Water Droplets as Template for Next-Generation Self-Assembled Poly

J. Phys. Chem. B 2008, 112, 10483–10496

10483

Water Droplets as Template for Next-Generation Self-Assembled Poly-(etheretherketone) with Cardo Membranes Annarosa Gugliuzza,*,† Marianna Carmela Aceto,†,‡ Francesca Macedonio,†,‡ and Enrico Drioli†,‡ Research Institute on Membrane Technology-National Research Council (ITM-CNR), Via P. Bucci, Cubo 17C, I-87030 Rende (CS), Italy, and Department of Chemical Engineering and Materials, UniVersity of Calabria, Via Pietro Bucci 45A, I-87030 Rende, Italy ReceiVed: March 11, 2008; ReVised Manuscript ReceiVed: July 1, 2008

Next generation PEEK-WC membranes have been fabricated by using an innovative self-assembly technique. Patterned architectures have been achieved via a solvent-reduced and water-assisted process, resulting in honeycomb packed geometry. The membranes exhibit monodisperse pores with size and shape comparable to those left by templating water droplets. Influencing factors for the formation of self-assembled poly(etheretherketone) with Cardo [PEEK-WC] membranes have been evaluated, identifying the critical parameters for nucleation, growth, and propagation of the droplet-mobile arrays through the overall films. Structure-transport relationships have been discussed according to the results achieved from the implementation of membrane distillation processes, yielding indication about the suitability of self-assembled PEEK-WC films to work as interfaces in contactor operations. Introduction Regular and ordered patterns represent an attractive topic of research, especially for advanced branches of the membrane technology that include membrane contactors.1,2 Usually, the contactor technology utilizes devices equipped by microporous hydrophobic membranes, working as nonselective interfaces between two different phases through which mass transfer is promoted. Specifically, the membrane keeps in contact two different phases at the entrance of the pores, preventing the media from mixing and promoting transport by diffusion through the open channels of the interface. Well-sized and well-shaped pore density and narrow distribution influence positively the trans-membrane flux, because of the uniformity of the process through the overall membrane surface. Unfortunately, the fabrication of well-structured membranes working as interfaces for contactor applications still represents an important challenge. Despite many attempts, well-sized and well-shaped membranes remain an ambitious target. Recently, the breath figures (BFs) approach has been rediscovered as a powerful tool for the fabrication of well-defined 2D and 3D micro- and nanostructures.3-7 The technique strategically exploits the templating action of the moist air, resulting in self-assembled and well-structured architectures.8 The driving force of this water-assisted process is a gradient of temperature generated between the casting solution and humid atmosphere, caused by the solvent evaporation. The difference of temperature induces the water droplets to condense and arrange at the air-polymer dope interface, behaving as templates for porous layers. The growth and propagation of the water droplets through the liquid films usually lead to hexagonally packed regular arrays. During the formation of BFs, the water * To whom correspondence should be addressed: E-mail: a.gugliuzza@ itm.cnr.it. Address: Research Institute on Membrane Technology-Research National Council, ITM-CNR, Via Pietro Bucci 17/C, c/o University of Calabria, I-87030 Rende (CS), Italy. † ITM-CNR. ‡ University of Calabria.

droplets are prevented from coalescing by the precipitation of the polymer at the water-solution interface. Finally, the complete evaporation of both the solvent and water leaves airbubble arrays orderly sequenced. This simple method seems undeniably to offer an attractive opportunity to tailor smart membranes having suitable structural characteristics for equipping contactor devices. Drawbacks concerned with size, shape, and uniformity of the membrane pores could be successfully overcome at lower costs and in shorter time than traditional techniques.9,10 In addition, solvent reduced, large availability of a nontoxic template, fast removal, and feasible recovery of the solvents make the manufacturing process more environmentally friendly. The accomplishment of membrane contactor processes can be seriously affected by the membrane operated; thus, a wellcontrolled structure together with adequate physicochemical interfacial properties of the film represent crucial issues for the implementation of this advanced membrane technology. The membrane has to be accomplished by well-defined pores uniformly distributed on the surface, since high surface porosity combined with high bulk porosity can productively result in a high interfacial area per unit of volume and high volumetric mass transfer rate. Furthermore, mutual membrane-liquid interactions must be considered in order to prevent the liquid from wetting the pores. Indeed, the penetration of liquid inside of the pores should cause a drastic increase in the resistance to the transport, resulting in reduced productivity of the process. Thus, well-defined morphology and surface properties must be necessarily combined in the membranes equipping contactor devices. This target could be achieved by mimicking what is daily occurring in nature: the spontaneous tendency of the water droplets to condense on cold surfaces and their self-assembly in ordered lattices. This approach can lead to a new class of next-generation porous polymeric architectures where the polymer generates the network of the matrix and the air bubbles array the membrane pores.

10.1021/jp802130u CCC: $40.75  2008 American Chemical Society Published on Web 08/05/2008

10484 J. Phys. Chem. B, Vol. 112, No. 34, 2008

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Figure 1. BF PEEK-WC membranes: (a) chemical structure of the polymer; (b) picture of a membrane tailored by self-assembly approach; (c) AFM topography of a BF PEEK-WC membrane; (d,e) SEM images showing the honeycomb packed geometry of both the top surface and section of a BF PEEK-WC membrane, respectively; (f) SEM image showing the top surface of a PEEK-WC membrane prepared by dry-wet phase inversion.

TABLE 1: Physichemical Parameters Estimated for All Polymer Solutions C (wt %) 0.2 0.5 0.8 1.0

η (cP) 20 °C 25 °C 30 °C 20 °C 25 °C 30 °C 20 °C 25 °C 30 °C 20 °C 25 °C 30 °C

1.28 1.27 1.21 1.94 1.56 1.48 2.01 1.95 1.87 2.82 2.47 2.20

d (g/cm3)

γsol (mJ/m2)

1.4845 29.2 ( 0.6

ecoh δ (106J/m3) (103 J1/2/m3/2) 242.9

TABLE 2: Comparison between Dry and Wet Porosity Estimated for Membranes Prepared at Various Polymer Concentration, 25°C, 70%, 0.5 mL in Static and Dynamic Conditions statica

15.34

1.4849 29.4 ( 0.8

245.4

15.26

1.4851 29.4 ( 0.9

246.1

15.24

1.4855 29.5 ( 0.3

246.6

15.22

However feasible the technique appears, structure and morphology of the membrane channels depend critically on the nucleation, growth, and propagation rates of the water droplets.4 Indeed, each single stage can be significantly affected by multiple critical factors such as temperature and relative humidity of the cabinet as well as concentration, viscosity, density, surface free energy, and solubility parameters of the casting solution.5,11-13

dynamica

casting solutionsb (wt %)

dry (%)

wet (%)

dry (%)

wet (%)

1.0 0.8 0.5

40 ( 7 52 ( 2 64 ( 2

45 ( 6 50 ( 3 66 ( 4

34 ( 3 51 ( 7 63 ( 7

33 ( 4 52 ( 8 61 ( 6

a Manufacturing approach. b Volume of 0.5 mL of polymer dopes exposed to 70% of relative humidity at 25 °C.

Various attempts of explaining the general mechanisms that control the arrangement of ordered micropatterns have been proposed,4,8,14 although the identification of the crucial factors directing the formation and the extension of the bubble-arrays is still a matter of debate. Despite that the changes in the size and sequence of the pores continues being the subject of many studies,5-7 the penetration of the mobile array through the overall film and the formation of multilayered structures in the presence of solvent denser than water are events rarely observed and discussed.3,8,15 Finally, structure-transport relationships have been never investigated for BF patterns.

Fabrication of Next-Generation PEEK-WC Membranes

J. Phys. Chem. B, Vol. 112, No. 34, 2008 10485

SCHEME 1: Experimental Apparatus Used to Carry out MD Operationsa

a

(a) Schematic flow sheet of the lab plant: (A) distillate tank; (B) membrane module; (C) feed tank; (D) pump; (E) cooler; (F) heater. (b) Schematic representation of the BFs PEEK-WC membrane module utilized in the MD tests.

TABLE 3: Morphological and Physicochemical Properties of Commercial Poly-(propylene) and Manufactured Poly-(etheretherketone) with Cardo Group Membrane Modules acronyms

PP

PEEK-WC

code membrane form housing material

MD020CP-2N BFs capillary flat polypropylene modified poly(etherether)ketone contact angle ∼120° g120° no. of fibers 40 available area [m2] 0.1 2.27 × 10-4 porosity [%] 75-80 65 pore size [µm] 0.2 2.0 on top surface, 0.5 on bottom surface inner diameter [mm] 1.8 shell diameter [cm] 2.1 membrane thickness [µm] 120 25 length of each fiber [cm] 57 pore diameter [µm] 0.2

This work assesses the potentialities of the breath figure technique in the field of the membrane preparation, yielding important indication about the suitability of BF structures for contactor operations. With this regard, preliminary tests based on the membrane distillation technology (MDT) have been carried out to evaluate the adequacy of the proposed microporous layers to work as nonselective hydrophobic interfaces. The membrane distillation (MD) is an innovative process considered as a low cost and energy saving alternative to conventional separation operations such as distillation and reverse osmosis.16 The driving force of a MD process is a vapor pressure difference across the membrane, established by a difference of temperature between two contacting media. This driving force can be alternatively generated by applying vacuum, air gap, or sweep gas in the permeate side. A heated aqueous feed solution is usually put in contact with one side (feed side) of a hydrophobic microporous membrane establishing a vapor-liquid interface at each pore entrance. The most volatile compounds are vaporized at the liquid/vapor interface and diffused through the membrane pores. Considering the final application of the proposed BF layers, a morphological analysis of the membranes has been done,

examining the influence of various experimental parameters on the imprinting action of the water droplets. Mutual membraneliquid water interactions have been also investigated to estimate the wetting degree for the BF membranes addressed to prevent penetration and mixing of the contacting media. Water vapor permeation through the films has been measured, and membrane distillation experiments have been implemented to compare the performance of BF layers with that of commercial membranes. The polymer selected for this exploration is a poly(oxa-1,4phenylene-oxo-1,4-phenylene-oxa-1,4-phenylene-3,3-(isobenzofurane-1,3-dihydro-1-oxo)-diyl-1,4-phenylene) more commonly labeled as poly-(etheretherketone) with Cardo [PEEK-WC] (Figure 1a). The chemical architecture of this polymer is characterized by a phtaloyl moiety making it extremely soluble in common solvents differently from other trade PEEKs. Membranes based on PEEK-WC are normally prepared by using traditional phase inversion techniques,9,17,18 whereas the fabrication of honeycomb PEEK-WC membranes by self-assembly approach is unprecedented. Traditional PEEK-WC membranes exhibit usually a broad range of nonregular pores, reduced surface porosity, and low hydrophobic character owing to the hydrophilic pore formers used during the membrane preparation.17,18 On the contrary, honeycomb PEEK-WC membranes exhibit structural parameters that addressed both the surface and transport properties at satisfying the requirements of the contactor technology. Experimental Details Materials and Methods. PEEK-WC powder was used without further purification and dissolved in CHCl3 (Carlo Erba, 99%, water content e0.03%, d20°C ) 1.481 g/cm3, η20°C ) 0.56 cP) at 0.2, 0.5, 0.8, and 1.0 wt % in polymer. The homogeneous polymer dopes were cast on clean and dried stainless steel supports and located inside of boxes equipped with sensors of temperature and relative humidity (DELTA E Co.). Three different approaches were used for BF formation: static, dynamic without laminar flux, and dynamic with laminar flux. In the first case, the casting solutions were located inside of the box previously equilibrated at the temperature and humidity of interest. In the second case, a humid current was blown nonlinearly on the casting solutions. In the third case, a flux


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Figure 2. Pore size distribution calculated according to statistical method: (a) cumulative distribution function versus the ascending pore size; (b) effects of the polymer concentration; (c) effects of the temperature; (d) effects of the relative humidity; (e) effects of the direction of the humid air blown on the liquid surface/membrane prepared from casting solutions at 0.5 wt %, 25 °C and 70 RH%; (f) examples of pore distributions calculated on the bottom surfaces of membranes prepared at various experimental conditions.

with various percentage of humidity was tangentially blown on the surface of the liquid film. The viscosity of the various polymer dopes was measured at 200 rpm from 20 to 30 °C by using a programmable rehometer (Brookfield DV III Ultra),

(Table 1). Membrane morphology was investigated by scanning electron microscopy (ESEM, Quanta FENG 200, FEI Company). Pore size, pore distribution, and surface porosity have been estimated from SEM images by using ImageJ 1.37V



Figure 3. SEM images showing structural changes on the top surfaces of BFs PEEK-WC membranes prepared at various experimental conditions: (a-c) effects due to the rising polymer concentration (C, wt %); (c-e) effects due to the rising operating temperature (T, °C); (f,c,g) effects due to the rising relative humidity (RH%)

software. The pore size has a log-normal distribution for all membranes; the cumulative distribution function versus the ascending pore size is a straight line on log-normal probability paper. The mean pore size has been calculated from the lognormal plot, and the pore distribution has been expressed by the probability density function according to the procedure described in ref 19. The surface porosity has been calculated from the ratio between the area of the pores to the total membrane surface area.19 The overall porosity has been expressed as the percentage of the ratio between void volume fraction and total volume of the membrane, respectively.10 The void volume fraction has been estimated from (a, dry porosity) the apparent density of both the membrane and polymer, respectively, and (b, wet porosity) by weighing the amount of a fluorinated solvent at low surface free tension adsorbed inside of the membrane pores (Fluorinert FC40, 3 M Novec). The data obtained from the two different approaches have comparable results (Table 2). Membrane topography and roughness (Ra) were estimated by using atomic force microscopy (AFM, Nanoscope III Digital Instruments, VEECO Metrology Group). Tapping mode AFM was operated by scanning a tip attached to the end of an oscillating cantilever across 10 × 10 µm2 of sample surface at a rate of 1.0 Hz. The sessile drop method was used for the estimation of the water contact angle values (θ, deg) for all membranes and the calculation of the surface free energies (γsol, mJ/m2) for the casting solutions (CAM 200-KSV instrument LTD, Helsinki, Finland). The liquid probe droplets were dispensed and deposited on the membrane surface by using a microsy-

ringe with an automatic dispenser, and the images were captured by a digital camera. The energy cohesion density (ecoh, 106 J/m3) and the solubility parameters (δ, 103 J1/2/m3/2) were calculated for each polymer dope and water according to the relations reported in ref 10 (Table 2). Changes in the glass transition temperature (Tg, °C) of the PEEK-WC membranes were detected by thermal analysis (Diamond Pyris DSC, Perkin-Elmer Instruments). Each sample was heated from 50 to 260 °C, cooled down to 50 °C, and finally heated up to 260 °C at 15 °C/min to remove traces of solvents. The second heating runs were evaluated and compared with that measured for the polymer powder. The amount of sample weighed for each experiment was approximately 6.5 mg. The water vapor transmission rate was measured according to the gravimetric method (4818 UNI, part 26 of the procedure). The membrane-sealing flasks filled by ultrapure water were equilibrated for 1 h at 25 °C and then placed into a box containing desiccant. The loss of water as vapor through the membranes was estimated with time, and the transmission rate was expressed as ratio between the weight variation of the water and membrane area per day [g/(m2 · day)]. The membranes were tested in a MD apparatus where the difference of vapor pressure was generated by trans-membrane temperature difference (Scheme 1a). Retentate (continuous line) and distillate (broken line) streams converged, in a counter-current way, toward the membrane module (Scheme 1b) containing the BFs PEEK-WC membranes where the solvent (liquid water) evaporated. On the retentate side a pump was taking and sending the heated feed to the membrane module. Also on the distillate line, a second pump ensured the counter-current recycle of the cold stream in order


Figure 4. Relations between influencing controlling factors and final BF PEEK-WC membrane morphology: (a) increase in bigger pore size with temperature at various conditions of polymer concentration and relative humidity; (b) linear correlation between the membrane surface porosity and overall porosity with rising temperature and relative humidity.

to remove from the solution the vapor that was diffusing through the membrane pores. The tank on the distillate side represents the drawing and picking tank for the distillate. The transmembrane flux was estimated by evaluating the volume variations in the feed and distillate tanks. The plant was supplied with the necessary tools for the control of the most significant parameters of the system: flow rate and temperature. The results were compared with those achieved for commercial poly(propylene) membrane modules (MD020CP-2N) supplied by Enka Microdyn, the characteristics of which are described in Table 3. Results and Discussion Influencing Factors for the Membrane Formation. In this work some influencing critical factors for the formation of BFs PEEK-WC membranes were evaluated in relation to the final structure of the films. Viscosity (η, cP), concentration (C, wt %), and surface free tension (γ, mJ/m2) of the casting solutions were examined in combination with operating conditions of temperature (T, °C) and relative humidity (RH, %). Spreading volume (V, mL) and solubility parameters (δ, 103 J1/2/m3/2) were also considered as crucial factors for the propagation of the water droplet lattice through the film, yielding multilayered porous films. Polymer dopes at various concentrations (0.2, 0.5, 0.8, 1.0 wt %) were exposed to static and dynamic moist atmosphere

Gugliuzza et al. (60, 70, and 80%), changing the temperature from 20 to 30 °C. Self-assembled patterns were formed over 3.5 cm2 of area (Figure 1b). Each single air bubble identifiable with the pore of the membrane appears generally surrounded by six other bubbles (Figure 1c). Noncoalesced pores are uniformly distributed on the membrane surface (Figure 1d). Pore sizes of approximately 2.0 um were calculated for the top membrane surface, whereas a reduction of 1 order of magnitude was estimated for the pores distributed on the bottom surface. The median ranks were plotted as a function of the measured pore size on log-normal probability paper. The data were fitted by straight lines with high correlation coefficients (R2 g 0.94) for all BF PEEK-WC membranes (Figure 2a). The formation of multilayered films was observed for all three fabrication methods, although a slight packing discontinuity can be sometimes appreciated at around half of the film thickness (Figure 1e). A hexagonally packed geometry distinguishes all PEEK-WC membranes (Figure 1d) with respect to films prepared by traditional approaches (Figure 1f), whereas the order of the patterns appears to be influenced by the strategy selected for condensing water molecules on the liquid surface. Surface porosity up to 85% was achieved for the most part of the membranes, while bulk porosity of 75-80% was estimated for patterns formed from casting solutions at 0.2 wt % in polymer. Unfortunately, these last films appear too brittle to be used as membranes in contactor operations. Structural changes have been investigated by means of SEM and AFM analyses. High enlargements of the pictures have revealed the significant influence of various factors on the final size, shape, sequence, density, and order of the pores forming well-architectured PEEK-WC membranes with different degrees of surface and overall porosity. A detailed description of the influencing factors for the formation of BFs PEEK-WC membranes is reported hereafter. Pore Size and Distribution. Concentration and Surface Free Tension. Hexagonal arrays have been achieved for all polymer dopes, although smaller air-bubbles appear massively distributed along the edges of the bigger pores as the polymer concentration is decreased (Figure 3a-c). The final stage of the process concerns the complete evaporation of the residue solvent; thus, the formation of pores within a size range can be plausibly explained according to the larger amount of solvent in the polymer solution. Indeed, the excess of solvent residue in polymer-rich regions can prolong the time of evaporative cooling before the overall system rises to room temperature. Little gradients of temperature are thus locally maintained, inducing secondary nucleation of smaller-sized water droplets along the edges of the bigger bubbles (Figure 3a). Because the viscosity of the medium is just increased, the growth and rearrangement of the droplets germinated lately and their following penetration into the solution are delayed, resulting sometimes in coalesced bubbles. The result is a broadness of the log-normal pore distribution at lower polymer concentration as a direct consequence of succeeding nucleation (Figure 2b). On the contrary, unimodal pore size is observed at higher polymer concentration (Figure 2b). Fast, uniform, and simultaneous nucleation occurs at all regions of the polymer surface, while a succeeding slow growth leads to a hexagonal uniform and semicrystalline lattice of the water droplets. A significant reduction of the smaller bubbles and the formation of more regular patterns are, therefore, observed as concentration and viscosity of the solution increase (Table 1). This can be explained according to the reduced time of evaporative cooling and the enhanced encapsulation and stabilization of the water



Figure 5. Influence of the humid air direction on the final order of the hexagonal patterns. SEM images showing the top surfaces of membranes prepared from casting solutions at 1.0 wt %, 20 °C, and 70% RH in (a) static conditions; (b) dynamic without laminar flux; (c) dynamic with laminar flux. AFM images collected on a surface area of 10 × 10 um2 and showing the topography of membranes prepared from casting solutions at 1.0 wt %, 25 °C, and 70% of RH in (a′) static conditions; (b′) dynamic without laminar flux; (c′) dynamic with laminar flux.

droplets by the polymer. In this regard, Pitois et al. 8 have suggested that a local precipitation of the polymer takes place forming an envelope at the droplet-polymer solution interface when the water droplet sinks into the polymer solution. Quicker polymer precipitation produces, therefore, more pronounced stabilization of the droplet arrays. Favorable intermolecular interactions are meant to occur between the liquid water and PEEK-WC, resulting in a stabilization of the mobile lattice, especially as the surface free tension and energy cohesion density (ecoh, 106J/m3) of the casting solution increase (Table 1). This suggests a reduction of the difference between the

solubility parameters (δ, 103 J1/2/m3/2) of the water and the polymer dopes, indicating higher reciprocal affinity. Temperature. Changes in temperature from 20 to 30 °C produce a reduction of the distance between the pores as a direct effect of a gradual growth of the water droplet diameter and, then, of the penetration degree into the polymer solution (Figure 3c-e). The driving force of the water molecular self-assembly is a gradient of temperature between the polymer solution and the humid atmosphere blown over the surface. An increase in temperature produces, therefore, a higher rate of evaporative cooling, resulting in a larger difference of temperature (∆T).


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Figure 6. SEM images showing the propagation of the water droplet mobile lattice through films (0.5 wt %, 25 °C, 70%) prepared in: (a) static conditions; (b) dynamic without laminar flux; (c) dynamic with laminar flux; (d) AFM image captured on surface area of 10 × 10 um2 and showing the formation of multilayered film (0.5 wt %, 25 °C, 70%).

Quicker growth of the diameter of the water droplets is consequently generated, resulting in a reduced thickness of the polymer layer enveloping the template droplets (Figure 3c-e). In addition, bigger water droplets submerge more easily into a less resistant liquid layer, the viscosity of the casting solution being lower at higher temperature (Table 1). The crucial role of the temperature during the stages of the nucleation and growth has been largely confirmed by experiments carried out at conditions considered off-limits for PEEK-WC. The exposition of solutions at 1.0 wt % of polymer to atmosphere with 60% of relative humidity have led to PEEK-WC membranes with fewer, smaller, and nonregular pores on the surface as the operated temperature was 20 °C (Figure 3f). On the contrary, a gradual increase in the population of bigger hole size was observed at the same conditions of concentration and humidity as the temperature was raised up to 30 °C. This confirms that the driving force of the process is the evaporative cooling generated by the solvent evaporation. Variations of the temperature produce wellbalanced changes in the droplet growth rate and in the solution viscosity, resulting in modulated imprinting action of the water droplets. Because the hexagonal round hole corresponds to the cross-section shape of the top hemisphere of the water droplet in contact with air,20 an increase of its diameter with rising temperature reflects a rapid growth of the droplet volume and a facilitated penetration into the solution. The result is increased size and reduced interdistance of the pores independently of the operating conditions of concentration and relative humidity

Figure 7. Water-polymer interactions: reduction of the glass transition temperature estimated for BF PEEK-WC membranes with increasing (a) temperature, (b) concentration, (c) relative humidity.

(Figure 4a). Surface and overall porosity enhance with rising temperature (Figure 4b), while unimodal pore distributions are preserved (Figure 2c). Unfortunately, an undesired effect of the rising temperature is the reduction of the degree of order of the arrays, as shown in Figure 3. Tian et al. 6 also observed similar behaviors with polymers having different architectures. An explanation could be given on the basis of one of the most recognized theories about the formation of BF, proposed by Srinivasarao.4 After nucleation, the water droplets arrange in hexagonal quasicrystalline islands, which move like rafts on the surface under thermocapillary forces and Marangoni convection. At higher temperature the array mobility rises and turbulent and fast collisions are exerted on the aggregated droplets, resulting in somewhat irregular bubble-arrays. RelatiWe Humidity. Another influencing factor for the BF formation is the relative humidity. The availability of condensable water particles depends strongly on the moisture blown over the casting solution. The nucleation is significantly limited as well as the growth of the droplet when the liquid film is exposed to low relative humidity (60%), especially if the rate of evaporative cooling is not enough and the viscosity is rather high. The result is the formation of PEEK-WC membranes with



Figure 8. Evaluation of the propagating mobile water-droplet array through the film: (a-c) effect of the polymer concentration on thickness and symmetry of membranes prepared at 70%, 30 °C, 0.8 mL; (d,e) effect of the spreading volume on the symmetry of BF PEEK-WC membranes prepared at 1.0 wt %, 70%, 25 °C, 0.4 mL.

slightly porous skin (Figure 3f) and low overall porosity (Figure 4b). Differently, PEEK-WC membranes with an extensive air bubble array and overall porosity around 60% are achieved as casting solutions at 1.0 wt % in polymer are exposed to 80% of relative humidity and the operating temperature is 20 °C. In this case, massive nucleation is promoted and the entire surface area of the solution is quickly covered, although semiordered patterns are formed (Figure 3f,c,g). Indeed, the velocity of two first stages prevents ordered self-assembly, since the neighboring droplets collide quickly leading to coalescence phenomena. This irregularity occurs independently of the fabrication approach selected and is more evident with rising temperature, owing to additional effects of thermocapillary forces exerted harshly on the droplet islands. The consequence is a proportional increase in the surface and overall porosity with increasing relative humidity (Figure 4b), although a broadness of the pore distribution is also appreciated (Figure 2d). Order and Sequence. Humid Air Flux Direction. BF PEEKWC membranes were prepared according to three different strategies in order to evaluate the effects of the humid air direction on the final structure of the patterns. Polymer dopes at various concentrations were exposed to a previously equilibrated humid atmosphere inside of a box (static method). At the same time, the solutions were placed in two other boxes and humid air was blown on the liquid surfaces. In the last cases, a carrier inert gas was bubbled through distilled water and fluxed tangentially (dynamic method with laminar flux) and randomly (dynamic method without laminar flux) on the liquid surface. Changes in concentration, temperature, and relative humidity produced comparable effects independent of the fabrication strategy selected (Figure 3). Differently, enhanced control of the order and sequence of the membrane pores was noted for layers prepared according to the static and dynamic with laminar

flux methods, respectively (Figure 5). Indeed, in static conditions the water droplets arrange themselves in a longer time, leading to more sequenced and regular geometry. Differently, the highest order observed for BF membranes prepared by tangentially fluxing the humid air on the liquid surface can plausibly be ascribed to the highest orientation of the water molecules. In both cases, the turbulence of the thermocapillary and convective forces exerted on the mobile arrays appears significantly reduced with respect to the films fabricated in the absence of laminar flux. As a result, higher control of the pore distribution is achieved on the membrane surfaces as both the static and dynamic (with laminar flux) conditions are used (Figure 2e). Differently, humid air nonlaminarly blown on the casting solutions yields a broadness of the log-normal pore size (Figure 2e). Instead, no significant differences are appreciated in terms of pore size, the air bubbles formed on the top surface being approximately 2 um. Multilayered Patterned PEEK-WC Membranes. Propagation of the Mobile Droplet Lattice. In literature, the breath figure is often indicated as a useful approach for the fabrication of regular porous layers that can find applications also like membranes. However, the debates are mainly about the mechanisms controlling the process formation,4,21 the order and regularity of the air-bubble patterns,15 and the polymer-water affinity.22 The experiments are often carried out by spreading a few microliters of the polymer dope on glass plates, and the event leading to the assembly of extensive self-supported porous layers is rarely discussed. In this work, unprecedented honeycomb PEEK-WC arrays are proposed like self-supported membranes suitable for equipping devices based on advanced technology such as the membrane contactors. Hexagonally selfassembled membranes with a surface area of 3.5 cm2 and a thickness up to 34 ( 2 um have been tailored by identifying some critical factors that control the propagation of the


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Figure 9. SEM images showing the bottom surfaces of various BF membranes. Enlargement at 20000×. (a,b) Effects of the rising relative humidity; (c) membrane prepared in static conditions from casting solutions at 1.0 wt %, 25 °C, 70% RH and spreading volume ) 0.6 mL; (d) AFM image of membrane prepared in dynamic conditions with laminar flux from casting solutions at 1.0 wt %, 25 °C, 70% RH and spreading volume ) 0.4 mL.

hexagonal arrays through the film. This is a crucial issue for assuring high interfacial area per unit of volume and reduced resistance to the mass-transfer. It is well known that a solvent denser than water forms only one 2D-layer of pores, whereas a solvent lighter than water produces 3D-architectures, as well stated by Srinivasarao.4 Because PEEK-WC membranes have been prepared from chloroform solutions (Table 1), a monolayer of an air-bubble array should be, therefore, expected to form. Really, PEEKWC films exhibit multilayered and well-distinct porous arrays with hexagonally structured geometry through the overall thickness, independently of the strategy selected for the BF membrane fabrication. SEM and AFM images well emphasize the coexistence of sublayers due to the submersion of water droplets crystallized in the hexagonal lattice (Figure 6), although a slight packing discontinuity can be sometimes appreciated at around half of the film thickness. These discontinuities can be plausibly ascribed to collisions occurring among the water droplets, yielding coalescence. Indeed, larger submerged air bubbles are frequently visible in membranes prepared in

dynamic conditions, because higher kinetics move the water droplets during the self-assembly resulting in higher probability of collisions. In any case, the penetration of the water array is well observed in membranes prepared in static and dynamic conditions, respectively. The propagation of the droplet mobile lattice through the entire membrane thickness may be well interpreted considering both the kinetic and thermodynamic aspects of the phenomenon. Currently, the role of kinetics factors controlling the mobility of self-assembled water droplets throughout denser solutions is the subject of some theoretical studies still in progress. However, thermodynamic factors turned to the casting solution-water droplet affinity could offer new insights into the interpretation of the phenomenon. Indeed, water-polymer moiety interactions concur to stabilize and propagate the droplet-islands leaving 3D-architectures. The increase in a thermodynamic indicator such as the energy cohesion density (ecoh, 106J/m3) reflects an enhanced polymer dope-water attraction. Indeed, the reduction of the differences calculated between the solubility parameters (δ, 103 J1/2/m3/2) of the liquid water and polymer solutions provides significant


Figure 10. Evaluation of the wetting degree: (a) linear relationship between the surface roughness and surface porosity; (b) kinetic experiments for membranes having different surface porosity; (c) kinetic experiments for membranes prepared according to different static and dynamic approaches (0.5 wt %; 25 °C; 70%; RH, 0.5 mL).

Figure 11. Relationships between the water vapor transmission rate and overall porosity at 25 °C after 24 h.

indication about the larger attraction of two contacting media as the polymer concentration raises (Table 1). The strong tendency of the water droplets to condense, selfassemble, and propagate through PEEK-WC solutions takes place because of significant intermolecular interactions with the polymer chains. This datum is further reflected by changes in the glass transition temperature (Tg, °C) of the patterned PEEKWC membranes. Thermal analysis have provided indication about the influence of the water on the polymer packing and,

J. Phys. Chem. B, Vol. 112, No. 34, 2008 10493 therefore, about the segment polymer moiety-water affinity. A slight reduction of the thermal transition has been appreciated for all BF PEEK-WC membranes at various experimental conditions (Figure 7). This suggests that the water droplets exert on the polymer chains a plasticization action, resulting in a slight softness of them. This effect is the consequence of attractive affinity between the polymer and water-droplet array. The liquid layer viscosity and the evaporating time are two other factors influencing the degree of penetration of the bubble arrays, respectively. The polymer concentration significantly influences thickness, symmetry, and overall porosity of the membrane (Figure 8), yielding asymmetric structure and sometime packing discontinuity at around the half-section. At higher concentration, the membrane exhibits increased thickness and relevant asymmetry (Figure 8a-c). The highest viscosity of the solution is suggested to inhibit the penetration of the water droplets into the sublayers of the polymer solution, especially as the time of evaporative cooling ends before the submersion is completed. The chloroform and water evaporation therefore leave the bottom part of the film more compacted (Figure 8c). Alternatively, lower polymer concentrations improve the overall porosity of the film, yielding uniform thickness and symmetric structures (Figure 8a). This suggests that the time of evaporative cooling is enough to complete the propagation of the bubble arrays through the overall film. Thus, a direct proportionality between the time of evaporative cooling and the amount of solvent is found. Nevertheless, asymmetry and compactness occurring at a higher polymer concentration can be prevented by reducing the height of the spreading volume of the solution. Indeed, symmetric membranes with a hexagonal pattern uniformly distributed along the overall thickness of the film can be also formed from solutions at 1.0 wt % in polymer, as the spreading volume of the solution is reduced (Figure 8d,e). Symmetry and porosity of the films can be enhanced as lower columns of spreading volume are exposed to humid atmosphere. This means that the pathway is shorter and the gradual rising of viscosity of the solution during the solvent evaporation is not enough to prevent the complete penetration of the water droplets. Although the penetration of the droplet array into denser liquid is unexpected, a similar behavior was already observed for other systems. Park et al. 15 detected the formation of multilayered air bubbles for solutions of cellulose acetate in chloroform, although it was only limited to the top layers of the film and closed air bubbles were yielded.14 On the contrary, PEEK-WC membranes exhibit multilayered patterns throughout the entire thickness of the film with interconnected bubbles, resulting in attractive membranes. Despite that SEM and AFM images well emphasize the interconnectivity of the bubbles (Figure 6), further experiments have been done to confirm the presence of open air bubbles. All membranes were wetted by a fluorinated solvent and all pores were successfully filled. The void volume fraction (wet porosity) of the patterns was calculated from the ratio between the density and weight of the solvent. The values resulted in good agreement with those calculated from the apparent densities of the membranes and polymer, respectively (dry porosity) (Table 2). The high degree of interconnectivity of the bubbles can be explained by the presence of further nanosized pores distributed massively on the walls of the larger air bubbles and on the bottom layer of the membranes (Figure 89). The formation of these smaller pores can be plausibly attributed to local phase separation between water-rich regions and water-lean regions, taking place during the breath figure propagation. The event


Gugliuzza et al.

Figure 12. (a) Trans-membrane flux vs time for two different experimental tests carried out in the same conditions: feed temperature ) 39 °C, permeate temperature ) 25 °C, feed flow rate ) 100 mL/min, permeate flow rate ) 84 mL/min; (b) trans-membrane flux vs time for the PK membrane and the MD020CP-2N membrane module (feed temperature ) 32 ( 1 °C, permeate temperature ≈ 19 ( 1 °C).

occurs extensively in the sublayers of the membrane, leaving on the bottom surface pore size lower than 1 order of magnitude with respect to the top membrane surface (Figure 9). These nanosized pores continue to exhibit well-defined shape and geometry, suggesting arrangements of the water-rich phases leading to smaller no coalesced air-bubbles. The distribution of these pores on the bottom surface is much more massive as polymer concentration and spreading volume decrease as well as relative humidity rises (Figure 9). Structure-Property Relationships. Membrane Hydrophobicity. The investigation of the structure-property relationships gains a relevant importance if the role of interface of the membrane is considered. The high hydrophobic character of the membrane is, for example, one of the most important requirements for processing the film in contactor applications. The reason is the necessity of preventing the membrane from wetting and to promote the formation of a liquid-vapor interface at the entrance of the pores. BF PEEK-WC membranes exhibit a low wetting degree, since water contact angle values of 130 ( 3° have been measured. An increase of around 40° has been estimated with respect to PEEK-WC membranes (88 ( 2°) prepared by traditional techniques (Figure 1f).23 The enhanced water repellence of BF PEEK-WC membranes is specifically concerned with the surface topography. The membranes exhibit a surface entirely covered by regular pores uniformly dispersed and sequenced. This generates the surface roughness responsible for the capture of hydrophobic air inside the pores.10,24 Indeed, a linear relation between the density of surface pores and the membrane surface roughness has been found (Figure 10a). The formation of an interface between the probe polar liquid and the layer of hydrophobic air entrapped into the pores consequently provides enhanced water repellence, yielding apparent contact angle values up to 130 ( 3° at time zero. Despite a slight spreading of the water droplet being observed with time, the strong hydrophobic character is

preserved (Figure 10a,b). No substantial differences are appreciated between the membrane surfaces prepared by static and dynamic (with laminar flux) methods, the surface porosity being comparable for both sets of patterns. Differently, a slightly higher spreading behavior can be appreciated for surfaces of membranes prepared by the dynamic (without laminar flux) method, because of higher irregularity of the array. Transport Properties. Tests of permeation to water vapor were addressed at evaluating the suitability of BF PEEK-WC membranes to work as no selective interfaces for contactor operations. Two kinds of experiments were implemented to measure the water vapor mass transfer through the membranes; in both the cases the driving force of the processes was a transmembrane pressure difference (∆P). Water Vapor Transmission Rate (WVTR): Static Experiments. Permeation experiments were carried out in static conditions and the loss of water vapor through the membranes was gravimetrically measured with time. A trans-membrane ∆P was generated placing the membranes, sealing flasks filled by ultrapure water, inside of equilibrated boxes containing fresh desiccant. An increase in water vapor permeation [WVTR, gm-2 day-1] was estimated for membranes with rising overall void volume fraction [ε, %], confirming the degree of interconnectivity of the structures (Figure 11). Open and interconnected pores of the honeycomb patterns promote trans-membrane flux, while a reduction of the resistance to the mass transfer is appreciated through membranes fabricated from casting solutions at lower polymer concentration. MD Experiments: Dynamic Conditions. MD experiments have been implemented to evaluate the water vapor transport through BF PEEK-WC membranes according to the requirements of the most common contactor operations. The driving force of the process has been generated by means of a difference of temperature (∆T) between feed and permeate streams. Transmembrane water vapor flux [J, Lh-1 m-2] has been measured at different feed flow rates and at various thermal differences,



Figure 13. (a) BF PEEK-WC membrane module: trans-membrane flux vs time at two different feed flow rates (permeate flow rate ) 5 L/h, feed temperature ) 39 °C, permeate temperature ) 25 °C); (b) MD020CP-2N membrane module: trans-membrane flux vs time at four different feed flow rates (feed temperature ) 32 ( 1 °C, permeate temperature ≈ 19 ( 1 °C); (c) BF PEEK-WC membrane module: trans-membrane flux vs time at two different thermal differences (DT) between the hot and cold streams (feed flow rate ) 6 L/h; perm. flow rate ) 5 L/h).

resulting in a good reproducibility with low standard deviation and errors less than 0.02 (Figure 12a). The results have been compared with those achieved for a commercial poly-(propylene) membrane (MD020CP-2N) having comparable overall porosity and hydrophobicity, but different structural parameters and module configurations (Table 3). The trans-membrane fluxes estimated for MD020CP-2N and BF PEEK-WC patterns at the same experimental conditions of temperature both on permeate and feed side are displayed in Figure 12b. Lower flow rates have been used for BF PEEK-WC membranes, because the effective area was smaller than that of the commercial membrane module. In both cases, apart from an initial transitory stage, almost constant trends can be observed at steady state. This means that neither polarization effects nor membrane wetting occur in the membranes, resulting in a good operation. It is well-known that flat sheet membrane modules are generally less productive than tubular or capillary membranes, because of the reduced effective surface area. Despite that PEEK-WC membranes exhibit a flat sheet configuration, a higher transmembrane flux has been measured through BF patterns than through hollow fiber MD020CP-2N assembled in a tubular module (Figure 12b). This suggests higher productivity of

PEEK-WC membranes, excluding also additional resistance of the support to the mass transfer. Indeed, a stainless steel support has been used in order to prevent deflections and breaks of the films. No cracks occurred during the experiments, indicating enough resistance of the films. Unfortunately, an estimation of the membrane mechanical resistance was not allowed, because of the undersized surface area of the samples. Effects of both the fluid-dynamic and temperature on the MD operation have been also evaluated for both modules in relation to the trans-membrane flux (Figure 13a-c). The water vapor permeation increases as the feed flow rate rises (Figure 13a). This is expected to favor mass and energy transport, since higher feed flow rates mean higher Reynolds number and transport coefficients. Considering the less effective area of the BF PEEKWC, a higher productivity is once more confirmed for the patterned arrays (Figure 13a,b). Finally, an increase in the transmembrane flux is observed as the thermal difference (∆T) between the hot and cold streams rises (Figure 13c). An enhancement of around 64% has been appreciated through BF PEEK-WC layers as ∆T rose only 2 °C, confirming the significant dependence of MD operation on the temperature.

10496 J. Phys. Chem. B, Vol. 112, No. 34, 2008 The successful outcomes of these processes are undoubtedly ascribed to the particular morphology of the BF structures. The big pores densely and uniformly distributed on the top surface of membranes promote large interfacial areas at the entrance of the pores on the feed stream. Simultaneously, the high waterrepellence of the membrane surfaces prevent the films from wetting and the contacting media from mixing, resulting in the formation of a liquid-vapor interface stable with time. The open and highly interconnected air-bubbles further reduce the resistance to the water vapor transport, enhancing the diffusion through the membrane channels and, consequently, the volumetric mass transfer rate. Undoubtedly, MD experiments have provided preliminary data indicating the BF PEEK-WC membranes as potential attractive interfaces to be used in sophisticated contactor operations. Larger membrane areas are expected to further improve the productivity of the process with respect to traditional systems. Conclusion BF PEEK-WC membranes have been fabricated for the first time by using a self-assembly approach. The water-assisted procedure has been adopted for templating polymeric layers in a honeycomb packed geometry. Shape, size, distribution, and ordered sequence of the pores have been controlled by modulating specific operative parameters. Multilayered arrays have been unusually achieved, although the solvent of the casting solution was denser than water. Intermolecular interactions seem to thermodynamically promote the propagation of the mobile droplet-array through the liquid film, because of favorable intermolecular interactions between water droplets and polymer segment chains. The propagation through the entire thickness of the liquid film is facilitated at lower concentration and reduced path of the casting solution. A difference of around 1 order of magnitude has been estimated between the pores distributed on the top and bottom membrane surfaces. Open and interconnected air bubbles characterize all hexagonal patterns, resulting in reduced resistance to the trans-membrane transport. Low wetting degree combined with good water vapor transmission rate make the BF PEEK-WC membranes significantly adequate for contactor applications. Preliminary experiments of membrane distillation have demonstrated enhanced performance of the flat sheet self-assembled PEEK-WC membranes in comparison with traditional commercial hollow-fiber modules. The achievements in terms of trans-membrane water vapor flux suggest these layers as promising interfaces for contactor applications, because of their intrinsic structural properties. The water self-assembly appears, therefore, a powerful tool for the fabrication of next generation membranes.

Gugliuzza et al. Acknowledgment. This work has been supported by Ministero dell’Istruzione dell’Universita` e della Ricerca Fondo Investimenti per la Ricerca di Base, FIRB-MIUR (Contract N.RBNE03JCR5 CAMERE). The authors are grateful to Dr. Giovanni Desiderio from Department of Physics of the University of Calabria for the use of his technical facilities and Miss Rita Costabile for her partial contribution to the experimental work. References and Notes (1) Bunz, U. H. AdV. Mater. 2006, 18, 973. (2) Song, L.; Bly, R. K.; Wilson, J. N.; Bakbak, S.; Park, J. O.; Srinivasarao, M.; Bunz, U. H. F. AdV. Mater. 2004, 16/2, 115. (3) Widawski, G.; Rawiso, M.; François, B. Nature 1994, 369, 397. (4) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79. (5) Zhao, B.; Li, C.; Lu, Y.; Wang, X.; Liu, Z.; Zhang, J. Polymer 2005, 46, 9508. (6) Tian, Y.; Jiao, Q.; Ding, H.; Shi, Y.; Liu, B. Polymer 2006, 47, 3866. (7) Yabu, H.; Tanak, M.; Ijiro, K.; Shimomura, M. Langmuir 2003, 19, 6297. (8) Pitois, O.; François, B.;. Eur. Phys. J. B 1999, 8, 225. (9) Gugliuzza, A.; Clarizia, G.; Golemme, G.; Drioli, E. Eur. Polym. J. 2002, 38, 235. (10) Gugliuzza, A.; Drioli, E. J. Membr. Sci. 2007, 300, 51. (11) Peng, J.; Han, Y.; Li, B. Polymer 2004, 46, 447. (12) Xu, Y.; Zhu, B.; Xu, Y. Polymer 2005, 46, 713. (13) Gugliuzza A.; Aceto M. C.; Drioli E. Proceedings of 70th PMM 46th Microsymposium on Nanostructured Polymers and Polymer Nanocomposites, Prague, 2007. (14) Govor, L. V.; Bashmakov, I. A.; Kiebooms, R.; Dyakonov, V.; Parisi, J. AdV. Mater. 2001, 13/8, 588. (15) Park, M. S.; Kim, J. K. Languimur 2004, 20, 5347. (16) Lawson, K. W.; Lloyd, D. R. J. Membr. Sci. 1997, 124, 1. (17) De Bartolo L.; Gugliuzza A.; Cirillo B.; Morelli S.; Drioli E. In Membranes Preparation, Properties and Applications; Burganos V. N., Noble, R. D., Asaeda, M. A., Ayral, LeRoux J. D. Eds.; Materials Research Society: Warrendale, PA, 2003(Mat. Res. Soc. Symp. Proc. 2003, 752, AA14.2.1). (18) De Bartolo, L.; Gugliuzza, A.; Morelli, S.; Cirillo, B.; Gordano, A.; Drioli, E. J. Mater. Sci.: Mater. Med. 2004, 15, 877. (19) Khayet, M.; Feng, C. Y.; Matsuura, T. J. Membr. Sci. 2003, 213, 159. (20) Nishikawa, T.; Ookura, R.; Nishida, J.; Arai, K.; Hayashi, J.; Kurono, N.; Sawadaishi, T.; Hara, M.; Shimomura, M. Langmuir 2002, 18, 5734. (21) Pitois, O.; François, B. Colloid Polym. Sci. 1999, 277, 574. (22) Karthaus, O.; Maruyama, N.; Cieren, X.; Shimomura, M.; Hasegawa, H.; Hashimoto, T. Languimur, 2000, 16, 6071. (23) Aceto M. C. Experimental Thesis carried out at ITM-CNR, Italy, 2006. (24) Young, T. H.; Cheng, L. P.; Lin, D. J.; Fane, L.; Chuang, W. Y. Polymer 1999, 40, 5315.

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