AQUEOUS SIZE EXCLUSION CHROMATOGRAPHY OF NON-DERIVATIZED CELLULOSE: APPLICATION OF EXCLUDED VOLUME CONCEPTS TO CALIBRATION
Abstract
A method for determining the molecular weight distribution of non-derivatized cellulose by aqueous size exclusion chromatography (SEC) has been developed. It uses Sepharose CL-6B as the column packing material, 0.5 N NaOH as the eluent and cadoxen as the cellulose solvent. The traditional approach to SEC of cellulose has been to convert the cellulose to its nitrate, thereby making it soluble in organic solvents that are commonly used as eluent in SEC. The circumvention of the need to derivatize the cellulose in the proposed system results in significant reduction in analysis time. The configuration and the operating characteristics of the system are discussed. The effect of particle size distribution of the column packing material on separation efficiency is commented upon. Well-characterized primary standards for cellulose are not available for calibrating a SEC system. Hence, the need to use secondary standards e.g., sodium polystyrene sulfonates (NaPSS) or dextrans as calibrants for SEC. To use the calibration curve constructed from secondary standards for interpreting the elution data of other polymers, the mechanism of separation in SEC must be understood. Molecular size governs separation in SEC. The mean-square radius of gyration of a molecule is a measure of its molecular size. Benoit's calibration method uses the hydrodynamic volume as a measure of size, the implicit assumption being the constancy of the Flory viscosity parameter, (phi). This procedure did not lead to a single calibration curve for the NaPSS and dextrans at various NaOH strengths, nor for the NaPSS in NaCl solutions. An alternate method for SEC calibration is that of Coll and Prusinowski. The statistical theory, on which their development is based, allows for asymmetrical shape of molecule and partial draining of solvent through the polymer coil. The parameter (phi) now becomes a function of the polymer solvent interaction. The Coll-Prusinowski procedure led to an unique calibration curve for the NaPSS and dextrans irrespective of the strength of NaOH solutions. Aqueous solutions of polyelectrolytes such as NaPSS exhibit additional excluded volume effects due to electrostatic interactions causing the value of their reduced viscosity parameter, (phi)/(phi)(,0), to deviate from unity. The Coll-Prusinowski method does allow for deviations in the reduced viscosity parameter and hence would provide a better fit for the elution data of polyelectrolytes. However, it is inadequate for the NaPSS-NaCl system, especially in solutions of very low ionic strength. Interactions between the polymer and the chromatographic support become predominant in eluents of low ionic strength--calibration methods based on molecular size are no longer valid. The additional excluded volume effects in polyion solutions must be carefully considered and adequately corrected for, whenever necessary, in the methods suggested for calibrating aqueous SEC systems. SEC of nonderivatized cellulose in conjunction with acid hydrolysis may be used to follow structural changes in cellulose on solvent pretreatment. Crystallites of native cellulose exist in the cellulose I form. Solvent pretreatment with cadoxen or 70% H(,2)SO(,4) transforms the crystallite structure to that of cellulose II. The effect of solvent pretreatment may be described in terms of the folding chain model for cellulose. Native cellulose with a fold length of about 160 DP (degree of polymerization) is converted to a form with a fold length of 30 DP on solvent pretreatment. The extent of reduction in fold length on pretreatment is a measure of how effective the solvent is as a pretreatment agent.
Degree
Ph.D.
Subject Area
Chemical engineering
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