Biomedical Engineering Reference
In-Depth Information
solvent of the activator, which can make the solvent have a larger polarity to
dissolve cellulose. Therefore, the mechanism of cellulose dissolved in a nonaqueous
solvent system cannot be easily explained by swelling theory, as in aqueous
solvents. The detailed mechanism of this process can be expressed as follows:
①
An oxygen atom and a hydrogen atom of cellulose hydroxyl participate in the
interaction of the EDA; the oxygen atom and the hydrogen atom act as a
-
electron donor and a
The active agent in the
solvent system has an electron donor center and an electron receiving center; the
spatial location of these two centers is suitable for interaction with the oxygen
atom and hydrogen atom of cellulose hydroxyl.
•
-electron acceptor, respectively.
②
There is necessarily a suitable
scope for the interaction strength of the EDA, causing the centers of the donor
and acceptor to interact in polar organic solvents. When the hydroxyl charge
separates to some extent, the complex of cellulose molecular chains is separated
and dissolved.
Several
③
①
Paraformaldehyde/dimethyl sulfoxide (PF/DMSO) is an excellent new solvent
system that is not biodegradable. PF resolves into formaldehyde by heating,
and then formaldehyde reacts with the hydroxyl group of cellulose to generate
hydroxymethylcellulose, which is dissolved in DMSO.
different
systems
of
nonaqueous
solvents
of
cellulose
exist:
Dinitrogen tetroxide/
dimethylformamide (N
2
O
4
/DMF or DMSO) is an intermediary derivative of the
reaction of N
2
O
4
with cellulose to generate nitrite esters; it can be dissolved in DMF
or DMSO.
②
③
Amine oxides directly dissolve cellulose without the intermediate
derivatives.
Liquid ammonia/ammonium thiocyanate restricts the dissolution of
the cellulose; the solvent consisting of 72.1 % (w/w) NH
4
SCN, 26.5 % (w/w)
NH
3
, and 1.4 % (w/w) H
2
O has the maximum dissolving ability.
④
Lithium
chloride/dimethylacetamide (LiCl/DMAC) also directly dissolves cellulose without
the intermediate derivatives. At room temperature, the LiCl/DMAC solution is
stable and can be used for reeling off raw silk and film forming. Recently, research
on nonaqueous solvents of cellulose has been active; they not only can be used to
produce artificial fiber and films but also can be available for processing cellulosic
materials and for the use of cellulose in homogeneous conditions to produce
cellulose derivatives. The problems of cellulose solvents are the low solubility
of cellulose, high price and low recovery of solvents, and environmental pollution.
Thermal decomposition of cellulose is in the narrow temperature range of 300-
375
ı
C. Different products depend on different temperatures. Heated at a low
temperature (200-280
ı
C), cellulose mainly dehydrates into dewatering cellulose
and then forms charcoal and gas products. Heated at higher temperatures, cellulose
separates into flammable volatile products (tar). The most important intermediate
product of cellulose high-temperature thermal degradation is laevoglucose, which
can be further degraded into low molecular products and tar-like products. Tar-
like products can be polymerized into an aromatic ring structure similar to graphite
structure at high temperature (400
ı
C or higher). Mechanical degradation of the
cellulose occurs because cellulose in the mechanical process can effectively absorb
⑤
