CHAPTER- I :: INTRODUCTION
1.1 General Introduction:
Of late, electrochemical energy storage devices have taken a significant
role in technical applications like communication and computational devices,
industrial controls, automation, space vehicles and sophisticated laboratory
equipments etc. Moreover, they meet some of the stringent needs of the portable
electronic market like solar cells, pacemakers, remote controls and toys etc. In
these emerging technologies, there is a special demand for long life,
environmentally friendly, low cost, reliable rechargeable batteries with specific
energy or power expectations [1]. Such power requirement, to some extent, may
be met by using the liquid electrolytes which have hectically moving charge
carriers. Solid polymer electrolytes are extensively studied in the past decades to
overcome the disadvantages of liquid electrolytes such as leakage, reaction with
the electrode and poor electrochemical stability. Solid polymer electrolytes due
to their excellent mechanical, thermal stability and high ionic conductivity have
drawn the attention of many researchers.
This paved way towards the
identification and development of many useful and suitable solid polymer
electrolytes in large number of applications pertaining to computer memory back
up, smart windows, photovoltaic cells, fuel cells, electric vehicle traction and
space power applications etc.
Good mechanical stability is one of the important properties expected of
useful polymer electrolytes in order to remain structurally stable during
1 manufacturing, cell assembly and to avoid leakage from cell container.
Necessary pre processing is done to incorporate this stability and the polymer is
prepared to form a thin polymer electrolyte membrane with adequate toughness
for various applications. Also, Berthier et al. [2] established that ionic conductivity
in polymer electrolytes is associated with amorphous phase of studied samples.
To produce high ionic conduction, flexibility, considerable mechanical strength
and amorphous nature, polymers are complexed with appropriate organic and
inorganic acids or salts which provide mobile cations and are processed to
prepare membranes of these polymer electrolytes. The blend-based polymer
electrolyte complexes are also exemplified as some of the promising and feasible
approaches by a few research groups [3-5].
1.2 Polymers:
Molecules of compounds are made of more than one type of atom. A
simple example of a small molecule is water which contains two hydrogen atoms
and one of oxygen. Polymers are very large molecules with many more atoms
ranging from 10,000 to 100,000 atoms per molecule. The word polymer is derived
from the Greek root poly-, meaning is many and mer- meaning is part or
segment. Many of the same units (or mers) are connected together to form a long
chain called polymer. Polymers can be extremely large, often made up of
hundreds of thousands of atoms. So, they are also referred to as
macromolecules.
Partly because of their size, polymers have interesting behavior and
special properties. Small molecules of water do not tend to get tangled with each
2 other and each one of them is separate and distinct from the others. Whereas,
the large molecules of polymers become enmeshed with each other, much like a
single strand of cooked spaghetti (noodles) gets tangled up with other strands.
The long polymer chains (molecules) intermingle with one another and become
entangled. It is very difficult to separate one chain from the remaining chains.
This structure gives polymers some unusual properties, including their resistance
to breakage and ability to stretch and recover.
The behavior of small molecules can be understood in terms of 3 states:
solid, liquid and gas. The behavior of polymers is much more complex. Some
polymers exhibit long-range order. That is, portions of the chains can arrange
themselves into small 3-dimensional structures called the crystallites (small
crystals).
These crystallites formed are microscopic in size.
Most polymers
however, cannot assume the close-range packing needed to form stable crystals.
Most of them are chaotic, disordered and entangled masses of chains with no
long-range order. Based on the ability of the individual chains to move about, the
response of this entangled mass of chains to an outside stress varies from
situation to situation. When the temperature of a polymer is raised, the internal
energy increases. The increased energy allows the polymer segments to rotate
and slip past one another, which results in large-scale molecular rearrangements.
Of course, larger molecules rearrange more slowly than smaller ones.
Each type of polymer has a characteristic point called as the Glass
Transition Temperature (abbreviated as Tg). Below this temperature, there is
insufficient energy for bond rotation and the polymer chains cannot rearrange.
3 Therrefore, it is said that the
t
polyme
ers are rigid
d and quite
e brittle below Tg. This
rigid state is ca
alled a glasss. As an example,
e
this phenomenon can be
b observe
ed
durin
ng the winte
er time with
h the sub-zzero temperratures in g
garbage wh
hen a plastic
can or
o sled sha
atters. Abov
ve the Tg, tthe materia
al is called a melt where molecula
ar
rearrrangements
s are possib
ble.
Polymers
s with crosss-links or network hav
ve interestin
ng rearrang
gements. As
A
temp
perature is one form of
o outside stress. The
e increasing
g outside sttress cause
es
the segments
s
between th
he cross-lin
nks to movve around.
Howeverr, when this
outsiide stress iss removed,, the cross--linked poin
nts return to
o their norm
mal positionss.
As a consequence, the se
egments alsso return back
b
to theiir unstresse
ed locationss.
Polym
mers which
h exhibit th
his type off behavior are thereffore called as rubberss.
Some of these cross
c
linked
d segmentss are shown
n in the figu
ure-1.1.
Figurre 1.1:: Cro
oss linked segments of a polym
mer materia
al.
1.3 Poly Viny
yl Alcoho
ol (PVA):
Poly (vin
nyl alcohol)) (PVA), a water-solu
uble polyhyydroxy polyymer, is th
he
large
est volume synthetic resin produ
uced in the
e world. Polyvinyl
P
alcohol is no
ot
know
wn to occur as a natural producct. Polyvin
nyl alcohol was first prepared by
b
4 Herm
mann and Haehnel
H
in 1924 by hydrolyzing
h
polyvinyl a
acetate in ethanol witth
potasssium hydrroxide. Polyvinyl alco
ohol is classified into
o two class
ses namelyy:
partia
ally hydrolyyzed and fu
ully hydrolyyzed. Partia
ally hydrolyzzed PVA iss used in th
he
foodss.
or food use
e is an odo
ourless and tasteless, translucen
nt,
Polyvinyll alcohol fo
white
e or cream colored gra
anular pow
wder. It is soluble
s
in w
water, slighttly soluble in
ethan
nol, but in
nsoluble in other org
ganic solve
ents. Typiccally a 5% solution of
o
polyvvinyl alcoho
ol exhibits a pH in the range of 5.0 to 6.5. Polyvinyl alcohol
a
has a
meltiing point off 180 to 190°C. It hass a molecular weight o
of between
n 26,300 an
nd
30,00
00, and a degree
d
of hyydrolysis off 86.5 to 89
9%.
Polyvinyll alcohol iss unique among polyymers in th
hat it is not built up in
molecules known as
polym
merization reactions from sin
ngle-unit precursor
p
a
mono
omers. The
e chemical structure of
o the vinyl alcohol
a
repeating units
s is:
(–[–C
CH2–CHOH
H–]n–).
PVA is one of the few linear, non
n halogena
ated aliphatic
polym
mers. PVA
A has a two dimension
nal hydroge
en-bonded network
n
she
eet structurre
as sh
hown in figu
ure.1.2.
Fig
gure 1.2 :: Typical sttructural arrrangemen
nt of PVA.
5 Polyvinyl alcohol is produced commercially from polyvinyl acetate, usually
by a continuous process. The acetate groups are hydrolyzed by ester
interchange with methanol in the presence of anhydrous sodium methylate or
aqueous sodium hydroxide. The primary raw material used in the manufacture of
polyvinyl alcohol is vinyl acetate monomer. Generally, it is manufactured by the
polymerization of vinyl acetate followed by partial hydrolysis. The process of
hydrolysis is based on the partial replacement of ester group in vinyl acetate with
the hydroxyl group and is completed in the presence of aqueous sodium
hydroxide. Following gradual addition of the aqueous saponification agent, poly
vinyl alcohol is precipitated, washed and dried.
The degree of hydrolysis is
determined by the time point at which the saponification reaction is stopped. The
physical and chemical properties of PVA depend to a great extent on its method
of preparation. That is, the physical characteristics and its specific functional
uses depend on the degree of polymerization and the degree of hydrolysis.
PVA is a polymer with exceptional properties such as water solubility,
biodegradability,
biocompatibility,
non-toxicity
and
non-carcinogenity
that
possesses the capability to form hydrogels by chemical or physical methods [611]. The excellent chemical resistance, physical properties and complete biodegradability of PVA resins have led to their broad practical applications [12-17].
Poly vinyl alcohol films have high tensile strength and abrasion resistance and
hence used as binder in electrochemical windows, blood prosthetic devices, fuel
cells and double layer capacitors etc. [18]. Polyvinyl alcohol is also used as a
moisture barrier film for food supplement tablets and for foods that contain
6 inclusions or dry food with inclusions that need to be protected from moisture
uptake.
Its fields of applicability were widely broadened during the most recent
years due to factors like the development of medicine, the increase of the
utilization in new biomaterials and the introduction of new concepts in medication
by creating the controlled drug release systems.
Also, the application areas
increased due to the need of environmental protection aiding sustainable
development by designing of new ecological systems for water purification
(membranes or absorbent materials) and requirement of improvised conductive
systems for renewable energy sources, etc.
[19–21].
PVA is a non-expensive and versatile polymer adaptable to various needs
with minor modifications of the synthetic procedures [6-11, 19-22].
During the
recent years, the attention of many research groups is drawn towards the study
of PVA films or gels obtained by the simple addition of salts or acids to the
aqueous PVA solution and very fascinating properties have been found. For
example, Every et al. [23] reported that PVA based lithium electrolytes have
conductivity in the range 10−8to 10−4 S cm−1. The conductivity of PVA polymer
complexes also show high values by blending PVA with other suitable polymer.
Very interestingly, the addition of NaCl to the PVA water solution and by
freezing of the obtained solution at liquid nitrogen temperature led to the increase
of the hydrogel crystallinity and as a consequence the rigidity modulus is
increased [24]. The same increase of the rigidity modulus corresponding to the
decreasing of relative swelling for PVA hydrogels has been evidenced by other
7 researchers [25] by repeated freezing- thawing processes performed at –
20°C/20°C.
Along with all these above properties, PVA shows additional important
properties by copolymerization, plasticization. Blending and addition of ceramic
fillers have been used to modulate conductivity of the PVA polymer electrolytes.
Blending of PVA polymers is a useful tool to develop new polymeric materials
with improved mechanical stability. Main advantages of the blend system are
simplicity of preparation and ease of control of physical properties by
compositional change.
PVA may be
blended with many other polymers such as PEG, PEO,
PANI, Sulfonated Polystyrene and Poly vinyl alcohol-co-ethylene, polypyrrole,
polyacrylamide(PAM), polycaprolactone(PCL), poly (vinyl pyrrolidone), poly(3hydroxybutyrate)PHB, Sodium alginate(NaAlg) etc. to increase conductivity and
mechanical strength of the membranes to fit for different applications.
By virtue of all these useful properties of PVA, this is chosen as basic
polymer for studies in this research.
Instead of inorganic salts, organic acids
particularly three of the dicarboxylic acids such as Oxalic, Malonic and Succinic
acid are added (doped) to PVA. Practically, no work or very scanty work is
reported in this promising area and hence is chosen in the present research work
to study the proton conduction mechanism.
Oxalic, Malonic and Succinic acids can donate a loosely bonded proton.
It is observed that the hydroxyl group of PVA picks up protons of Oxalic acid;
8 these protons are loosely attached to the alternate carbons, having OH groups
over the entire polymer layer.
Figure 1.3 :: Loosely bound protons in PVA-Oxalic acid composite which
participate in conduction
These loosely held protons make this PVA-Oxalic acid composite as a
proton conducting material. Figure.1.3 shows the details of PVA composite with
conducting protons. Hydroxyl group of PVA takes proton from Oxalic acid forming
oxonium ion and the proton attached to oxonium ion over the entire polymeric
layer acts as proton conducting polymer, thus improving the conductivity of the
polymer.
1.4 Review of earlier work:
The polymers are insulating materials, which are having excellent
electrical properties, as well as variety of mechanical, chemical and physical
9 properties. Even though these are insulators, some polymers can give scope for
ion transportation. However, it would appear that the electrical conduction of
polar polymer in a relatively low electric field is ionic. Faraday’s law verification in
a gas evolution experiment with an applied voltage is an example of this ionic in
nature and the dependence of electrical conductivity on pressure free volume and
degree of crystallinity [25-30].
The free volume is a central concept in
considering both equilibrium thermodynamic properties and transport phenomena
in liquids or polymers.
S. Pat¸Achia, C. Flore, Chr. Friedrich, Y. Thomann [31] aimed to study the
possibility to modify the properties of poly vinyl alcohol(PVA) hydrogels prepared
in the presence of different salt types (Na2SO4, NaCl and NaNO3). Their
investigations helped to extend the nature of the salts already used in obtaining
films or gels, to expand their concentration domains and to explain the increase
of film strain associated with the increase of their crystallinity.
M. Sivakumar, R. Subadevi, S. Rajendran, N.-L. Wu, J.-Y. Lee[32] studied
the PVA–PMMA-based electrolyte films containing fixed LiBF4 salt. The
complexation has been confirmed from XRD and FTIR spectral studies. The ac
impedance studies are performed to evaluate the ionic conductivity of the
polymer electrolyte membranes in the range 302–373K and the temperature
dependence seems to obey the VTF relation.
The influence of blend
compositions on the ionic conductivity has also been discussed.
M. Helen, B. Viswanathan, S. Srinivasa Murthy [33] studied the functional
properties of the composite membrane generated from polyvinyl alcohol,
10 zirconium phosphate and silicotungstic acid are described. The fabricated
membranes were characterized by using FTIR, XRD, TGA, DSC and SEM
techniques. These fabricated membranes showed reduced methanol cross over
(for possible application in DMFC) relative to that of Nafion® 115. A maximum
proton conductivity of 10−2S cm−1 at 60% RH was attained with 30 wt% STA
incorporated composite membrane.
The scope of the their study was to
investigate a composite membrane made of polyvinyl alcohol (PVA) and
zirconium phosphate (α-ZrP) with silicotungstic acid (SWA) as an active moiety.
Kadir MF, Aspanut Z, Majid SR, Arof AK [34] studied Fourier transform
infrared (FTIR) spectroscopy studies of poly vinyl alcohol(PVA) and chitosan
polymer blend doped with ammonium nitrate (NH4NO3) salt. This is plasticized
with ethylene carbonate (EC). Their investigations have been performed with
emphasis on the shift of the carboxamide, amine and hydroxyl bands.
A Lewandowski, M Galinski [35] fabricated solid state electric double layer
capacitor using a PVA-H2SO4-H2O polymer electrolyte and activated carbon
powder (ACP) as an electrode material. The polymer electrolyte served both as a
separator as well as a binder of carbon powder.
The PVA-H2SO4-H2O
(separator) as well as PVA-H2SO4-H2O-ACP foils were prepared by the solution
cast technique. The electric performance of the capacitors was investigated by
cyclic
voltammetry,
galvanostatic
charging/discharging
and
impedance
spectroscopy.
Jen Ming Yang, Hung ZenWang, Chun Chen Yang [36] studied the
modification of semi crystalline poly vinyl alcohol by UV radiation with acrylic acid
11 monomer to get interpenetrating poly(acrylic acid) modified poly(vinyl alcohol),
PVAAA membrane. The stability of various PVAAA membranes in water, 2M
CH3OH, 2M H2SO4, and 40 wt% KOH aqueous media were evaluated by various
tests like XRD, DSC, TGA etc.
Andrzej Lewandowski, Katarzyna Skorupska, Jadwiga Malinska [37]
prepared a series of alkaline thin-film solid electrolytes, based on poly vinyl
alcohol(PVA),
potassium hydroxide(KOH) and water (PVA–KOH–HO).
Their
conductivity was studied using complex impedance method. The most of the
conducting foils were composed of ca. 40 wt.% of PVA, 25–30 wt.% of KOH, and
30–35 wt.% of water.
Typical conductivity of such foils reached the level of
nearly 10-3 Ω-1 cm-1 at room temperature. Temperature dependence of the
conductivity was found to be in agreement with the Arrhenius expression with the
activation energy of the order of 28–22kJ mol, depending on the electrolytes
composition.
C. Uma Devi, A.K. Sharma, V.V.R.N. Rao [38] have studied the electrical
conductivity, current–voltage (I–V) characteristics and optical absorption of pure
and AgNO3-doped polyvinyl alcohol(PVA) films under different conditions. The
electrical conductivity increased with increasing dopant concentration up to
0.5wt.% of the dopant and then showed a decrease beyond this concentration.
N. Nagaraj, Ch.V. Subba Reddy, A.K. Sharma, V.V.R. Narasimha Rao [39]
studied the current–voltage characteristics of pure polyvinyl alcohol (PVA) films
and those doped with potassium thiocyanate (KSCN)
temperature and dopant concentration.
12 as a function of film
CH Linga Raju, J L Rao, B C V ReddyY and K Veera Brahman [40] probed
into the thermal transitions and thermal degradation of copper doped polyvinyl
alcohol samples with respect to copper concentration. They used differential
scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) for their
studies. Also they observed FTIR spectrum of PVA doped Cu2+ ions which
indicate the presence of O–H, C–H, C=C and C–O groups.
Bin Ding, Hak-Yongkim, SE-C Lee, C-Lu Shao, Douk-Rae Lee, Soo-Jin
Park, Gyu-Beom Kwag, Kyung-Ju Choi[41] have prepared and Characterized
the
Nanoscale Poly vinyl alcohol Fiber Aggregate Produced by an
Electrospinning Method.
G.Filoti, V. Kuncser, H.Kardinahl and G.Manivannan [42] studied the
change in the refractive index of ferric chloride doped polymers under UV
irradiation. The Fe:PVA is sensitive to ferric chloride concentration, molecular
weight of PVA and recording beam power. They tried to correlate the Fe2+/Fe3+
ratio, as well as the temperature dependence of the energetic parameters with
the assumed mechanisms which involve both the valence state by Mossbauer
investigations.
Jun-Seo Park, Jang-Woo Park, Eli Ruckenstein [43] prepared and studied
the Poly vinyl alcohol(PVA) films chemically crosslinked with glutaraldehyde(GA)
in the presence of HCl by casting from aqueous solutions.
These were
investigated by differential scanning calorimetry (DSC), thermo-gravimetric
analysis (TGA), and dynamic mechanical analysis (DMA); their swelling
characteristics and tensile strength were also determined.
13 M Krumova, D López, R Benavente, C Mijangos, J.M Pereña [44] have
reported cross- linking Poly vinyl alcohol with hexamethylene diisocyanate in
solution. They observed that the variation of the thermal and mechanical
properties of PVA with the cross linking density show an initial decrease due to
the diminution of the crystallinity of the system, caused by the cross linking. After
an abrupt rise at about 20%, the properties tend to level off independently on the
increase of the cross linking.
V. M. Mohan, Weiliang Qiu, Jie Shen and Wen Chen [45] prepared Li ion
conducting polymer electrolyte films based on poly vinyl alcohol(PVA) with 5, 10,
15, 20, 25 and 30 wt% lithium iron phosphate (LiFePO4) salt using a solutioncasting technique. XRD, DSC, ionic conductivity and dielectric studies were
performed.
M. E. Fernández, J. E. Diosa , W. O. Bucheli1, R. A. Vargas , T.M.W.J.
Bandara , B.-E.Mellander [46] prepared the polymer electrolyte with polyvinyl
alcohol - AgI –H2O and studied using techniques of impedance spectroscopy
(IS) and differential scanning calorimetry (DSC). The impedance measurements
were carried out in the frequency range from 106Hz to 109 Hz over the
temperature range 22 to 65°C.
The polymer electrolyte exhibited an ionic
conductivity of the order 10-3 Scm-1 at room temperature for the hydrated
samples. The permittivity curves reveal two dielectric relaxations at around the
frequencies 106Hz and 109 Hz. The DSC results show for dry samples that the
glass transition phase is no sensible to the AgI content and appears at around
the temperature 75oC.
Two more phase transitions appear at about the
14 temperatures 150o C and 200o C corresponding to the superionic silver phase
transition and the PVAL melting point, respectively.
S K Patel, R B Patel, A Awadhia, N Chand and S L Agrawal[47] attempted
to combine gel and composite polymer electrolyte routes together to form a
composite polymeric gel electrolyte that is expected to possess high ionic
conductivity with good mechanical integrity. Polyethylene glycol (PEG) based
composite gel electrolytes using polyvinyl alcohol (PVA) as guest polymer have
been synthesized with 1 molar solution of ammonium thiocyanate (NH4SCN) in
dimethyl sulphoxide (DMSO).
electrical studies.
This was characterized by XRD, DSC and
They observed that the ionic conductivity measurements
indicate that PEG: PVA: NH4SCN-based composite gel electrolytes are superior
(σmax = 5.7×10−2 S cm−1) to pristine electrolytes (PEG: NH4SCN system) and
conductivity variation with filler concentration remains within an order of
magnitude. They have correlated the conductivity maxima of PEG: PVA:
NH4SCN- and PVA: NH4SCN-type complexes. Temperature dependence of
conductivity profiles exhibits Arrhenius behavior in low temperature regime
followed by VTF character at higher temperature.
Yuan-Hsiang Yu, Ching-Yi Lin, Jui-Ming Yeh, Wei-Hsiang Lin [48] have
prepared a series of nanocomposite materials that consist of PVA and layered
montmorillonite (MMT) clay by effectively dispersing the inorganic nanolayers of
MMT clay in organic PVA matrix via an in-situ free radical polymerization of vinyl
acetate monomer followed by direct-hydrolysis with NaOH solution. Synthesized
PCN materials are characterized by FTIR spectroscopy, XRD and TEM. Surface
15 morphological images of as-synthesized PCN materials are studied through SEM
and OPM. The crystalline morphology of pure PVA converts to amorphous state
as the MMT clay loading increases.
This is found to be consistent with the
observation of XRD patterns.
1.5 Motivation for the Research:
PVA is a polymer with exceptional properties such as water solubility,
biodegradability, biocompatibility, non-toxicity and non-carcinogenity besides
excellent chemical resistance. Poly vinyl alcohol films have high tensile strength
and abrasion resistance.
Also, dicarboxilic acids like Oxalic, Malonic and
Succinic acids can donate a loosely bonded proton.
It is observed that the
hydroxyl group of PVA picks up protons of these acids when doped in a
controlled way.
However, no significant work is reported in the literature to
explore the properties of complexed electrolyte polymers consisting of PVA
polymer and any of the dicarboxilic acids.
Keeping these aspects in view, in the present thesis, the author reports the
results obtained on Poly Vinyl Alcohol (PVA) based polymer electrolytes obtained
by adding separately three different organic dicaboxylic acids such as Oxalic
acid, Malonic acid and Succinic acid in varying proportions and studying their
characterization to fit into suitable applications.
16 1.6 Organization of the Thesis:
The thesis entitled “PREPARATION AND CHARACTERIZATION OF PROTON
CONDUCTING PVA POLYMER COMPLEXED WITH DICARBOXYLIC ACIDS” contains six
chapters containing the following aspects.
1. General introduction and review of research wok done.
2. Various experimental methods available are discussed.
3. Preparation of Poly Vinyl Alcohol (PVA) based polymer electrolyte
complexes. Pure PVA, (PVA + Oxalic acid), (PVA + Malonic acid) and
(PVA + Succinic acid) polymer electrolytes have been prepared by the
solution cast method and the results obtained are presented.
4. To confirm the complexation of dicaboxylic acid into the PVA polymer
X-Ray diffraction (XRD), FT- IR spectroscopy, Scanning Electron
Microscopy(SEM) and Differential Scanning Calorimetry (DSC) techniques
have been employed on these electrolytes. From the results it is observed
that the complete dissolution of acid into the polymer is confirmed.
5. The optical properties of these polymer electrolytes are studied using UVVIS spectra.
6. The DC conductivity measurement in the temperature range 303 - 373 K
for all these polymer electrolytes has been carried out. From the results
obtained,
the
conduction
mechanism
is
explained
conductivity temperature plots.
Finally the results obtained are presented and discussed.
17 by
analyzing
References
[1]
B. Scrosati,
Electrochim. Acta 45 (2000) 246.
[2]
C. Berthier, W. Gorecki, M. Miner, M.B. Armand, J.M. Chabagno,
D.Rigand,
Solid State Ionics 36 (1989) 165.
[3]
B. Scrosati,
J. Electrochem. Soc. 139 (1992) 2776.
[4]
N. Tsutsumi, G.T. Davis, A.S. Dereggi,
Macromolecules 24 (1991)
6392.
[5]
W. Wieczorek, J.R. Stevens,
J. Phys. Chem. B 101 (1997) 1529.
[6]
Pat¸achia S.: Blends based on poly(vinyl alcohol) and the products based
on this polymer. in: ‘Handbook of polymer blends and composites’
(eds.: Vasile C., Kulshreshtha A. K. Rapra Technology, Shawbury,
288–365 (2003).
[7]
Hennink W. E., van Nostrum C. F.:
Novel crosslinking methods to design hydrogels Advanced Drug
Delivery Reviews, 54, 13–36 (2002).
[8]
Peppas N. A., Bures P., Leobandung W., Ichikawa H.
Hydrogels in pharmaceutical formulations.
European Journal of Pharmaceutics and Biopharmaceutics,50,27–46
(2000).
[9]
Ratner B., Hoffman A. S., Schoen F. J., Lemons J. E.:
Biomaterials science: An introduction to materials in medicine.
Elsevier Academic Press, San Diego (2004).
[10]
Hassan C. M., Peppas N. A.
Structure and applications of poly (vinyl alcohol) hydrogels produced by
conventional crosslinking or by freezing-thawing methods.
Advances in Polymer Science, 153, 37–65 (2000).
18 [11]
Hoffman A. S.
Hydrogels for biomedical applications.
Annals of the New York Academy of Science, 944, 62–73 (2001).
[12]
Martien, F. L.
Encyclopedia of Polymer
York,1986; pp167–73.
Science
and
Engineering;Wiley:New
[13]
Stejskal, J.; Kratochvil, P.; Helmstedt, M. Langmuir 1996, 12, 3389– 3403.
[14]
Mark, H. F.; Bikales, N. N.; Overberger, C. G.; Nenges, G.
Encyclopedia of Polymer Science and Engineering;
[15]
Wiley: Chichester, England, 1989; pp 62–73.
[16]
Suzuki, T.; Ichihara, Y.; Yamada, M.; Tonomura, K.
Agric Biol Chem 1973, 37, 747–759.
[17]
Finch, C. A.
Poly(vinyl alcohol) Development; Wiley:Chichester, England,1992; pp18-32.
[18]
S. Rajendran, O. Mahendran,
Ionics 7 (2001) 463.
[19]
Werner B., Bu H. T., Kjonisken A-L., Sande S. A., Nyström B.
Characterization of gelation of aqueous pectin via the UGI multicomponent condensation
reaction. Polymer Bulletin, 56, 579–
589 (2006).
[20]
de Jong S. J., De Smedt S. C., Demeester J. M., van Nostrum C. F.,
Kettenes-van den Bosch J. J., Hennink W. E.
Biodegradable hydrogels based on stereo complex formation between
lactic acid oligomers grafted to dextran.
Journal of Controlled Release, 72, 47–56 (2001).
[21]
dos Reis E. F., Campos F. S., Lage A. P., Leite R. C., Heneine I. G.,
Vasconcelos W. L., Lobato Z. I. P., Mansur H. S.
Synthesis and characterization of poly (vinyl alcohol) hydrogels and
hybrids for RMPB70 protein adsorption.
Materials Research, 9, 185–191 (2006).
19 [22]
Pat¸achia S., Valente A. J. M., Papancea A., Lobo V. M. M. V.
Poly (vinyl alcohol) [PVA]-based polymer membranes: Synthesis and
applications in ‘Organic and Physical chemistry using chemical
kinetics’
(eds.: Medvedevskikh Y. G., Valente A., Howell R. A.,
Zaikov G. E.
Nova Publishers, New York, 103–166 (2007).
[23]
H.A. Every, F. Zhou, M. Forsyth, D.R. Macfarlane,
Electrochim.Acta 43 (1998) 1465
[24]
Nuget M. J. D., Hanley A., Tomkins P. T., Higginbotham C. L.
Investigation of a novel freeze-thaw process for the production of drug
delivery hydrogels.
Journal of Materials Science:Materials in Medicine,16,1149–1158, 2005.
[25]
Lozinsky V. I., Domotenko L. V., Zumbov A. L., Simenel I. A.
Study of cryostructuration of polymer systems. XII. Poly (vinyl alcohol)
cryogels: Influence of low-molecular electrolytes.
Journal of Applied Polymer Science, 61, 1991–Y1998 (1996).
[26]
Murphy.E.J., Can.,
J.Phys. 41(1963) 1022
[27]
Zazhin. B.I., and Podessenova.N.G.,
Sov.Phys.Solid State.1(1965) 1755.
[28]
Saito.S., Sasabe.H., Nakajima.T and Tada.k.
J.Polym.Sci.A-Z 6(1968) 1297.
[29]
Miyamoto.T.Shibayama.K., and Kubunshi Kagaku.29 (1972) 301.
[30]
Miyamoto.T.Shibayama.K.,
J. App.phys. 44(1973) 5372.
[31]
S. Pat¸achia, C. Florea, Chr. Friedrich, Y. Thomann
Express Polymer Letters
Vol.3, No.5 (2009) 320–331
[32]
M. Sivakumar , R. Subadevi , S. Rajendran , N.-L. Wu , J.-Y. Lee
Materials Chemistry and Physics 97 (2006) 330–336
[33]
M. Helen , B. Viswanathan , S. Srinivasa Murthy
Journal of Membrane Science 292 (2007) 98–105
20 [34]
Kadir MF, Aspanut Z, Majid SR, Arof AK
Spectrochim Acta A Mol Biomol Spectrosc. 2011 Mar;78(3):1068-74.
Epub 2010 Dec 21.
[35]
A Lewandowski, M Galinski Polish
J chem (2001) volume: 1920, Issue: 12, Pages: 1913- 1920
[36]
Jen Ming Yang, Hung ZenWang, Chun Chen Yang
Journal of Membrane Science 322 (2008) 74–80.
[37]
Andrzej Lewandowski, Katarzyna Skorupska, Jadwiga Malinska
Solid State Ionics 133 (2000) 265–271.
[38]
C. Uma Devi, A.K. Sharma, V.V.R.N. Rao
Materials Letters 56 (2002) 167–174.
[39]
N.Nagaraj, Ch.V. Subba Reddy, A.K. Sharma, V.V.R. Narasimha Rao
Journal of Power Sources 112 (2002) 326–330
[40]
Ch Linga Raju, J L Rao, B C V Reddy and K Veera Brahmam
Bull Mater. Sci., Vol. 30, No. 3, June 2007, pp. 215–218. © Indian
Academy
of Sciences
[41]
Bin Ding, Hak-Yong Kim, Se-Chul lee, Cchang-lu Sshao, Douk-Rae
lee, Soo-Jin
Ppark,
Gyu- beom kwag, kyung-ju choi Inc.
J Polym Sci Part B: Polym Phys 40: 1261–1268, 2002
[42]
G.Filoti, V. Kuncser, H.Kardinahl and G.Manivannan
Journal of Radio analytical and Nuclear Chemistry, Articles, Vol.190,
No.2(1995) 315-320.
[43]
Jun-Seo Park, Jang-Woo Park, Eli Ruckenstein
Journal
of
Applied
Polymer
Science
pages 1816– 1823, 14 November 2001.
Volume
82, Issue7,
[44]
M Krumova, D López, R Benavente, C Mijangos, J.M Pereña
Polymer Volume 41, Issue 26, 15 December 2000, Pages 9265–927
[45]
V. M. Mohan, Weiliang Qiu, Jie Shen and Wen Chen
Journal of polymer research Volume17, Number1,143-150.
21 [46]
M.E. Fernández, J.E. Diosa, W.O. Bucheli, R.A. Vargas,
T.M.W.J.Bandara, B.-E. Mellander
Suplemento de la Revista Latinoamericana de Metalurgiay Materiales
2009; S1 (3): 1251-1253
[47]
S K Patel, R B Patel, A Awadhia, N Chand and S L Agrawal
PRAMANA — Journal of physics. Vol. 69, No. 3 September 2007 pp.
467–475
[48]
Yuan-Hsiang Yu, Ching-Yi Lin, Jui-Ming Yeh, Wei-Hsiang Lin,
Polymer 44 (2003) 3553–3560. 22