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Polymer International 42 (1997) 315È320
Influence of Syndiotacticity on Poly(vinyl
alcohol)–Iodine Complex Formation
Hiroji Noguchi, Hiroshi Jodai, Yoshihiko Ito, Shoji Tamura & Shuji Matsuzawa*
Faculty of Textile Science and Technology, Shinsyu University, Ueda, Nagano 386, Japan
(Received 3 October 1996 ; accepted 3 November 1996)
Abstract : The extent of colour development in an aqueous poly(vinyl alcohol)
(PVA)Èiodine complex solution increased with increasing syndiotacticity up to a
certain syndiotactic content, then decreased with increasing syndiotacticity. The
complex formation depended on the content of the syndiotactic pentad sequence
length in the PVA molecules. The most suitable content of the syndiotactic
pentad for forming complexes was around 12% in the case of reaction at 30¡C. A
0É5% solution of syndiotactic PVA (S-PVA) with an syndiotactic diad (s-(diad))
content of 63É8% changed into a sol, whereas that with an s-(diad) content of
58% did not change after standing for 2 h at 30¡C, and the colour due to the
iodine complex in the latter was deeper than in the former. The gel prepared
from the 0É5% solution of S-PVA with an s-(diad) content of 63É8% was hardly
coloured. In the solutions containing two kinds of s-PVA having di†erent syndiotacticities, additivity for the extent of colour development did not hold, and
the extent was lower than the sum of that of the components. The decrease in the
extent of colour development at higher syndiotacticity was due to the formation
of microgels. The absorption maximum shifted to a longer wavelength with
increasing syndiotacticity. The length of polyiodine increased with increasing
syndiotacticity.
Key words : poly(vinyl alcohol) (PVA), syndiotactic PVA, PVAÈiodine complex,
syndiotactic sequence, aggregate, microgel, PVA blend.
bonds,11,14 short branches,15 degree of hydrolysis and
degree of polymerization.9 Among these, the inÑuence
of the syndiotacticity of PVA is considered to be the
greatest. Imai & Matsumoto9 have suggested that
PVAÈiodine complex formation is inÑuenced by stereoregularity, using PVAs derived from poly(vinyl acetate)s
polymerized at several temperatures. Yamaura et al.10
have ascertained that PVAÈiodine complex formation
increases with increasing syndiotactic diad content of
PVA,
using
PVA
derived
from
poly(vinyl
triÑuoroacetate)s. Kikukawa et al.11 have reported a
large hypsochromic shift in the absorption maximum
for the syndiotactic sequence distribution in PVA molecules. We have reported in a previous paper12 that
PVAÈiodine complex formation increases with increasing syndiotacticity, reaches a maximum at a content of
the syndiotactic diad of 56È58%, and then decreases
with increasing syndiotacticity.
INTRODUCTION
Since its discovery in 1927,1,2 poly(vinyl alcohol) (PVA)
has been well-known for forming blue-coloured complexes with iodine. The iodine molecules in the PVAÈ
iodine complexes were reported to exist as polyiodine
with a period of 3É1 Ó in solid PVA.3 Two models, the
aggregate4 and the helix,5 were proposed in 1965. Since
then, the following observations regarding the structure
of polyiodine in the complexes, have been reported : the
presence of I~ using resonance Raman scattering ;6 the
5
presence of I~ , which is held by aggregated PVA mol10
ecules ;7 and the separation of I~ and I Æ I~.8 Investiga5
2 3
tions of PVAÈiodine complex formation have revealed
the inÑuence of the microstructure of the PVA molecules, such as stereoregularity,9h13 1,2-glycol
* To whom all correspondence should be addressed.
315
Polymer International 0959-8103/97/$09.00 ( 1997 SCI. Printed in Great Britain
H. Noguchi et al.
316
various solvents at temperatures between 60 and
[40¡C in accordance with the method described in our
previous paper.16 Atactic PVAs (A-PVA) were commercial PVAs and were used after complete saponiÐcation and puriÐcation by methanol extraction. The
syndiotactic diad (s-(diad)), triad (s-triad)) and pentad (spentad)) and pentad (s-(pentad)) contents were determined from the 13C nuclear magnetic resonance (NMR)
spectrum of the methine carbons of PVA in dimethylsulphoxide (DMSO-d ) using a Brucker DRX-500
6
NMR spectrometer.
Iodine (I ) of reagent grade and potassium iodide
2
(KI) of special grade were purchased from Wako Pure
Chemical Industries Co.
In this paper, we clarify the inÑuence of syndiotactic
sequence length in PVA molecules on PVAÈiodine
complex formation in aqueous solutions, using PVAs
with di†erent syndiotacticities and blends of two PVAs
with di†erent syndiotacticities.
EXPERIMENTAL
Materials
The PVAs used are shown in Table 1. Syndiotactic
PVAs (S-PVA) were prepared from poly(vinyl
triÑuoroacetate)s (PVTFA) polymerized radically in
TABLE 1. PVA samples used
Sample no.
Polymerization
conditions
Solvent
S-PVA-1
S-PVA-2
S-PVA-3
S-PVA-4
S-PVA-5
S-PVA-6
S-PVA-7
S-PVA-8
S-PVA-9
S-PVA-10
S-PVA-11
S-PVA-12
S-PVA-13
S-PVA-14
S-PVA-15
S-PVA-16
S-PVA-17
S-PVA-18
S-PVA-19
S-PVA-20
S-PVA-21
S-PVA-22
S-PVA-23
S-PVA-24
S-PVA-25
S-PVA-26
S-PVA-27
A-PVA-1
A-PVA-2
None
Pentane
Hexane
Heptane
Octane
Nonane
Acetone
Methyl ethyl ketone
Methyl n -propyl ketone
M-iPKc
M-iBKd
Propyl aldehyde
n B-Alde
i -Butyl aldehyde
Heptaldehyde
Nonef
Commercial PVA
Syndiotacticityb
DPa
r
rr
rrrr
rrrr
mmmm
D
l
max
57·8
63·0
61·5
58·5
60·0
58·7
62·7
58·4
58·7
61·3
55·9
56·8
56·4
56·6
55·7
57·4
57·1
56·3
57·8
61·6
61·0
56·0
58·4
61·6
58·0
61·0
63·8
52·5
52·9
35·6
40·4
38·0
34·4
35·6
34·6
39·2
34·3
34·8
38·6
31·8
32·5
31·2
31·9
32·0
32·8
32·8
33·0
33·1
38·0
37·2
31·3
34·1
38·1
33·8
37·5
40·8
28·1
26·5
12·2
16·8
15·2
13·2
13·9
12·5
15·1
13·2
11·7
16·6
10·4
10·6
9·94
11·0
10·6
11·1
11·2
10·6
11·1
14·4
14·7
11·0
12·7
14·2
10·8
15·7
—
7·8
7·3
3·14
5·26
5·12
4·01
4·98
4·15
6·45
6·32
3·99
4·83
2·38
2·79
2·42
3·01
2·68
3·01
3·27
3·05
3·52
1·52
6·10
3·65
3·31
5·38
3·67
6·02
—
1·57
2·58
4·89
2·00
2·35
4·47
3·65
5·00
2·82
4·00
4·93
1·73
1·01
1·97
1·67
4·55
1·45
4·02
4·62
3·10
0·48
2·69
3·36
0·49
1·65
3·12
0·92
3·92
1·19
0·00
0·00
605
607
606
602
612
599
613
599
600
610
607
603
597
602
594
600
596
596
580
594
593
571
575
588
579
589
606
—
—
Temp. (¡C)
60
É40
É40
60
30
60
É40
60
30
É40
60
30
60
30
60
30
30
30
60
É40
É40
60
30
É40
60
É40
É78
2070
2340
2480
514
1160
590
2440
760
1490
2160
1340
1350
580
930
540
330
550
210
90
90
100
40
60
80
380
170
1150
2550
190
Absorbance
a S-PVA from intrinsic viscosity of acetylated PVA using : ÍiË ¼ 8·91 Ã 10É3 DP0Õ62 (benzene, 30¡C). A-PVA from intrinsic
viscosity of PVA using : ÍiË ¼ 7·50 Ã 10É3 DP0Õ64 (water, 30¡C).
b From 13C NMR spectra of PVA in DMSO-d : r ¼ s-(diad)%, rr ¼ s-(triad)%, rrrr ¼ s-(pentad)%, mmmm ¼ isotactic
6
pentad%.
c Methyl i -propyl ketone.
d Methyl i -butyl ketone.
e n -Butyl aldehyde.
f Photopolymerized using benzophenone–tetrahydrofuran system.
POLYMER INTERNATIONAL VOL. 42, NO. 3, 1997
InÑuence of syndiotacticity on complex formation
317
Formation and measurements of properties of
PVA –iodine complexes
Most of the experiments were carried out as follows.
The reaction mixtures were prepared by adding
2É5 g l~1 PVA aqueous solution, which was allowed to
stand at 30¡C for 30 min after dissolution, to an equal
volume of an I ÈKI aqueous solution containing
2
2 ] 10~3 mol l~1 iodine and 8 ] 10~3 mol l~1 potassium iodide. Reaction was carried out at 30¡C for 24 h
to form the complexes. The dissolution of PVA was
carried out at 120¡C in a sealed tube for 2 h.
The aqueous solutions containing two PVAs with different syndiotacticities were prepared by dissolving the
PVAs simultaneously.
Sol and gel fractions were prepared from the 0É5%
S-PVA aqueous solution by allowing it to stand for predetermined times at 30¡C in test tubes. The colour reactions were carried out at 30¡C for 24 h by adding to
an I ÈKI aqueous solution containing 2 ] 10~3 mol l~1
2
iodine and 8 ] 10~3 mol l~1 potassium iodide in the
test tubes.
Visible light absorption spectra of aqueous solutions
of the complexes were taken using a Shimadzu UV-160
spectrometer. The absorbances of PVAÈiodine complexes were measured over a range of wavelengths from 400
to 800 nm using an I ÈKI aqueous solution containing
2
1 ] 10~3 mol l~1 iodine and 4 ] 10~3 mol l~1 potassium iodide as a reference. The light path of the cell
used was 10 mm.
RESULTS AND DISCUSSION
At low PVA concentration (1É25 g l~1) in aqueous solution and a reaction temperature of 30¡C, S-PVAs easily
formed a blue-coloured complex with iodine, but atactic
PVAs did not form the complex and the colour was not
developed. The absorbance and the absorption
maximum of PVAÈiodine complexes prepared from
various PVAs are also summarized in Table 1. Figures
1, 2 and 3 show the relationship between the absorbance at the absorption maximum (around 600 nm) and
the syndiotactic diad, triad and pentad contents, respectively. When the syndiotacticity of PVA increased to
more than s-(diad), s-(triad) and s-(pentad) contents of
53%, 28% and 8%, respectively, the absorbances
increased sharply with increasing syndiotacticity and
reached a maximum at s-(diad), s-(triad) and s-(pentad)
contents of about 58%, 34% and 12%, respectively.
After that, the absorbance decreased with increasing
syndiotacticity. The scatter of points around the curves
of absorbance relative to the syndiotactic diad and triad
contents were comparatively large (Figs 1 and 2), but
that relative to the syndiotactic pentad content was
fairly small (Fig. 3). From these Ðgures, PVAÈiodine
complex formation is found to be remarkably depenPOLYMER INTERNATIONAL VOL. 42, NO. 3, 1997
Fig. 1. Relationship between the absorbance and the content
of the syndiotactic diad for di†erent PVAs : L, bulk polymerization ; L
9 , polymerized in alkanes ; L
I , polymerized in
ketones ; …, polymerized in aldehydes ; K, commercial PVA.
dent on the quantity of the syndiotactic sequences
having a length beyond the pentad in PVA molecules.
Amiya & Fujiwara17 reported that the iodine molecules
interact with the syndiotactic portion of the PVA molecules, but not with the isotactic portions. This was
based on an investigation of the NMR spectra of PVA
and iodine in aqueous solution. Maeda et al.18 have
recently ascertained that the syndiotactic sequences in
PVA molecules form intermolecular hydrogen bonds
and the isotactic sequences form intramolecular hydrogen bonds. This was based on an investigation using
Fig. 2. Relationship between the absorbance and the content
of the syndiotactic triad for di†erent PVAs ; symbols as in
Fig. 1.
318
Fig. 3. Relationship between the absorbance and the content
of the syndiotactic pentad for di†erent PVAs ; symbols as in
Fig. 1.
electron-spin resonance for stereoregular PVAs in
aqueous solutions. Accordingly, the increases in absorbances in Figs 1È3 may be due to an increase in intermolecular hydrogen bonds among the PVA chains and an
increase in the interaction between the iodine molecules
and the hydroxyl side-groups of PVA molecules with
increasing syndiotacticity. If, however, helical PVAÈ
iodine complexes are formed, the increase in absorbance
with increasing syndiotacticity of PVA is difficult to
explain, because the formation of the helix is due to
intramolecular hydrogen bonds. Therefore, the PVAÈ
iodine complex is considered to be formed by an association between the PVA chains through intermolecular
hydrogen bonds. In other words, PVA molecules in
aqueous solution associate more easily because of
increasing intermolecular hydrogen bonds with increasing content of long syndiotactic sequences, and iodine
molecules enter into the aggregates formed to yield
polyiodine.
However, after maximum absorbance is reached, the
absorbance decreases with increasing syndiotactic
sequence. Go et al.19reported that the PVA aqueous
solutions gelled more easily with increasing syndiotacticity of PVA, based on the heats of fusion of
the gels. Ogasawara et al.20 showed that even a 1%
aqueous solution of S-PVA easily formed a gel with
crystallite junctions. Matsuzawa et al.21 discovered turbidity in S-PVA aqueous solutions at concentrations of
0É3È0É5 g l~1 at 40¡C. Moreover, Yamaura et al.22
showed that microgels are formed more easily in
aqueous solutions of PVA having a higher degree of
syndiotacticity. Accordingly, these decreases in absorbance were considered to be due to an increase in
gelation ability with increasing syndiotacticity.
H. Noguchi et al.
Figure 4 shows the variation in the colour of the sol
and the gel prepared from the S-PVA aqueous solutions. An aqueous solution of S-PVA-1 having a s-(diad)
content of 57É8% allowed to stand for 2 h was still a
solution (A). The S-PVA-27 aqueous solution having a
s-(diad) content of 63É8% allowed to stand for 2 h was a
sol (B), and that allowed to stand for 48 h was a gel (C).
Sample (A) coloured instantly after adding an I ÈKI
2
aqueous solution and became deep blue after 24 h.
Sample (B) did not colour immediately after adding an
I ÈKI aqueous solution, but coloured gradually and
2
became blue after 24 h. Sample (C) also did not colour
immediately after adding an I ÈKI aqueous solution
2
and was barely coloured even after 24 h. These results
support the belief that the decreases in absorbances in
Figs 1, 2 and 3 originate from the formation of microgels in aqueous solution, due to the increase in the
intermolecular hydrogen bonds with increasingly long
syndiotactic sequences in the PVA molecules.
In Figs 1È3, the absorbances of the PVAÈiodine complexes of S-PVAs prepared from PVTFA polymerized
in various aldehydes are shown by the dotted lines. The
absorbance increases with syndiotacticity and the
increase is smaller than that shown by the solid lines ; a
maximum was not observed in the experimental range
of syndiotacticity. These PVAs had a very low degree of
polymerization due to chain transfer to the aldehydes.
An increase in aggregates and microgels with increasing
syndiotacticity is considered to be inhibited by decreasing intermolecular hydrogen bond formation, due to
thedecrease in the number of syndiotactic sequences
beyond pentad in the PVA chains and by the e†ect of
the end-groups in the PVA chains produced by chain
transfer to aldehyde.
Figure 5 shows the relationship between j
and the
'
syndiotactic pentad content. The j
shifted to a longer
max
wavelength with increasing syndiotactic pentad content ;
i.e., the length of polyiodines formed is considered to
become longer. This means that the length of aggregate
portions also becomes longer with increasing syndiotactic pentad content. The j
for S-PVAs prepared
'
from PVTFA polymerized in aldehydes was shorter by
about 20 nm in comparison with that of the polymers
prepared in other solvents. This is considered to be due
to the same reason as described above for the di†erence
in the change in absorbances of S-PVAs prepared from
PVTFA polymerized in aldehydes and in other solvents.
Figure 6 shows the relationship between the absorbance and the blend ratio of S-PVA-1 and S-PVA-2,
which have di†erent syndiotacticities. Figure 7 shows
the change in absorbance with the concentration of
S-PVA-1 and S-PVA-2. The Ðve points of concentration
in Fig. 7 for S-PVAs correspond to the blend ratio of
S-PVA-1/S-PVA-2, 100/0, 75/25, 50/50, 25/75 and
0/100, in Fig. 6. For the blends of S-PVA-1 and S-PVA2, the absorbance decreased with increasing amounts of
S-PVA-2 with higher syndiotacticity. The dotted line in
POLYMER INTERNATIONAL VOL. 42, NO. 3, 1997
InÑuence of syndiotacticity on complex formation
319
Fig. 5. Relationship between j
and the content of the syn'
diotactic pentad for di†erent PVAs ; symbols as in Fig. 1.
Fig. 6 shows the absorbance calculated from the
absorbance in Fig. 7 corresponding to the individual
S-PVA concentrations in the reaction mixtures, i.e. it is
the sum of the absorbance for each component of the
S-PVAs in the blend ratio. If there is no interaction
between the two S-PVAs, the absorbances of the PVA
blend are considered to be equal to the absorbances of
the dotted line. However, the absorbances of the S-PVA
blend were lower than those of the dotted line and the
di†erence for the S-PVA-1-rich blend ratio is larger
than that for the S-PVA-2-rich blend ratio. It is considered that S-PVA-1, which on its own hardly forms
microgel, forms microgels more easily by interaction
Fig. 4. Variation in the colour for the gel on adding an I ÈKI
2
aqueous solution. (A) Solution ; (B) sol ; (C) gel. (a) Before
adding an I ÈKI aqueous solution ; (b) just after adding an
2
I ÈKI aqueous solution ; (c) 48 h after adding an I ÈKI
2
2
aqueous solution.
POLYMER INTERNATIONAL VOL. 42, NO. 3, 1997
Fig. 6. Absorbance for the blend ratio of S-PVA-1 and
S-PVA-2 : L
9 , S-PVA-1 (DP 2070 ; s-(pentad) content 12É2%) ;
L
I , S-PVA-2 (DP 2340 ; s-(pentad) content 16É6%) ; K, S-PVA1/S-PVA-2.
H. Noguchi et al.
320
formation due to the blend of the S-PVA having higher
syndiotacticity. Therefore, it is considered that
poly(vinyl alcohol)Èiodine complex formation increases
with increasing intermolecular hydrogen bonding due
to the increase in syndiotactic content up to a certain
point, but decreases with increasing microgel formation
due to further increase in the syndiotactic content.
REFERENCES
Fig. 7. Relationship between the absorbance and the S-PVA
concentration in aqueous solution ; symbols as in Fig. 6.
with S-PVA-2, which easily forms microgels. The quantity of the microgel formed increases and the quantity of
aggregates decreases in the reaction solution ; hence, the
absorbance becomes lower than the sum of the absorbance for each individual S-PVA at the concentration
corresponding to the blend ratio.
CONCLUSIONS
PVAÈiodine complex formation depends on the content
of the syndiotactic sequence length beyond pentad contained in the PVA molecules. A syndiotactic pentad
content of around 12% is most suitable for forming
complexes, with a colour-forming reaction at 30¡C. The
gel prepared from an S-PVA aqueous solution is hardly
coloured. In a blend of two S-PVAs, complex formation
is reduced in comparison with the sum of that for each
S-PVA alone at the concentration corresponding to the
blend ratio, because of the inÑuence of the S-PVA
having higher syndiotacticity. The microgels in solution
increase with increasing intermolecular hydrogen bond
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POLYMER INTERNATIONAL VOL. 42, NO. 3, 1997
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