Cellulose is the main load-bearing component in plant fibre due to its covalent β-1→4- link that bonds glucose molecules into a flat ribbon and tight network of intra- and
intermolecular hydrogen bonds. It is possible to manipulate the intra- and intermolecular
hydrogen bonds in order to embed highly crystalline cellulose in a matrix of non-crystalline
cellulose, thereby creating self-reinforced cellulose composites. Cellulose is an excellent choice
of raw material for the production of sustainable and high-strength composites by selfconsolidation
of cellulose since it is readily biodegradable and widely available. Nowadays, the
cellulose industry makes extensive use of solvents. A multitude of solvents for cellulose is
available but only a few have been explored up to the semi-industrial scale and can qualify as
"sustainable" processes. An effective solvent for cellulose is a mixture of the LiCl salt and
organic solvent N,N-Dimethylacetamide (DMAc). Once cellulose has been dissolved, the
cellulose/LiCl/DMAc mixture can be precipitated in water. Preliminary results showed that a
solution of 1 wt.% kraft cellulose in 8 wt.% LiCl/DMAc that was precipitated in water formed
an hydrogel where cellulose chains were held in their amorphous state and in which no
crystalline phase was detected by wide angle X-ray diffraction (WAXD). The initially
amorphous cellulose started crystallizing by cross-linking of hydrogen bonds between the
hydroxyl groups of the cellulose chains when the cellulose gel was dried and the water to
cellulose ratio reached 7 g/g. The final form was poorly crystalline but distinct from amorphous
cellulose. In order to study all-cellulose composites at a fundamental level, model all-cellulose
composite films were prepared by partly dissolving microcrystalline cellulose (MCC) powder
in an 8% LiCl/DMAc solution. Cellulose solutions were precipitated and the resulting gels
were dried by vacuum-bagging to produce films approximately 0.2-0.3 mm thick. Wide-angle
X-ray scattering (WAXS) and solid-state ¹³C NMR spectra were used to characterize molecular
packing. The MCC was transformed to relatively slender crystallites of cellulose I in a matrix
of paracrystalline and amorphous cellulose. Paracrystalline cellulose was distinguished from
amorphous cellulose by a displaced and relatively narrow WAXS peak, by a 4 ppm
displacement of the C-4 ¹³C NMR peak, and by values of T₂(H) closer to those for crystalline
cellulose than disordered cellulose. Cellulose II was not formed in any of the composites
studied. The ratio of cellulose to solvent was varied, with greatest transformation observed for
c < 15%, where c is the weight of cellulose expressed as percentage of the total weight of
cellulose, LiCl and DMAc. The dissolution time was varied between 1 and 48 h, with only
slight changes occuring beyond 4 h. Transmission electron microscopy (TEM) was employed
to assess the morphology of the composites. During dissolution, MCC in the form of fibrous
fragments were split into thinner cellulose fibrils. The composites were tested in tension and
fracture surfaces were inspected by scanning electron microscopy (SEM). It was found that the
mechanical properties and final morphology of all-cellulose composites is primarily controlled
by the rate of precipitation, initial cellulose concentration and dissolution time. All-cellulose
composites were produced with a tensile strength of up to 106 MPa, modulus up to 7.6 GPa
and strain-to-failure around 6%. The precipitation conditions were found to play a large role in
the optimisation of the mechanical properties by limiting the amount of defects induced by
differential shrinkage.
Dynamic mechanical analysis was used to study the viscoelasticity of all-cellulose
composites over temperatures ranging from -150℃ to 370℃. A β relaxation was found
between -72 and -45℃ and was characterized by an activation energy of ~77.5±9.9 kJ/mol,
which is consistent with the relaxation of the main chain through co-operative inter- and
intramolecular motion. The damping at the β peak generally decreases with an increase in the
crystallinity due to enhanced restriction of the molecular motion. For c≤15%, the crystallinity
index and damping generally decreased with an increasing dissolution time, whereas the size
distribution of the mobile entities increases. A simple model of crystallinity-controlled
relaxation does not explain this phenomenon. It is proposed that the enhanced swelling of the
cellulose in solution after higher dissolution times provides a more uniform distribution of the
crystallites within the matrix resulting in enhanced molecular constriction of the matrix
material. For c = 20%, however, the trend was the opposite when the dissolution time was
increased. In this case, a slight increase in crystallinity and an increasing damping were
observed along with a decrease in the size distribution of the mobile entities. This phenomenon
corresponds to a re-crystallisation accompanied with a poor consolidation of the composite. A
relaxation ɑ₂ at ~200℃ is attributed to the micro-Brownian motion of cellulose chains and is
believed to be the most important glass transition for cellulose. The temperature of ɑ₂
decreased with an increase in crystallinity supposedly due to enhanced restriction of the mobile
molecular phase. A high temperature relaxation which exhibited two distinct peaks, ɑ₁﹐₂ at
~300℃ and ɑ₁﹐₁ at ~320℃, were observed. ɑ₁﹐₂ is prevalent in the cellulose with a low
crystallinity. A DMA scan performed at a slow heating rate enabled the determination of the
activation energy for this peak as being negative. Consequently, ɑ₁﹐₂ was attributed to the
thermal degradation onset of the surface exposed cellulose chains. ɑ₁﹐₁ was prevalent in higher
crystallinity cellulose and accordingly corresponds to the relaxation of the crystalline chains
once the amorphous portion starts degrading, probably due to slippage between crystallites.
The relative ɑ₁﹐₁/ɑ₁﹐₂ peak intensity ratio was highly correlated to the amount of exposed chains
on the surface of the cellulose crystallites.
Novel aerogels (or aerocellulose) based on all-cellulose composites were also prepared
by partially dissolving microcrystalline cellulose (MCC) in an 8 wt.% LiCl/DMAc solution.
Cellulose gels were precipitated and then processed by freeze-drying to maintain the openness
of the structure. The density of aerocellulose increased with the initial cellulose concentration
and ranged from 116 to 350 kg.m⁻³. Aerocellulose with relatively high mechanical properties
were successfully produced. The flexural strength and modulus of the aerocellulose was
measured up to 8.1 MPa and 280 MPa, respectively.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/2147 |
| Date | January 2008 |
| Creators | Duchemin, Benoît Jean-Charles |
| Publisher | University of Canterbury. Mechanical Engineering |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Benoît Jean-Charles Duchemin, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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