Size Characterisation of Water-Soluble Polymers Using Asymmetrical Flow Field-Flow Fractionation

Abstract: Size, as expressed by molar mass, radius of gyration or hydrodynamic radius, for example, is a fundamental parameter of polymers in solution. Thus, there is a strong need for the characterisation of both the size and shape of polymers, particularly of those utilised in various industrial applications. In this thesis, a relatively novel technique, asymmetrical flow field-flow fractionation (flow FFF), was employed for the size charaterisation of water-soluble polymers of varying origin. The applicability of flow FFF for the size characterisation of a charged amphiphilic graft copolymer was tested first. The micellisation and aggregation behaviour of this copolymer was studied measuring hydrodynamic size in different salts at differing salt concentrations and pH by means of flow FFF. Hydrodynamic size was shown to increase dramatically when ionic strength was increased, due to the formation of large micelles and aggregates. Flow FFF was able here to rapidly characterise both single polymer molecules, of small hydrodynamic diameter (approximately 5 nm), and large aggregates, more than 100 nm in diameter. Combined with a multi-angle light scattering (MALS) detector, it could also be used for determining molar mass and radius of gyration. This possibility was first demonstrated for two model polysaccharides, dextrans and pullulans, of known molar mass and polydispersity. Flow FFF - MALS provided here measurements of molar mass in good agreement with the manufacturer's data. Flow FFF - MALS was then used for the characterisation of modified celluloses and carrageenans. For these polymers, displaying a complex solution behaviour and thus very diffucult to characterise, distributions of molar mass and radius of gyration were readily obtained. The importance of choosing appropriate experimental conditions so as to avoid artifacts was clearly demonstrated. For the carrageenans, very large structures, with a molar mass of several million, were found to be eluted in a reverse elution order in flow FFF.