Structure, accessibility and 'reactivity' of cellulose I as revealed by CP/MAS13 C-NMR spectroscopy and atomic force microscopy.
The dissolving pulps used in this thesis are high-grade cellulose pulps, with low amounts of hemicellulose, degraded cellulose and lignin, produced by acid bi-sulphite pulping of a fast growing South African hardwood Eucalypt clone. Microcrystalline cellulose (MCC) grade, viscose grade and cellulose acetate grade dissolving pulp were produced using a 4 stage bleaching process. MCC, viscose and cellulose acetate are the cellulose derivatives of 91% α-cellulose, 92% α-cellulose and 96% α-cellulose respectively. The key properties of the dissolving pulp considered for cellulose derivatisation are: (1) Structure (2) Accessibility and (3) ‘Reactivity’. The ‘reactivity’ depends to a large extent on the supra-molecular structure of cellulose I. Supra-molecular structure deals with the arrangement of cellulose I molecules into cellulose fibrils which then make up the cellulose fibril aggregate. The accessibility of cellulose I depends on the surface area, as determined by the size of the cellulose fibril aggregates, that are accessible; the structure of the cellulose molecules, which will determine which hydroxyl groups are accessible; and the size and type of reagent used during derivatisation. Supra-molecular changes in cellulose fibril aggregation of cellulose I, in hardwood acid bi-sulphite pulp, during bleaching and drying were studied using Atomic Force Microscopy (AFM) and Cross-polarization/Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance Spectroscopy (CP/MAS 13C-NMR – Solid state NMR) in combination with spectral fitting. There was a marked increase in cellulose fibril aggregation (i.e. supra-molecular structure) during bleaching of hardwood acid bi-sulphite pulp using 96% α-cellulose conditions. In contrast there was no increase in cellulose fibril aggregation pulp bleached using 91% α-cellulose and 92% α-cellulose bleaching conditions. An increase in hemicellulose and degraded cellulose / short chain glucan was shown to correlate with a decrease in cellulose fibril aggregation recorded using solid state NMR. Further changes in supra-molecular structure were noticed when each of the dissolving pulp samples were dried. First time drying of hardwood acid bi-sulphite pulp samples induces a significantly different degree of irreversible cellulose fibril aggregation in the 92% α-cellulose and the 96% α-cellulose pulp samples. The irreversible increase in cellulose fibril aggregation correlates with the estimated amount of hemicellulose and degraded cellulose / short chain glucan present in the pulp. The percentage increase in cellulose fibril aggregation upon drying is as follows: 96% α-cellulose > 92% α-cellulose > 91% α-cellulose. Hemicellulose and degraded cellulose / short chain glucan are among the wet chemical properties that influence cellulose fibril aggregation and the presence in dissolving pulp samples could provide steric hindrance preventing the aggregation of fibrils. Reactivity studies were carried out on the 91% α-cellulose, 92% α-cellulose and 96% α-cellulose grades of dissolving pulp. During 91% α-cellulose reactivity studies, there was no relationship between cellulose fibril aggregation, acid hydrolysis or MCC preparation. Other possible techniques for 91% α-cellulose reactivity evaluation such as the degree of polymerization (DP) determination using AFM have been discussed. Size exclusion chromatography with multi-angle laser light scattering detection was shown as a more suitable method of estimating the reactivity of 92% α-cellulose pulp samples. 96% α-cellulose reactivity studies were carried with the aid of a model system consisting of the acetylation of high purity pulp samples viz. cotton linters cellulose and 96% α-cellulose. Results indicate that the initial reaction rate constant is proportional to the specific surface area for the two cellulose pulp samples showing that specific surface area is directly related to initial reactivity of the performed acetylation. This work has shown that it is possible to control the cellulose fibril aggregation and hence specific surface area in laboratory produced 91% α-cellulose, 92% α-cellulose and 96% α-cellulose by the method in which the pulp is dried. Thus controlling cellulose fibril aggregation can probably be one viable route for controlling the initial reactivity of dissolving pulp towards acetylation.