Biomedical Engineering Reference
In-Depth Information
is divided into swellability in the crystalline regions and swellability between
crystalline regions. The former refers to the fact that the swelling agent can only
reach the surface of crystalline and amorphous regions, and the X-ray pattern of
cellulose does not change. The latter refers to the fact that the crystallization regions
of microfilaments are permeated with the swelling agent and then swell to generate
new crystalline lattice and display a new X-ray pattern. Unlimited swelling of the
cellulose is dissolution. The hydroxyl groups in the cellulose have polarities. As a
swelling agent, the greater polarity the liquid has, the greater degree of swelling the
cellulose has. The metal ion in the alkali solution is usually in the form of aquo ions,
which is more favorable for entering the crystallization region. Usually, 15-20 %
NaOH will cause swelling within crystalline regions. If the alkali concentration
is increased, the radius of aquo ions is reduced because the ion density is too
high, resulting in the drop of swellability. Except for alkali, the swellability of
other swelling agents, sorted from strong to weak, is as follows: phosphoric acid,
water, polar organic solvents, and so on. Cellulose is saturated in a concentrated
solution of NaOH to generate alkali cellulose. Although alkali cellulose is washed
with water and dried, such changes cannot restore it to its original condition. Alkali
cellulose may have a crystalline form of hydrate cellulose that is more stable than
that of natural cellulose, which would increase its absorbability and make it easy to
react with a variety of reagents. Using alkali to impregnate celluloses is also called
mercerization
. In addition, alkali cellulose is the important intermediate product for
the production of viscose fibers and derivatives of cellulose ether [
9
].
Characteristics of polymer compounds are high molecular weight and a strong
cohesive force. They have movement difficulties in the system and a poor diffusion
capacity, so they cannot be dispersed in a timely manner in the solvent. The
solvent dissolved with celluloses is not the real cellulose solution, but the mixed
product is obtained by mixing celluloses and components in liquids. The solvents
of the cellulose can be divided into two categories: aqueous and nonaqueous.
Aqueous solvents include the following:
Inorganic acids, such as H
2
SO
4
(65-
80 %), HCl (40-42 %), H
3
PO
4
(73-83 %), and HNO
3
(84 %) can lead to the
homogeneous hydrolysis of cellulose. Concentrated HNO
3
(66 %) does not dissolve
the cellulose but forms an addition compound with cellulose.
①
Lewis acids, such
as LiCl, ZnCl
2
,Be(ClO
4
)
2
, thiocyanate, iodide, bromide, and others, could dissolve
celluloses with a low DP.
②
Inorganic bases, such as NaOH, hydrazine and sodium
zincate, NaOH, and others can only dissolve cellulose with a low DP.
③
Organic
bases, such as quaternary ammonium bases (CH
3
)
4
NOH, amine oxides, and others,
are also aqueous solvents. The application of amine oxide solvent to dissolve
cellulose can be used to manufacture the man-made fibers.
④
Complexes, such as
copper oxide ammonia (cuoxam), copper ethylenediamine (cuen), cobalt hydroxide
ethylenediamine (cooxen), zinc ethylenediamine (zincoxen), cadmium ethylenedi-
amine (cadoxen), and the iron-tartaric acid-sodium complex (EWNN, an aqueous
alkaline solution of iron sodium tartrate) are included as aqueous solvents [
9
].
A nonaqueous solvent of cellulose refers to a nonaqueous or less-aqueous solvent
that is based on the organic solvents. It consists of activators and organic liquids.
The organic solvents can be used as a component of the active agent and as a
⑤
