Indian Journal of Fibre & Tcxtile Rescarch
Vol . 30, June 2005, pp. 207-2 1 0
S tudy on superabsorbent polyacry lonitri le­
based fibre
X i aoyu H u
& Changfa X i ao"
Department of Material Science, Tianj i n Polytechnic University,
300 I 60 Tianjin, People's Republ ic of China
Received I I March 2004;
revised received and accepted 25 A ugllst 2004
Superabsorbent polymer fibres with maxi mum water absorb­
'
ency were produced using acrylonitrile and methyl methacrylate
as I"ilOnOmers, N-hydroxymethyl acrylamide as potential
crossl inking agent, dimethyl sulfoxide as solution and a7.0-bis­
isobutyronitrile as initiator. The copolymer solution was thcn
spun using dry-wet spinning method with water as coagUlation
bath. The fibres thus produced were heated to get crosslinking
structure and thei r surfaces were hydrolyzed hy .:l kaline solution.
The i nfluence of hydrolyzing conditions, such as temperature and
concentrations of alkali nc solution and N-hydroxymcthyl ac­
rylamide, on the fibre structure and properties was also studied
using FrIR, DSC, DMA and SEM techniques. The changes in
storage modulus, Tg and surface structures of fibres were also
studied. The superabsorbent polymer of about 40g/g water ab­
sorbency was obtained using N-hydroxymethyl acrylamide con­
centration of equal to about 1 0wt%monomer, alkali concentration
of about 1 5wt% and hydrolyzing time of about 5 min.
Keywords: Polyacrylonitrile fibre, Superabsorbent polymer,
Water absorbcncy
IPC Code: lnt. CI.7 A6 I L l 5/00, D06M 1 3/00
Superabsorbent polymers (SAPs) are water-insoluble
hydrophilic polymers having the ability of absorbing
large amount of water (> 10 times of their own
weight). Nowadays, though SAP particles have been
widely used in various fields l •2 , their shape restricts
their applications. To overcome this disadvantage, the
production of fibrous S APs draws great attention of
the researchers over the world. SAP fibres not only
have the characteristic of high absorptive speed, but
also can be converted into a wide range of textile
structures, which make them far more suitable for
specialist applications3 . Recently, there are some
reports about the preparation of SAP fibres4.5 , but
work on polyacrylonitrile-based SAP fibre is
relatively less.
"To whom ali the correspondence should be addressed.
Phone: 24528 1 38; Fax: +86-22-24528000;
E-mail: cfx i ao @ ljpu.edu.cn
In the present work, a kind of SAP fibre based on
polyacrylonitrile (PAN) whose surface can absorb
water evenly (water absorbency, 40g/g) and core can
keep the fibre ' s form and strength has been produced.
Acrylonitrile (AN) was purified by decompressed
distillation. Azo-bis-isobutyronitrile (AIBN) was puri­
fied by recrystallization from aqueous alcohol
(95wt%). N-hydroxymethyl acrylamide (NHMA)
(analytical grade) as potential crosslinking agent and
dimethyl sulfoxide (DMSO) as solution were com­
mercially procured and used as such. Methyl­
methacrylate (MMA) (analytical grade) was used as
monomer and NaOH (analytical grade) was used to
make the solution alkaline to hydrolyze the fibres. All
solutions were prepared with distilled water.
AN, NHMA, MMA, DMSO and AlBN were put
into a three-open flask (500m1) in a certain ratio under
N2 atmosphere. The mixture was heated to 65°C and
the temperature was maintained as such for 4h to ob­
tain a yellow and viscous solution. After spinning by
the dry-wet spinning method with water as coagula­
tion bath, the fibres were put into an oven at about
1 70° - 1 80°C for 10 min to obtain proper crossli nking
structure. They were then hydrolyzed by the alkaline
solution of different concentrations for different time
at 1 00°C to obtain SAP fibres.
A sample from the SAP fibres was immersed in
water at room temperature until equilibrium had
reached. Absorbability was determined by weighing
the swollen fibres that were allowed to drain for 1 0
min. The water absorbency (Q ) was calculated using
the following equation:
Q(g/g)=( W2- W I )/ Wt
where WI and W2 are the weights of fibres before and
after water absorption respectively. Absorbency is
expressed as the ratio of retained water i n the fibres to
the weight of the dried fibres.
The infrared transmission spectra of the samples
were recorded on a B ruker Vector-22 spectroscope
using a method of ATR.
The D M A (Dynamic Mechanical Analysis) curves
were obtained by Netzsch D M A242. The temperature
range covered in this analysis was 40°-340°C at a
heating rate of 5°C/mi n and the stress frequency was
1 0Hz.
INDIAN 1 . FIBRE TEXT. RES., JUNE 2005
208
The DSC (Differential Scanning Calorimetric)
curves were obtained using Perkin Elmer DSC-7. The
temperature range covered in this analysis was 40°340°C at a heating rate of 20°C/min.
The morphology of the sample was observed by
KYKY-2800 (China) scanning electron microscope.
Whether PAN i s crosslinked or not, it has no abil­
ity to absorb water. Only after getting hydrolyzed and
after the -CN group on the chain changed partly (or
all) into hydrophilic -CONH2 and -COOH, it be­
comes SAPs. The major reaction is gi ven below 6 :
Fig. I confirms the structural change i n hydrolyz­
ing process. Peak at 2240cm" is due to -C=N­
stretching and wide peak at 3274cm" is due to
stretching of O-H from -COOH 7 . The alkaline hy­
drolysis of the copolymer could be verified by the
disappearance of -C=N- stretching band and the ap­
pearance of carboxamide and carboxylate bands at
1 660 cm' l and 1 549 cm, I respectively 8 (Fig. l b) .
Fig. 2 confirms the change i n fibre' s surface mor­
phology before and after hydrolyzing. Hydrolyzation
influences the fibre ' s surface morphology greatly. The
surface of fibre is found to be smoother before hy­
drolyzing than after hydrolyzing, which has porous
structure (Fig. 2b). This is because the fibres are
partly crosslinked. The parts crosslinked perfectly
would not be destroyed easily but the uncrosslinked
parts would be destroyed or even dissolved by alka­
line solution. The change in Tg is shown in Eg. 3. I n
D M A curves, Tg can b e denoted b y the x-coordinate
of the tano peak. Fig. 3 shows that the hydrolyz�r�s
decreases the Tg of polymer. Thi s may be due to thl
fact that: (i) the hydrolyzing decreases the number of
1 02
u
C
e<:
�
.E
E
if)
c
'"
�
�
o
1 00
Fig. 2-Surface morphologies of cross l i nked fibres rCa) unhy­
. drolyzed and (b) hydrolyzed]
r------,
(0)
c.o
\:l
ro
f--
98
96
94
-CN group and hence the forces between macromole­
cules decrease; (ii) hydrolyzing destroys parts of the
fibre ' s crosslinking structure; and (iii) hydrolyzing
temperature ( 1 00°C) makes some parts of the oriented
chai I 1 S 1 I l1orien lccl .
3500
3 000
2 500
20 00
-I
Wavenumber, em
1 50 0
1 000
Fig . J-ATR-FTIR spectra of fibres [Ca) unhydrolyzed and
(b) hydrolyzed)
50
1 00
1 50
Temperature, oc
200
Fig. 3-Curves of tan b from DMA of crossli nked fibres rCa) un­
hydrolyzed and (b) hydrolyzed)
209
SHORT COMMUNICATION
The circulation of PAN ' s macromolecules causes
the exothermic peaks (Fig. 4). For the longer hydro­
lyzing time, the peak moves towards lower tempera­
ture. This phenomenon might also be attributed to the
destruction of crosslinking structure.
Fig. 5 shows that the water absorbency i ncreases as
the NaOH concentration (CNaOH) i ncreases and finally
reaches a certain value. This could be ascribed to an
increase in hydrophilic functional groups caused by
the increase in CNaOH in the same hydrolyzing
time.Fig. 6 shows that the effect of hydrolyzing time
on water absorbency is similar to the effect of CNaOH
on water absorbency. The decrease in water absorb­
ency after the maximum could be attributed to the
over-hydrolyzing of the polymer that partly destroys
the fibre's polymer gel network.
Potential crosslinking agent is such an agent that
could merely be (partly or not) crosslinked during
polymerization. After the copolymer solution is spun
into fibres, the crosslinking structure could be ob­
tained by heating. Fig. 7 shows the I R spectra of
polyNHMA and the fibres before and after crosslink­
ing. Peak at 1 023cm- 1 is due to the c-o stretching of CH3 -OH group9 . After crosslinking, the peak is weak­
ened (Fig. 7c). The crosslinking mechanism might be
the cause of disappearance of -CH3-OH and the ap­
pearance of -CHrO-CHr. Liu and Rempel lO studied
the polymer similar to polyNH M A and found that
there is no peak at 1 040cm- 1 before crosslinking,
while after crosslinking the peak appears. The reasons
for the difference between the two results are still not
clear.
Fig. 8 shows the effect of NHMA concentration on
water absorbency. With the increase in NHMA con­
centration, the water absorbency i ncreases because of
the more perfect polymer gel network. The decrease
(a)
J:!l
50
OJ)
�
c:
40
12
II.)
30
o
CI)
20
�
...
�
1
•
10
:3
/
. ---
---­
./
/
•
25
2�
0--�
1�
5 �
10
�
�
°0
�--�
5--�
CN•OH'
wt%
Fig. 5-Effect of CNaOH on water absorbency [NHMA 1 5wt%
monomer, hydrolyzed for 8min]
G'
c:
i
20
/
15
.
/
:
i'
•
./ . - 11 ...... .
,/
_
0
1�
B--�
�
-6
�
�
°0L-�2
��4
Hydrolyzing time ,min
Fig. 6-Effect of hydrolyzing time on water absorbency [NHMA
7wt% monomer, CNaOH 1 5wt%]
100
8
c::
�
'E
90
80
70
� 60
�
'#.
50
r-------�
h.r---....
r-.---__
4a
0�
3 � 2��
S�
1�
O -1�
7�
5�
7�
5O
0-�
1 S�
1�
0�
0 �
0 2�
0�
0�
0-0�
2 S�0�
-1
Wavenumber (em )
Fig. 7-ATR-FTIR spectra of polyNHMA (a), and the fibres
uncross linked (b) and crosslinked (c) [NHMA 1 5wt% monomer]
60 .-------,
o
On
01)
;;:..
(b)
]
(c)
u.l
1 50
1 75
<..>
c
II.)
.0
....
200
Temperature, °C
Fig. 4---DSC curves of crosslinked fibres [(a) unhydroJyzed
(b) hydrolyzed for 4min and (c) hydrolyzed for I l min]
0
CI)
.0
'"
tIl.)
�
:3
50
40
30
20
10
o ....
o
.
2
."....
4
/�
.
/
.
6
B
10
12
14
NHMA conc. ,wt%monomer
Fig. 8- Effect of NHMA concentration on water absorbency
[CNaOH 15%. hydrolyzed for 4min]
I N DIAN J . FI B R E TEXT. R ES . , J U N E 2005
210
25
50
75 1 00 125 150 1 75 200 225
Temperature, "C
Fig. 9-Curves of tan () from D M A of tibres I (a) without N H M A
and ( b) containing 1 5wt% monomer N H M A ( u ncrosslinke d l l
L
I
Telllpc r<l!ure,
,
:il
"C
100
Fig. 1 0-E' and tan 0 curves from D M A of fibres [(a) un­
crosslinked and (b) crossl inked] (NHMA 1 5wt% monomer)
in water absorbency after maximum could be attrib­
uted to the over-crosslinking of fibre polymer gel
network, which hinders the stretching of polymer
chains in the network. This conclusion is consistent to
the Flory ' s theory " .
Figs 9 and 1 0 show the influence of NHMA on the
thermal behaviour of fibre polymer. It can be ob­
served from Fig. 9 that the copolymer' s Tg is higher
by using NHMA as crosslinker. This shows that some
parts of the copolymer are crosslinked during polym­
erization, thus the movement of chains is hindered.
Fig. 1 0 also shows that the Tg of copolymer increases
again after the copolymer is heated. This proves that
the crosslinking process takes place during heating.
At the same time, the E' (storage modulus) of the fi­
bre i ncreases.
Enhancillg alkaline concentration has the same ef­
fect as it was with increasing hydrolyzing time on
improving watcr absorbency; the excessi ve hydrolysis
would however decrcase water absorbency. Hydro­
lyzing makes the Tg of the copolymer tB move to­
wards lower temperature and finally a kind of fibre
with porous surface is obtained. I n addition, the
mechanism of the crosslinking process invol ves the
appearance of -CH 2-0-CHT and crosslinking makes
the 7� of copolymer and E' of the fibre higher. The
study reveals that to obtain the SAP fibres with max
water absorbency of about 40g/g, the NHMA con­
centration is found to be equal to about
1 0wt%monomer and hydrolyzing time and CNaOH are
controlled to be about Smin and 1 5wt% respectively.
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