Chemistry Reference
In-Depth Information
H
H
1
6
4
H
OH
O
O
CH
2
OH
O
HO
H
H
H
H
HO
1
4
O
O
CH
2
OH
OH
H
6
H
Fig. 2.1
Cellulose formula [
4
]
glycosidic linkages [
1
-
3
]. In one repeating unit of cellulose molecule, there are one
methylol and two hydroxyl groups as functional groups [
4
].
Due to absence of side chains or branching, cellulose chains can exist in
an ordered structure. Therefore, cellulose is a semicrystalline polymer, and it contains
both crystalline and amorphous phases. Although it is a linear polymer and
contains two types of hydroxyl groups, primary hydroxyl in methylol group
(-CH
2
OH) at C-6 and secondary hydroxyl groups (-OH) at C-3 and C-4, both of
which are hydrophilic, it does not dissolve in water and in common solvents due to
strong hydrogen bondings between the cellulose chains. The hydrogen bondings
between the cellulose chains and van der Waals forces between the glucose units
lead to the formation of crystalline regions in cellulose [
5
]. In cellulose, there are two
types of hydrogen bondings: intramolecular and intermolecular [
6
]. Intramolecular
hydrogen bondings are of two types, i.e., O-2-H
O-5 links [
6
]. It
can be dissolved in DMSO-PF (dimethyl sulfoxide-paraformaldehyde), LiCl-DMAc
(lithium chloride-dimethylacetamide) [
7
],
N
-methylpyrrolidone-lithium chloride
(NMP-LiCl) [
7
], etc. Cellulose also dissolves in strong concentrated acids such as
H
2
SO
4
and H
3
PO
4
and certain metal complexes such as cuprammonium (cuam) or
cupriethylenediamine (cuen) that they break the hydrogen bonds between the cellu-
lose chains.
Cellulose has two crystal forms: cellulose I and cellulose II [
4
]. In the former,
cellulose chains are oriented in parallel conformation, and in the latter, antiparallel
one [
4
] (Fig.
2.1
). Cellulose has other polymorphic forms of cellulose III and
cellulose IV. According to XRD data [
8
], the crystallinity of cotton cellulose is
60 %which is in cellulose I form, and the rest is amorphous cellulose. The degree of
crystallinity (CrI) of cellulose, namely, the fraction of crystalline content in the
cellulose sample, is calculated from XRD patterns by using the peak intensities at
2
O-6 and -3-H
18
(
I
am
: intensity due to amorphous portion) and at 2
22.5
(
I
200
: inten-
θ ¼
θ ¼
sity attributed to both amorphous and crystalline phases) [
6
,
9
].
¼½ðI
200
I
am
Þ=I
200
CrI
100
(2.1)
Hermans et al. [
10
] calculated the crystallinity of cellulose (
X
c
) by another
equation given below:
X
c
¼ I
c
=ðI
c
þK þ I
am
Þ
(2.2)
