Exploiting Fluoropolymers Immiscibility to Tune Surface Properties

Article Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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Exploiting Fluoropolymers Immiscibility to Tune Surface Properties and Mass Transfer in Blend Membranes for Membrane Contactor Applications Carmen Meringolo,† Teresa Poerio,*,† Enrica Fontananova,*,† Teresa F. Mastropietro,† Fiore P. Nicoletta,‡ Giovanni De Filpo,§ Efrem Curcio,∥ and Gianluca Di Profio†

ACS Appl. Polym. Mater. Downloaded from pubs.acs.org by WEBSTER UNIV on 02/27/19. For personal use only.

†

National Research Council of Italy (CNR), Institute on Membrane Technology (ITM), Via P. Bucci Cubo 17C, 87036 Rende (CS), Italy ‡ Department of Pharmacy, Health and Nutritional Sciences, University of Calabria, Via P. Bucci Edificio Polifunzionale, 87036 Rende (CS), Italy § Department of Chemistry and Chemical Technologies (DCTC), University of Calabria (UNICAL), Via P. Bucci Cubo 15/C, 87036 Rende (CS), Italy ∥ Department of Environmental and Chemical Engineering (DIATIC), University of Calabria (UNICAL), Via P. Bucci Cubo 45/C, 87036 Rende (CS), Italy S Supporting Information *

ABSTRACT: Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) and polyvinylidene fluoride (PVDF) blend membranes, with interconnected channels decorated with polymer crystallites, were produced by a two-step phase separation technique, using nontoxic solvents and without any chemical additive as pore forming. Results demonstrated that the mass-transport and the interface properties of the membranes can be tailored by a synergic combination of immiscible blend components and an adequate manufacturing procedure, that exploit the slow diffusion of the nonsolvent by vapor induced phase separation. In this way, multilevel hierarchical surfaces, with raspberry- and cauliflower-like substructures, were obtained. The optimal blending ratio between the two components was assessed by chemical physical and mass-transport property characterizations of the prepared membranes. The beneficial effect of blending was evidenced for all the properties investigated (crystallinity, surface roughness, wettability, surface charge, strength, toughness, and flux), indicating a good physical interaction between the two components, despite their thermodynamically stated immiscibility. This physical interaction was also proved by the presence of PVDF crystallites embedded in the PVDF-HFP network of blend membranes, which act as reinforcing agents rather than as independent fillers with respect to the pure copolymer, providing enhanced mechanical properties. KEYWORDS: PVDF, PVDF-HFP, polymers blending, solubility parameters, membrane contactor, miscibility, surface roughness, wettability

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INTRODUCTION

More recently, membrane contactor operations, such as membrane distillation (MD) and membrane crystallization (MCr),6,7 come into light as potential innovative tools for advanced separation processes, especially in the biopharmaceutical sector.8 One of the main challenges for the membrane contactor applications is to solve the membrane wetting during long-term operations. Therefore, high mechanical resistance and hydrophobicity of the membranes are required to ensure the highest transmembrane flux and rejection coefficient in

Partially fluorinated polymers represent the most investigated class of hydrophobic polymers for membrane preparation by solution casting for membrane contactor applications. Among commercially available materials, polyvinylidene fluoride (PVDF) has received much attention as a preferential choice for hydrophobic polymeric membrane preparation.1,2 This is because, in addition to its excellent chemical and thermal resistances,3 PVDF can be easily processed via conventional nonsolvent induced phase inversion techniques from vapor or liquid phases.4 PVDF membranes have therefore been the subject of many studies for various industrially relevant separations.5 © XXXX American Chemical Society

Received: November 7, 2018 Accepted: February 14, 2019

A

DOI: 10.1021/acsapm.8b00105 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Polymer Materials

namic principles of miscibility, surface, and interfacial properties allowed for a better control of membrane properties in terms of mechanical stability, hydrophobicity, surface roughness, and pore size. Furthermore, blend membranes have been characterized and tested in MD/MCr, thus highlighting the positive outcomes of the selected strategy in terms of surface/ interface and transport properties.

MD/MCr processes. Some attempts aiming to improve PVDF membranes performance in terms of hydrophobicity, mechanical properties, and flux involved surface chemical treatments,9 grafting polymerization,10 and plasma treatment.11 However, the complexity of these chemical procedures is the main bottleneck for their practical application for membrane production on industrial scale. More recently, great attention was devoted to some particular fluoro-copolymers, such as polyvinylidenefluorideco-hexafluoropropylene (PVDF-HFP).12,13 Compared to PVDF, PVDF-HFP is a potentially higher attractive material because of the presence of an amorphous phase in its structure, due to the presence of the hexafluoropropylene groups, which reduce crystallinity and increase the fluorine content. Recent papers report several studies about the application of this copolymer as a membrane material. Lalia et al.14 developed a new technique to coat membrane surfaces with tetrafluoroethylene oligomer (OTFE) particles able to improve the hydrophobicity and roughness of PVDF-HFP membranes by exploiting the “dust effect” of OTFE particles. Kyoungjin An et al.15 enhanced membrane hydrophobicity by coating the PVDF-HFP membrane surface with polydimethylsiloxane microspheres. An interesting strategy to improve the membrane properties is based on the formation of blending between two or more polymeric materials16−21 to obtain membranes with tuned mechanical and physical properties representing a concrete and cost-effective solution with respect to the development of a new material.22−24 The increased interest toward polymer blends is due to the possibility to obtain properties which differ from those of the individual starting materials. The easy preparation procedure (by mechanical mixing, solution mixing, fine powder mixing, etc.) coupled with the possibility to vary the blend composition permits to accomplish a wide range of properties for different end-use applications.25 In this work, we demonstrated that highly hydrophobic membranes can be prepared by blending two fluorinated polymers, i.e., PVDF homopolymer and its copolymer PVDFHFP, by a two-step sustainable casting method previously developed for PVDF membranes.26 To the best of our knowledge, only two recent examples of such blending can be found in literature.27,28 However, the employed manufacturing procedures were based on a traditional liquid-induced phase separation (LIPS) process that, as already discussed in our previous work,26 is not able to produce membranes with elevated roughness and hydrophobicity. On the contrary, the application of the vapor-induced phase separation (VIPS) methodology allows for a better control of the membrane properties in terms of hydrophobicity, roughness, and pore size. Moreover, in these articles, toxic solvents, like N-methyl2-pyrrolidone (NMP),27,28 tetrahydrofuran (THF), and N,Ndimethylformamide (DMF),27 and pore forming additives28 were used to prepare the casting solutions. Here, on the contrary, the distinctive properties of each component of the PVDF-HFP/PVDF blend were exploited in combination with a specific expertise on the hydrophobic fluorinated membrane preparation by VIPS26 in order to optimize the resultant membranes. Since the higher fluorine content of the PVDF-HFP polymer can increase the membrane hydrophobicity, while the PVDF crystalline regions can help to maintain the mechanical integrity of the films, membranes with a different blending ratio were prepared. Polymer blend optimization on the basis of the thermody-

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EXPERIMENTAL SECTION

Materials. PVDF-co-HFP Solef 21510 (Mw, 290−310 kDa; Mw/ Mn, 1.8−2.1; 12 wt % HFP; and density, 1.75−180 g/mL) and PVDF Solef 6010 (Mw, 300−320 kDa; Mw/Mn, 2.1−2.6; and density, 1.75− 1.80 g/mL) were kindly supplied by Solvay Solexis.29 Analytical grade dimethyl sulfoxide (DMSO) was purchased from Sigma-Aldrich. Casting Solutions and Membrane Preparation. Blend membranes of various compositions as well as pure polymeric membranes (Table 1) were prepared by VIPS and LIPS methods. All

Table 1. Membrane Composition membrane code

PVDF-HFP (wt %)

PVDF (wt %)

PVDF-HFP A B C D E PVDF

100 71 67 50 33 29 0

0 29 33 50 67 71 100

the casting solutions consisted of homogeneous polymer (17 wt %) solution in DMSO (83 wt %) at 60 °C and were left under magnetic stirring overnight. The solutions were cast with a 350 μm thickness onto a nonwoven fabric by using an automatic casting machine (TQC AB3120) inside a box with controlled temperature (30 ± 1 °C) and relative humidity (RH). The final dry membrane thickness is reported in Table S1. On the basis of a previous investigation,26 the VIPS exposition time and relative humidity were fixed at 5 min and 50%, respectively, and then immersed in a water coagulation bath to complete the membrane formation by LIPS process. The membranes were subsequently dried at 25 °C before characterization tests. The degree of crystallinity and melting temperature were measured by differential scanning calorimetry (DSC-NETZSCH 200) using a sample of about 10.0 mg and heating/cooling rates of 10 °C min−1 under a nitrogen flux of 20 mL/min (temperature range 30−300 °C). The viscosity of the casting solutions was measured at 30 ± 1 °C and with a share rate of 1.4 s−1 by using a digital viscometer (Brookfield, Viscometer model: LVDV-III). The average of five measurements for each solution was reported. The mechanical properties were measured in stress−strain elongation experiments using a ZWICK/ROELL Z 2.5 instrument. The size of each specimen was of 50 mm × 10 mm, and each sample was stretched unidirectionally at a constant rate of 5 mm min−1; the initial distance between the clamps was of 50 mm. Five specimens were tested for each sample. For the mechanical test, membrane samples were produced without the nonwoven support in order to eliminate its contribution to the mechanical properties. A scanning electron microscope (SEM, EVO MA10 Zeiss) was employed to examine the morphology of the membrane surface and cross section. For surface observations, a small piece of membrane sample was fixed with carbon conductive double side tape to steal the stubs, while for the cross-sectional views, membranes were cryofractured. All samples were sputtered with a thin layer of gold. The surface roughness of the membranes was measured by a Nanoscope III atomic force microscope (AFM) (Digital Instruments, VEECO Metrology Group) in air, with contact mode imaging. The average of four different measurements of membrane surfaces (10 × 10 or 14 × 14 μm2) was reported. Roughness analysis was performed B



χ1,2 ij ϕ yz ij ϕ yz ΔGm = jjj 1 zzzln ϕ1 + jjj 2 zzzln ϕ2 + (ϕ ϕ ) j z j z RTV V 1 2 k V1N1 { k V2N2 {

by WSxM 5.0 Develop 6.1 software (Nanotec Electronica S. L.)30 by calculating root-mean-square roughness (RMS). Fourier transform infrared spectroscopy analyses in attenuated total reflectance (ATR-FT-IR) were performed using a PerkinElmer Spectrum One ATR-FT-IR spectrometer on the upper surface of each membrane. ATR-FT-IR analyses were used for the quantification of β-phase content.31,32 The surface charge properties of the membranes were measured by a zeta potential analyzer Surpass3 (Anton Paar). Zeta potentials were measured using 0.005 mol L−1 KCl aqueous solution in the pH range of 2−6 at room temperature. Two measurements were performed for each sample. The water contact angle (WCA) of membrane surfaces was measured using a CAM 200 device (contact angle meter, KSV instrument Ltd.). The measurement was repeated five times for each membrane sample. The mean pore diameter was measured by a capillary flow porometer (PMI, Porous Materials Inc. Ithaca, NY) using a wetting liquid (3MFluorinert Electronic Liquid FC-40, supplied by Essegie Srl) and nitrogen as the pressurizing gas. Membrane porosity was measured using a gravimetric method with 3MFluorinert Electronic Liquid FC-40. The liquid entry pressure of water (LEP), defined as the pressure difference at which liquid water penetrates into the membranes pores, was determined using the Smolders and Franken’s method.33 A dry membrane sample was placed in a membrane module where the compressed gas (nitrogen) on the water reservoir was allowed to increase stepwise the pressure until observing the starting of permeate flow. For each membrane sample, the mean value of five measurements was reported. The membrane transport properties were tested in a direct contact MD/MCr plant as detailed elsewhere.34 Performances were evaluated in terms of trans-membrane flux and rejection to NaCl. Operating conditions were feed and distillate temperatures of 53 ± 2 and 20 ± 2 °C, respectively; feed and distillate solution flow rates of 12 L h−1 (axial velocity 6.1 m h−1); membrane area of 2.4 × 10−3 m2; initial feed composition of NaCl 1 g L−1(0.017 M); and initial distillate composition of ultrapure water. The flux (J) is taken as the average flux under steady conditions (normally after the first hour of operation and until the end) and calculated as J=

(∂ 2ΔGm /∂Φ2 2)P , T = (∂ 3ΔGm /∂2 3)P , T = 0

χcr =

1 ijjj 1 + j 2 jjk Ν1

1 Ν2

yz zz zz z {

2

(6)

where χcr is the critical parameter above which the blend separates into two distinct phases. Equation 6 gives the miscibility condition for systems with the component of different molecular weights. Considering that the immiscibility can be overcome if attractive interactions between the two components are present, eq 4 can be rewritten as a function of the solubility parameters of Hildebrand, δ1 and δ2, obtained as the square root of the cohesive energy densities35 χ1,2 =

VR (δ1 − δ2)2 RT

(7)

Another approach, used to predict the miscibility, is the solubility parameters model proposed by Hansen.35 According to this model, all the intermolecular forces (London dispersion forces between nonpolar molecules, repulsive forces between nonpolar molecules, Coulombic ion/ion interactions, dipole/dipole interactions between the permanent dipoles, permanent dipole/ion interactions, induced dipole/ion interactions, permanent dipole/induced dipole interactions, charge-transfer forces, hydrogen bonds, metallic bonds, and coordination bonds) can be combined into three types of interactions, corresponding to dispersive (δd), polar (δp), and hydrogen bonds (δh). The total solubility parameter can be defined as follows

(1)

Δδ = (Δδd2 + Δδp2 + Δδ h2)1/2

(8)

Consequently, two substances would be miscible only when their solubility parameters are within the sphere with the critical radius given by eq 8.34 The Hansen solubility parameters of a blend membrane can be calculated using the fraction of each component and the solubility parameter value for pure polymers as follows35−37

(2)

where Cd and Cf are the distillate and the feed concentrations, respectively, measured in continuous by an electrical conductivitymeter (Jenway, Bibby Scientific, UK).26 Theoretical Background of Polymer Blend Miscibility. The rule “like dissolves like” is not applicable for the mixing of polymers, since the mixing of two polymers forms a single phase when the Gibbs free energy of mixing (ΔGm) is negative ΔGm = ΔHm − T ΔSm < 0

(5)

where the binodal and spinodal curves meet the critical point, and considering the so-called binary interaction parameter, χcr, as a constant, the critical conditions for the phase separation can be expressed as

where Mp is the mass of distillate, t is the permeation time, and A is the active membrane area. The membrane rejection to NaCl (R) was determined as follows ij C yz R = jjj1 − d zzz × 100 j Cf z{ k

(4)

where φ1 and φ2 are the volume fractions of component 1 and 2 in the mixture, N1 and N2 are the number of monomer in the corresponding chains, V1 and V2 are the volume of each monomer of component 1 and 2, and V is an arbitrary reference volume. Then, considering that for polymer blends V1 and V2 are large, the entropy is small. Consequently, the miscibility or immiscibility of the system mainly depends on the value of the last term (χ1,2/V) φ1φ2. Applying to eq 4 the conditions for phase separation

Mp tA

Article

δd,Blend = δd,1ϕ1 + δd,2ϕ2

(9)

δp,Blend = δp,1ϕ1 + δp,2ϕ2

(10)

δ h,Blend = δ h,1ϕ1 + δ h,2ϕ2

(11)

Accordingly, eq 7 can be rewritten as follows

(3)

χ1,2 =

Despite the thermodynamic definition of miscibility being clear, there are a lot of methods to predict miscibility. It is noteworthy, that all miscibility prediction methods have to be considered as an estimation under the restrictions of using polymers with molecular mass polydispersity. The Huggins−Flory theory is the simplest theoretical approach for modeling the free energy of binary polymer mixtures. It was initially employed for solvent−solvent and polymer−solvent mixtures.25 For binary systems, it is expressed as follows

VR (ϕ Δδ1 − ϕ2Δδ2)2 RT 1

(12)

It has been reported, that polymer pairs tend to be immiscible if χ1,2 ≥ χcr and phase separation occurs, instead if χ1,2 ≤ χcr, the polymers are miscible and no phase separation occurs. 38

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RESULTS AND DISCUSSION The immiscibility of PVDF-HFP and PVDF was first assessed under general thermodynamic framework, using eqs 7 and 12. C


Article

ACS Applied Polymer Materials Table 2. Hansen Solubility Parameters and Flory−Huggins Interaction Parameters of the Pure Polymers and Blend Membranes PVDF-HFP A B C D E PVDF

φ1

φ2

δd (MPa1/2)

δp (MPa1/2)

δh (MPa1/2)

Δδ (MPa1/2)

1 0.71 0.67 0.5 0.33 0.29 0

0 0.29 0.33 0.5 0.67 0.71 1

19.9 19.1 19.0 18.6 18.1 18.0 17.2

12.8 12.7 12.7 12.7 12.6 12.6 12.5

11.6 10.9 10.8 10.4 9.99 9.90 9.20

26.4 25.4 25.3 24.7 24.2 24.1 23.2

χ12

ref

0.0590 0.0411 0.0010 0.0191 0.0318

35,37 this work this work this work this work this work 36

istic signals of α and β polymorphs, with a dominance of β phase varying from 0.90 to 0.98 (Figures S3 and S4). β phase is often associated with important properties of the polymer, such as ferro- and piezo-electric effects and, in particular, improved mechanical strength.40 These results are in agreement with our previous work,26 in which the use of a polar solvent (i.e., DMSO) and LIPS and VIPS methods favor the formation of the more thermodynamically stable β phase. The total enthalpy of melting reflects the contribute of the two components, it increases, in absolute value, by increasing the amount of PVDF, from −26 J/g for the pure PVDF-HFP to −77 J/g for the sole PVDF (Figure 1). In addition to chemical effects, the influence of the PVDF content on the crystallinity of blend membrane is also due to kinetic effects during phase separation. Increasing the amount of PVDF, solution viscosity increases as well (see Figure S5), with the consequent delay of the phase separation, which favors polymer crystallization through the slower solvent/nonsolvent exchange. These kinetic effects also influence the membrane morphology, including both skin and substructure formation,41 as a function of the blend ratio. The increasing of the solution viscosity causes the slower diffusion of the water vapor during the LIPS step, favoring the formation of crystallites, as assessed by SEM observation of the membrane surfaces (Figure 2). The SEM image of the pure PVDF-HFP membrane (Figure 2, first panel) showed a stretched and relatively smooth surface morphology. On the contrary, the membrane prepared using the pure homopolymer PVDF showed a rough surface structure with a spherulitic morphology indicative of partial polymer crystallization (Figure 2, last panel). These specific features are maintained also in the membrane cross section, with the presence of the crystallites along the length of the membrane thickness in the case of the PVDF polymer, and with the presence of smooth interconnected porous structures in the case of PVDF-HFP (Figure S6). Blend membranes with PVDF content equal or higher than 50% (samples C, D, and E) showed an open porous structure with dominant crystallite morphology on the surface. The number of crystallites is more significant as the content of PVDF increases. The spherulitic morphology of the PVDF membrane is indicative of a partial polymer crystallization, which mainly occurs during the VIPS stage,26 thus enhancing the roughness of the membrane surface (see below). The hierarchical substructure of the blend membranes is visible in SEM images obtained at high magnification, which highlight a typical morphology with raspberry- or cauliflowerlike microprotrusions (Figure 3). The multiscale hierarchical structures of blend membranes are associated with some peculiar physical properties. Enhanced surface roughness of intrinsically hydrophobic

For all the blend membranes, the value of χ1,2 (Table 2) was higher than χcr = 0.000442, indicating the immiscibility between the two polymers. Despite the two polymer components are thermodynamically not miscible and give phase separation at the molecular level, when mixed, they macroscopically look like a homogeneous system.25 This behavior could be also due to a higher affinity of the blended system for DMSO in comparison with the pure components, as evident from the lower difference of Hansen solubility parameters (Figure S1). DSC analyses experimentally confirmed the immiscibility of the two components, with the presence of two separated endothermic peaks in the thermograms of the blend membranes (Figure S2), assigned to the melting of the crystalline portions of the two individual polymers (Tm1= 169 for PVDF and Tm2= 128 °C for PVDF-HFP). A shift in the melting temperature of the two components was also observed in the blends with respect to the pure polymeric membranes (Figure 1). When the PVDF content in the blend membranes

Figure 1. (Left axis) Melting points of pure and blend membranes. (Right axis) Enthalpy of melting. The red- and green-dashed lines evidence the position of the pure homo- and copolymer membrane melting points, respectively.

increases, its melting temperature (Tm1) shifts to values closer to that of pure PVDF (from 165 to 167 °C, from A to E). Similarly, by increasing the fraction of PVDF-HFP in the blend membranes, Tm2 shifts to values closer to pure PVDF-HFP (from 131 to 129 °C, from E to A) (Figure 1). These shifts are indicative of the presence of intermolecular interactions between the homo- and the copolymer in the blend membranes,39 in agreement with the expectations for two quite chemically similar fluorinated polymers. Concerning the crystalline phases composition, ATR-FT-IR spectra of all of the prepared membranes show the characterD


Article


Figure 2. SEM (left) and 3D AFM height (right) images of the surface of the pure polymeric (PVDF or PVDF-HFP) and blend membranes (A− E). SEM images are taken at 5 K× magnification.

Figure 3. Particular of the up surface of the blend membranes C (a) and D (b).

surfaces is consistent with the increase of WCA, according to the Cassie−Baxter wetting theory.42 In the case of blend membranes, for PVDF content up to 67% (samples from A to D), the increase in WCA up to 138° is observed (Figure 4). However, with the further increase of the homopolymer content (sample E), a drastic decrease of surface roughness is observed, with the consequent reduction of WCA. This effect is due to the formation of a huge number of crystallization nuclei of smaller dimensions, which reduced consistently the average height of microprotrusions and the overall membrane surface roughness. This effect is even more pronounced for the

pure PVDF membranes, whose WCA is lower (131°). Despite the presence of HFP groups with higher fluorine content in the PVDF-HFP polymer, which are expected to improve the membrane hydrophobic character, contact angle is lower for PVDF-HFP (113°) than PVDF membranes (Figure 4). The presence of two different blocks in the PVDF-HFP copolymer determines a minor tendency of the polymer to crystallize, consequently, a smooth surface is obtained (Figure 2). Accordingly, lower contact angles were measured for the blend membranes with PVDF content

Exploiting Fluoropolymers Immiscibility to Tune Surface Properties

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