theta solvent
Molar mass distributions of poly(methyl methacrylate) samples with molar masses in the range between 50 000 and 20 000 000 g·mol−1 and with low polydispersities were measured by sedimentation analysis in the analytical ultracentrifuge (Beckman Model E) with UV optics (λ = 226 nm) using acetonitrile as solvent at the theta temperature (35 °C).
We found good agreement of the weight average molar masses Mw with those determined by light scattering and by size exclusion chromatography (SEC), respectively; however, there are somewhat larger discrepancies in the polydispersities (AUC vs. SEC). In contrast to literature data, we found that with 35°C, we were slightly below the theta temperature, so that the coefficients in the relationship between sedimentation constant and molar mass had to be fitted with the light-scattering results.
Further experiments will aim at an improved signal/noise ratio by employing a very fast analogue/digital converter for the data acquisition.
key wordsAnalytical ultracentrifuge molar mass distribution poly(methylmethacrylate) sedimentation analysis theta system UV scanner light scattering size exclusion chromatography
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