Structure, accessibility and 'reactivity' of cellulose I as revealed by CP/MAS13 C-NMR spectroscopy and atomic force microscopy.
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Date
2009
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Abstract
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.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2009.
Keywords
Cellulose--Synthesis., Theses--Chemistry.