Preparation and application of dense poly(phenylene oxide) membranes in pervaporation

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1 Journal of Colloid and Interface Science 278 (2004) Preparation and application of dense poly(phenylene oxide) membranes in pervaporation M. Khayet a,, J.P.G. Villaluenga a, M.P. Godino a, J.I. Mengual a,b.seoane a, K.C. Khulbe b, T. Matsuura b a Department of Applied Physics I, Faculty of Physics, University Complutense of Madrid, Av. Complutense s/n, Madrid, Spain b Industrial Membrane Research Institute, Department of Chemical Engineering, University of Ottawa, 161 Louis Pasteur, P.O. Box 450, Stn. A, Ottawa, Ontario K1N 6N5, Canada Received 19 November 2003; accepted 2 June 2004 Available online 28 July 2004 Abstract Dense flat-sheet membranes were prepared from poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) using the casting solvents chloroform and 1,1,2-trichloroethylene. X-ray diffraction, tapping mode atomic force microscopy (TM-AFM), and contact angle studies were used to characterize the membranes. The surface energy and the solubility parameters of the PPO membranes were determined from the measured contact angles and compared with the predicted ones from the group contribution method. Swelling experiments and pervaporation separation of methanol from its mixture with ethylene glycol over the entire range of concentration, 0 100%, were conducted using these membranes. Flory Huggins theory was used to predict the sorption selectivity. The results are discussed in terms of the solubility parameter approach and as function of the morphological characteristics of the membranes. It was found that PPO membranes prepared with chloroform exhibited better pervaporation performance than PPO membranes prepared with 1,1,2-trichloroethylene Elsevier Inc. All rights reserved. Keywords: Tapping mode atomic force microscopy; X-ray diffraction; Contact angle; Polymer; Poly(phenylene oxide); Dense membrane; Pervaporation; Separation; Methanol/ethylene glycol 1. Introduction The separation process pervaporation is an attractive alternative to conventional techniques for the separation of aqueous organic mixtures and organic liquid mixtures having an azeotropic point and similar physical and chemical properties. The mass transport in pervaporation through dense membrane is generally described by the solution diffusion model [1]. According to this theory, the mechanism of transport is considered to be a three-step process consisting of (i) sorption of the permeants at the liquid upstream side of the membrane, (ii) diffusion of the permeants through the membrane, and (iii) desorption at the low-pressure side * Corresponding author: Fax: +34(91) address: khayetm@fis.ucm.es (M. Khayet). of the membrane. Thus, the permeation rate is a function of solubility and diffusivity. The membrane selectivity is affected by both solubility, which is a thermodynamic property, and diffusivity, which is a kinetic property. The differentiation of sorption of different compounds in a given polymer can be obtained by solubility parameters [2]. The diffusion, on the other hand, is generally governed by the molecular size, shape, and mass of permeants. When both the sorption and diffusion processes favor a given component, very high pervaporation selectivity for this component may be obtained. A good overview of literature sources on pervaporation has been given in Ref. [1]. The main application areas for pervaporation are (i) dehydration of organics, (ii) removal of organics from aqueous solutions, and (iii) separation of an organic from its mixture with another organic compound. Due to the fact that /$ see front matter 2004 Elsevier Inc. All rights reserved. doi: /j.jcis

2 M. Khayet et al. / Journal of Colloid and Interface Science 278 (2004) methanol is one of the major chemicals used in different fields such as the pharmaceutical and paint industries, it is often necessary to separate and recycle methanol from other organic mixtures. Recently various studies were carried out on the separation of methanol from aromatic hydrocarbons, alkanes, ethers, esters, etc. [3 8]. However,despite its importance, pervaporative separation of methanol is not as advanced as pervaporative dehydration. In this study, methanol ethylene glycol mixtures are used as example of the pervaporation separation of organic mixtures. These two components have different molecular masses, sizes, and solubility parameters. In fact, the separation of methanol from ethylene glycol is industrially important in the manufacture of poly(ethylene terephthalate) in the polyester industry. Ghosh et al. [6] proposed the use of a cellophane membrane of thickness 25 µm for the pervaporation separation of the methanol ethylene glycol mixture. It was found that methanol permeated preferentially through the membrane, with a separation factor less than 59 and permeate fluxes lower than 0.4 kg/m 2 h at a feed temperature of 30 C. Later on, Ray et al. [7] used acrylonitrile copolymerized membranes of thickness 50 µm and found that the copolymers hydroxyethyl methacrylate and methacrylic acid showed selectivities of 14.7 and 11.3 with fluxes of and kg/m 2 h, respectively, whereas the copolymer vinyl pyrrolidone showed comparable flux but poor methanol selectivity. The objective of this work is to study the applicability of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) membranes in pervaporation. PPO is one of the aromatic glassy polymers having a high glass transition temperature and a high permeability to gases due to the absence of polar groups in the polymer backbone. The high free volume and the ease of the rotational motion of the phenyl rings contribute to the high gas diffusivity and permeability of PPO dense films. In addition, PPO possesses excellent mechanical and thermal properties. The phenyl rings make PPO hydrophobic and resistant to a number of chemical reagents including acids, bases, and alcohols. A valuable reference on poly(phenylene oxide) and modified poly(phenylene oxide) membranes is the recent published book by Chowdhury et al. [9]. Itmust be pointed out that while much effort has been devoted to the use of homogeneous, asymmetric dense or modified PPO membranes in gas separation, less attention has been paid to the use of PPO membranes in pervaporation [4,10,11]. In fact, this paper is a continuation of previous studies on the development of dense PPO membranes used for gas separation. A great deal of attention has been given to studying the morphology of PPO membranes and attempts have been made to correlate the surface and bulk membrane characteristics with the performance of these membranes [9]. Khulbe and co-workers studied the effect of various casting parameters used during preparation of dense PPO membranes including casting temperature, type of solvent, and membrane thickness [12 18]. Electron spin resonance (ESR), Raman spectroscopy, atomic force microscopy (AFM), X-ray diffraction, and gas permeation techniques were used. From AFM analysis it was observed that the surface morphology of the membranes (i.e., membrane roughness and nodule size) and the membrane selectivity depended on the temperature (22, 4, and 10 C) at which solvent was evaporated [13]. The permeability showed a minimum for each pure gas (i.e., O 2,N 2,CO 2,andCH 4 ) and the permeability ratio showed a maximum at 4 C for the gas pair O 2 /N 2 and CO 2 /CH 4. It was stated that the properties of the solvents (i.e. volatility, surface tension and viscosity) used for casting PPO dense membranes, affect the gas permeation of these membranes [14]. Khulbe et al. [15] reported on the X-ray diffraction of PPO membranes prepared from different casting solvents. The selectivity and the permeability data for O 2,N 2,CO 2,andCH 4 obtained for these membranes were correlated with the obtained membrane characteristics from X-ray diffraction analysis. Moreover, it was concluded that, in general, the mean roughness of the PPO membranes measured by AFM was directly related to the gas permeability of dense PPO membranes [16]. It was also stated that membrane selectivity decreased as surface roughness increased and when nodules are packed less compactly [17]. The effect of membrane thickness on surface morphology has been studied for the case of PPO membranes prepared using the solvent 1,1,2-trichloroethylene [18]. Both roughness and nodule size decrease with increasing membrane thickness up to a certain point after which they begin to increase. Minima in roughness and nodule size were observed for films 9 11 µm thick. These minima depend on initial polymer concentration in the casting solution. At a membrane thickness greater than 11 µm, supernodular aggregates were formed. In this study PPO membranes are applied to the separation of methanol ethylene glycol mixtures by pervaporation over the entire concentration range, 0 100%, at a temperature of 30 C. The membranes were prepared from PPO solutions in two different solvents; chloroform and 1,1,2- trichloroethylene, while the polymer concentration, the casting temperature, and the casting surface remained the same. The PPO membranes so prepared were characterized by X-ray diffraction, surface images by AFM, contact angles, swelling in various binary mixtures of methanol and ethylene glycol, and pervaporation experiments. The overall and the preferential sorption characteristics of methanol/ethylene glycol liquid mixtures in the PPO membranes were studied. The preferential sorption was predicted based on the Flory Huggins theory. The surface energy and solubility parameters of PPO films were determined. The pervaporation transport properties of methanol ethylene glycol are explained using the relative values of the solubility parameters of the components, which seem to govern sorption and membrane morphology that dominates diffusion. An attempt is made to correlate membrane surface morphology with the properties of casting solvents used for membrane preparation.

3 412 M. Khayet et al. / Journal of Colloid and Interface Science 278 (2004) Table 1 Solvent-related properties: molecular weight of the solvent, M; molar volume, V ; boiling point, T bp ; vapor pressure at casting temperature, P v ; surface tension, σ Solvent M (g/mol) V (cm 3 /mol) T bp ( C) P v (kpa) Chloroform TCE a a 1,1,2-Trichloroethylene. 2. Experimental 2.1. Membrane preparation σ (10 3 N/m) Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) powder of intrinsic viscosity 1.57 dl/g in chloroform at 25 C and density 1.04 g/cm 3 was supplied by General Electric (GE). Analytical grade (AR) chloroform and 1,1,2- trichloroethylene were purchased from Aldrich Chemicals and used as solvents for the preparation of the PPO membranes. Table 1 shows the properties of these solvents. The method used for preparation of PPO membranes was similar to that described in details elsewhere [12]. In this study, the concentration of the PPO casting solution was 8% by weight both in chloroform and in 1,1,2-trichloroethylene. Four milliliters of the PPO solution was spread smoothly over a leveled glass plate inside an O-ring of diameter about 10 cm made of stainless steel. The casting ring was then covered with a filter paper to keep out dust. After 24 h at a temperature of about 25 C, the membranes became solid and relatively free of solvent. The membranes were removed very cautiously from the glass plate by immersing the whole plate in a water bath and then transferred onto a filter paper. After drying the membranes at room temperature for 24 h in a fume hood, the membranes were further dried for 72 h in vacuum at room temperature to remove last traces of solvent. The PPO membranes prepared by chloroform and 1,1,2- trichloroethylene are named, hereafter, PPO CH and PPO TC, respectively. The mean thickness of each membrane, measured by a Millitron micrometer (Mahr Feinpruf, type 1202 IC), was basedontheaverageofatleast25spotswithina28cm 2 area of the membrane. The obtained thicknesses of PPO CH and PPO TC membranes were 25.3 ± 1.8 and 24.5 ± 1.7 µm, respectively Membrane characterization Contact angle measurements Direct measurements of both the advancing and receding contact angles of distilled water and formamide (BDH chemicals, 99%) on the air-side surface of the prepared PPO membranes were conducted at room temperature using a contact angle meter 14 Horizontal Beam Comparator (SCHERR ST TUMICO, Model SERIES). The procedure used to carry out the contact angle measurements was explained in our previous papers [19,20]. Initial drops of about 2 µl were deposited on the membrane surface. The volume of the drop was slowly increased and then decreased by adding the liquid to and withdrawing it from the surface using a tight syringe. Direct measurements of the advancing and receding contact angles were performed at both left and right sides of each drop. More than 15 readings were obtained for each PPO membrane sample and the average value together with the standard deviation were calculated and reported in this study X-ray diffraction The X-ray diffraction spectra of both PPO powder and the two PPO membranes prepared from it were obtained using an X pert Philips diffractometer. Continuous scans were carried out with step size 2θ equal to 0.04 and scan speed 0.04 /s between 2θ equal to 5 and 90. The radiation wavelength CuK α (λ = nm) was used as an X-ray source at a generator voltage of 45 kv and a current of 40 ma employing a 0.15-mm slit in front of the detector. The polymer samples were mounted on a sample holder plate without using back tape and the fitted profiles of the spectra were obtained by the X pert software Tapping mode atomic force microscopy (TM-AFM) The prepared PPO membranes with two different solvents have been structurally characterized by a tapping mode atomic force microscope (TM-AFM) on a Nanoscope III equipped with 1553D scanner from Digital Instruments, Santa Barbara, CA, USA. The mode of operation to take the AFM pictures was extensively explained in previous works [13,14,16]. Small pieces of approximately cm 2 in area were cut from each membrane and fixed over a magnetic holder by using double-sided adhesive tape. All the TM-AFM images were made in air at room temperature over different areas of each membrane sample. In this study, scans were made on areas of 5 5 µm and areas of 1 1µmwere selected randomly on the membrane surface for analysis. The membrane nodule size and roughness parameters were determined. To obtain the nodule sizes, cross-sectional line profiles were selected to traverse micrometer scan surface areas of the TM-AFM images and the diameters of nodules (i.e., high peaks) were measured by a pair of cursors along the reference line. The horizontal distance between each pair of cursors was taken as the diameter of the nodule. The AFM software program allows quantitative determination of nodules by use of the images in conjunction with digitally stored line profiles. The minimum, average, and maximum nodule sizes of both PPO membranes are presented in this study. In addition, the AFM analysis software program allowed computation of various statistics related to the surface roughness on a predetermined scanned membrane area. The evaluation of the roughness parameters of each membrane sample was based on various micrometer scan areas (i.e., 1 1µm). It should be emphasized here that the roughness parame-

4 M. Khayet et al. / Journal of Colloid and Interface Science 278 (2004) Table 2 Solubility parameters for methanol, ethylene glycol (ETG), chloroform, and 1,1,2-trichloroethylene (TCE) taken from [25] and the calculated ones for PPO polymer: hydrogen-bonding component, δ h ; polar component, δ p ; dispersion force component, δ d ; total solubility parameter, δ; solubility parameter difference, δ Component δ h (10 3 J 1/2 /m 3/2 ) δ p (10 3 J 1/2 /m 3/2 ) δ d (10 3 J 1/2 /m 3/2 ) δ (10 3 J 1/2 /m 3/2 ) δ (10 3 J 1/2 /m 3/2 ) Methanol a ETG b Chloroform TCE PPO c a Molecular weight, g/mol and molar volume, 40.2 cm 3 /mol. b Molecular weight, g/mol and molar volume, 55.6 cm 3 /mol. c All the solubility parameters of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) are estimated from the group contribution using the method of Hoftyzer and Van Krevelen [2,25]. ters depend on the curvature and size of the TM-AFM tip as well as on the treatment of the captured surface data (plane fitting, flattening, filtering, etc.). If scanning probes of different size and shape were used, the variation in the roughness parameters, as well as in the diameter of nodules, could be significant because of tip-surface convolution [14]. Therefore, the roughness parameters obtained from TM-AFM images should not be considered as absolute roughness values. However, in the present study, the same tip was used for all membranes and all captured surfaces were treated in the same way. Thus, the comparison of the roughness data obtained in this study with literature data might therefore not be appropriate. The mean roughness, R a, the root-mean-square of Z data, R q, and the mean difference in height between the five highest peaks and the five lowest valleys, R z, were obtained from the TM-AFM images of different locations taken from each membrane sample and the average values are reported in this paper. The mean roughness, R a, represents the mean value of the surface relative to the center plane for which the volume enclosed by the images above and below this plane are equal. This parameter was calculated from the equation R a = 1 L x L y L x L y 0 0 f(x,y) dxdy, (1) where f(x,y) is the surface profile relative to the center plane and L x and L y are the dimensions of the surface in the x and y directions, respectively. The root-mean-square roughness, R q, is the standard deviation of the Z values within the specific area and is calculated using the equation (Zi Z m ) R q = 2, (2) N p where Z i is the current Z value, Z m is the average of the Z values and N p is the number of points within a given area. The average difference in height, R z, between the five highest peaks and the five lowest valleys is calculated relative to the mean plane, which is a plane about which the image data has a minimum variance Swelling measurements Various dried PPO CH and PPO TC membrane samples of known weights were immersed in closed bottles containing either methanol (Panreac Química S.A., 99.8%), ethylene glycol (Panreac Química S.A., 99%), or a mixture of these solvents. Table 2 shows the solubility parameters of methanol and ethylene glycol. The samples were allowed to equilibrate for about 72 h at a temperature of approximately 30 C. These membranes were then taken out from the solutions and weighed again after the surface liquid was quickly removed with a filter paper. The overall solubility is calculated from the weight of the swollen and the dry membrane sample and is expressed in units of grams of sorbed liquid per gram of dry membrane using the expression S = m W m D, (3) m D where m W is the mass of the swollen membrane and m D is the mass of the dry membrane, both in grams Pervaporation The pervaporation experiments were conducted using an apparatus described elsewhere [21]. In this study, PPO membranes were installed in the stainless steel pervaporation cell having an effective membrane area in contact with the liquid feed of 28 cm 2. The feed liquid containing methanol, ethylene glycol, or a mixture of both in the entire concentration range, 0 100%, was circulated through the pervaporation cell from a feed reservoir kept at a temperature of 30 C. The pressure at the downstream side was kept at approximately 1 mmhg by a vacuum pump. The permeate fluxes were determined by measuring the weight of liquid collected in the cold traps cooled by liquid nitrogen during a certain period at steady-state conditions. The feed and permeate compositions were analyzed at approximately 25 C by measuring the refractive indices with an Abbey-type refractometer Model 60/ED (Bellingham + Stanley Ltd.). More details were outlined in our previous work [21]. The pervaporation

5 414 M. Khayet et al. / Journal of Colloid and Interface Science 278 (2004) selectivity, α p, was calculated using the expression α p = y M/y E, (4) x M /x E where x and y represent the weight fraction in the feed and in the permeate, respectively. Index M refers to the more permeable component, methanol in this study, and index E refers to the less permeable one, ethylene glycol. 3. Theoretical 3.1. Estimation of surface energy and solubility parameter from contact angles The permeation of components through a dense polymer membrane is generally governed by the sorption diffusion mechanism. The relative sorption of a permeant in a polymeric membrane depends on its relative solubility in the membrane that can be explained by the solubility theory or by the interaction parameter or Flory Huggins theory [22]. To determine the solubility parameter of the prepared PPO membranes, the surface energy was first evaluated from the advancing and the receding contact angle measurements of two different liquids. The geometric-mean and harmonicmean approximations were employed to get the dispersive, γm d, and the nondispersive, γ nd m, contributions to the total surface energy, γ m, of polymeric membranes. According to Owens and Wendt [23,24], the surface energy of a given solid can be determined using the following equation (i.e., geometric-mean approximation) applied to two liquids, (1 + cos Θ)γ l = 2 ( γm d γ d ) 1/2 ( l + 2 γ nd m γ nd ) 1/2, l (5) where γ l is the surface free energy of pure liquid and the superscripts d and nd correspond to the dispersive and to the nondispersive contributions to the total surface energy, respectively. The contact angle, Θ, was calculated from the following expression [19] Θ = cos 1 ( cos Θa + cos Θ r 2 ), where Θ a and Θ r are the advancing contact angle and the receding contact angle, respectively. As an alternative to Eq. (5), the following expression (i.e., harmonic-mean approximation) was employed to evaluate the total surface energy [24]: ( ) γ d m γ d (1 + cos Θ)γ l = 4 l γ d m + γ d l + γ m ndγ l nd γm nd + γ nd By measuring the contact angles of two liquids on a membrane surface, two simultaneous equations will be obtained for Eq. (5) or Eq. (7), which can be easily solved for γm d and γm nd. Consequently, by assuming the linear additivity l. (6) (7) of the intermolecular forces (i.e., dispersive and nondispersive forces), the sum of the two components, γm d and γ nd m, should provide an estimate value of the total surface free energy, γ m [23,24]. In this study, the surface energy of the PPO membrane was estimated from the contact angle measurements to water (γ l = 72.8 mj/m 2, γl d = 21.8 mj/m 2, γl nd = 51.0mJ/m 2 ) and formamide (γ l = 58.5mJ/m 2, γl d = 39.5mJ/m 2, γl nd = 19.0mJ/m 2 ) [25]. On the other hand, the solubility parameter of the membrane, δ, was related to the cohesive energy density of the membrane, e coh, as stated in [20], δ = (e coh ) 1/2, (8) where e coh can be determined from the surface free energy, γ m, by using the following equation: γ m = 0.75(e coh ) 2/3. It must be pointed out that in Eq. (9), to obtain a value of e coh in 10 6 J/m 3,thevalueofγ s must be in mj/m 2. For mutual solubility of two components, penetrant and polymer, their Gibbs free energy of mixing, G, should be negative, G = H T S= n S n P V( δ) 2 T S, (9) (10) where H and S are the enthalpy and the entropy of mixing, respectively; T is the absolute temperature; n S and n P are the volume fractions of the penetrant and the polymer, respectively; V is the molar volume; and δ is the solubility parameter difference between the polymer and the penetrant. In Eq. (10),since S is positive, H must be reduced as much as possible to ensure a more negative G. Therefore, for higher affinity between the polymer and the penetrant δ should be as small as possible. Preferential sorption of a penetrant will occur if the difference in the individual contributions of the solubility parameter for the penetrant and the polymer is low Interaction parameter The affinity between the polymer and a solvent can be expressed in terms of interaction parameter, χ ip. According to Flory Huggins theory the free energy of mixing, G, of a binary mixture consisting of solvent (S) and polymer (P) is given by the expression [22] G/RT = ln(ϕ S ) + ϕ P + χ ip ϕ 2 P, (11) where R is the gas constant, T is the temperature, ϕ S is the volume fraction of solvent in the polymer, and ϕ P is the volume fraction of the polymer. In the case of equilibrium sorption of pure solvent in a polymer, the energy of mixing is zero and the binary interaction parameter can be calculated as χ ip = ln ϕ S ϕ P ϕp 2. (12)

6 M. Khayet et al. / Journal of Colloid and Interface Science 278 (2004) As affinity or interaction between the polymer and the penetrant increases, the amount of liquid inside the polymer increases and χ ip decreases. In contrast, with decreasing affinity between the polymer and the penetrant, the value of χ ip increases. Therefore, a lower value of χ ip implies higher sorption Prediction of preferential sorption The sorption of a binary liquid mixture in a polymer is characterized by the overall sorption and the preferential sorption. The overall sorption represents the total amount of liquid inside the polymer, where as the preferential sorption is the difference of the liquid composition in the binary liquid phase from that in the polymer phase. The preferential sorption of the methanol ethylene glycol binary liquid mixture into a polymer membrane can be described by the following expression, which has been derived from the Flory Huggins thermodynamics [1], ( ) ( ) ϕm vm ln ln ϕ E v E ( ) ϕe [ = (l 1) ln χ ME (vm v E ) + (ϕ E ϕ M ) ] v E ϕ P (χ MP lχ EP ), (13) where the indices M and E refer to the binary liquid components (M for methanol and E for ethylene glycol) and the index P refers to the polymer membrane, v i represents the volume fraction of liquid i in the binary liquid mixture, and l is the ratio of volume fractions of methanol and ethylene glycol (i.e., v M /v E ). The volume fraction of component i in the ternary polymeric phase is denoted by ϕ i, and therefore ϕ M + ϕ E + ϕ P = 1. (14) The polymer volume fraction, ϕ P, can be obtained from the overall sorption measurements. For the sake of clarity, the volume fraction of the component i in the liquid mixture present in the membrane is denoted by u i = ϕ i /(ϕ M + ϕ E ). (15) The binary interaction parameters between methanol and the polymer and ethylene glycol and the polymer, χ MP and χ EP, respectively, were assumed to be concentrationindependent and were calculated from the single-liquid sorption data using Eq. (12), as has been described previously [21]. On the other hand, the parameter of binary interaction between methanol and ethylene glycol, χ ME, can be calculated from the following equation [1,26], χ ME = 1 x M u E [ x M ln ( xm u M ) + x E ln ( xe u E ) ] + GE, RT (16) where x M and x E are the mole fractions of methanol and ethylene glycol in the mixture, respectively; and G E the excess free energy of mixing data for the methanol ethylene glycol system, have been calculated using the UNIFAC group contribution method [26]. The composition of the sorbed liquid can then be determined from Eq. (13) and the sorption selectivity (α S ), defined in the same way as the pervaporation selectivity, can be obtained from the equation α S = z M/z E, (17) x M /x E where x and z represent the concentration in the binary liquid mixture and in the sorbed liquid, respectively. Index M refers to the preferentially soluble component, methanol in this study, and index E refers to ethylene glycol X-ray diffraction analysis The d-space, which is the distance between diffracting planes, was calculated by substituting the scattering angle, 2θ, of the peak into the Bragg equation, d = λ (18) 2sinθ, where λ is the wavelength of the CuK α ray and 2θ is the diffraction angle. The full width at half maximum (FWHM) of the diffraction peaks were calculated by fitting the X-ray diffraction data with a Gaussian Lorentzian function as stated in [15] and the crystallite size was calculated according to the Scherrer formula, D = Kλ (19) B cos θ, where K is the Scherrer constant, which depends upon lattice direction and crystallite morphology (0.9 is used in this study), and B is the FWHM given in radians. 4. Results and discussion The measured advancing, θ a, and receding, θ r, contact angles on the surface of PPO CH and PPO TC membranes for water and formamide are summarized in Table 3.For both PPO membranes the obtained contact angles are almost the same. This suggests that the effect of the casting solvent on the PPO membrane surface characteristics is very small since the same PPO polymer was used for the preparation of Table 3 Water and formamide contact angles of PPO TC and PPO CH membranes Membrane Water Formamide Θ a ( ) Θ r ( ) Θ ( ) Θ a ( ) Θ r ( ) Θ ( ) PPO TC 93.9 ± ± ± ± PPO CH 91.3 ± ± ± ±

7 416 M. Khayet et al. / Journal of Colloid and Interface Science 278 (2004) Table 4 Surface energy, cohesive energy density, and solubility parameter of PPO TC and PPO CH membranes Membrane γm nd ) γm d (mj/m2 ) γ m (mj/m 2 ) γm nd/γ m (%) e coh (10 6 J/m 3 ) δ (10 3 J 1/2 /m 3/2 ) Geometric-mean approximation PPO TC PPO CH Harmonic-mean approximation PPO TC PPO CH both membranes. This fact will be discussed later more in detail based on the TM-AFM analysis. As stated previously, the surface energy of the PPO membranes was evaluated by using both the geometric-mean and the harmonic-mean expressions, Eqs. (5) (7). Table 4 shows the values of the dispersive and nondispersive contribution to the surface energy together with the degree of surface polarity, calculated as the ratio of the nondispersive surface energy to the total surface energy (i.e., γm nd/γ m). The dispersive component, γm d, was practically equal for both membranes; while the nondispersive component of the PPO CH membrane is 20.8% and 8.3% higher than that of the PPO TC membrane when the geometric-mean and the harmonic-mean equations were used, respectively. The γ m values are greater, 5.6% (PPO TC) and 4.9% (PPO CH), when calculated with the geometric-mean equation than with the harmonic-mean values, and the γm nd values obtained from the geometric-mean approximation are lower, 76.6% (PPO TC) and 73.9% (PPO CH). Moreover, the degree of surface polarity is very small when the geometric-mean method is used. One possible explanation might be an inappropriate selection of the liquid pair (water, formamide). However, when other liquids were used, similar results were obtained for polyetherimide and polyvinylidene fluoride membranes [19,20]. It should be noted that, in this study, the purpose of determining the surface energy is to compare the effect of the solvent in the casting solution on the PPO membrane surface. Furthermore, the cohesive energy density and the solubility parameter of the PPO membranes were estimated from Eqs. (8) and (9). The results are also given in Table 4. It can be observed that the solubility parameters of the two membranes are almost the same using either the geometric-mean or the harmonic-mean approximations. However, the values obtained from the geometric-mean equation are slightly higher (i.e., 4.2 and 3.6%). Matsuura [2] put together a comprehensive list of group contributions, which allows estimation of the solubility parameter and its components for a wide variety of polymers. In this study, the solubility parameter components (i.e., polar, hydrogen-bonding, and dispersive forces) of PPO material were estimated from the group contribution using the method of Hoftyzer and Van Krevelen [25]. The obtained values are reported in Table 2. The predicted solubility parameter considering the three types of interactions, dispersion Fig. 1. X-ray diffraction patterns of the PPO powder, PPO TC membrane, and PPO CH membrane. The dotted lines refer to the position of the diffraction angles of PPO polymer given by JCPDS file forces, polar forces, and hydrogen-bonding contribution, is about J 1/2 /m 3/2, which is close to the experimental values obtained from the geometric-mean expression. A value of J 1/2 /m 3/2 was also reported in [25]. However, the solubility parameter of PPO estimated from the cohesive energy, J/m 3, calculated by means of group contributions and applying Eq. (8) is higher ( J 1/2 /m 3/2 ). This difference illustrates the possible uncertainty associated with the estimation of the solubility parameters for polymers and the necessity of experimental tests to determine the solubility parameters of polymeric films. Fig. 1 shows the X-ray diffraction (XRD) spectra of PPO powder and the dense flat-sheet membranes, PPO CH and PPO TC, prepared from it. As can be observed, the obtained X-ray diffraction pattern of PPO powder matches well with JCPDS card No corresponding to the polymer poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) with chemical formula (C 8 H 8 O 2 ) n, M w = 23,000, and M n = 8000 from General Electric (GE). There are basically four peaks that can be observed in the XRD pattern of PPO powder, 2θ = 7.7, 13.0, 16.0, and The XRD patterns of the two PPO membranes are different from that of PPO powder. According to Wijmans et al. [27] PPO is a semicrystalline polymer and contains a very low crystalline content after membrane formation. This is in accordance with the spectra presented in Fig. 1. As can be observed PPO membranes exhibit a typical broad peak that appeared at 2θ about It seems that the two peaks of PPO powder that appeared at 2θ = 13.0 and 16.0 merged

8 M. Khayet et al. / Journal of Colloid and Interface Science 278 (2004) into one peak at 2θ = 14.6 for both PPO membranes. However, the clear crystalline diffraction peaks in PPO powder at 7.7 and at 21.7 practically disappeared for the two prepared PPO membranes in this study. The above results imply that the crystalline structure depends on the procedure of membrane preparation, but the solvent types used in this study did not alter the average intersegmental distance of polymer chains significantly. In fact, almost no change in d-spacing was observed when either chloroform or 1,1,2-trichloroethylene was used as solvent for the preparation of dense PPO membranes; spacing was 6.04 Å for PPO CH and 6.05 Å for PPO TC. From the appearance of the XRD spectra in Fig. 1 it is obvious that both PPO membrane samples examined have very small particle sizes because of their broad peaks. The decrease in the peak width of the PPO CH membrane suggests a higher crystalline size of this membrane. In fact, the FWHM of PPO CH is lower (5.47 ) than that of PPO TC (5.74 ). The Scherrer equation, i.e., Eq. (19), was employed in order to get a rough estimate of the crystallite size of the membrane samples. The obtained values are 14.6 and 13.9 Å for the membranes PPO CH and PPO TC, respectively. Nevertheless, it can be observed that the peak intensity is higher for the PPO TC membrane. This indicates the higher density of crystallites of this membrane. These observations may be attributed to the lower volatility of 1,1,2- trichloroethylene at 25 C in comparison to that of chloroform as the formation of crystalline region depends on the time allowed for the polymer to crystallize from the polymer solution and the solvent evaporation rate is expected to be lower when the solvent 1,1,2-trichloroethylene is used [27]. During fast evaporation of a polymer solution, the viscosity of the materials rises, solidification begins and the polymer molecules find it difficult to move about and arrange their long chains in a regular pattern needed for crystal formation. Since permeation sites may be of either amorphous material or interstices between crystallites and the crystallinity region is regarded as not being suitable for penetrants to transport through the membrane matrix, the total flux of PPO TC membrane is expected to be lower than that of PPO CH because of the reduced space available for diffusion of this membrane. Two-dimensional TM-AFM pictures of the prepared PPO membranes are presented in Fig. 2. The membrane surfaces are not smooth and possess a nodule-like structure. The nodules are seen as bright spots on the TM-AFM images. The roughness parameters and the nodule sizes were determined as stated earlier and the results are shown in Table 5.The membrane prepared with 1,1,2-trichloroethylenecasting solvent exhibits smaller nodule sizes and lower roughness parameters in comparison to the membrane prepared from chloroform. This observation could be caused by different rate of solvent evaporation from the membrane surface (i.e., the evaporation rate of chloroform solvent at the top surface was faster than that of 1,1,2-trichloroethylene) since chloroform and 1,1,2-trichloroethylene have similar solubility parame- (a) (b) Fig. 2. Surface images 1 1 µm scan size obtained by AFM in tapping mode of the prepared PPO membranes: PPO TC (a) and PPO CH (b). ters (Table 2) [14]. In fact, as the solvent evaporates, the viscosity of the casting film near the surface will increase sharply, resulting in high viscosity gradients mainly due to the increase in local polymer concentration. This will lead to an increase of the polymer polymer interaction on the membrane surface and the formation of large nodules should be favored. Ruaan et al. [28] studied the effect of various factors on the surface nodule size of poly(methyl methacrylate) membranes, including the type of solvent in the casting solution. It was concluded that at the place where there was noninterfacial tension, the nodule size was mainly governed by the probability of interchain entanglement; but at the surface of the membrane, where interfacial tension could not be avoided, the nodule size was controlled by the interfacial tension. In addition, Kesting [29] suggested that the increase of the surface tension of the casting solution tended to decrease the surface nodule size when membranes were formed by the dry process. In this study, the surface tension is higher for 1,1,2-trichloroethylene and hence lower nod-

9 418 M. Khayet et al. / Journal of Colloid and Interface Science 278 (2004) Table 5 Surface nodule sizes and roughness parameters of PPO membranes Membrane Nodules Roughness parameters Minimum ν l (nm) Average ν a (nm) Maximum ν h (nm) R a (10 2 nm) R q (10 2 nm) R z (10 2 nm) PPO TC PPO CH Fig. 3. Overall solubility of methanol ethylene glycol mixtures in PPO TC and PPO CH membranes at 30 C as a function of methanol content in the mixture. ule sizes are expected to be formed for PPO TC membrane (Table 5). In addition, Kesting [29] has discussed nodule and supernodule aggregate formation on the surfaces of the membranes. It was concluded that there are four structural elements in integrally skinned phase-inversion membranes, namely, macromolecules, nodules (approximately 20 nm in diameter), nodule aggregates ( nm in diameter), and supernodular aggregates (0.1 2 µm in diameter). Therefore, it must be stated that the surface of PPO CH membrane also contains nodule aggregates with size higher than 40 nm. The roughness parameters were higher for the membrane PPO CH in comparison to those of the membrane PPO TC. This may be explained, since the roughness parameters depend on the Z values and the PPO CH membrane exhibits higher nodule size. It may be stated that the fast evaporation of chloroform tends to generate instability, which happens so fast and rigorously that rougher and irregular surface is formed [12]. Furthermore, it was reported that the membrane permeability was closely related to the mean roughness of the membrane measured by AFM [16,30]. Therefore, in this study, it is expected that a higher permeation flux will be obtained for the PPO membrane prepared from chloroform, which exhibits higher roughness parameters and larger nodules. This expectation is consistent with the conclusion obtained earlier from the X-ray diffraction results. Fig. 3 shows the overall solubility of methanol ethylene glycol mixtures in the PPO membranes at 30 C versus methanol content in the liquid organic solution. It is evident from the figure that both methanol and ethylene glycol are sorbed by the PPO membranes, and when pure liquids are considered, methanol sorption is higher than that of ethylene glycol. It is further observed that with increasing methanol concentration in the liquid mixture, total equilibrium sorption in the PPO membranes increases and is almost identical for both PPO membranes. This is attributed to an increase in swelling with the increase in methanol in the mixture. The relative sorption of methanol and ethylene glycol in the membrane depends on their relative solubilities. As mentioned earlier, the closer are the values of the solubility parameters for a polymer and a solvent, the higher will be their mutual solubility. The calculated solubility parameter difference between methanol and PPO is smaller than that of ethylene glycol and PPO (Table 2). Therefore, methanol is sorbed more preferentially than ethylene glycol by PPO membranes as observed in Fig. 3. The calculated interaction parameters from the sorption data of pure solvents using Eq. (12) are 2.63 and 2.55 for ethylene glycol in PPO TC and PPO CH membranes, respectively, whereas for methanol the obtained values are lower, 1.37 and 1.42 in PPO TC and PPO CH, respectively. For both PPO membranes, the interaction parameters between ethylene glycol and PPO membranes are more than 1.8 times higher than the interaction parameters between methanol and PPO membranes. Thus, affinity of methanol towards PPO is higher than that of ethylene glycol. Consequently, sorption of methanol in PPO films is higher. On the other hand, Mandal and Pangarkar [5] reported that the sorption of a solvent not only depends on polarity and solubility parameters but also on the free volume of the membrane, the extent of amorphous regions available in the membrane and the size of the penetrant. In this paper, however, the effect on sorption of the degree of crystallinity of the PPO membranes could not be detected. The trend of the curves in Fig. 3 can be explained by the fact that the presence of hydroxyl groups in both methanol and ethylene glycol may result in self-association among the same type of sorbed molecules. In this case, the mutual interaction between the sorbed molecules is greater than their interaction with the PPO polymer, resulting in sorption resembling a type III sorption isotherm described by Roger (Fig. 3) [31]. In fact, for different types of mixtures, different types of sorption isotherms may arise. Ray et al. [7] who used acrylonitrile copolymerized membranes also found sorption isotherms of both methanol and ethylene glycol closely simulating Roger s type III sorption isotherm. Their membranes were methanol selective but both methanol and ethylene glycol were sorbed by these

10 M. Khayet et al. / Journal of Colloid and Interface Science 278 (2004) Fig. 4. Predicted composition of sorbed liquid, methanol and ethylene glycol, in PPO TC and PPO CH membranes vs methanol concentration in the methanol ethylene glycol liquid mixture. Fig. 5. Estimated sorption selectivity of PPO TC and PPO CH membranes vs methanol content in methanol ethylene glycol mixture. membranes (from to g of methanol and from to g of ethylene glycol per gram of membrane were sorbed at 30 C). It may be explained that the sorption of both methanol and ethylene glycol may be attributed to the comparable polarity and solubility parameters of both ethylene glycol and methanol in the membranes. Ghosh et al. [6], using cellophane membranes, obtained solubilities of methanol and ethylene glycol of 0.45 and 0.31 g per gram of dry membrane, respectively, at 30 C. The composition of the liquid sorbed in the membrane was estimated from Eq. (13). Fig. 4 shows the calculated composition of the sorbed liquid in the swollen PPO CH and PPO TC membranes as a function of the composition of the liquid mixtures. Methanol content in the sorbed liquid was much higher than that of ethylene glycol over the entire range of liquid mixture composition. It is also observed that the composition is nearly the same in both PPO membranes. Moreover, Fig. 5 shows the calculated sorption selectivity, α s, for methanol ethylene glycol in PPO CH and PPO TC membranes at different concentrations and at 30 C. It is seen that the sorption selectivity of PPO TC membrane is slightly higher than that of PPO CH membrane at low methanol content and the sorption selectivity of methanol in both PPO membranes decreases drastically with the increase Fig. 6. Variation of the concentration of methanol in the permeate and in the membrane with its feed concentration for PPO TC and PPO CH membranes at 30 C. of methanol in the liquid mixture. This may be due to the increase in swelling, which causes loosening of the polymer matrix and results in an easier sorption of both permeants. Fig. 6 presents the concentration of methanol in permeate obtained in the pervaporation experiments of PPO membranes at a temperature of 30 C versus the concentration of methanol in the methanol ethylene glycol feed mixture. The methanol concentration of the sorbed liquid in the membrane is also shown. From this figure, it is clear that both PPO CH and PPO TC membranes are methanol-selective. Furthermore, the pervaporative feed-permeate composition line is above the sorption lines. This may be due to much greater diffusion of methanol through PPO membranes. In fact, the diffusion cross section of methanol, 14.4 Å 2, is lower than that of ethylene glycol, 23.4 Å 2 [7]. For pervaporation of methanol ethylene glycol mixture, Ghosh et al. [6] and Ray et al. [7] obtained similar trends when using cellophane and copolymerized acrylonitrile membranes, respectively. It is also seen from Fig. 6 that at lower methanol concentration in the feed mixture (i.e., 20 wt%) the weight percent of methanol in the permeate is slightly higher when using chloroform as solvent for the preparation of PPO membranes. However, from the sorption selectivity results given in Fig. 5, the sorption selectivity of methanol is higher for PPO TC membrane. This suggests that the diffusion selectivity is higher for PPO CH membrane since permeation selectivity is a combination of sorption selectivity and diffusion selectivity. Fig. 7 shows the total permeation flux and the methanol selectivity of the PPO membranes as functions of the methanol concentration in the feed mixture. With increasing methanol concentration in the feed solution, total permeation flux increases, while the methanol selectivity decreases. When methanol concentration in the feed mixture is higher than 20 wt%, the permeation flux of the PPO CH membrane becomes greater than that of the PPO TC membrane, while the selectivities are nearly the same for both membranes. The higher permeation rate of PPO CH was suggested earlier from the X-ray diffraction and the TM- AFM analysis. For both PPO membranes the permeation selectivity for methanol follows the same trend obtained for the sorption selectivity; i.e., the selectivity decreases with

11 420 M. Khayet et al. / Journal of Colloid and Interface Science 278 (2004) (a) (b) Fig. 7. Pervaporation flux (a) and permeation selectivity (b) of PPO TC and PPO CH membranes vs methanol concentration in feed mixture of methanol ethylene glycol at 30 C. increasing methanol content in the liquid mixture (Fig. 5). When the concentration of methanol in the feed is low, almost all methanol molecules try to interact with the membrane and sorption selectivity becomes high. Besides, it is likely that there is an intermolecular hydrogen bonding between the ethylene glycol molecules, which further retards the diffusion of ethylene glycol molecule through the membranes. When the concentration of methanol is high in the feed, high sorption of the permeants may increase the free volume of the membrane (i.e., density of the swollen PPO membrane decreases) and diffusion of both methanol and ethylene glycol through the membrane become easier, which leads to a low selectivity. It is worth mentioning that, compared to cellophane membrane used for methanol/ethylene glycol separation, higher methanol selectivity with almost similar fluxes are achieved by the PPO membranes prepared for this study [6], while in comparison to the copolymerized acrylonitrile membranes with hydroxyethyl methacrylate, methacrylic acid or vinyl pyrrolidone, both flux and methanol selectivity obtained by PPO membranes are higher [7]. 5. Conclusions The X-ray diffraction pattern of dense poly(2,6-dimethyl- 1,4-phenylene oxide) (PPO) membranes prepared with the solvents chloroform and 1,1,2-trichloroethylene is different from that of PPO powder and both membranes exhibit very small particle sizes and equal d-spacing. The decrease in the peak width of the PPO membrane prepared with chloroform suggests a higher crystalline size of this membrane. The peak intensity is higher for the PPO membrane prepared with 1,1,2-trichloroethylene. This indicates the higher density of crystallites of this membrane. Based on atomic force microscopy analysis, it was found that the membrane prepared with 1,1,2-trichloroethylene exhibits smaller nodule sizes and lower roughness parameters than the membrane prepared with chloroform. The contact angles, the cohesive energy density and the solubility parameter of the two PPO membranes are almost the same. Both PPO membranes are methanol selective when methanol ethylene glycol mixtures are separated by pervaporation. The total permeation flux increases while methanol selectivity decreases with increasing methanol content in the feed mixture. This was associated with the increase in sorption of the permeants at higher methanol concentration in the feed mixture, which increases the free volume of the PPO membranes and favors the diffusion of both methanol and ethylene glycol. When methanol concentration in the feed methanol ethylene glycol mixture is low (i.e., 20 wt%), the membranes prepared with chloroform as solvent exhibited higher selectivities than the membranes prepared with 1,1,2-trichloroethylene and both membranes have similar permeation rates. At higher methanol concentration, the membrane prepared with chloroform exhibited higher permeation rate with almost the same selectivity as the membrane prepared from 1,1,2-trichloroethylene. Acknowledgments The authors of this work gratefully acknowledge the financial support of the University Complutense of Madrid through its Project PR78/ and the MCYT (Spain) through its Project PPQ The authors also express their gratitude to Dr. J. Velázquez Cano (CAI Difracción de Rayos X, Faculty of Chemistry, University Complutense of Madrid) for taking the X-ray diffraction spectra. Appendix A. Nomenclature Roman letters B full width at half maximum, FWHM, in the X-ray spectra (radians) d d-space defined in Eq. (15) (Å) e coh cohesive energy density of the membrane (J/m 3 ) G Gibbs free energy change of mixing (J/mol) H enthalpy change of mixing (J/mol) K Scherrer constant