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ISSN 10637834, Physics of the Solid State, 2011, Vol. 53, No. 6, pp. 1302–1306. © Pleiades Publishing, Ltd., 2011.
Published in Russian in Fizika Tverdogo Tela, 2011, Vol. 53, No. 6, pp. 1234–1238.
1302
1
1. INTRODUCTION
One problem on the way of polymeric conductors
and semiconductor circuits production is an obtaining
of certain molecular superstructure of polymers. It is
known that flexiblechain polymeric molecules are
inclined to the stowage with the formation of different
superstructures [1]. An electron transfer between the
macromolecular chains is suppressed in such materi
als. This restricts their usage. It is desirable to produce
polymeric materials where the macromolecules were
unwrapped in their full length. There are some ways to
do it [1]. In our research an attempt was made to pro
duce macroscopic PVA whiskers with molecules
unwrapped along whisker axis. An organic semicon
ductor, i.e., polyacetylene can be obtained then as a
result of PVA dehydration. Its molecular superstruc
ture should be similar to that of initial material:
To achieve this aim it was supposed to produce a
clathrate of urea and PVA. It is known that urea and
thiourea form clathrates with quite longchain
unbranched organic molecules [2]. In such com
pounds unlimited long channels exist where guest
molecules are placed. There are restrictions on the
minimal guestmolecule length but not on its maximal
length. It was reported, for example, about synthesis of
urea and polyethylene oxide clathrate with molecular
weight of 40000 [3]. We will not discuss the features of
clathrates formation. We only state that in the case of
PVA such a possibility is not obvious. There is also
1
The article was translated by the authors.
–[CH–CH
2
]
n
–
–
OH
–
n
H
2
O
–[CH=CH
2
]
n
–.
information concerning another type of urea and PVA
interaction that leads to PVA solubility change [4].
Having PVA and urea clathrate it could be possible to
decompose it thermally and produce PVA whiskers
with unwrapped macromolecules what was the aim of
this research.
2. SAMPLE PREPARATION
AND EXPERIMENTAL TECHNIQUE
A 10% PVA water solution was added into the satu
rated urea water solution in 2 : 1 ratio. This blend was
hold for some days in Petry dish at room temperature.
Just before an urea crystallization the appearance of
POLYMERS
Influence of Urea on Polyvinyl Alcohol Molecular
Superstructure Formation
1
I. Yu. Prosanov*, A. A. Matvienko, and B. B. Bokhonov
Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch, Russian Academy of Sciences,
ul. Kutateladze 18, Novosibirsk, 630128 Russia
*email: prosanov@mail.ru
Received May 31, 2010; in final form, October 26, 2010
Abstract
—Whiskers up to 1 cm in length were grown in polyvinyl alcohol (PVA) and urea solution. Raman
and IR spectra discover an interaction between PVA and urea molecules. Optical and electronic microscopy
data show that urea influences on PVA molecular superstructure formation. PVA whiskers prepared in urea
solution can be used for organic semiconductors production which properties are determined by arrangement
of polymer macromolecules.
DOI:
10.1134/S1063783411060278
1 cm
Fig. 1.
Photo of PVA whiskers extracted from the growth
solution.
PHYSICS OF THE SOLID STATE
Vol. 53
No. 6
2011
INFLUENCE OF UREA 1303
whiskers was observed which were extracted and inves
tigated with different techniques.
An “11/2” grade PVA made in Russia and PVA
“1888” produced by “BDH Chemicals Ltd.” were
used. The results obtained on different reagents were
similar. IR spectra were recorded by means of Fourier
transform “Infralum FT801” spectrometer and IR
microscope “Micran.” Thin layers of PVA and urea
were prepared for IR measurement. PVA whiskers
were squeezed with press at uncontrolled conditions
for the preparation of specimens suitable for IR
research. The measurements were carried out on the
20
×
5
μ
m area. The control measurement of IR
absorption of pristine PVA powder pressed with KBr
was carried out with the routine practice [5]. These
results coincide with known data [5] and does not rep
resented in this paper. Raman measurements were car
ried out with Bruker RFS 100/S spectrometer. A
Nd:YAG laser with 1.064
μ
m wavelength was used for
excitation. Resolution was 4 cm
–1
.
The structure of produced whiskers was investi
gated with scanning electron microscope Hitachi TM
1000 and transmission electron microscope JEM
2000FX.
3. RESULTS AND DISCUSSION
It was observed that in urea (and thiourea) and PVA
solution the whiskers grow up to 1 cm length. The
photo of such whiskers extracted from the solution is
100
µ
m
(а)
100
µ
m
30
µ
m
10
µ
m
100
µ
m
10
µ
m
10
µ
m
(b)
(e)
(f)
(g)
(c)
(d)
Fig. 2.
Photos of PVA whiskers made with scanning electron microscope: (a–c) before heating, (d, e) after heating for 30 min in
vacuum at 470 K, and (f, g) after heating for 2 h in vacuum at 470 K.
4000300020001000 Wavenumber, cm
−
1
Transmittance, arb. units
4
3
2
1
Fig. 3.
Absorption spectra of (
1
) pristine PVA, (
2
) thicken
ing on the PVA whisker, (
3
) PVA whisker, and (
4
) urea.
1304
PHYSICS OF THE SOLID STATE
Vol. 53
No. 6
2011
PROSANOV, et al.
presented at Fig. 1. It was discovered using optical and
electron microscopy that some whiskers have thicken
ings (Fig. 2). Junctions were not observed. Whisker
thickness was nearly 10
μ
m. After half an hour heating
in vacuum at 470 K whiskers thickness decreased
approximately 3 times and their structure developed.
It was particularly evident at the whisker thickening. It
can testify that thickenings consist of polymer chains
which did not unwrap completely. Decrease of whisker
thickness can be explained by the evaporation of urea
which deposited on the whisker at its extraction from
the growth solution. After the second two hour heating
in vacuum at 470 K the further less prominent
(а)
(b)
Fig. 4.
Photos of the thermally treated PVA whisker in
polarized light. Whisker axis is (a) parallel and (b) perpen
dicular to the direction of electric field vector oscillation.
20 nm
(а) 50 nm
200 nm 20 nm
(b)
(c) (d)
Fig. 5.
TEM images of PVA extracted from urea solution.
PHYSICS OF THE SOLID STATE
Vol. 53
No. 6
2011
INFLUENCE OF UREA 1305
decrease of whisker thickness was observed (Fig. 2)
that can be attributed to PVA dehydration.
The absorption bands typical for pristine PVA and
urea were observed in whisker IR spectra (Fig. 3).
Some bands are shifted and their intensities changed
as it can be seen at these spectra. The presence of urea
is more evident at whisker thickening spectrum.
According to [6] the bands at 916 and 850 cm
–1
are
attributed to cyndio and isotactic PVA structures cor
respondingly. In whisker spectrum the only 897 cm
–1
band is presented in this region. PVA crystallinity is
determined by 1144 cm
–1
band [6]. It is clearly identi
fied in pristine PVA powder pressed with KBr. In PVA
film spectrum given for the comparison at Fig. 4 this
band is week and it is absent in whisker spectrum. On
the whole it can be concluded based on IR absorption
spectra that produced whiskers contain PVA and they
have no crystallinity areas in contrast to PVA film.
At PVA dehydration the polyacetylene can be pro
duced in ideal case. It is known that its molecules have
an anisotropic polarizability in electric field [7]. We
used this peculiarity to confirm polymer molecules
orientation in the produced whiskers. They were dehy
drated for 20 h in vacuum at 470 K. Then the whiskers
were studied with polarization microscope at parallel
orientation of analyzer and polarizer and two positions
of whisker: parallel and perpendicular to analyzer and
polarizer axes. It was observed that before dehydration
the whisker does not polarize the light but after dehy
dration it does. The most transparency was observed
when a whisker was oriented perpendicular to the
plane of electric field oscillations (Fig. 4). It can be
predicted based on the assumption of higher polariz
ability along molecular axis.
The influence of urea on the molecular superstruc
ture formation was studied with transmission electron
microscopy (TEM) as well. To prepare the specimens
suitable for the investigations we used the solution with
urea concentration less then that in the solution for the
PVA whiskers growth. This solution was kept for some
days at room conditions, then, TEM sample holder
was dipped in this solution. Further, the holder was
dried and fried from excess of urea by one hour heating
in vacuum at 470 K. The structure of PVA deposited
on the holder finally was observed with microscope
(Fig. 5). It can be seen at the picture that after such a
treatment a PVA film stripped in some direction was
formed. The similar image is observed with optical
microscope in a large scale. Such a structure is not
observed on the PVA films produced by the similar way
from the solution without urea. This result indicates
that urea promotes the PVA fibrillar superstructure
formation.
To confirm the formation of PVA and urea clath
rate Xray analysis of PVAurea solution precipitate
and PVAu rea mel t was carried out. There were no dif
fraction bands different from that of pure crystalline
urea. It makes doubtful the way of whiskers production
through PVAurea clathrate formation, but it cannot
be completely excluded. Unfortunately, it is impossi
ble to carry out Xray analysis of whiskers due to their
smallness.
An interaction between PVA and urea molecules
was observed by means of Raman spectroscopy
(Fig. 6). The spectra of urea in water or ethyl alcohol
solution are very similar but quite different from spec
trum of solid urea. As a rule of thumb they have the
same groups of bands but shifted to lower frequencies
on 100–300 cm
–1
. It can be explained by hydrogen
bonds formation between the molecules. In the spec
trum of PVA and urea solution both of their bands
were observed. Most of them are shifted in the range of
10 cm
–1
, some of them to 50 cm
–1
. This is unusual
result. It would be reasonable if the spectrum of urea in
PVA would be alike with its spectra in water or ethyl
alcohol solution but not with the spectrum of solid
urea. Seemingly, such a result can be attributed to the
interaction of urea molecules with two adjacent
hydroxyl groups which are attached to PVA main
chain and are not mobile like water molecules or
hydroxyl groups of low molecular alcohols. Men
tioned above disappearance of PVA 916 and 850 cm
–1
bands and appearance instead of them 897 cm
–1
band
testifies for this interaction as well.
Seemingly, an interaction between urea and PVA
molecules affects the polymer molecular superstruc
ture formation. In pristine PVA this structure forms
due to intra and intermolecular hydrogen bonds for
mation between hydroxyl groups. At PVA dissolving
these bonds are destroyed and instead the bonds with
water molecules appear. It was suggested that in
diluted water solutions PVA molecules exist as the
spheres filled with water [6]. According to [1] there are
two possible ways.
3000200010000Raman shift, cm
−
1
Intensity, arb. units
1
2
4
3
Fig. 6.
Raman spectra of (
1
) PVA and urea solution, (
2
)
solid PVA, (
3
) solid urea, and (
4
) ethyl alcohol urea solu
tion.
1306
PHYSICS OF THE SOLID STATE
Vol. 53
No. 6
2011
PROSANOV, et al.
(1) Molecular superstructure is determined by
intermolecular interaction and final molecule condi
tion is unwrapped.
(2) Intramolecular interaction dominates and final
molecular condition is wrapped. One or another way
of process development is determined by socalled
β
parameter which is equal to the ratio of distance
between edges of molecule to its full length. There is a
critical value
β
*
≈
0.25 when a structure changes.
Based on our results we suggest that urea addition
leads to increase of
β
above its critical value and hence
to change of PVA structure.
4. CONCLUSIONS
PVA whiskers of 10
μ
m thickness and 1 cm length
can be formed in PVA and urea solution. It is suggested
to use this material for the production of organic semi
conductor—polyacetylene fibers with oriented mole
cules arrangement by PVA dehydration.
REFERENCES
1. V. A. Marikhin and L. P. Myasnikova,
Supramolecular
Structure of Polymers
(Khimiya, Moscow, 1977) [in
Russian].
2.
NonStoichiometric Compounds
, Ed. by L. Mandelcorn
(Academic, New York, 1964; Khimiya, Moscow, 1971).
3. F. E. Bailey, Jr. and H. G. France, J. Polym. Sci.
49
(152), 397 (1961).
4. Ch. Lei, Q. Wang, and L. Li, J. Appl. Polym. Sci.
114
,
517 (2009).
5. L. I. Tarutina and F. O. Pozdnyakova,
Spectral Analysis
of Polymers
(Khimiya, Leningrad, 1986) [in Russian].
6. M. E. Rozenberg,
Polymers Based on Vinyl Acetate
(Khimiya, Leningrad, 1983) [in Russian].
7. T. Jeon, G. Kim, H. Lee, and Y. W. Park, Curr. Appl.
Phys.
5
, 289 (2005).
