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Polymer International 47 (1998) 428È432
Formation of Poly(vinyl alcohol)–Iodine
Complex in Aqueous Solution: a SEM
Study of the Freeze-dried Substances
Hiroji Noguchi, Hiroshi Jodai, Kazuo Yamaura* & Shuji Matsuzawa
Faculty of Textile Science and Technology, Shinshu University, Ueda-city, Nagano-prefecture 386, Japan
(Received 12 September 1997 ; revised version received 24 December 1997 ; accepted 21 April 1998)
Abstract : The freeze-dried samples prepared from dilute PVA aqueous solutions
and PVAÈiodine complex aqueous solutions have been examined using a scanning electron microscope. The samples prepared from syndiotacticity-rich PVA
(S-PVA) solutions were found to have a network structure due to the formation
of intermolecular hydrogen bonds, whereas in the case of atactic PVA (A-PVA) a
network structure was not found. The network structure became more Ðnely
structured with increasing syndiotacticity. The structure of the freeze-dried
sample of the complex solution prepared from S-PVA having a syndiotactic diad
content of 63É8%, and iodine, was coarse in comparison with that of the freezedried sample of the S-PVA solution. In addition, the formation of spherical
bulges, which are considered to correspond to microgels in the aqueous solution,
were observed in several places. In the S-PVA having a syndiotactic diad content
of 57É8%, the spherical bulges were not observed, whereas the absorbance of the
aqueous solution was the highest. Although A-PVA did not form a PVAÈ
iodine complex at 30¡C in solution, the frozen solution turned blue due to the
formation of aggregates. These phenomena were conÐrmed by the degree of
crystallinity estimated from IR spectra, and the amount of iodine estimated from
X-ray microanalysis of the freeze-dried samples. The PVAÈiodine complexes are
formed by the interaction of the aggregates of PVA molecules with iodine molecules. However, the PVA microgels do not interact with iodine. ( 1998 Society
of Chemical Industry
Polym. Int. 47, 428È432 (1998)
Key words : poly(vinyl alcohol) (PVA) ; syndiotactic PVA ; PVAÈiodine complex ;
freeze-drying ; network structure ; aggregates ; microgel
bonding, because of the increase in the syndiotactic
content of PVA up to a certain point, but decreases
with increasing microgel formation arising from a
further increase in the syndiotactic content ;3 the aggregates of PVA molecules formed because of intermolecular hydrogen bonding of the syndiotactic sequence
portions, play an important role in forming the complexes in solution.4
Here, the relationship between complex formation
and structure of the freeze-dried substances prepared
from very dilute aqueous solutions containing PVA or
INTRODUCTION
Poly(vinyl alcohol) (PVA) has been known to
form blue-coloured complexes with iodine since its
discovery in 1927.1,2 Since then, many researchers have
studied PVAÈiodine complexes. In previous papers,
the following information regarding the formation of
PVAÈiodine complexes was reported : complex formation increases with increasing intermolecular hydrogen
* To whom all correspondence should be addressed.
428
( 1998 Society of Chemical Industry. Polymer International 0959È8103/98/$17.50
Printed in Great Britain
Formation of PV A-iodine complex in aqueous solution
PVAÈiodine complexes, has been examined in order to
clarify the inÑuence of the aggregates on the PVAÈiodine
complex formation.
EXPERIMENTAL
Materials
The PVAs used are shown in Table 1. S-64-PVA was
prepared from poly(vinyl triÑuoroacetate)s (PVTFA)
photopolymerized
using
the
benzophenoneÈ
tetrahydrofuran system in bulk at [78¡C ; S-58-PVA
was prepared from PVTFA polymerized radically using
2,2@-azobis-(2,4-dimethyl valeronitrile) in bulk at 60¡C.
A-PVA was commercial PVA (Unitika Poval UF-250G),
which was used after complete saponiÐcation and puriÐcation by methanol extraction. The contents of the
syndiotactic diad and triad were determined from the
13C NMR spectrum of the methine carbons of PVA in
DMSO-d using a Brucker DRX-500 NMR spectro6
meter.
Iodine (I ) of reagent grade and potassium iodide
2
(KI) of special grade having a purity of over 99É5% were
purchased from Wako Pure Chemical Industries Co.
Sample preparation and measurements
Samples were prepared by drying after freezing PVA
aqueous solutions and PVAÈiodine complex aqueous
solutions in liquid nitrogen. The PVA aqueous solutions used contained PVA at a concentration of
1É25 g l~1 and were prepared by allowing them to stand
at 30¡C for 30 min after dissolution. The PVAÈiodine
complex aqueous solutions were prepared by reaction
at 30¡C for 24 h, by the addition of 2É5 g l~1 PVA
aqueous solutions, which were allowed to stand at 30¡C
for 30 min after dissolution, to an equal volume of an
I ÈKI aqueous solution containing 2 ] 10~3 mol l~1
2
iodine and 8 ] 10~3 mol l~1 potassium iodide. The dissolution of PVA was carried out at 120¡C in a sealed
tube for 2 h.
Visible light absorption spectra of aqueous solutions
of the PVAÈiodine complexes were taken using a ShiTABLE 1. PVA samples used
Sample
S-64-PVA
S-58-PVA
A-PVA
Degree of
polymerization
1150b
2070b
2550c
Stereoregularitya
s-(diad) (%)
s-(triad) (%)
63·8
57·8
52·5
40·8
35·6
28·1
a From 13C NMR spectra of PVA in DMSO-d .
6
b From
intrinsic
viscosity
of
acetylated
PVA
using
ÍiË ¼ 8·91 Ã 10É3 DP0Õ62 (benzene, 30¡C).
c From intrinsic viscosity of PVA using ÍiË ¼ 7·50 Ã 10É3 DP0Õ64
(water, 30¡C).
POLYMER INTERNATIONAL VOL. 47, NO. 4, 1998
429
madzu spectrometer UV-160. The absorbances of PVAÈ
iodine complexes were measured at maximum near
600 nm using an I ÈKI aqueous solution containing
2
1 ] 10~3 mol l~1 iodine and 4 ] 10~3 mol l~1 potassium iodine as a reference. The light path of the cell
used was 10 mm.
Infrared (IR) spectra of the freeze-dried samples were
taken using a Nicolet Maguna-IR 560 spectrometer by
the KBr pellet method.
Scanning electron microscopy (SEM) of the freezedried samples was carried out using a TOPCON a-30A.
Samples were sputtered with gold using SEM sputtering
equipment.
X-ray microanalysis was undertaken using SEM and
a Philips energy dispersive X-ray analyser EDAX9800
(detector ECON3). Samples were used after being washed
with methanol to remove unreacted iodine and then
dried at room temperature.
RESULTS AND DISCUSSION
It has already been reported by us that the extent of
colour development in aqueous PVAÈiodine complex
solutions increases with increasing syndiotacticity up to
a syndiotactic diad content of around 58%, then
decreases with increasing syndiotacticity in the case of
reaction at 30¡C,3 and that the colour development
depends on the standing time of the PVA solution
before the addition of the I ÈKI aqueous solution.4
2
Horii et al.5 conÐrmed that the intermolecular hydrogen bonds of the solid state PVA increases with increasing syndiotacticity, by measurements using CP/MAS
13C NMR spectra. Maeda et al.6 reported that the
S-PVA forms intermolecular hydrogen bonds, and that
isotacticity-rich PVA formed intramolecular hydrogen
bonds, based on an investigation of segment mobility in
aqueous solutions for stereoregular PVAs, using
electron-spin resonance (ESR). Thus, we considered that
the formation of aggregates and microgels are due to
the formation of intermolecular hydrogen bonds in the
syndiotactic sequences of the PVA molecules.
Figure 1 shows scanning electron micrographs of the
freeze-dried samples of 1É25 g l~1 aqueous solutions of
S-64-PVA, S-58-PVA and A-PVA. The freezing of the
aqueous solutions was carried out rapidly in liquid
nitrogen because of the need to maintain the state of the
PVA molecules in aqueous solutions at 30¡C. Clearly,
the structures of the three samples di†ered greatly
depending on the di†erent syndiotacticities of the PVAs.
The sample of S-64-PVA, having the highest syndiotacticity of the three PVAs, showed a regular Ðne
network structure. This network structure became
coarse with decreasing syndiotacticity, and the sample
of A-PVA having the lowest syndiotacticity showed the
structure to be like that of a thin Ðlm rather than
forming a network structure. These structures are considered to be determined by the number of junction
H. Noguchi et al.
430
TABLE 2. Absorbances and colours of PVA–iodine
complexes
Sample
Absorbancea
(Colour)
Colour of
frozen state
S-64-PVA
S-58-PVA
A-PVA
1·19 (Blue)
4·89 (Blue)
0·00 (None)
Blue
Blue
Blue
a Values at l
near 600 nm measured at 30¡C after reactmax
ing at 30¡C for 24 h.
Fig. 1. Scanning electron micrographs of the freeze-dried
samples of PVA aqueous solutions : (a) S-64-PVA ; (b) S-58PVA ; (c) A-PVA. PVA concentration, 1É25 g l~1.
points resulting from the formation of intermolecular
hydrogen bonds between PVA molecules in aqueous
solution, and their distribution, i.e. the PVA having
higher syndiotacticity easily forms intermolecular
hydrogen bonds and the resulting network structures
are Ðner in comparison with those of the PVA having
lower syndiotacticity such as A-PVA.
Table 2 shows the absorbances and colours of
PVAÈI ÈKI aqueous solutions after reaction at 30¡C for
2
24 h and the colours of the frozen state of these aqueous
solutions. The reaction at 30¡C of PVAÈI ÈKI aqueous
2
solutions of S-PVAs rapidly formed blue PVAÈiodine
complexes. The absorbance of S-58-PVA had the
highest value of the three samples, that of S-64-PVA,
despite its higher syndiotacticity, being lower. In the
case of A-PVA, the aqueous solution did not colour
upon reaction at 30¡C. These results indicate that, of
the three samples, S-58-PVA has the most suitable syndiotacticity for the formation of a PVAÈiodine complex
upon reaction at 30¡C. In the case of S-PVAs, the
colours did not change due to the freezing process,
whereas in the case of A-PVA the colour changed to
blue upon freezing. Takamiya et al.7 reported that
A-PVA does not colour upon reaction with iodine at
30¡C ; however, it became coloured upon reaction with
iodine at 4É8 ¡C. Accordingly, in the case of A-PVA the
aggregates are considered to be formed rapidly during
the cooling process to form complexes instantaneously,
i.e. the formation of intermolecular hydrogen bonds was
facile because of the decreased mobility of PVA molecules in aqueous solution at lower temperature ; even
A-PVA, having lower syndiotacticity, formed intermolecular hydrogen bonds.
Figure 2 shows scanning electron micrographs of the
freeze-dried samples of PVAÈI ÈKI aqueous solutions
2
for three types of PVA. The colours of the frozen state
of these samples were blue, as shown in Table 2, and
turned brown upon freeze-drying. These colours
changed to blue again from brown after washing with
methanol. Therefore, the surfaces of these samples were
considered to be coated with unreacted iodine. In the
case of S-64-PVA, the network structure became coarse,
and spherical bulges were observed in several places.
These are considered to be areas of high density intermolecular hydrogen bonds, i.e. to correspond to the
microgels formed, which scarcely turn into complexes in
aqueous solutions. However, in the case of S-58-PVA
having high complex forming ability, spherical bulges
were almost absent.
Figure 3 shows IR spectra of the freeze-dried
samples of each aqueous solution. The degree of crystallinity for each freeze-dried sample, determined by the
method of Kenney and Willcokson8 from IR spectra, is
shown in Table 3. The other methods of estimating
POLYMER INTERNATIONAL VOL. 47, NO. 4, 1998
Formation of PV A-iodine complex in aqueous solution
431
Fig. 3. IR spectra of the freeze-dried samples of PVA and
PVAÈI ÈKI aqueous solutions : È È È, PVA ; ÈÈ, PVAÈI ÈKI.
2
2
Fig. 2. Scanning electron micrographs of the freeze-dried
samples of PVAÈI ÈKI aqueous solutions : (a) S-64-PVA ;
2
(b) S-58-PVA ; (c) A-PVA. PVA concentration, 1É25 g l~1 ; I
2
concentration,
1 ] 10~3 mol l~1 ;
KI
concentration,
4 ] 10~3 mol l~1.
crystallinity, such as density and X-ray methods, give
more accurate results than the IR spectral method but
are inÑuenced by iodine. In the case of the IR spectral
method, the peak based on the crystallinity of PVA is
not inÑuenced by iodine, and the results are considered
to be a suitable basis for a comparison between PVA
and PVAÈiodine complexes, though the values are
rather high. The peaks near 1145 cm~1, based on the
crystallinity of PVA, increased with increasing syndiotacticity ; also the degree of crystallinity increased
POLYMER INTERNATIONAL VOL. 47, NO. 4, 1998
with increasing syndiotacticity. The peak and degree of
crystallinity for each PVAÈiodine complex were higher
than those of the respective values for pure PVA, and
increased with increasing syndiotacticity. This is considered to result from the increase in intermolecular
hydrogen bonding for the elapsed reaction time. The
degree of crystallinity of the S-64-PVAÈiodine complexes had very large values in comparison with the other
PVAs ; this is considered to result from the formation of
microgels, and coincides fairly well with the results of
SEM, and the formation of complexes in aqueous solution.
In the IR spectra, the peaks near 1380 cm~1 for PVAs
shifted to a shorter wavelength by about 10 cm~1
because of the formation of complexes with iodine.
Although the reason for this could not be clariÐed in
this study, the interaction between the hydroxyl groups
of PVA molecules and iodine molecules is considered to
be likely.
Figure 4 shows the intensity of the detected La ray of
iodine (I La) from the freeze-dried samples of three complexes obtained by X-ray microanalysis. In these meaTABLE 3. Degrees of crystallinity of the freezedried samples
Sample
S-64-PVA
S-58-PVA
A-PVA
Degree of crystallinitya (%)
PVA
PVA–I –KI
2
25·1
20·9
14·7
49·4
27·0
19·4
a Calculated from IR spectra by the method of Kenney and
Willcokson.8
H. Noguchi et al.
432
spaces for holding iodine molecules, because the
number of intermolecular hydrogen bonds is too high.
Therefore, the PVAÈiodine complexes are scarcely
formed in the microgels. Accordingly, the PVAÈiodine
complex formation is explained by the aggregate model
as reported in previous papers.3,4,7,9,10
CONCLUSIONS
Fig. 4. X-ray microanalysis of the freeze-dried PVAÈiodine
complexes by energy dispersive X-ray analyser (samples used
after removing unreacted iodine).
surements, samples which had been washed with
methanol to remove unreacted iodine were used. The
intensity of I La of the complex containing S-64-PVA
was about 1/4 of that of the complexes containing S-58PVA. These results correspond fairly well to the
results of absorbances shown in Table 2, indicating that
the formation of the aggregates and complexes
increased only slightly with the freezing of the solutions
in the case of S-PVAs. However, the intensity of I La of
the A-PVAÈiodine complex was nearly equal to that of
S-58-PVA, which does not correspond to the results in
Table 2. It is considered that when the solution was
cooled with liquid nitrogen, the aggregates were formed
instantaneously in aqueous A-PVAÈI ÈKI solution
2
because of the rapid cooling. However, in this result
there is the potential to overestimate the intensity of I
La, because the unreacted iodine might not be removed
completely, due to the structure of the freeze-dried
sample of A-PVAÈI ÈKI aqueous solution shown in
2
Fig. 2.
It is assumed from the above results that the formation of PVAÈiodine complexes is inÑuenced greatly by
the PVA aggregates and microgels, which are formed
because of intermolecular hydrogen bonds present
between PVA molecules. Thus, the complexes are
formed by the interaction of iodine, which enters into
the spaces of the aggregates, with the free hydroxyl sidegroups in aggregates of PVA molecules. However, in the
case of S-PVA having a syndiotacticity higher than a
certain value, the microgels are formed because of the
increase of the junction points arising from intermolecular hydrogen bonding. These microgels do not have the
Increasing syndiotacticity of PVA the freeze-dried
samples of PVA aqueous solutions exhibited a very Ðne
network structure. In the case of PVAÈiodine complex
aqueous solutions, the network structures became
coarse because of the growth of the aggregates. For the
complex of S-64-PVA having the highest syndiotacticity, the spherical bulges observed in several
places in the structures are considered to correspond to
microgels formed with the further growth of the aggregates. With the S-58-PVA complex these were not
observed, and the extent of colour development of the
complex solution was higher than that for S-64-PVA.
Thus, the formation of microgels increases steeply with
increasing syndiotacticity up to a certain point, and the
amount of PVAÈiodine complex decreases. Therefore,
the PVAÈiodine complexes are formed by interaction of
the aggregates of PVA molecules with iodine molecules.
REFERENCES
1 Herrmann, W. O. & Haehnel, W., Ber. Deutsch. Chem. Ges., 60
(1927) 1658.
2 Staudinger, H., Frey, K. & Starck, W., Ber. Deutsch. Chem. Ges., 60
(1927) 1782.
3 Noguchi, H., Jodai, H., Ito, Y., Tamura, S. & Matsuzawa, S.,
Polym. Int., 42 (1997) 315.
4 Noguchi, H., Jodai, H. & Matsuzawa, S., J. Polym. Sci., Polym.
Phys. Ed., 35 (1997) 1701.
5 Horii, F., Hu, S., Ito, T., Kitamaru, R. & Odani, H., Polymer, 33
(1992) 2299.
6 Maeda, H., Morishima, Y. & Kamachi, K., Rep. POV AL Comm.,
105 (1994) 62.
7 Takamiya, H., Tanahashi, Y., Matsuyama, T., Tanigami, T.,
Yamaura, K. & Matsuzawa, S., J. Appl. Polym. Sci., 50 (1993)
1807.
8 Kenney, J. F. & Willcokson, G. W., J. Polym. Sci. A, 1 (1966) 679.
9 Oishi, Y., Yamamoto, H. & Miyasaka, K., Polym. J., 19 (1987)
1261.
10 Yokoyama, T., Kaneyuki, K., Sato, H., Hamamatsu, H. & Ohta,
T., Bull. Chem. Soc. Jpn., 68 (1995) 469.
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