Chemistry Reference
In-Depth Information
determined in the grafting reactions performed by the initiators CAN-HNO
3
[
95
,
96
,
101
,
103
,
124
], ceric ammonium sulfate [
49
], persulfates [
39
,
40
,
52
], and
KHSO
3
-CoSO
4
[
78
]. In the grafting of AAm-MA onto cellulose by CAN-HNO
3
initiator system, Gupta and Khandekar [
103
] determined that the disappearance rate
of Ce
4+
ions did not change with the variation of monomer concentration from 0.1
to 0.5 M and concluded from this finding that the Ce
4+
ions do not directly create
active radicals on the monomers. The high efficiency of grafting with Ce
4+
ions was
attributed to the creation of active radicals by CAN initiator preferentially on the
cellulose backbone than the monomers [
92
,
101
,
125
]. In addition, they [
101
]
observed that true grafting percentage (
G
T
%) increased with the increase in Ce
4+
concentration from 1.5
10
3
M to 7.5
10
3
M, but the higher concentrations
10
3
M led to decrease in
G
T
% due to hydrolysis of CAN and
being the hydrolysis product inactive for the creation of active sites in the absence
of sufficient amount of nitric acid (HNO
3
). They [
101
] have also determined that
the rate of grafting of these comonomers onto cellulose is dependent on the square
root of CAN concentration [
103
]. The similar findings were observed for the
grafting of EA and AAm onto cellulose by CAN-NHO
3
. Kim and Mun [
98
]
investigated the effect of the concentration of CAN in the solution pretreated
with wood pulp (WP) on the grafting of AAm in the presence of BAAm as cross-
linker. They [
98
] determined that grafting percentage increases with the increase in
CAN concentration of
of CAN than 7.5
10
3
the solution pretreated with WP from 1
to
10
3
M because the number of grafting sites on the cellulose backbone
increases. The further increase in the concentration of CAN solution to 5
2
10
3
10
3
M decreased the grafting percentage. The similar relationship was
also determined between the water absorbency of AAm-grafted Ce
4+
·WP and the
concentration of CAN in the pretreatment solution [
98
]. The maximum amount of
water absorption (2,700 g/g) was observed for the copolymer with grafting percent-
age of ca. 240 % which was prepared from Ce
4+
·WP treated with 5
and 10
10
3
M CAN
solution before the grafting. The water absorbency of AAm-grafted Ce
4+
·WP is
very high, but it should be taken into consideration that the grafting was performed
in the presence of cross-linker (the opinion of G¨rda˘). Although the authors [
98
]
have reported that they extracted the ungrafted homopolymer PAAm by successive
washings of graft product with the mixture 70 % isopropanol-water, it cannot be
completely removed from the reaction mixture since the grafting was performed in
the presence of cross-linker MBAAm, and the cross-linked homopolymer will not
be extracted by any solvent (the opinion of G¨rda˘). The increase in CAN concen-
tration leads to decrease in grafting yield, but the increase in homopolymer forma-
tion [
96
]. CAN prefers to form complex with cellulose over the monomer [
96
].
However, at higher concentrations of CAN, Ce
4+
ions form complex with the
monomer in addition to that with cellulose, and homopolymer formation can also
occur. The termination of growing polymer radicals is also accelerated with Ce
4+
concentration, and it leads to the decrease in grafting yield. When CAN was used as
initiator, the acid, mostly HNO
3
, has an important effect on the efficiency of
initiator for grafting. As known, the reaction of CAN with aqueous HNO
3
occurs
as written below:
