Molar mass characterization of cellulose acetates over a wide range of high DS by size exclusion chromatography with multi-angle laser light scattering detection

Abstract

A series of cellulose acetate (CA) samples with an average degree of substitution (DS) ranging from DS = 1.5 to DS = 2.3 was synthesized by partial saponification of a high DS sample (DS = 2.6). The comparison of the theoretical DS-values with ones determined by 1H NMR showed that the deacetylation reaction can be well controlled by the amount of sodium hydroxide (NaOH) added. The average molar masses of the samples and their molar mass distributions were characterized by size exclusion chromatography (SEC) with multi-angle laser light scattering (MALLS) detection in N,N-dimethyl acetamide (DMAc) containing 250 mmol/L lithium chloride (LiCl) in order to effectively suppress aggregation of the samples. The dependences of molar mass versus elution volume for samples of different DS can be well described by a common calibration curve. This allows using the same calibration curve for determination of molar masses of unknown samples, irrespective of their DS. A comparison of the absolute molar masses determined by light scattering with the molar masses obtained using a poly(methylmethacrylate) (PMMA) calibration curve revealed that the PMMA equivalent molar masses overestimated the absolute molar masses by a factor of approximately 3. Correction factors were determined making it possible to convert a PMMA calibration curve into a CA calibration curve. This procedure allows determination of correct CA molar masses based on commercially available PMMA-standards without using a LS instrument for samples in the DS range investigated.