Development of new analytical methods to characterize the heterogeneity of cellulose acetates

Cellulose is the most abundant natural polymer worldwide. The importance of cellulose does not arise only from the outstanding physical properties of cellulose itself, but also from the fact that cellulose can be converted to yield cellulose derivatives having interesting properties. Cellulose derivatives are regarded as complex copolymers which are heterogeneous at least in molar mass and chemical composition. This heterogeneity critically affects many of the properties of these derivatives such as adhesion strength, solubility, viscoelasticity and ability to control drug release from hydrophilic tablets and needs to be investigated. Despite the large number of applications, today’s characterization strategies for cellulose derivatives include mainly characterization of molar masses and their distribution, average degree of substitution (DS) and the distribution of the substituents within the individual anhydroglucose units (AGUs), i.e. partial DS in positions of O-2, O-3, and O-6 atoms. Size exclusion chromatography (SEC) is an established technique for the characterization of molar masses and their distribution of cellulose and cellulose derivatives. NMR techniques are employed to determine the average DS and the distribution of substituents on the monomer level. However, the use of cellulose derivatives for a particular application is strongly dependent on the distribution of the substituents among the polymer chains (heterogeneity of 1st order) and along the polymer chains (heterogeneity of 2nd order). Frequently, the polymer chains are (partially) degraded by using acids or enzymes. The degradation of the sample results in a mixture of monomers and/or oligomers present in different molar ratios. The resulting products are separated and characterized subsequently in detail. For this purpose, various analytical techniques, alone or in combination, such as SEC, anion exchange chromatography (AEC), gas liquid chromatography (GLC) and mass spectrometry (MS) are employed on the hydrolyzed products to provide information on both monomer composition and the substituent distribution on the oligomer levels. This procedure yields the distribution of the differently substituted AGUs along the polymer chain (2nd order heterogeneity). In general, the information is lost on whether the different monomeric or oligomeric units result from the same or from different chains, if the sample is partially or fully degraded. Therefore, establishing methods to characterize the chemical heterogeneity of cellulose derivatives on the level of intact polymeric chains is still a highly challenging task. Gradient HPLC has been shown to be a powerful tool for separating (co)polymer molecules according to chemical composition. The separation methods allow calculating the chemical composition distribution (CCD) of the copolymers. Nevertheless, knowledge on the separation of cellulose derivatives, particularly cellulose acetates (CA), with respect to DS is very limited and attempts to perform separations of such polymers according to chemical heterogeneity are scarcely found in literature. The present research work was mainly focused on the applicability of liquid chromatography for the analysis and characterization of molecular heterogeneity of CAs. The following significant results were obtained: 1. From the synthesis, it was shown that alkaline partial saponification of a high DS CA is a suitable method for preparing CAs of different average DS without altering the degree of polymerization. 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 added. 2. FTIR / ATR could be used to determine DS for very small sample amounts. This made it possible to characterize chromatographic fractions with respect to their DS. 3. From solubility tests, dimethyl sulfoxide (DMSO) and dimethylacetamide (DMAc) / lithium chloride (LiCl) were identified to dissolve the CAs, irrespective of DS. Therefore, these two solvents were utilized for chromatographic experiments. 4. An SEC method for characterization of molar masses and their distribution of the CAs in the DS-range of DS = 1.5-2.9 was developed. SEC light scattering investigations in pure DMSO and DMAc revealed aggregates which could be completely suppressed by the addition of LiCl. However, the significantly lower refractive index increment (dn/dc) of CA in DMSO as compared to DMAc made DMAc the preferred choice for SEC experiments. Samples differing in DS but being prepared from the same parent material showed nearly the same elution profile and nearly identical calibration curves, indicating that variations of the DS within the DS-range of DS = 1.5-2.9 do not alter remarkably the hydrodynamic volume. As a consequence, all the CA samples can be evaluated based on the same calibration curve, irrespective of their DS. The comparison of the true molar masses obtained by light scattering with PMMA equivalent molar masses revealed that the latter overestimated the absolute ones significantly. Therefore, correction factors were determined allowing calculating the true molar masses based on a PMMA calibration curve. 5. A gradient HPLC method for separating CAs within the DS range of DS = 1.5-2.9 was developed. By coupling liquid chromatography with infrared spectroscopy it was proved that a separation by DS was achieved not only for samples clearly differing in their average DS, but also within a single sample. Thus, the developed method for the first time allowed determination of the DS distribution of intact cellulose acetate chains, irrespective of DS. 6. Having established separations by DS and by molar mass, the correlations between DS and molar mass were examined. For this purpose, two dimensional separations were performed. First, a chromatographic separation according to DS was carried out using the gradient method developed before. The fractions, which were assumed to be homogenous in composition, were subsequently separated by SEC. The newly developed 2D LC system allowed new insights into the heterogeneity of the CAs