Preparation of jujube-cake structure membranes through in situ polymerization of hyperbranched polysiloxane in ethylene-vinyl acetate matrix for separation of ethyl acetate from water

Abstract

Ethylene-vinyl acetate (EVA)/hyperbranched polysiloxane (HPSiO) hybrid pervaporative membranes for separation of ethyl acetate (EA) from water with a “jujube-cake” structure were prepared by in situ polymerization. The morphology of the EVA/HPSiO membranes was characterized by using a scanning electron microscope (SEM) and transmission electron micrographs (TEM). The mechanical properties, contact angle, density, void volume and swelling behavior of the membranes were investigated. We also studied the effect of HPSiO content, operating temperature and feed concentration on the separation properties of EA/water mixtures. The results show that the addition of appropriate HPSiO could improve the hydrophobicity of the membranes, which is helpful to recover EA from water. The membrane, loading 20 wt% HPSiO, exhibited the highest separation factor and permeability for a feed concentration of 1 wt% at 30 °C.

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1. Introduction

Many flavor compounds of foods and beverages are recovered from fruit juices by thermal evaporation and distillation processes in the traditional food and cosmetics industries. However, most of these compounds are heat sensitive, which could lead to undesirable changes in the flavor profile and thus lower their market values.^1–3 Among these, ethyl acetate (EA) is a typical aromatic compound that can be found in most kinds of fruits and has been widely used in the manufacture of varnishes, thinners, nitrocellulose lacquers and various drugs because of its low toxicity, good volatility and favorable solubility.^4–8

Pervaporation (PV) is one of the most promising membrane separation processes for aroma compounds' recovery in the food industry due to its low energy consumption, moderate cost, compact and modular design.^9–17 Whereas, a major hurdle that limits PV commercialization for this purpose is lacking suitable membrane materials with both high flux and separation factor. Owing to the relative hydrophobic nature of EA than water, many kinds of elastic polymers with flexible chains are used as pervaporation membrane.^{1,6,7,18–20} Ethylene-vinyl acetate (EVA) copolymer is a versatile commercial polymer material that can vary greatly in their chemical and physical properties, such as chemical composition, branching, polarity and crystallinity. So the variation of the vinyl acetate (VA) content in EVA could alter many properties of EVA, such as degree of crystallization, glass transition temperature, mechanical strength, thermal stability, etc. Hence, EVA copolymers have a broad range of application early in hot melt adhesives, coatings, paints, plasticizers, modifiers, processing aids, film, packaging, agriculture,^21–23 etc. In the previous studied, we once reported that the EVA copolymer membranes with different vinyl acetate content was used for recovery of ethyl acetate from aqueous ethyl acetate solutions and the separation factor was 118 with 2.5 wt% ethyl acetate at 30 °C.²⁴ But the separation performances are not satisfied. Blending or composite membranes can easier improve their pervaporation performances than homo-polymer membranes.²⁵ For example, Vane et al. prepared ZSM-5 filled polydimethylsiloxane (PDMS) membranes and showed that the separation factor of ethanol from water increased monotonously from 8.7 to 43.1 when ZSM-5 loading increased from 0 to 65 wt%, and the ethanol flux also increased from about 40 to 300 g m⁻² h⁻¹.²⁶ In addition, Bai et al. reported that hyperbranched polysiloxane (HPSiO) crosslinked with PDMS membranes for recovery of n-butanol from aqueous n-butanol solutions, the selectivity were 6.42 and 5.99, the permeability of n-butanol were 75 000 and 224 000 barrer.²⁷

Hyperbranched polymers have recently attracted considerable attention due to their particular properties that greatly differ from conventional linear or moderately branched polymers, such as lower package density, larger inter-molecular free volume, intra-molecular micro-void and so on.^28–30 Most importantly, hyperbranched polymers have high reaction activity resulted from the large number of reactive end-groups within a molecule, approximately spherical molecular shape and the absence of chain entanglement. Introduction of hyperbranched structure into membrane material could obtain both high flux and separation factor.³¹ However, to the best of our knowledge, the incorporation of hyperbranched polymers as additive in EVA matrix has not been reported before.

In the present study, a new type of “jujube-cake” structure membrane was fabricated by in situ polymerization of hyperbranched polysiloxane in EVA matrix to improve membrane separation performance and mechanical strength. The pervaporation performance and mechanical properties of EVA/HPSiO blend membranes with different HPSiO content on pervaporation performance for separation ethyl acetate from water were intensively investigated.

2. Experimental

2.1 Materials

Sodium hydroxide (NaOH), acetic acid (CH₃COOH) and EVA40 were purchased from Shanghai Chemical Company, China and used as received. Vinyltriethoxysilane (VTES) of analytical grade was purchased from Sigma Chemical Co., Ltd. Toluene, ethyl acetate and tetrahydrofuran were purchased from National Pharmaceutical Group Chemical Reagent Co., Ltd. China and used as received. The number average molecular weights (M_n) and molecular weight distribution are 35 000 and 2.42, respectively.

2.2 Preparation of HPSiO

In this paper, hyperbranched polysiloxane (HPSiO) was synthesized by a modified literature procedure.^32,33 HPSiO was synthesized by a three-step process. Firstly, sodium vinyldiethoxysilanolate was prepared by a reaction of pulverized sodium hydroxide and VTES in toluene at 5 °C with stirring for 2 h, followed by the distillation off the toluene and the by-product ethanol in the reaction using a rotational evaporator evacuated with a vacuum pump. Secondly, the obtained sodium vinyldiethoxysilanolate was re-dissolved in toluene and a stoichiometric amount of acetic acid was added drop wisely for acidization at 0 °C with stirring. After filtering off the precipitates, a yellow solution (vinyldiethoxysilanol) was obtained. Finally, the catalyst, dibutyltin dilaurate (DBTDL), with a weight ratio of 0.01 to vinyldiethoxysilanol, was then added into the above solution to react for 8 h at 60 °C, after that, the toluene was distilled off to get a yellow liquid, which was HPSiO. The detailed synthetic processes of HPSiO were given in Scheme 1.

Scheme 1 Synthetic process of HPSiO.

2.3 Characterization of HPSiO

²⁹Si NMR spectrum of HPSiO was obtained from Advance 500 (Bruker) at room temperature performing 2000 scans with a pre-scan delay of 12.5 s using 20 wt% in toluene (99.5% D GmBH) solutions containing 0.015 mol L⁻¹ chromium(III) acetylacetonate (purum, Fluka) as a paramagnetic relaxing agent. Tetramethylsilane (puriss, Fluka) was used as an internal standard. The spectrum was corrected for the background signal from the probe by numerical subtraction of a baseline that was recorded for the pure solvent.

The molecular weights of HPSiO were analyzed by gel permeation chromatography (GPC) using a Waters 1515 isocratic pump, a column set consisting of three Waters Styragel® columns (7.8 mm × 300 mm) HR4, HR3, HR1 and a Waters 2414 differential refractive index detector. Tetrahydrofuran (TEDIA, HPLC grade) was used as fluent at 0.6 mL min⁻¹. Calibration of GPC equipment was carried out with narrow polystyrene standards (Shodex® Standard, peak molecular weights range 1200–538 000 g mol⁻¹).

2.4 Preparation of EVA/HPSiO membranes

EVA/HPSiO membranes were prepared by the following procedure. Firstly, appropriate amounts of HPSiO mixed with EVA40 were dissolved in tetrahydrofuran and then the catalyst, DBTDL, with a weight ratio of 0.01 to HPSiO, was added into the above solution at 40 °C to react for 3 h with magnetically stirring. The low-viscosity dispersions were subsequently cast onto Teflon dishes and left them 1 day at room temperature. Volatile products of the reaction and excess of solvent were removed by enduring specimens at 80 °C for 24 h. A membrane with EVA without any HPSiO was also prepared for comparison purposes. The membrane thickness detected by digital micrometer (Mitutoyo) were on the order of 200 μm. Samples with 5, 10, 15, 20 and 30% (w/w) HPSiO loading was designated as EVA/HPSiO-5, EVA/HPSiO-10, EVA/HPSiO-15, EVA/HPSiO-20 and EVA/HPSiO-30 respectively.

2.5 Characterization of EVA/HPSiO composites

2.5.1 Morphological characterization and network structure. The distribution of HPSiO phase into the EVA matrix was studied using a HRTEM (JEOL2000) operated at an accelerated voltage of 200 kV. The samples were sectioned into 100 nm thin sections at −140 °C using an ultra-cryomicrotome (Ultracut R, Leica) equipped with a glass knife. These sections were then transferred to the copper grid and were observed through the microscope.

The surface morphology of cross-section of membranes was studied using scanning electron microscope (SEM) (Hitachi S4800). For this purpose, the membrane samples were fractured in liquid nitrogen and coated with gold using an ion sputter (JFC-1100).

2.5.2 Mechanical properties. Tensile tests on the membranes were carried out at room temperature on a Zwick/Roell single column Universal Testing Machine (model Z2.5) equipped with a 50N load cell and flat pneumatic clamps. Specimens with an effective length of 40 mm (distance between the clamps) and a width of 5 mm were tested at a deformation rate of 20 mm min⁻¹. The average value of the tensile strength and elongation at break was determined on a series of 4–6 samples.

2.5.3 Contact angle measurement of EVA/HPSiO membranes. Static contact angles of water on membranes were measured by the sessile drop method using a contact angle meter (OCA 20, Dataphysics Instruments GmbH Germany) at 25 °C and at about 65% relative humidity. The volume of the water drops used was always 2 μL. All reported values were average of 10 measurements taken at different locations of the film surface and had standard deviation of ±1°.

2.5.4 Density of EVA/HPSiO membranes. The density of EVA/HPSiO membranes were determined by specific pycnometer at 30 °C and three parallel measurements were carried out for each sample. Prior to density measurement, the films samples were dried in a vacuum oven at 30 °C for 3 days.

2.5.5 Swelling measurement and adsorptive property of EVA/HPSiO membranes. The EVA/HPSiO membranes were dried at 40 °C for 24 h in vacuo directly before immersed in different concentration of ethyl acetate solution at 30 °C, respectively. At regular intervals, the swollen membranes were wiped out carefully with tissue paper to remove superficial liquid and weighted in a tightly closed bottle. The degree of swelling (DS) of the membrane was then determined from following equation:^34–36where m₀ and m_s are the weights of dry and swollen membranes, respectively.

The schematic diagram of adsorptive property measurement is shown in Fig. 1. First, membrane samples were desorption, then collected of stripping liquid with a liquid nitrogen cold trap. The composition of stripping liquid was detected by gas chromatography (GC). Sorption selectivity and diffusion selectivity were determined from following equations:³⁷

α = α_sα_d (3)

Fig. 1 The device for sorption selectivity: (1) digital constant temperature bath, (2) specimen, (3) sample cell, (4) cold trap, (5) drying column, (6) vacuum meter, (7) vacuum pump.

2.6 Pervaporation performances of EVA/HPSiO membranes

The schematic diagram of pervaporation process is shown in Fig. 2. In testing cell, the effective pervaporation area (S) of membrane was 35.24 cm². Before being fixed into testing cell, EVA/HPSiO membranes were soaked in the feed solution to a swelling saturation. In pervaporation operation, the upstream side (feed side) of the cell was filled with water/ethyl acetate solution and kept at an atmospheric pressure. The pressure of downstream side (permeate side) was maintained at 200 ± 10 Pa. The permeated mass was collected in a condensation trap cooled by liquid nitrogen. The composition in feed mixture and corresponding infiltration was detected by gas chromatography (GC) equipped with a packed column and a thermal conductivity detector (TCD). The permeation flux (J) and the separation factor (β) for all membranes were calculated according to the following equations:^38–40where Q_i is the weight of compound i in the permeation collected in time t, and A is the effective membrane area (35.24 cm²). x and y are the weight fractions of components in the feed and permeate samples, respectively.

Fig. 2 Schematic diagram of experimental set-up for pervaporation.

More meaningful in sight into a membrane's permeation properties can be obtained through analyses of permeability and selectivity. The permeability (P^G_i) is given as

where P^G_i is the membrane permeability for a compound i (Barrers, 1 barrer = 1 × 10⁻¹⁰ cm³ (STP) cm cm⁻² s⁻¹ cmHg), the selectivity (α) is defined as the ratio of the permeability of components i and j through the membrane, m_i is the molecular weight of component i and V^G_i is the molar volume of gas i (22.4 L mol⁻¹, STP), p_io and p_il are the partial pressures of component i on the feed (o) and permeate (l) side of the membrane.⁴¹

3. Results and discussion

3.1 Characterization of HPSiO

HPSiO was synthesized from an AB₂-type monomer vinyldiethoxysilanol and the ²⁹Si NMR spectrum of HPSiO was presented in Fig. 3. The ²⁹Si NMR spectrum of HPSiO showed four groups of peaks, which can be assigned to three types of repeating units classified as dendritic, linear and terminal ones depending on the number of un-reacted ethoxy functional groups and residual un-reacted VTES, respectively. For detailed, the four different groups were designated as Q_X, where X indicated the number of un-reacted ethoxy groups, i.e., Q₀, no ethoxy groups (−108 ppm, 45%, dendritic units), Q₁, one ethoxy groups (−91 ppm, 26%, liner units), Q₂, two ethoxy groups (−74 ppm, 26%, terminal units), and Q₃, three ethoxy groups (−66 ppm, 3%, VTES). The areas of the individual peaks allow the estimation of the relative amounts of terminal, linear, and dendritic units in the different polymers. The degree of branching (DB), which was usually used as a factor to explain the structure of hyperbranched polymers was calculated according to the eqn (6).⁴²where D is the number of dendritic units, T is the number of terminal units and L is the number of linear units. The calculated DB of HPSiO was 0.73.

Fig. 3 ²⁹Si NMR spectra of HPSiO.

As listed in Table 1, the results from GPC analysis based on linear polystyrene standards gave a weight-averaged molecular weight (M_w) of HPSiO 3513 and the relative large polydispersity 2.48, which is owing to the fact that the molecular chain contains a lot of branched structure.

Table 1 Molecular weight of HPSiO

Samples	Molecular weight		PDI^c
	Mw^a	Mn^b
a Mw, weight average molecular weight.b Mn, number average molecular weight.c Polydispersity index (PDI) = Mw/Mn.
HPSiO	3513	1417	2.48

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3.2 Curing of HPSiO in EVA matrix

After heat treatment, the phase of HPSiO is distributed uniformly in EVA matrix. Curing reaction of HPSiO in EVA matrix was presented in Scheme 2. The hydrolysis of HPSiO by catalyst leads to the transformation of free ethoxy groups to hydroxyl groups with the following inner cyclization and dimensional network structure formation according to Scheme 2, the crosslinked HPSiO can be defined as C–HPSiO.

Scheme 2 Crosslinking process of HPSiO.

3.3 Characterization EVA/HPSiO membranes

3.3.1 Structure and morphology of EVA/HPSiO membranes. Morphology of EVA/HPSiO membranes were investigated by SEM and TEM. Fig. 4 presents SEM micrographs of the cross-section of EVA/HPSiO-5 and EVA/HPSiO-30 membranes. It can be seen that the spherical particle was uniformly dispersed in the continuous matrix and the average diameter of the dispersed particles in EVA/HPSiO blends was varied from 1 to 5 μm by changing HPSiO content of blending from 5 to 30 wt%.

Fig. 4 SEM images of the EVA/HPSiO membrane: (a) EVA/HPSiO-5; (b) EVA/HPSiO-30.

Fig. 5 shows transmission electron micrographs (TEM) of the cross section of EVA/HPSiO-5 and EVA/HPSiO-30 membranes. Since image contrast results from the difference in electron density between silicon and carbon, the dark part in Fig. 5 is assigned to the HPSiO phase with domain sizes ranging from less than 1 μm (Fig. 4a) to 5 μm (Fig. 4b), which was in a good agreement with the results of SEM.⁴³

Fig. 5 TEM images of the EVA/HPSiO membrane: (a) EVA/HPSiO-5; (b) EVA/HPSiO-30.

3.3.2 Mechanical properties of EVA/HPSiO membranes. As shown in Fig. 6, tensile measurement results illustrated that incorporating HPSiO into EVA significantly enhanced mechanical strength of EVA membranes. The Young's modulus of EVA/HPSiO membranes increased with HPSiO content from 0 to 20 wt%, and decreased when HPSiO content was larger than 20 wt%. The maximum of the Young's modulus 1.25 MPa could be obtained when HPSiO content was 20 wt%. The tensile stress of the EVA/HPSiO membrane has the same changing tendency as the Young's modulus, and the maximum value 10.32 MPa was obtained when HPSiO content was 20 wt%.

Fig. 6 Mechanical properties of EVA/HPSiO blend membranes.

The enhancement of mechanical stress for the EVA/HPSiO membranes may be explained that HPSiO with hydroxyl groups played a role as cross-linker. When HPSiO content is lower than 20 wt%, HPSiO molecules in the presence of large amounts of branched structure, it is curing to form network structure, molecular chain segments motility decreased. These evenly distributed rigid structures in the EVA matrix inhibit the movement of EVA polymer segments. When HPSiO content is higher than 20 wt%, HPSiO particles can interconnect and the continuity of EVA in the membrane is destroyed. It can be confirmed by the TEM images in Fig. 5. When HPSiO content is too high, the network of the membrane was severely spoiled and mechanical strength appears to decline. Hence, there is an optimum HPSiO content in the EVA membranes for the enhancement of mechanical performance.

3.3.3 Contact angle of EVA/HPSiO membranes. Fig. 7 shows effect of HPSiO content on the contact angle for water of EVA/HPSiO membranes at room temperature. As can be seen from Fig. 7, the contact angle for water of EVA/HPSiO membranes increased with increasing HPSiO content. This result suggests that the hydrophobicity of EVA/HPSiO membranes were enhanced with increasing HPSiO content. This is because the HPSiO segment is more hydrophobic than EVA segment in EVA/HPSiO membranes. The more hydrophobic membrane surface favors absorption of ethyl acetate while repelling water from its surface. In other words, the introduction of HPSiO could enhance the affinity to ethyl acetate of EVA/HPSiO membranes.

Fig. 7 Effect of HPSiO content on the static contact angle of EVA/HPSiO membranes.

3.3.4 Density and void volume of EVA/HPSiO membranes. The results indicated that the composites composed of embedded HPSiO in EVA polymer tend to be lower than the calculated density as shown in Fig. 8. From Fig. 8, it was clearly seen that the experimental density of EVA/HPSiO membranes decreased with the content of HPSiO from 5 wt% to 20 wt%. There are two primary reasons for this result. Firstly, the significant void space can be created by the accumulation of hyperbranched structure in HPSiO after hydrolysis. Another important reason is the molecular aperture created by the rapid volatility of solvent during preparation of EVA/HPSiO membranes and this can be proved by the density of pure EVA change from 0.9334 to 0.8704 g cm⁻³ after preparation process.

Fig. 8 Effect of HPSiO content on density of EVA/HPSiO membranes.

The fractional free volume (FFV) of the membranes was determined from:⁴⁴

FFV = 1 − 1.3ν_Wρ (9)

where ρ represents the density of the pure EVA membrane or EVA/HPSiO membrane and ν_W is the van der Waal's volume of the repeat unit of EVA in cubic centimeters per gram, which was calculated using the group contribution method of Bondi considering only the vinyl and vinyl acetate segments in the structure of EVA.⁴⁵ Table 2 lists van der Waal's volumes (cubic centimeters per mole) and molecular weight (grams per mole) of different groups constituting the repeat unit of EVA Molecular weight (M_w) of the repeat unit was found to be 51.3 g mol⁻¹ and the van der Waal's volume 31.44 cm³ mol⁻¹ assuming 40% vinyl acetate and 60% vinyl groups.⁴⁶ Thus, ν_W in cubic centimeters per gram was 0.6129. The density of the membrane depends upon the concentration of HPSiO in the polymer. Table 3 shows density and FFV data.

Table 2 van der Waal's volume and molar volume of chemical groups constituting the EVA

Chemical group	Mw (g mol^−1)	νW (cm^3 mol^−1)
–CH2–	14.03	10.23
–O–	16.0	5.5
–Co–	28.01	11.7
–CH3–	15.03	13.67
-CH-	13.02	6.8

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Table 3 Density, and fractional free volume of EVA and EVA/HPSiO membranes

Membrane	Density (g cm^−3)	FFV (%)
EVA	0.87134	30.6
EVA/HPSiO-5	0.84707	32.5
EVA/HPSiO-10	0.81694	34.9
EVA/HPSiO-15	0.76743	38.9
EVA/HPSiO-20	0.70778	43.6

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Samples containing 5–20 wt% HPSiO have large density deviations from their theoretical ones and increasing FFV, which mean the membrane with higher loading of HPSiO has a looser structure. With spherical particles (C-HPSiO) are incorporated into the polymer phase, the membrane structure apparently transforms to a less dense structure due to the disruption of polymer–polymer packing by spherical C–HPSiO. As the HPSiO content increases, the dispersed C–HPSiO particle in EVA/HPSiO was varied from 1 to 5 μm, at the same time the dominant phase gradually changes to C–HPSiO when its content increased to 30 wt%, which can be observed from the TEM image. On the basis of the above analysis, the sketch maps of physical structures for EVA/HPSiO-5 and EVA/HPSiO-20 were proposed in Fig. 9.

Fig. 9 Tentative illustration on the relationship between the microphase-separated structures of EVA/HPSiO-5 (a) and EVA/HPSiO-20 (b) membranes.

3.3.5 Swelling behavior of EVA/HPSiO membranes in ethyl acetate aqueous solutions. The equilibrium DS values of EVA/HPSiO membranes in different concentration of ethyl acetate solution at 30 °C were shown in Fig. 10. It can be clearly seen that the equilibrium swelling degree of EVA/HPSiO membranes increased with the increase of ethyl acetate concentration in the feed solution, which indicates that EVA/HPSiO membranes had preferential selective adsorption for ethyl acetate. Also, the equilibrium swelling degree increased with the HPSiO content. This is because the swelling of a polymer material in a solvent is proportional to its interaction or affinity with the solvent.^47–50 All prepared EVA/HPSiO membranes had more affinity of ethyl acetate than pure EVA membrane. So, it was reasonable to predict that the concentration of ethyl acetate would cause the increasing in swelling of EVA/HPSiO membranes and then enhancing the permeation flux in pervaporation process.

Fig. 10 DS of EVA/HPSiO membranes in ethyl acetate solutions at 30 °C.

3.4 PV performances of the EVA/HPSiO membranes

3.4.1 Effect of HPSiO content. Fig. 11 shows the effect of HPSiO content on the PV performances of EVA/HPSiO membranes with a 1.0 wt% ethyl acetate aqueous solution at 30 °C. As shown in Fig. 11a and b, both flux of water and ethyl acetate, separation factor continuously increased with increasing HPSiO content.

Fig. 11 Effect of HPSiO content on PV performance of EVA/HPSiO membrane, (a) flux, (b) separation factor, (c) permeability and (d) selectivity.

For pervaporation process, the current generally accepted theory for sorption–diffusion theory. It is divided into the following three steps: (1) permeate components dissolve in the membrane surface through adsorption; (2) diffusion of the permeate components; (3) components separate through the membrane. Thus, in the process of permeate components through the membrane, adsorption and diffusion process are main influencing factors. As shown in Fig. 12, with the increasing of the content of HPSiO, EVA/HPSiO blend membrane adsorption selectivity increased, and diffusion selectivity declined. This is because that HPSiO is more hydrophobic than EVA. The incorporation of HPSiO made EVA/HPSiO blend membrane more preferential to adsorb ethyl acetate. The main reason for the declining of diffusion selectivity can be ascribed to the loose structure of C–HPSiO, which lead to the more easily permeation of both EA and water molecules, thus the selectivity lost. As shown in Fig. 8 and Table 3, the density of EVA/HPSiO blend membrane decreased, and the fractional free volume increased continuously with increasing HPSiO content. It indicated that the internal structure of that film was more loosing. This is in consistent with the results of pervaporation performances. From Fig. 11c, the fluxes (ethyl acetate and water) were normalized to permeability by taking into the account of the driving forces for permeation and membrane thickness, the permeability of ethyl acetate increased from 4801 barrer to 37 112 barrer, but the permeability of water increased from 132 058 barrer to 405 862 barrer. Thus, the increased permeability of ethyl acetate significantly greater than the growth in water because ethyl acetate tends to swell up EVA/HPSiO easier which can be supported by the DS measurement (Fig. 10). This directly led to the increase of the separation factor and selectivity.

Fig. 12 Effect of HPSiO content on the sorption selectivity and the diffusion selectivity of EVA/HPSiO membranes.

3.4.2 Effect of operating temperature. Fig. 13 shows the variation of flux (a), separation factor (b), permeability (c) and selectivity (d) of EVA/HPSiO-20 membrane with feed temperature (the feed concentration is 1.0 wt%). From Fig. 13c, it can be seen that both ethyl acetate and water permeability decreased with increasing the feed temperature. But, contrary to the variation of permeance versus temperature, ethyl acetate and water flux increased with feed temperature (Fig. 13a). It can be explained that the flux of pervaporative cannot reflect the temperature influence on transport performance accurately, reported by Gao et al.⁵¹ The flux contains both the intrinsic membrane properties as well as external operational factors, such as activity coefficient and saturated vapor pressure. However, the permeance can reflect the true transport of the membrane because of removing the effects of operational factors. Richard et al. compared permeance with the flux of silicone rubber membranes.⁵² They found permeance was decreased as temperature increased. They explained that the enhancement of vapor pressure driving force result in the increase of flux with increasing temperature. On the one hand, at higher temperatures, the side of the material liquid infiltration component of saturated vapor pressure increased, the mass transfer driving force increased, lead to the flux increased consequently. On the other hand, the high temperature, enhanced the polymer molecular chain segment, free volume increased further, a corresponding increased of the flux.

Fig. 13 Effect of operating temperature on PV performance of EVA/HPSiO-20 membrane, (a) flux, (b) separation factor, (c) permeability and (d) selectivity.

Generally, in the operation of the pervaporation process, with the increase of temperature, permeate flux will increase while membrane material swelling degree will increase in the feed solution and separation factor will be reduced. This phenomenon is known as the “trade-off”. However, for the EVA/HPSiO-20 membrane, with the temperature change in the process, separation factor of rising trend could be a breach of the law (Fig. 13d). It also suggested that the introduced rigid hyperbranched polymer into EVA matrix inhibited the swelling of the membrane. The most direct reason for the increasing of separation factor, the flux increased margin of ethyl acetate was greater than the growth in water flux. Activation energy data is on behalf of the components of the permeate flux of temperature sensitivity. From Fig. 14, for ethyl acetate and water respectively, the activation energy E_EA = 29.6 kJ mol⁻¹, E_Water = 12.55 kJ mol⁻¹. Visible, for EVA/HPSiO-20 membrane, pervaporation performance of ethyl acetate was affected more than that of water with the increase of temperature.

Fig. 14 Arrhenius plot of EVA/HPSiO-20 membrane for an aqueous solution of 1 wt% ethyl acetate.

3.4.3 Effect of feed concentration. Fig. 15 shows the variation of flux (a), separation factor (b), permeability (c) and selectivity (d) of EVA/HPSiO-20 membrane with the feed concentration from 1.0 to 2.5 wt% at 30 °C. Through the analysis shown above that the dominant process of permeate components through the EVA/HPSiO membrane was adsorption process. From Fig. 10, with the feed concentration increased, the swelling degree of EVA/HPSiO-20 membrane was from 20.52% to 30.98%. This meant that ethyl acetate concentration changes had much effect on the adsorption process. Thus, the flux of ethyl acetate increased from 48.57 to 198.86 g m⁻² h⁻¹ with the feed concentration from 1.0 to 2.5 wt% from Fig. 15. However, the flux of water decreased from 186.67 to 123.37 g m² h⁻¹. Due to the concentration of the feed solution was very low in the experiment, ethyl acetate and mass transfer driving force increased with the concentration of ethyl acetate. But relatively, the concentration of the water changed inconspicuous, so the flux change was very small, thus caused the separation factor was also rising.

Fig. 15 Effect of feed concentration on PV performance of EVA/HPSiO-20 membrane, (a) flux, (b) separation factor, (c) permeability and (d) selectivity.

4. Conclusions

A “jujube-cake” structure composed of hyperbranched polysiloxane and EVA copolymer was prepared by in situ polymerization of hyperbranched polysiloxane in EVA matrix. The observation of SEM and TEM show that the phase of HPSiO was uniformly dispersed in the continuous matrix. Both the mechanical strength and pervaporation performance of EVA/HPSiO membranes were enhanced due to the introduction of hyperbranched structure. The permeance and the selectivity of all the EVA/HPSiO membranes were improved with increase of HPSiO content in EVA matrix. The permeability of the EVA/HPSiO membranes decreased with the increase of feed temperature, but the selectivity increased. As the feed concentration increased, the permeability of ethyl acetate and selectivity of EVA/HPSiO membranes increased. The best PV performance, in terms of the permeability and the selectivity, of EVA/HPSiO-20 (with the content of HPSiO was 20 wt%) membrane were 442986 barrer and 0.091, respectively, with a feed concentration of 1 wt% at 30 °C.

Acknowledgements

This research was financially supported by the National Nature Science Foundation of China (Grant No. 21576114), the Open Project Program of the Key Laboratory of Food Colloids and Biotechnology Ministry of Education, Jiangnan University (No. JDSJ2013-05) and the Fundamental Research Funds for the Central Universities (JUSRP51513).

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