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Preparation and characterization of novel porous PMMA-SiO2 hybrid membranes Muhammad Ali Zulfikara, A. Wahab Mohammada*, Nidal Hilalb a
Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan, Malaysia Tel. +60 (3) 8921-6410; Fax: +60 (3) 8921-6148; email:
[email protected] b Centre for Clean Water Technologies, School of Chemical, Environmental and Mining Engineering, The University of Nottingham, NG7 2RD, UK Received 17 March 2005; accepted 15 September 2005
Abstract Poly(methyl methacrylate) (PMMA)-SiO2 composite membranes were prepared by casting ternary solutions obtained by adding additional amounts of TEOS to PMMA solutions. Methods to obtain an intimate dispersion of the inorganic particles in the PMMA solutions were developed. Various samples of hybrid membranes were prepared by varying some of the preparative parameters such as the PMMA solvent (tetrahydrofuran, and N,N-dimethyl formamide) and the PMMA/TEOS ratio in the ternary solution. Membranes were also cast from a binary solution of PMMA and solvent for comparison. The membranes were characterized by water permeability, scanning electron microscope observations, DSC and TGA analysis and ultrafiltration tests. The effects of the above-mentioned preparative parameters on the structure, thermal behavior, flux and dextran retention properties of the membranes are discussed. Keywords: Organic-inorganic membranes; Poly(methyl methacrylate); TEOS; Ultrafiltration properties
1. Introduction The possibility of combining properties of organic and inorganic materials was explored several years ago. The presence of finely dispersed inorganic particles in the polymer matrix *Corresponding author.
has proven to be very useful in the improvement of membrane performance. Many new types of organic/inorganic hybrid materials have the potential to combine the desired properties of inorganic and organic systems, improving the mechanical and thermal properties of inorganic ones with the flexibility and ductility of organic
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved
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polymers. These hybrid materials can be readily prepared by using a sol-gel process, which offers several advantages over other techniques. The micro- and macrostructure of a hybrid composite can be controlled by the optimization of several synthetic parameters, for example, pH, concentration, water-to-alkoxide ratio, temperature, pressure, type of catalyst, and solvent at low temperature [1–5]. Of these hybrid composites, poly(methyl methacrylate) (PMMA) as an organic matrix has been of special interest for its exceptional thermal and mechanical stability which has been applied to a wide range of industries such as bone substitution and optical devices [6–8]. PMMA-silica hybrids with covalent bonds between the organic and inorganic segments have been synthesized by polymerization of the methyl methacrylate (MMA) monomer in the presence of inorganic precursors, with some data of its properties being reported [5–14]. Landry et al. [2] studied the effect of pH and coating temperature on the properties of PMMA/silica films. The PMMA/silica films were prepared by adding tetraethoxyorthosilane (TEOS) and water to a PMMA polymer solution before thermal treatment. Two effects were reported to dominate the reinforcing nature of the SiO2 phase: the concentration of adhesion points between the PMMA and SiO2 surface, which depends on particle size and concentration; and the chemical character of the surface which is a function of pH. It has also been shown that the composite morphology and degree of phase separation are extremely important in determining the ultimate material properties, and these can be controlled with such variables as temperature and pH. In other studies, Landry et al. [3] found that formation of a molecular composition by in situ polymerization of TEOS in various organic polymers such as poly(methyl methacrylate), poly(vinyl acetate), poly(vinyl pirrolidone) and poly(N,N-dimethyl acrylamide) shows a highly homogenous, trans-
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parent, higher mechanical and solvent-resistant composite. There were strong interactions between the SiO2 networks and these polymers, and data analysis suggest hydrogen bonding between residual hydroxyls on the SiO2 and carbonyl groups on the polymer chains as a major source of these interactions. Chan et al. [15] studied the effect of heat treatment on properties of the PMMA/silica films. The films were prepared by the addition of TEOS and water to PMMA solution at various ratios. Each hybrid film was heated at various temperatures. It was observed that heat treatment at 180EC caused further condensation and diminished the content of silane group. Further condensation reactions consumed the silane group and diminished the hindrance effect from the ethyl group of unreacted silane. Thus, hydrogen bonds can be formed from carbonyl group and silanol group between organic–inorganic phases. With the higher interfacial interaction from hydrogen bonding, heat-treated hybrid materials exhibited higher miscibility as compared to untreated materials. Analogous procedures as above have been applied by Silveira et al. [16] who studied the mechanism of phase separation of PMMA/silica hybrid materials, and a spinodal decomposition mechanism was observed for intermediary compositions for the low molecular weight of a PMMA system and also for those with a higher PMMA content with high molecular weight. The nucleation and growth mechanism was detected for low and high contents in systems with low molecular weight PMMA. However, there has been no report on the study of a PMMA/silica hybrid for membrane materials, especially for liquid separation. In our previous work [4], PMMA/silica hybrid composites were obtained when TEOS is polymerized in situ in a PMMA under different solvent types. It was observed that these hybrids can be used as membrane material, and the type of solvent used
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plays an important role in their morphological, degradation behavior and membrane properties. In this study, the organic–inorganic concept is extended to the preparation of novel porous membranes from a system based on TEOS and PMMA. Hybrid membranes cast from PMMA– solvent–TEOS ternary solutions were prepared by in situ polymerization, and the effect of some preparative parameters on the structure and UF performance of the membrane is discussed. 2. Experimental In general, the membranes were prepared as previously described [4] from binary solutions of PMMA (350,000, Aldrich) in tetrahydrofuran (THF, Merck) and N,N-dimethyl formamide (DMF, Merck) as well as from ternary solutions composed of PMMA, THF and DMF. PMMA was thoroughly dissolved in the respective solvent at 15% concentration polymer (w/w) in solution first. TEOS was added directly to the solution under continuous agitation. Then acidic solutions (pH 2) were added in stoichiometric amounts (TEOS:H2O = 1:4) and mixed for 24 h more at ambient temperature. The final composition solution of PMMA/TEOS was 90/10, 85/15, 80/20, and 75/25 (w/w). The homogeneous solution above was left for 24 h to release air bubbles. Solutions were poured onto a drawing paper placed on a glass plate and cast as thin films (thickness ca. 350 µm) and was allowed to stand at room temperature for 20 days for gelation. Finally, the gelled samples were obtained and then dried at 60EC in vacuum for 5 h. Permeation measurements were carried out using distilled water. A stirred dead-end Amicon UF cell model 8200 was used with a volume capacity of 200 mL. The effective area of the membrane was 28.7 cm2. The operating pressure of the experiment was 1–5 bar. The permeate volume (20 mL) was collected in a graduated cylinder. Membrane surfaces and cross-sections
were observed under a scanning electron microscope (SEM, Oxford Instrument, 7353, England). All specimens were coated with a thin layer of gold before SEM observation. Cross-sections were prepared by fracturing the membranes at liquid nitrogen temperature. A differential scanning calorimeter (DSC 822e, Mettler Toledo) was used for the investigation of glass transition temperature variation of synthesized hybrid materials. An appropriate amount of samples (ca. 3 mg) were ground into fine powder and then were sealed in aluminum sample pans. DSC analyses of these hybrid materials were then conducted under dry nitrogen at a heating rate of 10EC/min from 20 to 250EC. The thermogravimetric data were obtained from a thermogravimetry analyzer (TGA, TA 951) performed under dry nitrogen atmosphere over a temperature range of 35 to 800EC at a heating rate of 10EC/ min. The measurements were taken using 3–5 mg samples. Weight-loss/temperature curves were recorded. Membrane samples (surface 28.7 cm2) were tested at an UF laboratory unit with an aqueous solution of dextran with nominal average molecular mass, 70 kDa (Sigma, Dextran T70). Operating conditions were: temperature, 25EC; transmembrane pressure, 2 bar; stirring speed, 400 rpm. Dextran concentration in the feed (200 ppm) and permeate were determined by the calorimetric method [17]. 3. Results and discussion Permeation measurements were carried out using the constant volume. The results of the permeation measurement are shown in Table 1, which shows that silica content affects permeation. Also, water permeability of hybrid membranes increases with silica content. As described in the SEM section below, H-90/10 have a nodular structure, while the H-85/15 and 80/20 membranes have dense structure which caused
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(a)
(b)
(c)
(d)
(e)
(f)
Fig. 1. SEM micrographs of cross sections (a–c) and surfaces (d–f) of binary solution (PMMA–THF–100/0) (a,d); ternary solution (H-THF-80/20) (b,e); ternary solution (H-THF-85/15) (c,f). Table 1 Water permeability of the hybrid membrane Membranes
Lp (L/m2.h.bar)
H-100/0 H-90/10 H-85/15 H-80/20 H-75/25
1.89 7.15 1.97 3.69 5.47
the water diffusion to decrease compared to H-90/10 membrane. Fig. 1 shows SEM micrographs of the cross section and skin layer surface of the hybrid membranes prepared from a PMMA–THF binary solution (Fig. 1a and d) and PMMA–SiO2–THF ternary suspensions (Fig. 1b, c,e,f), the latter obtained by adding to PMMA solution (15 wt.%) increasing ratio (80/20 and 85/15, respectively) of PMMA/TEOS. It is apparent that membranes obtained from the PMMA solution in THF solvent have a co-continuous
structure [4] and are not affected by the addition of TEOS to the PMMA solution. SiO2 particles are generally hidden by the polymer matrix. The skin layer surface of the membrane cast from the binary solution (Fig. 1d) and membranes prepared from ternary (Fig. 1e,f) appears to be smooth. By using DMF as the PMMA solvent, membranes with very different structures from the ones reported in Fig. 1 were obtained. This is clearly shown in Fig. 2 where micrographs of a membrane cast from a binary solution are reported. Large cavities are present (Fig. 2a) and appear to have a cellular structure [4] as can be better seen from the high magnification micrographs (Fig. 2b) taken in the area of the membrane cross-section. As the presence of the SiO2, in the solution alters the mechanism of phase separation in the system, the structure of the membrane surface (Fig. 3a) and cross-section (Fig. 3b) are similar to that shown in Fig. 2. The SiO2 particles are not clearly visible in the high magnification cross-
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(a)
(b)
Fig. 2. SEM micrographs of (a) top surface and (b) cross section of binary solution PMMA–DMF (PMMA–DMF–100/0).
(a)
(b)
Fig. 3. SEM micrographs of (a) top surface and (b) cross section of ternary solution PMMA–TEOS–DMF (H–DMF–80/20).
section micrograph (Fig. 3b) due to the size of the cavities that are higher than the particles. Fig. 3b also shows that the skin layer surface differs in structure from the porous sublayer, the latter characterized by a relevant presence of cellulars with size in the range of tenths of micrometers. Glass transition temperatures (Tg) of these hybrid membranes were investigated by DSC. The thermo-scan profiles are sketched in Fig. 4a for hybrid membranes. It was found that neat
PMMA has an obvious single Tg at 116.27EC. Synthesized PMMA/SiO2 hybrid membranes of various solvents exhibited various Tg. The Tg of H-DMF-80/20 was 120.91EC. However, synthesized PMMA/silica hybrid materials of H-THF90/10 exhibited two Tgs. The second Tg (lower one) of this hybrid membrane was lower than that of neat PMMA, while the first Tg (higher one) was higher than that of neat PMMA. The first Tg was 80.14EC and the second one was 184.22EC.
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(a)
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(b)
Fig. 4. Thermal analysis of binary solution PMMA–THF (PMMA) and ternary solution PMMA–TEOS–THF (H-THF80/20, 85/15 and 90/10) and PMMA–TEOS-DMF (H-DMF-80/20). (a) DSC analysis. (b) TGA analysis.
It was postulated that this may be due to the selfcondensation of silanol generated by hydrolysis of silicon alkoxide and the other side molecular chains of PMMA (in silica matrix) to aggregate together to form a neat PMMA matrix in this hybrid [12,18]. This argument was supported by SEM analysis in previous work [4]. Due to the DSC instrument’s limitation and degradation of the materials, possible higher Tgs were not measured for H-THF-85/15 and H-THF80/20. This phenomenon was also found by Chan et al. [15]. H-THF-80/20 and 85/15 hybrid membranes exhibited only single but tremendously wide profile of glass transition in DSC measurement (118.40EC and 120.91EC, respectively). These results indicate that phase separation still existed in the hybrid materials [2,4,15,18]. Nevertheless, from the result of Tg measurements, it can be recognized that hybrid membranes of HTHF-80/20 and 85/15 have obvious higher miscibility of organic and inorganic phases, as seen in SEM analysis. Also seen is that the Tg of H-THF80/20 and 85/15 hybrid membranes was higher than that of neat PMMA, which indicates that the incorporation of minerals improved the thermal properties of the polymer, due to the existence of strong interactions (in terms of hydrogen bonding) between the silica network and the polymer matrix. These strong interactions limit the move-
ment/motions of the polymer chain segments due to the increased rigid structure of the polymer [2–4,10,15,19–21]. According to Mulder [20], factors, which lead to an increase in thermal and also chemical stability are an increase of Tg and Tm and crystallinity. Another factor that can also increase thermal stability is the presence of a resonance structure in the system. However, these Tgs of hybrid membranes increased with inorganic silica content. All these showed that the content of silicon dioxide is a key factor influencing the compatibility of the polymer chains and the inorganic SiO2 networks [12]. For the H-15-DMF hybrid membrane, although its Tg is higher than others, it was not due to the existence of interaction between the silica and polymer, but rather might be caused by the presence of a silicon oxide structure resulting from the self-condensation of silanols generated by hydrolysis, which then resulted in a decrease of molecular mobility and increase of Tg [4,12]. It is noteworthy to recall at this time that the catalyst was added to PMMA/TEOS solution 24 h prior to the coating process. Thus, the TEOS was allowed to polymerize and to develop some structure in the solution. More evidence and possible explanations will be presented and discussed for the data of thermogravimetric analysis.
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Fig. 4b shows the results of differential thermogravimetric analysis of neat PMMA and synthesized hybrid membranes. It is well known that for neat PMMA there was only a single degradation stage found on the TGA measurement, which occurred from 243.0EC to 418.5EC. Nevertheless, except for H-THF-80/20, the hybrid membranes have three degradation stages. It is interesting to note that an obvious difference was found between H-THF and H-DMF hybrid membranes. The first stage of all H-THF hybrid membranes started at around 52.0EC and ended at 137.5EC and approximately from 42EC to 93EC for H-DMF-80/20. This resulted from the elimination of ethanol and water generated from a further condensation process, which was not released during the vacuum drying process [4,15, 22,23]. The second stage for all hybrid membranes, except H-THF-80/20 that occurred from 181EC to 312.5EC, might be ascribed to a decomposition of the chain end from vinylidene ends [4,13,14]. For all membranes, the third stage with tremendous weight loss occurred from approximately 243EC to 437.5EC, corresponding to the decomposition of the main chain of PMMA [4,13,14]. The values of Td10 (the temperature of degradation at which the weight loss is 10%) for pure PMMA and hybrid membranes in H-THF90/10, 85/15, 80/20 and H-DMF-80/20 are 248.0, 271.5, 299.5, 267.0 and 299.5EC, respectively. The thermal stability of hybrid membranes at a low temperature is not superior to that of the pure PMMA; however, the char yield of hybrid membranes is higher than that of the pure PMMA at 800EC and increases with increasing TEOS content. The char yield of pure PMMA is 0.89% and that of hybrid membranes is 12.05, 17.79, 24.28 and 12.89% for H-THF-90/10, 85/15, 80/20 and H-DMF-80/20 at 800EC, respectively. Consequently, the thermal stability of hybrid membranes at high temperatures exceeds that of pure PMMA, which suggests the successful incorporation of the silica moiety into the hybrid membranes [4,7,8]. This increase in thermal
Fig. 5. Flux and retention of membranes as a function of the PMMA/TEOS ratio in a binary casting solution or ternary solution (total concentration: 15 wt.% in THF). • rejection; flux.
stability can be attributed to the existence and high thermal stability of network silica and the existence of the strong interaction between the silica particles and the polymer matrix [4,5,10,14, 19,21,22]. This interaction becomes higher with increasing content of TEOS, and thermal weight loss of the hybrid membranes decreases with increasing silica content [5,12,21]. As shown in Fig. 4b, the hybrid membrane with THF solvent has higher thermal stability than the other. This was due to the existence of the hydrogen bond between silica and polymer matrix, and also the presence of Si-O-Si linkages in this membrane, as seen in FTIR spectra previously reported [4]. In the other hybrid membranes, it was only due to the presence of Si-O-Si linkages because in both no hydrogen bonds are present in the hybrid systems [4]. However, it can be concluded that the type of solvents used play an important role in their degradation behavior. Fig. 5 compares the UF performance of membranes as a function of the PMMA/TEOS ratio in the casting solution in THF solvent. It is worth observing that the reported permeate flux and retention values are the ones taken after 15 min from the start. This is because during the UF tests, the membranes tend to compact, with con-
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sequent lowering of the permeate flux. This trend was also found for other types of organic– inorganic systems [24–26]. Fig. 5 indicates that with the exception of H-THF-90/10, an increase of the PMMA/TEOS ratio leads to an increase of flux and a lowering of retention. Obviously, the absolute values of permeate flux and retention for the five types of membranes are quite different since the structures of these membranes are different (Fig. 1a–c). 4. Conclusions A large variety of membranes with different permeate fluxes and dextran retentions was prepared from ternary suspensions of PMMA and SiO2 in two different polymer solvents, namely THF and DMF. The structure of the membrane cross-sections is not practically affected by the presence of the SiO2 but strongly depends on the PMMA solvent. From DSC analysis, the Tg value of the PMMA moieties in hybrids membranes was in the order H-THF-90/10 < Pure PMMA < H-THF-80/20 < H-THF-85/15 < H-DMF-80/20. Furthermore, from TGA analysis it was found that the hybrid membranes have higher thermal stability compared to pure PMMA, and the type of solvents used play an important role in their degradation behavior. The UF performance of the membrane can be properly varied by the PMMA/ TEOS ratio in the ternary casting solution. By using DMF as PMMA solvent, the skin layer surfaces were deeply altered as pores with typical sizes of microfiltration membranes were formed. Solutions composed of PMMA-TEOS-THF produced membranes with very low porosity. This fact, along with the modifications induced by the presence of the amphoteric oxide, may open new avenues towards the preparation of new PMMAbased NF membranes.
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