Full Paper
S796
Effect of Plasma Treatment on Polymer TrackMembranes
Lyubov Kravets,* Serguei Dmitriev, Gheorghe Dinescu, Andrada Lazea,Veronica Satulu
The effect of plasma treatment on the properties of poly(ethylene terephthalate) trackmembranes has been investigated. The influence of plasma treatment conditions and com-position of plasma-forming gas on basic membrane characteristics–pore size and shape,wettability, hydrodynamic, and electrochemical properties–were studied. It is shown thatthe plasma treatment of membranes can significantly improve their performance character-istics and give them a lot of useful properties.
Introduction
The membrane filtration is one of the most promising
technological separation processes for complex mix-
tures.[1] Of the materials used for this purpose, an
important part is played by track membranes (TM)
obtained by the irradiation of polymer filmswith energetic
heavy ions and subsequent etching of tracks of these
particles up to the through pores.[2,3] Owing to some
properties such as the small thickness and high uniformity
of pores, TM exhibit insignificant flow resistance to a feed
stream, high permselectivity, low adsorption of solutes,
and easy regeneration. All these properties are to the
advantage of TM as compared with other filtration
membranes and responsible for their wide use in
medicine, biotechnology, and other engineering areas.
The manufacturing technology of TM has recently
begun to develop a new line, the surface modification of
membranes, which is meant as a directed change in the
composition and structure of the surface layer aimed to
improve the membrane properties. There are various
methods formodification of polymermaterials by physical
and chemical means.[4] Plasma treatment is superior to
other methods, as it allows a wide variety of both non-
L. Kravets, S. DmitrievJoint Institute for Nuclear Research, Flerov Laboratory of NuclearReactions, Joliot-Curie Str. 6, 141980 Dubna, RussiaFax: (þ7)-49621-28933; E-mail: [email protected]. Dinescu, A. Lazea, V. SatuluNational Institute for Laser, Plasma and Radiation Physics, Ato-mistilor Str. 111, 77125 Magurele, Bucharest, Romania
Plasma Process. Polym. 2009, 6, S796–S802
� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
polymerizing in plasma gas (inorganic compounds) and
plasma polymerizable compounds to be used–organic,
elementorganic, and some inorganic compounds (for
example, silane). New properties of the membranes
produced in this way mainly depend on a type of the
chemical compounds used for modification.
The main processes at the effect of non-polymerizing in
plasma gas (for example, air) on track membranes from
poly(ethylene terephthalate) (PET) are gas-discharge etch-
ing of a polymeric matrix and its hydrophilization.[5] It has
been shown that the gas-discharge etching of TM in the
plasma causes of both increase in the pore diameter[6] and
decrease in the a mass part of the low-molecular products
in the membrane.[7] The observed process of hydrophiliza-
tion of the TM surface on exposure to a gas-discharge is
related to the formation of functional, in particular
carboxylic, groups.[5] The appearance of COOH groups
can be explained by the oxidation of the end groups
formed at the break chemical bonds due to the active
plasma particles effect.
The main process at the treatment of materials by
plasma polymerizable compounds is a deposition of the
film onto their surface.[8] The plasma deposition of a thin
polymer layer onto the surface of microfiltration mem-
branes is of particular interest for improving their
performance characteristics. Depending on the duration
of the plasma treatment, one can obtain membranes for
microfiltration, ultrafiltration, and reverse osmosis.[5] In
the last case, a thin semipermeable layer that fully covers
the pores is deposited on the membrane surface. The
possibility of adjusting the thickness of the polymerized
DOI: 10.1002/ppap.200932002
Effect of Plasma Treatment on Polymer Track . . .
layer and the wide range of organic compounds for
producing the membranes of this type make this method
very promising. So, using carbohydrates (cyclohexane, for
example) as a plasma-forming gas, a thin polymer
chemically stable film is formed on the surface exposed
to plasma of not possessing functional groups.[5] This
provides a way for producing hydrophobic composite
membranes possessing a chemical strength. While depos-
iting the polymer film from the discharge to hexamethyl-
disilazane, the membranes having bactericide properties
are formed.[5] A positive or negative charge appearing due
to the introduction of the functional groups is one more
thing allowing one to regulate the membrane character-
istics. For instance, the use of dimethylaniline[9] or acrylic
acid[10] as a plasma-forming gas serves as a basis for
creation of hydrophilic composite membranes. The use of
allyl alcohol as a plasma-forming gas allows one to
improve the hydrodynamic characteristics of the track
membranes.[11] Improvement of hydrodynamic properties
of the membranes formed at treatment of track mem-
branes in non-polymerizing gas plasma is observed, when
introducing cyclohexane in the composition of the plasma-
forming gas.[12]
In this paper, we investigated some new processes of
modification of the properties of track membranes from
poly(ethylene terephthalate) by RF-discharge in non-
polymerizing gases and plasma polymerizable com-
pounds.
Experimental Part
The object of the investigation was PET TMwith a thickness of 9.5
mm and an effective pore diameter of 215 nm (pore density was
2�108 cm–2). In order to produce themembrane, the PET filmwas
irradiated with krypton positive ions, accelerated at �3 MeV/
nucleon in the cyclotron U-400, and then subjected to the
physicochemical treatment on a standard procedure.[3] The pores
of this membrane are cylindrical channels, the cross-sections of
which are practically independent of the depth.
The treatment of the membrane samples and the deposition of
the polymer from organic compounds were performed in a
plasma-chemical setup realizing a RF discharge in parallel plate
configuration at the frequency of 13.56 MHz. Details on the
treatment procedure and the scheme of the plasma reactor set-up
are described previously.[10] The plasma treatment time and
discharge parameters were varied. Only one side of themembrane
was subjected to the plasma treatment.
The characteristics of the source and plasma modified
membranes were determined through a series of complementary
procedures. The amount of the polymer obtained by plasma and
graft polymerization on the membrane surface was defined by
relation [Equation (1)]:
Plasma
� 2009
Qg ¼ ðmg �moÞ � 100mo
(1)
Process. Polym. 2009, 6, S796–S802
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
where mo is the mass of source membrane and mg is the mass of
membrane with the polymer layer obtained by plasma poly-
merization or plasma-induced graft polymerization methods. The
study of the samplesmicrostructure aswell as the definition of the
pore diameter on the membrane surface was conducted by SEM
using the JSM-840 (JEOL) with the resolution of 10 nm. Before
scanning, a thin layer of gold was deposited by the thermal
evaporation of metal in vacuum. The change in the membrane
thickness was measured with an electronic counter of thickness
Tesa Unit (Austria), the precision of themeasuring being�0.1 mm.
The gas flow rate through the membranes–a flow of gas (air)
passing the membrane with a square 1 cm2–was defined at a
pressure drop of 104 Pa. Gas consumption was measured by a
float-type flow meter. On the basis of the values obtained
according to these experiments the gas-dynamical pore diameter
(an effective pore diameter) was determined. For calculation, we
used the Hagen–Poiseuille equation.[1] The error of measurements
at definition of the effective pore diameter was not more than 3%.
The water contact angle (as the sessile drop method) was
determined with a horizontal microscope equipped with a
goniometer. Six measurements were made at different places
on the membrane surface and averaged, the measurement
accuracy was �18. Permeability experiments for water solutions
with various pH values were carried out with the help of the
standard filtration installation FMO-2 (Russia) on membrane
samples with the area of 254 mm2. Before filtration, the
membranes were maintained in a relevant solution during
20 min. Then the prepared membranes were mounted on a cell
and placed below a water reservoir. Water was allowed to flow
through the membranes under the air atmosphere. The pH of the
permeating solutionswas adjusted by adding diluted hydrochloric
acid or sodium hydroxide. The water flow rate was determined by
measuring the volume of water solution that was able to pass
through the membrane per minute. Prior to each measurement,
the membranes were equilibrated with the test solution under
pressure until a stable flux was achieved with the difference
between two subsequent readings of less than 5%. Measurements
of the current–voltage characteristics of membranes were carried
out on a direct current in a two-chambered cell with Ag/AgCl
electrodes, containing awater solution of potassium chloride (KCl)
of identical concentration on both sides of the membrane. The
volume of each chamber was 2.5 ml, the working area of the
membrane was 0.5 cm2. The concentration of the solution was
varied. Each point of the measurement corresponded to the
average value of the current during first 5–10 s. Before the
measurements, the membrane samples were maintained in a
corresponding solution of electrolyte during 1 h.
Results and Discussion
The performed research on the effect of the plasma
discharge on the track membranes allows one to establish
the following rules. One of the basic processes at the
treatment of membranes in the non-polymerizing gas
plasma is the gas-discharge etching of the polymer matrix
which is accompanied by the reduction of the membrane
thickness and increase of the pore size (Figure 1b). The
www.plasma-polymers.org S797
L. Kravets, S. Dmitriev, G. Dinescu, A. Lazea, V. Satulu
Figure 1. Electronic micrographs of the surface of the source PET TM (a) and membranetreated by air plasma for 600 s at a gas pressure of 0.26 Pa and discharge power of100 W (b).
S798
treatment of TM in the RF discharge plasma thus can be
used as amethod of pore etching. A detailed research of the
process of pore etching in the plasma shows that its rate
depends on the discharge parameters–increase in the gas
pressure and discharge power lead to increase in the etch
rate (Table 1). The etch rate also depends on the plasma-
forming gas composition. So, introducing oxygen into the
plasma-forming gas and increasing its concentration lead
to increase in the etch rate (Table 1). The use of pure oxygen
as a plasma-forming gas allows one to increase essentially
Figure 2. Electronicmicrographs of themetallic replicas from the pores of the source PETTM (a) and membrane treated by air plasma for 600 s at a gas pressure of 46.5 Pa anddischarge power of 300 W (b).
the etch rate and to intensify the process
of the gas-discharge etching. This makes
the process of gas discharge etching
more intensive.
The treatment by plasma of a non-
polymerizing gas (any from nitrogen, air,
or oxygen) causes change in the shape of
track membrane pores. As the results
show, the effect of the non-polymerizing
gas plasma on track membranes leads to
the formation of asymmetric track
membranes–the pore shape of the mem-
Table 1. Changing characteristics of the PET TM at the plasma treatment for 300 s.
Plasma-
forming
gas
Treatment regime Relative increase
in the effective
pore diameter
Poros
Gas pressure Discharge power
Pa W % %
– – – – 7.
Nitrogen 9.3 200 6.5 8.
Nitrogen 22.6 400 10.8 8.
Air 22.6 200 15.4 9.
Air 22.6 400 18.2 10.
Oxygen 22.6 400 22.7 10.
Oxygen 46.6 400 27.3 11.
Plasma Process. Polym. 2009, 6, S796–S802
� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
branes formed during discharge etching
changes (Figure 2b). It is the basis for the
creation of asymmetric TM. Thus,
depending on the choice of discharge
parameters, it is possible to reach etching
conditions either in part of the channel
or along the whole length of the pore
channels. In both cases the track mem-
branes with a higher flow rate are
formed. The membranes in which only
a part of the pore channels was etched in
plasma are of special interest. As a result
of the gas-discharge etching in a layer of
such membranes, cone-shaped hollows
are formed thus increasing the volume porosity. The layer
not affected by plasma etching, whose structure remains
the same, will determine the separation properties of the
membranes. The use of such membranes allows one to
raise the effectiveness of the filtration processes. The
length of the conical parts of the pore channels and the
length of the unaffected layer can be checked by changing
the discharge parameters. This provides a way for
producing a wide variety of track membranes with
different characteristics.
ity Initial water
flow rate at
DP¼ 7T 104 Pa
Water contact
angle
ml/min cm2 Deg
2 4.25 65
2 5.50 50
5 6.10 45
7 8.43 35
1 9.15 30
9 11.25 25
8 13.10 20
DOI: 10.1002/ppap.200932002
Effect of Plasma Treatment on Polymer Track . . .
Figure 3. pH dependence of flow rate for the: (a) source PET TM and the membranemodified in air plasma for 120 s at a gas pressure of 13.5 Pa and discharge power of300W; (b) composite membranes with the grafted layer of poly(acrylic acid) and poly(2-methyl-5-vinylpyridine).
The other processes proceeding at
treatment of TM by non-polymerizing,
in particular oxygen-containing, gas
plasma and caused by occurring destruc-
tion of polymer macromolecules are
hydrophilizationofmembrane superficial
layer and formation of radicals in it. The
surface hydrophilization is a result of
formation of the carboxylic groups. The
appearance of COOH groups is explained
by theoxidationof theend groups formed
at the break of chemical bonds. Besides,
theeffectofnon-polymerizinggasplasma
causes a morphological change in in the
surface relief (Figure 1b). Numerous craters appear the
initially smooth membrane surface and it gets rough. This
phenomenon is explained by different etch rates in plasma
for crystalline and amorphous regions. If varying the
treatment conditions and duration, the sizes of the crates
change too. The development of the erosion of the
membrane surface and their hydrophilization result in
increasingtheTMwettability.Thevalueofthewatercontact
angleessentiallydecreases, thussignificantly improvingthe
performance characteristics of the membranes. Change of
the discharge parameters influences the size of limiting
value of water contact angle (Table 1). So, the reduction of
plasma-forming gas pressure and increase in the discharge
power promote the achievement of a smaller value ofwater
contact angle. However, as the results of research show the
wettability of the membrane surface achieved by the gas-
discharge treatment decreases at storing the samples on air:
the value of the water contact angle for all modified
membrane growswith the course of time. This is stipulated
by the relaxation processes causing re-orientation of
functional surface groups in the volume of the polymer
matrix. Nevertheless, even three months after the plasma
treatment, the surface of the membranes remains hydro-
philic. For example, for the membranes modified by air
plasma after storing the samples the water contact angle is
less 458, i.e., 30% lower than that of the source membrane.
The increase in the membrane porosity causes increase
in its productivity: the initial water flow rate of the
asymmetric membrane is higher than the source one
(Table 1). The detailed research in the hydrodynamic
characteristics of themembranemodified by plasma of the
non-polymerizing gas shows that its permeability depends
in a greater degree upon the pH of the filtered solution in
comparison with the source membrane (Figure 3a). It is
explained by changing the properties of the membrane
surface layer. So, according to Figure 3a, thewater flow rate
G is equal to [Equation (2)]
Plasma
� 2009
G ¼ A� B � pH (2)
Process. Polym. 2009, 6, S796–S802
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
where A and B are constants defined by the nature and the
structure of the membrane surface layer. Parameter A
corresponds to the flow rate of the membrane for
[Hþ]¼ 1 mol � L�1, and B characterizes the conformation
mobility of the macromolecule chains of the surface layer
of the membrane.[13] The calculated values of parameter B
for the membranes under study are equal to 3.0� 10�2 for
the source membrane and 8.2� 10�2 for the modified one.
The greater significance of B for the membrane treated by
plasma specifies a larger conformation mobility of the
macromolecule segments of the polymer. This stipulates
an easier re-orientation of the surface carboxylic groups in
a water solution. The conformation stability of polymer
macromolecules of the membrane surface layer is
determined by the pH value of the solution. It is connected
to a degree of ionization of the carboxylic groups. For
example, in the acidic medium the dissociation of surface
COOH groups is suppressed and the membrane goes to a
neutral stable form. As a consequence, the permeability of
the membrane remains at a high level. On increasing the
solution pH, there is a dissociation of COOH groups. The
dissociation grows with increase in the pH value. As a
result, on the membrane surface a negative charge
appears. The presence of the charge on the membrane
surface stimulates decreasing the initial water flow rate. A
similar dependence is also observed for the source
membrane, but it is expressed in a smaller degree. The
more significant changing of the flow rate for the
membrane modified by plasma is explained by a greater
concentration of surface functional COOH groups which
are formed during the plasma treatment.[8]
The radicals formed in the surface of the membranes at
plasma treatment allow one to perform the process of
polymer grafting. In this case, the composite membranes
so-called ‘‘smart’’ or ‘‘intelligent’’ with unique properties
can be prepared. That is, suchmembranes whose transport
properties can be regulated by changing environmental
conditions such as temperature, electrical, or magnetic
fields, solvent composition, solution pH, pressure.[14,15] As
our research shows, the graft polymerization of 2-methyl-
www.plasma-polymers.org S799
L. Kravets, S. Dmitriev, G. Dinescu, A. Lazea, V. Satulu
S800
5-vinylpyridine (MVP) or acrylic acid (AA) induced by the
air plasma on the PET track membrane surface leads to the
formation of the composite membranes, the water
permeability of which can be controlled by changing
the solution pH. These results can be explained by the
conformation transition of macromolecules of the grafted
polymer layer from an expanded state ‘‘coil’’ into a
compact state ‘‘globule’’, and vice versa. So, the research on
water permeability dependence on the solution pH of the
membrane with a grafted layer of poly(2-methyl-5-
vinylpyridine) with a grafting yield of 7.2% demonstrates
its abnormal behavior (Figure 3b). Note, in this case the
effective pore diameter decreases down to 160 nm. This
membrane is not penetrative in the region from 1 to 3 pH
at pressure drop up to 7� 104 Pa. If increasing pH, one can
observe a linear increase of the water flow rate. Such a
behavior of the membrane is explained by various
conformational states of grafted poly(2-methyl-5-vinyl-
pyridine) macromolecules which causes changing the pore
diameter. At low pH values of the solution due to
protonating the nitrogen atoms of pyridine groups, the
segments of the macromolecules of the grafted polymer
acquire a positive charge that results in its swelling –
formation of gel,[15] causing a membrane pore contraction.
The membrane pores are ‘‘closed’’ in this state (Figure 4a).
The macromolecules of poly(2-methyl-5-vinylpyridine)
have an extended conformation state ‘‘coil’’. Such a
conformational state of macromolecules resulting from
the electrostatic interaction of charged segments with
water molecules is permanent. It leads to the complete
contraction of pores in the acidic medium (pH from 1 up
to 3).
Increase of filtrate pH (the drop of ion concentration Hþ
in the solution) leads to the loss of the charge on the
nitrogen atoms, i.e. transition of segments of the poly(2-
methyl-5-vinylpyridine) macromolecules to a neutral
state. Therefore, the electrostatic interaction gets weaker.
With decrease in the Coulomb interaction, the non-
Figure 4. Schematic illustration of changing the conformational stamacromolecules on the track membrane surface. For the grafted layer5-vinylpyridine): (a) mediumwith low pH; (b) mediumwith high pH; fpoly(acrylic acid): (a) medium with high pH; (b) medium with low p
Plasma Process. Polym. 2009, 6, S796–S802
� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
electrostatic interaction of hydrophobic groups, in this
case, of non-polar CH3– and CH2– groups increases.[16] That
results in a collapse of gel–transition of macromolecules in
a compact conformational state ‘‘globule’’. The membrane
pores are ‘‘open’’ in this state (Figure 4b) that leads to
increase in the membrane pore diameter; thus, its water
permeability increases. The grafting of poly(2-methyl-5-
vinylpyridine) on the PET TM surface induced by plasma
thus results in forming a composite mechanochemical
membrane, the permeability of which is controlled by
changing pH of the solution. For the membrane with a
grafting yield of 7.2% at pH¼ 3 one can observe change-
over to an operation mode of a ‘‘chemical valve’’ i.e. at
smaller pH values of the filtrate the membrane gets
impermeable for water molecules. At higher pH values of
the filtrate the membrane gets permeable for water
molecules. Clearly, the boundary value of pH solution
where the membrane changes for this mode will be
determined by the properties of substrate and the grafted
polymer layer.
The grafting of poly(acrylic acid) from a gas phase at
elevated temperature on the membrane modified by
plasma results in formation of a mechanochemical
membrane with ‘‘chemical valve’’ too. For the membrane
that has a grafting yield of Qg¼ 7.4% and effective pore
diameter of 190 nm at pH values more than 8 the full pore
contraction is observed due to negative charge on the
segments of the macromolecules of the grafted polymer
macromolecules resulted from the dissociation of car-
boxylic groups. This leads to swelling the polymer layer
and formation of gel. Such a conformational state of the
macromolecules leads to the contraction of pores. The
membrane pores are ‘‘closed’’ under this condition
(Figure 4a). Decrease in the filtrate pH leads to loss of
the charge on the segments of the grafted polymer
macromolecules. That results in a collapse of gel –
transition ofmacromolecules in a compact conformational
state. The membrane pores are ‘‘open’’ in this condition
te of the graftedof poly(2-methyl-or grafted layer ofH.
(Figure 4b) that leads to increasing the
membrane pore diameter; thus, its water
permeability increases. Thus, for the
membrane with a grafted poly(acrylic
acid) layer change-over to an operation
mode of a ‘‘chemical valve’’ one can
observe at pH¼ 8, i.e., at higher pH values
of the filtrate themembrane gets imperme-
able for water molecules. At smaller pH
values of the filtrate, the membrane gets
permeable for water molecules.
Deposition of the thiophene (Th)
plasma polymer layer on the PET TM
surface leads to the formation of com-
posite membranes that possess asym-
metry of conductivity in electrolyte
DOI: 10.1002/ppap.200932002
Effect of Plasma Treatment on Polymer Track . . .
Figure 5. Current–voltage characteristics of source PET TM and the composite membranes with the PPTh layer in KCl solution withconcentration of 10�4 (a) and 10�3 mol�L�1 (b).
solutions–the effect of the current rectification. This effect
is similar to p–n transition in semiconductors. The ‘‘diode-
like’’ behavior of the modified membranes allows one to
use it as an electrical valve. Such a property can be
explained first of all that the formedmembranes have two
layers with an antipolar conductivity. Indeed, if the
polymer matrix of the source membrane is characterized
by the presence on the surface of cation-exchange
carboxylic groups. The surface of this layer has an average
hydrophilization level: the water contact angle of PET TM
is 658. In KCl (pH of which is 6.0) solutions there is a
dissociation of surface COOH groups. It leads to the
formation of anion segments on macromolecules of the
polymer. The presence of a negative charge on the
macromolecules segments causes swelling of a surface
layer of the membrane and formation of polyelectrolyte
gel.[15] The layer formed by the plasma polymerization of
thiophene (PPTh), as the research shows, contains an
anion-exchange sulfuric groups. However, the concentra-
tion of the anion-exchange sulfuric groups is very small. As
a result, this layer has a hydrophobic character. For the
membranes with a PPTh layer, the value of the water
contact angle is 888. Therefore, swelling this layer is
practically absent. The contact of those layers causes
asymmetry of conductivity for the composite membrane
in the electrolyte solution under the electric field (Figure 5).
Figure 6. Electronic micrographs of the surface of the PET track membranes treated bythiophene plasma for 60 (a) and 300 s (b).
It should be noted that the asymmetry of
conductivity for composite membranes
with a PPTh layer is observed only in the
case when a semipermeable layer of
polymer is formed that practically blocks
the pores on the surface of the source PET
TM. So, at plasma treatment of the
membrane for 60 s the thickness of the
deposited PPTh layer is quite small
(around 100 nm), and the pores are not
blocked (Figure 6a) and, as a conse-
quence, the asymmetry of conductivity
in the electrolyte solution does not arise
(Figure 5). On the contrary, at the plasma
Plasma Process. Polym. 2009, 6, S796–S802
� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
treatment of the membrane for 300 s, on its surface a
semipermeable polymer layer with the thickness of
400 nm is obtained that almost completely blocks the
pores (Figure 6b). The pores of this layer have a pore
diameter at a level of 10–30 nm. Formembrane of this type
the asymmetry of conductivity is observed (Figure 5). The
size of this effect can be characterized by rectification
coefficient (kr) which is calculated as a ratio of values of the
current in two mutually opposite directions at a potential
of 1 V. The conducted research shows that the rectification
coefficient for themembrane treated by plasma for 300 s in
KCl solution with concentration 10�3 mol�L�1 is equal to
1.8, and in the solution with concentration 10�4 mol�L�1
is 1.5.
The appearance of the asymmetry of conductivity for
the membrane with semipermeable PPTh layer on the
surface can be resulted from the distinction of the
resistance of the system at changing the current direction.
So, under direct current reverse bias condition when the
PPTh layer of the membrane is inverted to the anode,
significant decrease in its resistance is observed. This effect
is connected with the reduction of pH solution at the
surface of this layer due to occurrence of a gradient of
concentration of ions Hþ and protonating of sulfur atoms.
The formation of the positive charge on the segments of
PPThmacromolecules leads to the essential swelling of this
www.plasma-polymers.org S801
L. Kravets, S. Dmitriev, G. Dinescu, A. Lazea, V. Satulu
S802
layer that promotes carrying the current. Under direct
current forward bias condition, at the surface of the PPTh
layer of the composite membrane which is inverted to
the cathode, the concentration of hydroxyl ions grows. The
protonating of sulfur atoms in this case does not take
place, and the absence of the charge on the segments of
the polymer macromolecules leads to the dehydration of
the layer and, as a consequence, to its collapse.[16] It causes
increase in the resistance of the system and, accordingly,
decreasing the current. One cannot exclude the fact that
during the plasma treatment of the source PET TM, having
cylindrical pores, in the process of deposition of the PPTh
layer, on its surface the pore geometry changes. The
membrane pores, as the research shows, get an asym-
metric (conical) form. The pore diameter on the untreated
side of the membrane does not change, and it essentially
decreases on the side subjected to plasma treatment. For
PET TM with the conical form of pores the effect of
asymmetry of conductivity is known and described in
detail by Siwy.[17] According to those results, the
asymmetry of conductivity is caused not only by the pore
geometry, but also by the presence of the gel phase in a
narrow part of the pore produced due to swelling of the
surface layer of the membrane. In our case, we cannot
exclude the influence of changing the pore geometry of the
membrane on their electrochemical properties. In other
words, the appearance of the effect of conductivity
asymmetry for PET TM with a semipermeable PPTh layer
on the surface can be caused both by the contact of two
layers having chemically different functional groups and
by changing the pore geometry.
Conclusion
Thus, the presented examples of research in the process of
the track membranes treatment by non-polymerizing
gases show that the plasma modification method can be
successfully applied to alter their properties, such as
wettability, hydrodynamic characteristics, and increase in
its productivity. The plasma-induced graft polymerization
of 2-methyl-5-vinylpyridine and acrylic acid can be used to
obtaining pH-responsive membranes. Besides, the deposi-
tion of polymer on its surface by the plasma polymeriza-
tion of thiophene can be applied for the formation of
composite membranes possessing the asymmetry of
conductivity in the electrolyte solution–a rectification
Plasma Process. Polym. 2009, 6, S796–S802
� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
effect. The membranes with such properties can be used in
biotechnology and medicine. They can also be used for
controllable drug delivery, in biosensor control, for
modeling processes of regulation in the cell.
Acknowledgements: This work was supported by grants (No 06-02-90878 and No 08-08-12207) from the Russian Foundation forBasic Research.
Received: July 25, 2008; Revised: February 10, 2009; DOI: 10.1002/ppap.200932002
Keywords: non-polymerizing gas plasma; plasma polymerizablecompounds; plasma treatment; track membranes
[1] T. D. Brock, ‘‘Membrane Filtration’’, Science Tech., Inc.,Madison, WI 1983.
[2] H. W. Ballew, ‘‘Basics of Filtration and Separation’’, Nucleo-pore Co., Pleasanton, CA 1978.
[3] G. N. Flerov, Vestnik Akademii Nauk SSSR 1984, 4, 35 (inRussian).
[4] ‘‘Polymer Surfaces: From Physics to Technology’’, F. Garbassi,M. Morra, E. Ochiello, Eds., JohnWiley & Sons, New York, USA1994.
[5] S. N. Dmitriev, L. I. Kravets, N. V. Simakina, V. V. Sleptsov,Radiat. Meas. 1995, 25, 723.
[6] S. N. Dmitriev, L. I. Kravets, V. V. Sleptsov, Nucl. Instrum.Methods Phys. Res. B 1998, 142, 43.
[7] L. I. Kravets, S. N. Dmitriev, G. Dinescu, A. Lazea, V. V. Sleptsov,V. M. Elinson, J. Phys.: Confer. Ser. 2007, 63, 012032.
[8] H. Yasuda, ‘‘Plasma Polymerization’’, Academic Press,N.Y. – London 1985.
[9] L. I. Kravets, S. N. Dmitriev, A. B. Gilman, A. I. Drachev, G.Dinescu, J. Membr. Sci. 2005, 263, 127.
[10] A. Lazea, L. I. Kravets, S. N. Dmitriev, G. Dinescu, Roman. Rep.Phys. 2005, 57, 413.
[11] S. N. Dmitriev, L. I. Kravets, N. V. Levkovich, V. V. Sleptsov,V. M. Elinson, V. V. Potryasai, High Energy Chem. 1998, 32,275.
[12] L. I. Kravets, S. N. Dmitriev, V. V. Sleptsov, V. M. Elinson,Desalination 2002, 144, 27.
[13] Y. Osada, K. Honda, M. Ohta, J. Membr. Sci. 1986, 27, 327.[14] J. S. Turner, Yu.-L. Cheng, J. Membr Sci. 1998, 147, 207.[15] M. Nishino, J. Gong, Y. Osada, Bioseparation, 1999, 7, 269.[16] A. R. Khokhlov, E. E. Dormidontova, Physics – Uspekhi 1997,
167, 113 (in Russian).[17] Z. Siwy, P. Apel, D. Baur, D. Dobrev, Yu. Korchev, R. Neumann,
R. Spohr, C. Trautmann, K. Voss, Surf. Sci. 2003, 532–535, 1061.
DOI: 10.1002/ppap.200932002