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Enhancement of polycarbonate membrane permeability due to plasma polymerization precursors
Applied Surface Science 268 (2013) 28–36
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Enhancement of polycarbonate membrane permeability due to plasma polymerization precursors Dilek C¸ökeliler ∗ Biomedical Engineering Department, Bas¸kent University, Ba˘glıca Campus, 06530 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 12 August 2012 Received in revised form 10 October 2012 Accepted 24 November 2012 Available online 26 December 2012 Keywords: Plasma polymerization Polycarbonate membrane Permeability Allyamine
a b s t r a c t The diffusivity of different species through a membrane depends on several factors to illustrate the structure of the matrix, molecular size and concentration of the species and temperature. This study concerns the use of the low-pressure plasma process with different monomers to confer surface chemical character to polycarbonate membranes without altering their bulk properties for change membrane permeability. Track-etched polycarbonate membranes with 0.03 m pore sizes were modified by plasma polymerization technique with two precursors; acrylic acid and allylamine in radio frequency discharge at certain plasma process conditions (discharge power: 20 W, exposure time: 10 min, frequency: 13.56 MHz). The transport properties of model organic acid (citric acid) was studied through unmodified and modified polycarbonate membranes by using diffusion cell system. Such plasma treated membranes were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and surface energy changes were studied by static contact angle measurements. These results showed that the change of surface properties could be used to improve the transport properties of the target substrates. The diffusion of citric acid through plasma treated polycarbonate membrane was increased about 54.1 ± 3.5% with precursor: allylamine while it was decreased 48.7 ± 2.5% with precursor acrylic acid. It was observed that the presences of proper functional group (like amino) in surfaces of pores can raise the affinity to citric acid and improve its transport rate. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently there is an increasing interest toward the integration of two or more varied processes into one process yielding new and better results. To give an example of such an integrated process is extractive fermentation, which combines fermentation products, e.g. organic acids, during the process favors the efficiency of the latter due to reduced inhibiting effect of the products. The aim of this study was to show how different chemical surface structure alters the citric acid permeability of track etched polycarbonate membranes. As a citric acid was chosen for reference organic acid since it is importance for the food industry. Moreover end-product of some process is almost accompanied by waste and/or recycling streams containing acid–water mixtures. Thus, the separation of aqueous acid mixtures is of interest to the chemical industry. The facilitation of some ions and substrates in liquid membrane extraction instead of classical extraction allows the simultaneous recovery of the organic acid from the fermentation medium and its
∗ Corresponding author. Tel.: +90 3122466666x1325; fax: +90 3122341051; mobile: +90 5334465576. E-mail address: [email protected] 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.136
pre-concentration in another solution appropriate for subsequent treatment [1]. The leaching of organic compounds is also inconvenient since the final product, present in the receiving aqueous. Bulk liquid membranes were often used to simulate membrane transport of organic acids of biological interest. These membranes displayed a high selectivity due to a specific complex (hydrogen bonding, electrostatic or hydrophobic interactions) of the solute by the selective carrier, but presented a low stability [2]. Usually, the carrier, the solvent or both suffer from continuous leaching out from the membrane support to the aqueous phases, therefore short lifetime of the liquid membrane reduces the industrial exploitation of these membranes [3]. The review of stability of supported liquid membranes clearly indicates that the stability is a major problem for its industrial application. In this study the usage of hydroxyl group or amino containing chemicals as the vapor source at plasma polymerization was a new idea to improve basic transport characteristic of the polycarbonate membrane surface. Polycarbonate track-etched (PCTE) membranes were selected since novel applications have been proposed for these types with well-defined pore characteristics e.g. very sharp cut off allows their use in the filtration field, yet they were also useful as in vitro substrate in cell biology and also used to determine the reflection coefficients of rigid and partially flexible
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macromolecules [4]. PCTE membranes have been used in applications such as filtration, blood plasma separations, drug delivery, and flow control of reagents for assays. Several studies of these membranes have involved gas permeation/separation applications. PCTE membranes are unique in that they possess cylindrical pores in their fabrication process. Their pore morphology can be considered close to ideal with regard to the availability of uniform, straight cylindrical pores. Our objective was examine the utility of employing these membranes as mechanically simple, yet precise, approach to control acid transport behavior for ultimate use [5]. In order to create membranes with controllable transport properties, one can use the ability of the macromolecules at the surface layer to make reversible conformational transitions. The research in this direction is related to a goal-directed formation of a membrane surface with tailored chemical structure. For this purpose, various methods are used: chemical methods [6] or radiation-induced graft polymerization [7], preliminary activation of the surface etc. [8]. With the same goal, membranes are successfully modified by deposition on the surface of thin polymeric layers obtained by plasma polymerization [9] that was the direct process from the monomer vapors fabricates plasma polymers for the practical use. Even though there are many ways to alter the surface chemistry, plasma treatment appears to be the most effective and convenient one. Plasma polymerization provides ultra-thin membranes composed of cross-linked structure, and the functions of membranes are due to the surface as well as the bulks. It is well known that plasma treatment can alter the physicochemical properties of the polymer surface. Such goals as improvements of wettability, permeability, conductivity, printability, adhesion or biocompatibility are easily achieved within very short time. Plasma polymerized layer has better thermal stability and adhesion properties than the layer formed by conventional methods and we previously reported a plasma treatment studies for sensor surfaces, biomaterials that achieves new permanent surface properties [10]. The usage of plasma provides a number of additional advantages such as: the control of the thickness of the polymeric layer deposited on the membrane surface, the high adhesion of the layer, the short treatment time and the opportunity of using of a wide list of organic and element-organic compounds for modification. Thus, the surface properties of the formed membranes appreciably depend on the type of discharge and nature of the chemical compound used for modification. Focus of the presented work is on testing the differences from transport mechanism of citric acid from medium using thin film deposition on PCTE with monomers; acrylic acid and allyl amine as the precursors and understanding the mechanism. The modifications based on plasma polymerization of amino compounds are of particular interest for biomedical applications [11–19]. Membranes are rarely used as a substrate for such amino modifications yet in the literature, only membranes for gas separation [20–22] and filtration [15,23] are described, while not transporting properties of acids were tested previously. On the other hand, introduction of polar groups such as carboxylic acid has been interesting for the characteristic functions of adsorption, biomaterial and surfactants [24]. Fabrication of the acidic surfaces of substrates and the coating by plasma polymers containing the –COOH group was tested for the same motivations. PCTE membranes were modified in a glow discharge reactor by plasma polymerization technique at certain conditions with different monomers. Then, the effect of type of monomers on the membrane permeability was tested by a diffusion cell system. The chemical and physical structural characterization of the PCTE membranes before and after the surface modification is done by means of scanning electron microscopy and X-ray photoelectron spectroscopy. The surface wettability of the pure and plasma modified membranes were characterized using the surface contact angle of water.
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2. Experimental 2.1. Plasma polymerization process PCTE membranes with nominal pore size 0.03 m was obtained from Poretics (California, USA) and treated by plasma within a tubular type reactor containing a pair of plate copper electrodes separated by 5 cm (Fig. 1). A 13.6 MHz radio frequency generator was used to sustain glow discharge in the reactor and power losses were minimized by means of a matching network. PC membranes were located in the center of two electrodes. Monomer acrylic acid (AAc) of 99% purity was purchased from Merck Co. and allyl amine (AA) was obtained commercially (Sigma, Steinheim and Merck, Darmstadt, Germany). After the system was evacuated to a pressure of 10−3 Torr, first the AAc vapor was introduced to the system to a predetermined pressure of 5 Pa. Monomer vapors were allowed to flow at a rate of 25 ml/min. Then, RF power was adjusted to 20 W and the membrane was exposed to glow discharge for 10 min. At the end of the process the RF generator was turned off and the system was fed with Argon gas for 30 min to deactivate free radicals. After the one set of the plasma polymerization process, new polycarbonate membranes were inserted and monomer container was filled by new precursor; AA. The plasma was ignited at a frequency of 13.56 MHz with power 20 W for 10 min. All parallel modifications were repeated at the same plasma polymerization parameters (discharge power: 20 W, time: 10 min, flow rate: 25 ml/min). The monomers were chosen to provide surfaces differing greatly in their surface energies and stability. These factors were varied and their effects on acid permeability of PCTE membranes were studied using citric acid as permeate.
2.2. The characterization of PCTE membranes The surface morphologies of PCTE membranes were investigated by using AFM (Burleigh Metris 2001A-MS) and pore structures were characterized by SEM (JEOL, JSM 5600, USA). For the SEM analysis membranes were photographed in the electron microscope with 5000× magnification. The polymeric surface usually has no electrical conductance; hence a 10–15 nm gold layer must be provided by sputter coating, to remove electrostatic charge from the surface due to electron beam radiation. Analysis of polymer surface functional groups after plasma treatments was performed by X-ray photoelectron spectrometer in ADES-400 equipped by Al K␣ excitation sources and a hemispherical energy analyzer was used to obtain information about the composition and the chemical bonding of elements found in a surface region of untreated and plasma treated samples. The operating pressure was 1 × 10−7 Pa. Pass energies 50 and 15 eV were used for the survey and high resolution scans, respectively. The take-off angle was ˛ = 45◦ . The HR envelopes were analyzed and peak-fitted after subtraction of a Shirley background using a Gaussian–Lorenzian peak shapes obtained from the Casa XPS software package. The signal of C 1s, 285 eV, was selected for energy calibration. The water contact angles were measured using a NRL (Rame Hart) goniometry. The contact angle of a pure water (de-ionized with a resistivity of 18.2 MV cm) drop on the membrane surface was measured by sessile drop method. The membrane was placed over a micrometer bench and multiple droplets are formed on the surface using a syringe. The images of the water droplets on the membrane surface was captured by a high-resolution video camera and digitized. The contact angles were directly measured from the images using a software. For each specimen, the measurements were repeated in three different areas, 10 times each area. The measured values of the contact angles were averaged.
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Fig. 1. Plasma polymerization system.
2.3. The test of membrane permeability PCTE membrane permeability was tested by a diffusion cell technique [25]. The diffusion cell technique uses a diaphragm diffusion cell that consists of two identical chambers separated by a membrane with 5.3 cm2 of available membrane area in which the diffusivity will be measured. In our system has two compartments with 125 ml volume divided by a PCTE membrane. Initially, one chamber (A) was filled with a solution that contains the solutes of citric acid (0.005 M) and the other (B) with pure solvent as distilled water. Before the test, the membranes were maintained in a relevant solution during 20 min then prepared membranes were mounted on a cell. In both chambers, sufficient mixing will be provided to ensure a negligible external mass transfer resistance. Therefore the feed and strip solutions were mechanically stirred at 400 rpm (except otherwise stated) at ambient temperature (24 ◦ C) in a thermo stated room to suppress concentration polarization conditions at the membrane interfaces and in the bulk of the solutions. Since the membrane solvent was hold within the pores by capillary forces, it was expected that pore structure may play a major role in the stability and diffusion rate of the acid (Fig. 2). The permeability of citric acid through the PCTE membrane was monitored by periodically removing 200 l samples from both the chambers and equal volume of drug-free buffer solution was refilled to avoid errors arising from the resulting volume variation. The concentrations of citric were analyzed by an UV spectrophotometer (Ultrospec 1000E, Pharmacia Biotech, Sweden) from the peak absorbency at 204 nm. After an initial period of time, a quasisteady state of solute flux was attained. The concentration change in each experiment was evaluated to calculate the permeability of unmodified and modified polycarbonate membrane for citric acid. Each experiment was repeated three times and the results were expressed as the mean of the three results.
3. Result and discussion 3.1. The characterization of PCTE membranes 3.1.1. Physical characterization Plasma treatment is a powerful technique to modify only the surface. Briefly a glow discharge created by RF generator wave within a low pressure monomer vapor leads to etching, functionalizing, grafting of polymer layer on the touched material. First, vertical sections of PC membranes were observed in SEM (Fig. 3).
As seen in Fig. 3b, the thickness of the dense top layer was clear as the plasma precursor; AAc. For a rough estimation, the top layer thickness of the membrane was twice that of the one is plasma modified with AA. Because of high extend of molecular fragmentation occurring in the discharge and the variety of reactions running both in the volume and on the surface; the composition and deposition rate of plasma polymers is much more complex. Since plasma polymerization involves the fragmentation of precursor molecules, when the monomer contains different ratio of elements (C/N, C/H, C/O, etc.) they may combine into different chemical functionalities and deposition rate incorporated in a highly cross-linked matrix explains the difference of film thickness according to the change of precursors; AAc and AA in Fig. 3. Furthermore SEM observations indicate the absence of erosion of the membrane surface under plasma treatment and both modified membranes show similar flat appearance when it was compared with unmodified one in Fig. 3a. Track-etched membranes with well-defined pore characteristics were prepared by bombarding polycarbonate plastics with 161Dy ions of 13.0 MeV per nucleon energy and by bombarding polycarbonate plastics with fission fragments from a 252Cf source. Novel applications have been proposed for these tract etched membranes, e.g. the very sharp cut off allows their use in the filtration field, but they were also beneficial as in vitro substrate in cell biology. In the case of the membrane applications, plasma can change pore size and alter the character of surface. PCTE membranes were examined under SEM, in plan view, for the pore structure characteristics, including changes of pore sizes after polymer coating (Figs. 4–6). These images clearly reveal the physical pore size reduction which accompanies the polymer film deposition on the plasma modified membrane with precursor; AA. Pore size of pure PCTE was 37 ± 5 nm (Fig. 4) but it was 30 ± 2 nm for plasma modified PCTE membrane by precursor AA (Fig. 6) along with the conformal aspect of the coating process. Beside this, the enlargement in pore size resulting from plasma deposition of monomer AAc was verified by SEM in Fig. 5 (41 ± 2 nm). During plasma action three processes always compete; polymer deposition, substrate etching or polymer grafting. The former decreases pore size; the both latter two may increase it and additional chemical analysis of surface was added for the selection on one of these mechanisms. Despite the fact that these pore size change was clearly the consequences of plasma treatment, no visible pitting of the plasma polymerized membrane surface was observed, after the modifications (Fig. 7b and c). Both plasma modifications with precursors cause physical changes on the surface with an increase on
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Fig. 2. Diffusion cell system.
Fig. 3. The vertical sections of PCTE membranes (a) unmodified, (b) precursor, AAc and (c) precursor, AA.
roughness compare with unmodified PC membrane (Fig. 7a). Regarding SEM results there were no differences with modified PCTE membranes as the mean of front view. 3.1.2. Chemical characterization Two different monomers were used for the plasma polymerizations, each intended to incorporate different chemical functional groups into the surface. Acrylic acid and ally amine was used to fabricate films rich in carboxyl groups and nitrogen containing groups respectively. Fig. 8a indicates a typical wide scan spectrum of the
polymer deposited onto PCTE membrane and the X-ray photoelectron spectroscopy analysis of samples produced under the nominal conditions shows that C1s region of an acrylic acid plasma polymer showing five component peaks with C C (59.2%), C O (3.4%), C O (3.3%) and COOR (17.1%) (Fig. 8b) Plasma polymers deposited from ally amine plasma contain carbon, nitrogen and a small quantity of oxygen. A typical wide scan spectrum was shown in Fig. 9a. The C 1s region of an allyl amine plasma polymer was shown in Fig. 9b. There were difficulties in fitting the C 1s regions of nitrogen containing surface
Fig. 4. PCTE membrane, unmodified (a) 20,000× and (b) 40,000×
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Fig. 5. Modified PCTE membrane by precursor: AAc (a) 20,000× and (b) 40,000×
Fig. 6. Modified PCTE membrane by precursor: AA (a) 20,000× and (b) 40,000×
which relate to the large spread of chemical shifts that are associated with nitrogen-containing functional groups. For curve fitting these regions, four component peaks were used and a peak representing amine groups (primary, secondary and tertiary) was placed at a shift of +0.9 eV relative to the hydrocarbon. A third peak was placed with a shift of 1.7 eV includes contributions from imine and diamine functional groups. A fourth peak at +3.0 eV includes amide functionalities and any carbonyl groups. The chemical distribution using this curve fitting strategy shows all chemical functionality with precursor allyl amine, C C (68.6%), C N (19.8%), C N, C O (11%), C O, OCN (0.5%) For the chemical composition to see Si element onto unmodified PC membrane was susceptible due to in commercially PC membranes it was possible to get this chemical. The disappearing of Si is evident that there is plasma polymerizes film onto PC membrane surface in both plasma treatments (Table 1). Plasma polymer of acrylic acid contained slightly high oxygen than might be expected from the monomer composition (31.8%) and the amount of carboxylic acid functionality was depended on chemicals C O, C O and COOR. For polymer films deposited from plasmas containing a acrylic acid, there could be loss of material upon
exposure to an aqueous environment. Post treatment reactions between the water and carbon radicals trapped in the plasma polymer is processed and all data are obtained after immersion in water and drying at room temperature. Plasma polymers of allyamine contain a small amount of oxygen (15.1%) which is attributed to residual oxygen species within the plasma and to post polymerization reactions with atmospheric oxygen. The N/C ratio from the values in Table 1 is 0.21, amine content of the surface is likely to consist of a high proportion of secondary and tertiary amine groups.
Table 1 Surface elemental composition of PCTE membranes.
Table 2 Static contact angle values () of PCTE membranes.
Unmodified Plasma modifications
Precursor: AAc Precursor: AA
4. Surface wettability The contact angles of the plasma deposited surfaces were different to those of unmodified PCTE ( = 71.8 ± 4.8) membranes indicating that the surface chemistry has been modified by plasma depositions process (Fig. 10). Table 2 represents the static contact values of samples. Therefore the increase of wettability can be associated with the process of formation of additional hydrophilic groups on the surface. The water contact angle experiment was used to estimate the surface hydrophilicity and lower contact angle value indicates higher
C (%)
O (%)
N (%)
Si (%)
Sample
Mean (±s.d.)
n
73.6 68.2 69.9
20.4 31.8 15.1
– – 15
6.0 – –
Unmodified Acrylic acid Allylamine
71.8 ± 4.8 36.4 ± 2.6 62.7 ± 6.7
8 10 10
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Fig. 7. Topology of the membranes in front by AFM (a) unmodified, (b) precursor AAc and (c) precursor: AA.
hydrophilicity, suggesting that higher carbon radicals exist on the membrane surface, thus showing improved polymerization efficiency. The amino groups resulted in a great increase in membrane surface hydrophilicity, which bring the benefit to developing the
permeation of organic solvents, due to mainly polymerization reactions (Fig. 10c and Table 2). According to the chemical characterization, plasma polymerization with AAc causes C C intend as 59.2% which is lower amount than plasma polymerization with
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Fig. 8. XPS spectra (a) survey scan of AAc plasma treated PCTE membrane and (b) narrow scans of C 1s peaks.
Fig. 9. XPS spectra (a) survey scan of AA plasma treated PCTE membrane and (b) narrow scans of C1s peaks.
precursor AA. So, in AAc plasma create more hydrophilic surface when it was compared with AA plasma related to the lower C C content. So, static contact angle values are correlated with these critics as = 36.4 ± 2.6◦ for surface that plasma modified with AAc which is the lowest value (Fig. 10b and Table 2). 4.1. Test of membrane permeability The effect of different plasma polymerization precursors on the PCTE membrane permeability was reported by evaluating the data
from diffusion cell system experiments. Fig. 11 shows the time dependence of the cumulative amount of citric acyde permeation through unmodified and modified membranes with plasma polymerization technique. The transport characteristics of PCTE membranes were altered by applying low-temperature plasma process. The graph in Fig. 10 shows the correlate the precursor nature (acrylic acid or allyl amine) in the preparation of films by plasma polymerization technique with the transfer characteristics of the membranes. The experiments were intended to find out whether plasma
Fig. 10. Water droplets on PCTE membranes (a) unmodified (b) precursor, AAc and (c) precursor, AA.
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Fig. 11. Effect of plasma process for change on transport mechanism at PCTE membrane.
polymerization technique could enhance facilitation effect in the presence of proper precursor. The plasma modification of the PC membranes in general decreased the permeability of due to the deposition and polymerization of the monomers on the surfaces, which has been shown in our previous studies [26]. But in this study, we observe that when the PCTE membranes with 0.03 m pore size was modified with proper precursor the citric acid permeability could be increased compared with non-modified membrane. Independently on the plasma conditions performed and the type of the membrane support, the citric acid transport through modified membranes also could be decreased according the monomer type. Influence of various parameters which affect target molecule transport, viz. feed acidity, pore size, membrane thickness, carrier concentration, nature of strippant and the aqueous stirring rate have been investigated in previous studies. But, there is a few study shows the effect of plasma polymerization technique. Some research works to enhance the performance of membranes were attempted, e.g., surface modification by hydrogen peroxide and plasma process to increase its separation performance and chemical resistance. In our study, the transport slope obtained for citric acid is 48.7 ± 2.5% lower in the case of plasma modified PC membrane with precursor; AAc than the corresponding diffusivity in unmodified PC ones. On the other hand the transport coefficient was influenced by the change in precursor type. Higher diffusivity is obtained with plasma modified PC membrane with AA (54.1 ± 3.5% higher). The presence of AAc in the plasma reaction chamber has the opposite effect––the pore size increased but the transport of citric acid was reduced. This proved that one possibility; the chemical functionality of membrane surface was more important effect than the change of pore size in the transport phenomena. An expected result was observed for allylamine plasma; the pore size decreases but the transport of citric acid was enhanced. It is possible that the presences of proper functional group in surfaces of pores can raise the affinity to acid and improve its transport rate. 5. Conclusion In the fermentation broth media, the concentration of acids is very low and they are present in a mixture of different compounds. So, separation and purification steps are necessary during the down stream production of organic acids. The goal of this paper was to demonstrate some of the properties of acrylic acid and allylamine an plasma deposited layers. Finally, the influence of these deposits on citric acid filtration through porous membranes was considered.
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This approach can be a low-cost separation technique due to the relatively small inventory of extracting and low energy consumption. Since pores are well defined, track etched membranes were used as model supports. Plasma polymerization has been extensively studied to modify the surface of polymeric membranes for the enhancement of selectivity and fouling resistance. Plasma enhanced chemical vapor deposition, is a very powerful technique to deposit a thin, dense and pinhole-free layer, named plasma polymer, at the surface of a polymer substrate contacting a glow discharge of organic or organometallic monomers. The result of this paper was to demonstrate the possibilities of such a kind of plasma modified membranes for the facilitated transport of organic acids, where the citric acid used as a target molecule. The results obtained indicate clearly that systematic increases in liquid citric acid permeabilities were achieved with nitrogen content and this controllability did not require complete pore closure. We were able to get membranes with weakly basic groups on the surface. We concluded that it was caused by alteration of the modified surface in which the number of amine groups was induced. Determination of the membrane pore size seems to be a valuable tool in understanding plasma processes. Hence, monitoring the pore size offers an insight into the effects of plasma treatment. AA plasma evidently causes a significant decrease of the average pore diameter. This proves that AA plasma causes mostly deposition of a polymerized layer in the wall of pore surfaces. In the case of AA plasma, the increase in water flux after 30-s modification is conflicted with the pore size decrease (see Fig. 11). Hence, the enhancement of transport property was fully understandable in light of the surface tension and chemical change of surface. When amine containing monomer is used for plasma polymerization on other type of membranes, the surfaces become basic due to the presence of amino group. So, in our study the affinity of citric acid toward nitrogen could be responsible for the increase of selectivity with the acid–base phenomena. But, more characterization test and different type of nitrogen containing polymers need to be studied as future goals. If this is true, it implies that a high nitrogen-containing thin top layer is required for high citric acid selectivity. Alikeness and affinity between amino groups on surface and carboxyl acid groups on target molecule result in faster transportation of target molecule along inner surface of pore. On the contrary, it was found that the surface chemistry could be responsible for the decrease of permeability. The differences in these values suggest that some other factors, such as chemical composition of the pores, might affect the process of water permeation. For example, in the case of monomer; AAc some acidic moieties were formed. Subsequently, the transport velocity decreases as opposed to increase in pore size with the mechanism of plasma grafting. Plasma polymerization of acrylic acid has become an interesting research subject, since these coatings are expected to be beneficial for biomedical applications due to their high surface density of carboxylic acid functional groups. This functionality has a great importance for different application. During the glow discharge applications related to the COO, C C, C O ratio in plasma polymer, surface electronegativity could change and it may lead to change of selectivity for the separations. Beside this, presence of AAc in the plasma reaction chamber has the effect with the pore size increased but the transport of citric acid was reduced. This proved that one possibility; the chemical functionality of membrane surface was more important effect than the change of pore size in the transport phenomena. The increase in the separation selectivity was attributed to the formation of a thin film of plasma polymer on the PCTE surface membrane and it is not a simple hydrophilic change. Because of both AAc and AA plasma cause increasing the hydrophilicity meanwhile transport profiles were different.
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Enhancement of polycarbonate membrane permeability due to plasma polymerization precursors Dilek C¸ökeliler ∗ Biomedical Engineering Department, Bas¸kent University, Ba˘glıca Campus, 06530 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 12 August 2012 Received in revised form 10 October 2012 Accepted 24 November 2012 Available online 26 December 2012 Keywords: Plasma polymerization Polycarbonate membrane Permeability Allyamine
a b s t r a c t The diffusivity of different species through a membrane depends on several factors to illustrate the structure of the matrix, molecular size and concentration of the species and temperature. This study concerns the use of the low-pressure plasma process with different monomers to confer surface chemical character to polycarbonate membranes without altering their bulk properties for change membrane permeability. Track-etched polycarbonate membranes with 0.03 m pore sizes were modified by plasma polymerization technique with two precursors; acrylic acid and allylamine in radio frequency discharge at certain plasma process conditions (discharge power: 20 W, exposure time: 10 min, frequency: 13.56 MHz). The transport properties of model organic acid (citric acid) was studied through unmodified and modified polycarbonate membranes by using diffusion cell system. Such plasma treated membranes were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and surface energy changes were studied by static contact angle measurements. These results showed that the change of surface properties could be used to improve the transport properties of the target substrates. The diffusion of citric acid through plasma treated polycarbonate membrane was increased about 54.1 ± 3.5% with precursor: allylamine while it was decreased 48.7 ± 2.5% with precursor acrylic acid. It was observed that the presences of proper functional group (like amino) in surfaces of pores can raise the affinity to citric acid and improve its transport rate. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently there is an increasing interest toward the integration of two or more varied processes into one process yielding new and better results. To give an example of such an integrated process is extractive fermentation, which combines fermentation products, e.g. organic acids, during the process favors the efficiency of the latter due to reduced inhibiting effect of the products. The aim of this study was to show how different chemical surface structure alters the citric acid permeability of track etched polycarbonate membranes. As a citric acid was chosen for reference organic acid since it is importance for the food industry. Moreover end-product of some process is almost accompanied by waste and/or recycling streams containing acid–water mixtures. Thus, the separation of aqueous acid mixtures is of interest to the chemical industry. The facilitation of some ions and substrates in liquid membrane extraction instead of classical extraction allows the simultaneous recovery of the organic acid from the fermentation medium and its
∗ Corresponding author. Tel.: +90 3122466666x1325; fax: +90 3122341051; mobile: +90 5334465576. E-mail address: [email protected] 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.136
pre-concentration in another solution appropriate for subsequent treatment [1]. The leaching of organic compounds is also inconvenient since the final product, present in the receiving aqueous. Bulk liquid membranes were often used to simulate membrane transport of organic acids of biological interest. These membranes displayed a high selectivity due to a specific complex (hydrogen bonding, electrostatic or hydrophobic interactions) of the solute by the selective carrier, but presented a low stability [2]. Usually, the carrier, the solvent or both suffer from continuous leaching out from the membrane support to the aqueous phases, therefore short lifetime of the liquid membrane reduces the industrial exploitation of these membranes [3]. The review of stability of supported liquid membranes clearly indicates that the stability is a major problem for its industrial application. In this study the usage of hydroxyl group or amino containing chemicals as the vapor source at plasma polymerization was a new idea to improve basic transport characteristic of the polycarbonate membrane surface. Polycarbonate track-etched (PCTE) membranes were selected since novel applications have been proposed for these types with well-defined pore characteristics e.g. very sharp cut off allows their use in the filtration field, yet they were also useful as in vitro substrate in cell biology and also used to determine the reflection coefficients of rigid and partially flexible
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macromolecules [4]. PCTE membranes have been used in applications such as filtration, blood plasma separations, drug delivery, and flow control of reagents for assays. Several studies of these membranes have involved gas permeation/separation applications. PCTE membranes are unique in that they possess cylindrical pores in their fabrication process. Their pore morphology can be considered close to ideal with regard to the availability of uniform, straight cylindrical pores. Our objective was examine the utility of employing these membranes as mechanically simple, yet precise, approach to control acid transport behavior for ultimate use [5]. In order to create membranes with controllable transport properties, one can use the ability of the macromolecules at the surface layer to make reversible conformational transitions. The research in this direction is related to a goal-directed formation of a membrane surface with tailored chemical structure. For this purpose, various methods are used: chemical methods [6] or radiation-induced graft polymerization [7], preliminary activation of the surface etc. [8]. With the same goal, membranes are successfully modified by deposition on the surface of thin polymeric layers obtained by plasma polymerization [9] that was the direct process from the monomer vapors fabricates plasma polymers for the practical use. Even though there are many ways to alter the surface chemistry, plasma treatment appears to be the most effective and convenient one. Plasma polymerization provides ultra-thin membranes composed of cross-linked structure, and the functions of membranes are due to the surface as well as the bulks. It is well known that plasma treatment can alter the physicochemical properties of the polymer surface. Such goals as improvements of wettability, permeability, conductivity, printability, adhesion or biocompatibility are easily achieved within very short time. Plasma polymerized layer has better thermal stability and adhesion properties than the layer formed by conventional methods and we previously reported a plasma treatment studies for sensor surfaces, biomaterials that achieves new permanent surface properties [10]. The usage of plasma provides a number of additional advantages such as: the control of the thickness of the polymeric layer deposited on the membrane surface, the high adhesion of the layer, the short treatment time and the opportunity of using of a wide list of organic and element-organic compounds for modification. Thus, the surface properties of the formed membranes appreciably depend on the type of discharge and nature of the chemical compound used for modification. Focus of the presented work is on testing the differences from transport mechanism of citric acid from medium using thin film deposition on PCTE with monomers; acrylic acid and allyl amine as the precursors and understanding the mechanism. The modifications based on plasma polymerization of amino compounds are of particular interest for biomedical applications [11–19]. Membranes are rarely used as a substrate for such amino modifications yet in the literature, only membranes for gas separation [20–22] and filtration [15,23] are described, while not transporting properties of acids were tested previously. On the other hand, introduction of polar groups such as carboxylic acid has been interesting for the characteristic functions of adsorption, biomaterial and surfactants [24]. Fabrication of the acidic surfaces of substrates and the coating by plasma polymers containing the –COOH group was tested for the same motivations. PCTE membranes were modified in a glow discharge reactor by plasma polymerization technique at certain conditions with different monomers. Then, the effect of type of monomers on the membrane permeability was tested by a diffusion cell system. The chemical and physical structural characterization of the PCTE membranes before and after the surface modification is done by means of scanning electron microscopy and X-ray photoelectron spectroscopy. The surface wettability of the pure and plasma modified membranes were characterized using the surface contact angle of water.
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2. Experimental 2.1. Plasma polymerization process PCTE membranes with nominal pore size 0.03 m was obtained from Poretics (California, USA) and treated by plasma within a tubular type reactor containing a pair of plate copper electrodes separated by 5 cm (Fig. 1). A 13.6 MHz radio frequency generator was used to sustain glow discharge in the reactor and power losses were minimized by means of a matching network. PC membranes were located in the center of two electrodes. Monomer acrylic acid (AAc) of 99% purity was purchased from Merck Co. and allyl amine (AA) was obtained commercially (Sigma, Steinheim and Merck, Darmstadt, Germany). After the system was evacuated to a pressure of 10−3 Torr, first the AAc vapor was introduced to the system to a predetermined pressure of 5 Pa. Monomer vapors were allowed to flow at a rate of 25 ml/min. Then, RF power was adjusted to 20 W and the membrane was exposed to glow discharge for 10 min. At the end of the process the RF generator was turned off and the system was fed with Argon gas for 30 min to deactivate free radicals. After the one set of the plasma polymerization process, new polycarbonate membranes were inserted and monomer container was filled by new precursor; AA. The plasma was ignited at a frequency of 13.56 MHz with power 20 W for 10 min. All parallel modifications were repeated at the same plasma polymerization parameters (discharge power: 20 W, time: 10 min, flow rate: 25 ml/min). The monomers were chosen to provide surfaces differing greatly in their surface energies and stability. These factors were varied and their effects on acid permeability of PCTE membranes were studied using citric acid as permeate.
2.2. The characterization of PCTE membranes The surface morphologies of PCTE membranes were investigated by using AFM (Burleigh Metris 2001A-MS) and pore structures were characterized by SEM (JEOL, JSM 5600, USA). For the SEM analysis membranes were photographed in the electron microscope with 5000× magnification. The polymeric surface usually has no electrical conductance; hence a 10–15 nm gold layer must be provided by sputter coating, to remove electrostatic charge from the surface due to electron beam radiation. Analysis of polymer surface functional groups after plasma treatments was performed by X-ray photoelectron spectrometer in ADES-400 equipped by Al K␣ excitation sources and a hemispherical energy analyzer was used to obtain information about the composition and the chemical bonding of elements found in a surface region of untreated and plasma treated samples. The operating pressure was 1 × 10−7 Pa. Pass energies 50 and 15 eV were used for the survey and high resolution scans, respectively. The take-off angle was ˛ = 45◦ . The HR envelopes were analyzed and peak-fitted after subtraction of a Shirley background using a Gaussian–Lorenzian peak shapes obtained from the Casa XPS software package. The signal of C 1s, 285 eV, was selected for energy calibration. The water contact angles were measured using a NRL (Rame Hart) goniometry. The contact angle of a pure water (de-ionized with a resistivity of 18.2 MV cm) drop on the membrane surface was measured by sessile drop method. The membrane was placed over a micrometer bench and multiple droplets are formed on the surface using a syringe. The images of the water droplets on the membrane surface was captured by a high-resolution video camera and digitized. The contact angles were directly measured from the images using a software. For each specimen, the measurements were repeated in three different areas, 10 times each area. The measured values of the contact angles were averaged.
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Fig. 1. Plasma polymerization system.
2.3. The test of membrane permeability PCTE membrane permeability was tested by a diffusion cell technique [25]. The diffusion cell technique uses a diaphragm diffusion cell that consists of two identical chambers separated by a membrane with 5.3 cm2 of available membrane area in which the diffusivity will be measured. In our system has two compartments with 125 ml volume divided by a PCTE membrane. Initially, one chamber (A) was filled with a solution that contains the solutes of citric acid (0.005 M) and the other (B) with pure solvent as distilled water. Before the test, the membranes were maintained in a relevant solution during 20 min then prepared membranes were mounted on a cell. In both chambers, sufficient mixing will be provided to ensure a negligible external mass transfer resistance. Therefore the feed and strip solutions were mechanically stirred at 400 rpm (except otherwise stated) at ambient temperature (24 ◦ C) in a thermo stated room to suppress concentration polarization conditions at the membrane interfaces and in the bulk of the solutions. Since the membrane solvent was hold within the pores by capillary forces, it was expected that pore structure may play a major role in the stability and diffusion rate of the acid (Fig. 2). The permeability of citric acid through the PCTE membrane was monitored by periodically removing 200 l samples from both the chambers and equal volume of drug-free buffer solution was refilled to avoid errors arising from the resulting volume variation. The concentrations of citric were analyzed by an UV spectrophotometer (Ultrospec 1000E, Pharmacia Biotech, Sweden) from the peak absorbency at 204 nm. After an initial period of time, a quasisteady state of solute flux was attained. The concentration change in each experiment was evaluated to calculate the permeability of unmodified and modified polycarbonate membrane for citric acid. Each experiment was repeated three times and the results were expressed as the mean of the three results.
3. Result and discussion 3.1. The characterization of PCTE membranes 3.1.1. Physical characterization Plasma treatment is a powerful technique to modify only the surface. Briefly a glow discharge created by RF generator wave within a low pressure monomer vapor leads to etching, functionalizing, grafting of polymer layer on the touched material. First, vertical sections of PC membranes were observed in SEM (Fig. 3).
As seen in Fig. 3b, the thickness of the dense top layer was clear as the plasma precursor; AAc. For a rough estimation, the top layer thickness of the membrane was twice that of the one is plasma modified with AA. Because of high extend of molecular fragmentation occurring in the discharge and the variety of reactions running both in the volume and on the surface; the composition and deposition rate of plasma polymers is much more complex. Since plasma polymerization involves the fragmentation of precursor molecules, when the monomer contains different ratio of elements (C/N, C/H, C/O, etc.) they may combine into different chemical functionalities and deposition rate incorporated in a highly cross-linked matrix explains the difference of film thickness according to the change of precursors; AAc and AA in Fig. 3. Furthermore SEM observations indicate the absence of erosion of the membrane surface under plasma treatment and both modified membranes show similar flat appearance when it was compared with unmodified one in Fig. 3a. Track-etched membranes with well-defined pore characteristics were prepared by bombarding polycarbonate plastics with 161Dy ions of 13.0 MeV per nucleon energy and by bombarding polycarbonate plastics with fission fragments from a 252Cf source. Novel applications have been proposed for these tract etched membranes, e.g. the very sharp cut off allows their use in the filtration field, but they were also beneficial as in vitro substrate in cell biology. In the case of the membrane applications, plasma can change pore size and alter the character of surface. PCTE membranes were examined under SEM, in plan view, for the pore structure characteristics, including changes of pore sizes after polymer coating (Figs. 4–6). These images clearly reveal the physical pore size reduction which accompanies the polymer film deposition on the plasma modified membrane with precursor; AA. Pore size of pure PCTE was 37 ± 5 nm (Fig. 4) but it was 30 ± 2 nm for plasma modified PCTE membrane by precursor AA (Fig. 6) along with the conformal aspect of the coating process. Beside this, the enlargement in pore size resulting from plasma deposition of monomer AAc was verified by SEM in Fig. 5 (41 ± 2 nm). During plasma action three processes always compete; polymer deposition, substrate etching or polymer grafting. The former decreases pore size; the both latter two may increase it and additional chemical analysis of surface was added for the selection on one of these mechanisms. Despite the fact that these pore size change was clearly the consequences of plasma treatment, no visible pitting of the plasma polymerized membrane surface was observed, after the modifications (Fig. 7b and c). Both plasma modifications with precursors cause physical changes on the surface with an increase on
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Fig. 2. Diffusion cell system.
Fig. 3. The vertical sections of PCTE membranes (a) unmodified, (b) precursor, AAc and (c) precursor, AA.
roughness compare with unmodified PC membrane (Fig. 7a). Regarding SEM results there were no differences with modified PCTE membranes as the mean of front view. 3.1.2. Chemical characterization Two different monomers were used for the plasma polymerizations, each intended to incorporate different chemical functional groups into the surface. Acrylic acid and ally amine was used to fabricate films rich in carboxyl groups and nitrogen containing groups respectively. Fig. 8a indicates a typical wide scan spectrum of the
polymer deposited onto PCTE membrane and the X-ray photoelectron spectroscopy analysis of samples produced under the nominal conditions shows that C1s region of an acrylic acid plasma polymer showing five component peaks with C C (59.2%), C O (3.4%), C O (3.3%) and COOR (17.1%) (Fig. 8b) Plasma polymers deposited from ally amine plasma contain carbon, nitrogen and a small quantity of oxygen. A typical wide scan spectrum was shown in Fig. 9a. The C 1s region of an allyl amine plasma polymer was shown in Fig. 9b. There were difficulties in fitting the C 1s regions of nitrogen containing surface
Fig. 4. PCTE membrane, unmodified (a) 20,000× and (b) 40,000×
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Fig. 5. Modified PCTE membrane by precursor: AAc (a) 20,000× and (b) 40,000×
Fig. 6. Modified PCTE membrane by precursor: AA (a) 20,000× and (b) 40,000×
which relate to the large spread of chemical shifts that are associated with nitrogen-containing functional groups. For curve fitting these regions, four component peaks were used and a peak representing amine groups (primary, secondary and tertiary) was placed at a shift of +0.9 eV relative to the hydrocarbon. A third peak was placed with a shift of 1.7 eV includes contributions from imine and diamine functional groups. A fourth peak at +3.0 eV includes amide functionalities and any carbonyl groups. The chemical distribution using this curve fitting strategy shows all chemical functionality with precursor allyl amine, C C (68.6%), C N (19.8%), C N, C O (11%), C O, OCN (0.5%) For the chemical composition to see Si element onto unmodified PC membrane was susceptible due to in commercially PC membranes it was possible to get this chemical. The disappearing of Si is evident that there is plasma polymerizes film onto PC membrane surface in both plasma treatments (Table 1). Plasma polymer of acrylic acid contained slightly high oxygen than might be expected from the monomer composition (31.8%) and the amount of carboxylic acid functionality was depended on chemicals C O, C O and COOR. For polymer films deposited from plasmas containing a acrylic acid, there could be loss of material upon
exposure to an aqueous environment. Post treatment reactions between the water and carbon radicals trapped in the plasma polymer is processed and all data are obtained after immersion in water and drying at room temperature. Plasma polymers of allyamine contain a small amount of oxygen (15.1%) which is attributed to residual oxygen species within the plasma and to post polymerization reactions with atmospheric oxygen. The N/C ratio from the values in Table 1 is 0.21, amine content of the surface is likely to consist of a high proportion of secondary and tertiary amine groups.
Table 1 Surface elemental composition of PCTE membranes.
Table 2 Static contact angle values () of PCTE membranes.
Unmodified Plasma modifications
Precursor: AAc Precursor: AA
4. Surface wettability The contact angles of the plasma deposited surfaces were different to those of unmodified PCTE ( = 71.8 ± 4.8) membranes indicating that the surface chemistry has been modified by plasma depositions process (Fig. 10). Table 2 represents the static contact values of samples. Therefore the increase of wettability can be associated with the process of formation of additional hydrophilic groups on the surface. The water contact angle experiment was used to estimate the surface hydrophilicity and lower contact angle value indicates higher
C (%)
O (%)
N (%)
Si (%)
Sample
Mean (±s.d.)
n
73.6 68.2 69.9
20.4 31.8 15.1
– – 15
6.0 – –
Unmodified Acrylic acid Allylamine
71.8 ± 4.8 36.4 ± 2.6 62.7 ± 6.7
8 10 10
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Fig. 7. Topology of the membranes in front by AFM (a) unmodified, (b) precursor AAc and (c) precursor: AA.
hydrophilicity, suggesting that higher carbon radicals exist on the membrane surface, thus showing improved polymerization efficiency. The amino groups resulted in a great increase in membrane surface hydrophilicity, which bring the benefit to developing the
permeation of organic solvents, due to mainly polymerization reactions (Fig. 10c and Table 2). According to the chemical characterization, plasma polymerization with AAc causes C C intend as 59.2% which is lower amount than plasma polymerization with
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Fig. 8. XPS spectra (a) survey scan of AAc plasma treated PCTE membrane and (b) narrow scans of C 1s peaks.
Fig. 9. XPS spectra (a) survey scan of AA plasma treated PCTE membrane and (b) narrow scans of C1s peaks.
precursor AA. So, in AAc plasma create more hydrophilic surface when it was compared with AA plasma related to the lower C C content. So, static contact angle values are correlated with these critics as = 36.4 ± 2.6◦ for surface that plasma modified with AAc which is the lowest value (Fig. 10b and Table 2). 4.1. Test of membrane permeability The effect of different plasma polymerization precursors on the PCTE membrane permeability was reported by evaluating the data
from diffusion cell system experiments. Fig. 11 shows the time dependence of the cumulative amount of citric acyde permeation through unmodified and modified membranes with plasma polymerization technique. The transport characteristics of PCTE membranes were altered by applying low-temperature plasma process. The graph in Fig. 10 shows the correlate the precursor nature (acrylic acid or allyl amine) in the preparation of films by plasma polymerization technique with the transfer characteristics of the membranes. The experiments were intended to find out whether plasma
Fig. 10. Water droplets on PCTE membranes (a) unmodified (b) precursor, AAc and (c) precursor, AA.
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Fig. 11. Effect of plasma process for change on transport mechanism at PCTE membrane.
polymerization technique could enhance facilitation effect in the presence of proper precursor. The plasma modification of the PC membranes in general decreased the permeability of due to the deposition and polymerization of the monomers on the surfaces, which has been shown in our previous studies [26]. But in this study, we observe that when the PCTE membranes with 0.03 m pore size was modified with proper precursor the citric acid permeability could be increased compared with non-modified membrane. Independently on the plasma conditions performed and the type of the membrane support, the citric acid transport through modified membranes also could be decreased according the monomer type. Influence of various parameters which affect target molecule transport, viz. feed acidity, pore size, membrane thickness, carrier concentration, nature of strippant and the aqueous stirring rate have been investigated in previous studies. But, there is a few study shows the effect of plasma polymerization technique. Some research works to enhance the performance of membranes were attempted, e.g., surface modification by hydrogen peroxide and plasma process to increase its separation performance and chemical resistance. In our study, the transport slope obtained for citric acid is 48.7 ± 2.5% lower in the case of plasma modified PC membrane with precursor; AAc than the corresponding diffusivity in unmodified PC ones. On the other hand the transport coefficient was influenced by the change in precursor type. Higher diffusivity is obtained with plasma modified PC membrane with AA (54.1 ± 3.5% higher). The presence of AAc in the plasma reaction chamber has the opposite effect––the pore size increased but the transport of citric acid was reduced. This proved that one possibility; the chemical functionality of membrane surface was more important effect than the change of pore size in the transport phenomena. An expected result was observed for allylamine plasma; the pore size decreases but the transport of citric acid was enhanced. It is possible that the presences of proper functional group in surfaces of pores can raise the affinity to acid and improve its transport rate. 5. Conclusion In the fermentation broth media, the concentration of acids is very low and they are present in a mixture of different compounds. So, separation and purification steps are necessary during the down stream production of organic acids. The goal of this paper was to demonstrate some of the properties of acrylic acid and allylamine an plasma deposited layers. Finally, the influence of these deposits on citric acid filtration through porous membranes was considered.
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This approach can be a low-cost separation technique due to the relatively small inventory of extracting and low energy consumption. Since pores are well defined, track etched membranes were used as model supports. Plasma polymerization has been extensively studied to modify the surface of polymeric membranes for the enhancement of selectivity and fouling resistance. Plasma enhanced chemical vapor deposition, is a very powerful technique to deposit a thin, dense and pinhole-free layer, named plasma polymer, at the surface of a polymer substrate contacting a glow discharge of organic or organometallic monomers. The result of this paper was to demonstrate the possibilities of such a kind of plasma modified membranes for the facilitated transport of organic acids, where the citric acid used as a target molecule. The results obtained indicate clearly that systematic increases in liquid citric acid permeabilities were achieved with nitrogen content and this controllability did not require complete pore closure. We were able to get membranes with weakly basic groups on the surface. We concluded that it was caused by alteration of the modified surface in which the number of amine groups was induced. Determination of the membrane pore size seems to be a valuable tool in understanding plasma processes. Hence, monitoring the pore size offers an insight into the effects of plasma treatment. AA plasma evidently causes a significant decrease of the average pore diameter. This proves that AA plasma causes mostly deposition of a polymerized layer in the wall of pore surfaces. In the case of AA plasma, the increase in water flux after 30-s modification is conflicted with the pore size decrease (see Fig. 11). Hence, the enhancement of transport property was fully understandable in light of the surface tension and chemical change of surface. When amine containing monomer is used for plasma polymerization on other type of membranes, the surfaces become basic due to the presence of amino group. So, in our study the affinity of citric acid toward nitrogen could be responsible for the increase of selectivity with the acid–base phenomena. But, more characterization test and different type of nitrogen containing polymers need to be studied as future goals. If this is true, it implies that a high nitrogen-containing thin top layer is required for high citric acid selectivity. Alikeness and affinity between amino groups on surface and carboxyl acid groups on target molecule result in faster transportation of target molecule along inner surface of pore. On the contrary, it was found that the surface chemistry could be responsible for the decrease of permeability. The differences in these values suggest that some other factors, such as chemical composition of the pores, might affect the process of water permeation. For example, in the case of monomer; AAc some acidic moieties were formed. Subsequently, the transport velocity decreases as opposed to increase in pore size with the mechanism of plasma grafting. Plasma polymerization of acrylic acid has become an interesting research subject, since these coatings are expected to be beneficial for biomedical applications due to their high surface density of carboxylic acid functional groups. This functionality has a great importance for different application. During the glow discharge applications related to the COO, C C, C O ratio in plasma polymer, surface electronegativity could change and it may lead to change of selectivity for the separations. Beside this, presence of AAc in the plasma reaction chamber has the effect with the pore size increased but the transport of citric acid was reduced. This proved that one possibility; the chemical functionality of membrane surface was more important effect than the change of pore size in the transport phenomena. The increase in the separation selectivity was attributed to the formation of a thin film of plasma polymer on the PCTE surface membrane and it is not a simple hydrophilic change. Because of both AAc and AA plasma cause increasing the hydrophilicity meanwhile transport profiles were different.
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