ХИМИЯ ВЫСОКИХ ЭНЕРГИЙ, 2013, том 47, № 6, с. 497-504
- ПЛАЗМОХИМИЯ
SURFACE MODIFICATION OF POLYMERIC ULTRAFILTRATION MEMBRANES III. EFFECT OF PLASMA-CHEMICAL SURFACE
MODIFICATION ONTO SOME CHARACTERISTICS OF POLYACRYLONITRILE ULTRAFILTRATION MEMBRANES © 2013 T. Vladkova*, P. Atanasova*, S. Petrov**, P. Dineff***
*University of Chemical Technology and Metallurgy 8 "Kliment Ohridski" Blvd, 1756Sofia, Bulgaria E-mail: tgv@uctm.edu **University "Prof. Dr Asen Zlatarov" 1 "Prof. Iakimov" Str, 8008Burgas, Bulgaria ***Technical University, 1756Sofia Поступила в редакцию 01.04.2013 г. В окончательном виде 30.05.2013 г.
Plasma-chemical surface modification of polyacrilonitrile ultrafiltration membranes is presented here including surface pre-activation by treatment in cold plasma, obtained in dielectric barrier discharge at atmospheric pressure, and following chemical grafting of mono-functional polyethylene glycol (PEG) chains. The effect of such plasma-chemical modification onto the physico-chemical characteristics of the membrane surface as well as onto some basic working membrane characteristics such as productivity and selectivity was studied. XPS analysis was employed to control the chemical composition of the membrane surface. Contact angle measurement was used to characterize the hydrophilic/hydrophobic balance on the surface. Membrane structure was imaged by SEM observations.
DOI: 10.7868/S0023119713060124
Surface modification of ultrafiltration membranes by low temperature plasma treatment leading to surface etching, grafting of functional groups, deposition of thin plasma coating or to surface activation followed by chemical grafting of suitable polymer chains opens a way to improve the existing polymer membranes properties such as fouling resistance and productivity while keeping the selectivity and to create new composite membranes with unique characteristics [1—5]. The potential of the more easy for industrialization atmospheric pressure plasma for membrane surface modification is less studied as compared to the low pressure high frequency plasma. An attempt to improve some basic working characteristics of polyacry-lonitrile ultrafiltration membranes (PUF) by treatment in plasma, obtained at atmospheric pressure, 50 Hz dielectric barrier discharge in air, was presented in a former our communication [6]. However, scare information could be fined in the special literature about the ability of the atmospheric pressure dielectric barrier discharge [6, 18], to activate the surface of PUF membranes for following grafting of suitable functional groups, improving their performance in water purification and other applications. Polyethylene glycols (PEG)s are interesting candidates for membrane surface modification because they may
render the surface more hydrophilic and endow it with enhanced fouling resistance toward organic foul ants and especially proteins [7—10]. In water purification, PEG-based materials have been explored to modify the surface of ultrafiltration (UF) membranes, aimed at improvement of their fouling resistance [11—13]. For example, cross-linked PEG coating layer on polysulfone UF membrane increases simultaneously both, the water flux and rejection of organic in oil/water emulsion, leading to significantly improved fouling resistance against emulsified oil droplets [14, 15]. PEG-based coatings enhance also the fouling resistance of desalination membranes [16]. Sagle et al. [17] prepare fouling-resistant coatings for water purification membranes based on copolymer hydrogel networks, using PEG di-acrylates as a cross-linker. The modified membranes characterize with increased water permeability and low oil affinity to the surface that suggests improved fouling resistance toward, oils containing waste water. The antifouling properties of poly(acrylonitrile-co-maleic acid) membranes have been effectively improved by surface immobilization of PEG through an esterification reaction [17]. Here is described a brush type thin (monomolecular) PEG coating creation onto the surface of atmospheric plasma pre-activated PUF membranes by thermally-as-
Productivity G, l/m2 h 350
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Productivity G, l/m2 h 350
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Fig. 1. Water productivity, G at pressure of 0.1 to 0.5 MPa for membranes: non-modified and grafted with (a) PEG 300, (b) PEG 1100 and (c) PEG 2080 for 3, 6 and 12 hours; pressure forward — the light grew columns, and back — the dark grew columns.
sisted grafting of mono-functional PEG (PEG-mono-methyl methacrylate), this plasma-chemical modification aimed at preparation of high performance ultrafiltration membranes.
EXPERIMENTAL
Industrially produced ter-co-polymer of acryl nytril-methylmetacrylate-2- acrylamido-2-methyl-propansul-fonic acid (AN/MMA/AMPSA) (92 w %/7 w %/1 w %) (Luckoil, Burgas, Bulgaria) was used for preparation
of membranes. Dimethylformamide (Merk, Germany) was used as the solvent. A face inversed process was employed for membranes formation at room temperature onto polyester substrate (Velidon, FO 2403, Germany) with addition of lithium nitrate, LiNO3, as it is more detailed described in [6, 18].
Surface pre-activation was performed in air plasma, obtained in atmospheric pressure, 50 Hz dielectric barrier discharge at 10 kV for 5 min.
Chemical grafting of PEG methyl ether methacrylate (Mw of 300, 1100 and 2800, Sigma - Aldrich) was performed from 25% solution at 60°C for 3, 6 and 12 hours.
Contact Angle Measuring Instrument Easy Drop (Kruss, Germany) was used for contact angle measurement (angle resolution ±0.1°) using three liquids with known surface tension: water, ethylene glycol and n-hexadecan. Surface energy was calculated according to Fowkes method [19].
XPS surface analysis was carried out with ESCA-LAB II MK VG Scientific spectrometer. The excitation X-ray source was AlK (excitation energy 1486.6 eV). Complete spectral scans as well as detailed recordings of the main peaks were made at 10-8 Pa. The binding energy scale was fixed by assigning Eb = 285 eV to the -CH2 carbon (1s) peak. According to previous studies [16, 18, 19] and using this -CH2-reference peak, the carbon chemical shifts for different oxygen and nitrogen containing groups are: -C-O- from hydroxyl, hydroperoxide, ether, alkyl or sulphate ester, AEb = 1.5 eV; =C=O from carbonyle or amide, AEb = 3.0 eV; -COO from carboxyle or the corresponding ester, AEb = 4.2 eV The peaks area was computed graphically and corrected by using Scofields relative cross-sections [20].
The technological parameters, productivity at pressure of 0.1 to 0.5 MPa (G) and selectivity (9), and stability of the albumin productivity were determined on a laboratory module Sartorius SM 165-26, UK Bovine serum albumin (BSA) with molecular weight (Mm) of76000, Merk was used in the membrane selectivity testing. The BSA concentration was estimated with UV/VIS spectrophotometer, Unicam 8625, UK. SEM JSM-5510, Japan was employed for the membrane cross-sections observation.
RESULTS AND DISCUSSION
Water Productivity
Figure 1 represents the water productivity, G at pressure of 0.1 to 0.5 MPa of non-modified and plas-ma-chemically modified PUF membranes with surface grafted PEG chains of different length: (a) PEG 300; (b) PEG 1100 and (c) PEG 2080. It is evident that the PEG grafting influences significantly the membrane water productivity the effect depending on both, the PEG molecular weight and the grafting duration. The increase of the water flux, G is the best expressed at, for 3 hours, PEG 2080 grafted membrane (Fig. 1c)
followed by, for 6 hours, PEG 300 grafted one (Fig. 1a). The water flux, G (at 0.5 MPa) for the non-modified membrane of about 110 l/m2h increases up to about 320 L/m2h for the 3 hours PEG 2080 grafted membrane, i.e. of about 3 folds (Fig. 1c); an increase of the water flux, G is characteristics also for pressure of 0.1—0.4 MPa. These results indicate that the optimal duration grafting of relatively long PEG chains (3 h for PEG 2080) could beutilized for increasing the water productivity of PUF membranes.
Membrane Selectivity
Membrane selectivity, 9 is another very important membrane characteristic. Bovine serum albumin with known molecular mass was employed in this study for its estimation. In Fig. 2 is presented the selectivity of the non-modified and the studied PEG grafted membranes. It is evident that all 3, 6 or 12 hours PEG 2080, PEG 1100 and PEG 300 grafted membranes (Fig. 2, all groups of columns) demonstrate higher selectivity, 9 as compared to the non-modified membrane (Fig. 2, the first single column). Its good selectivity, 9 of 96.4% is additionally improved up to 97.5—99.7% depending on the PEG molecular weight and grafting duration. The highest selectivity, of 99.7% is characteristics of the PEG 300 grafted membrane for 3 hours.
Bovine Serum Albumin Productivity and Its Stability with the Progress of the Filtration
To evaluate the stability of the work (protein fouling resistance) of the modified membranes, the stability of their albumin productivity, GBSA with the progress of the filtration was estimated and compared to that of the non-modified membrane. The results, presented in Fig. 3 (a) demonstrate the albumin productivity and those presented in Fig. 3 (b) its stability with the progress of the filtration, i.e. the antifouling (protein rejective) effect of the PEG grafted membrane surfaces. The albumin productivity of all PEG grafted membranes (excluding PEG 1100 grafted for 6 and 12 h) is higher as compared to that of the non-modified membrane, as it is evident form Fig. 3 (a) — compare the three groups of columns to the first column. Fig. 3 (b) clearly demonstrates the keeping of the higher albumin productivity of the PEG grafted membranes with the progress of the filtration: all three curve
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