Solution Casting Blending: An Effective Way for Tailoring Gas

Apr 19, 2019 - The permeation of permanent gases was measured at a feed pressure of 1 bar. The data were interpreted with theoretical macroscopic mode...
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Article Cite This: J. Phys. Chem. C 2019, 123, 11264−11272

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Solution Casting Blending: An Effective Way for Tailoring Gas Transport and Mechanical Properties of Poly(vinyl butyral) and Pebax2533 Gabriele Clarizia,† Franco Tasselli,† Cataldo Simari,‡ Isabella Nicotera,‡ and Paola Bernardo*,† †

Institute on Membrane Technology, ITM-CNR, Via P. Bucci 17/C, 87036 Rende, Cosenza, Italy Department of Chemistry and Chemical Technologies, University of Calabria, Via P. Bucci 14/D, 87036 Rende, Cosenza, Italy

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‡

ABSTRACT: Polymer blending is a suitable approach for tailoring the gasseparation properties of polymeric membranes. Films based on poly(vinyl butyral)/poly(ether-block-amide) (PVB/Pebax) blends were prepared by the solution casting and solvent evaporation technique. The films were characterized for miscibility, mechanical and spectral properties by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), tensile testing and Fourier transform infrared (FT-IR) spectroscopy. The permeation of permanent gases was measured at a feed pressure of 1 bar. The data were interpreted with theoretical macroscopic models. Miscible blends were obtained, as shown by thermal and mechanical testing, while specific molecular interactions between the two polymers were revealed by FTIR analysis. DMA and DSC results indicated that the two components were miscible, while the permeation tests provided data consistent with a homogeneous blend system due to the interfacial interaction between the phases. The developed PVB/Pebax blend membranes provided an optimized performance with a good combination of permeability, selectivity, thermal and mechanical stability.

1. INTRODUCTION Blending of two or more polymers with different properties represents a convenient, effective and low-cost method to obtain tailored properties. This approach is successfully adopted also in membrane technology.1 Typical membrane preparation techniques start from polymeric solutions; thus, the identified components of the blends should have some mutual solvents. Solution casting guarantees a molecular-level mixing, improving adhesion between the phases and, thus, the resulting mechanical properties.2 Many studies are devoted to novel membrane forming materials for gas separation applications. Membrane gas separation processes are energyefficient and low-cost solutions for a range of technologically relevant applications, such as hydrogen recovery, air separation, the recovery of volatile organics from gas streams and carbon dioxide capture.3 Pebax, consisting of polyether (PE) rubbery blocks and hard polyamide (PA) semi-crystalline segments, are elastomers with interesting properties as membrane materials. The soft PE blocks, owing to their high chain mobility, are gas-permeable, while the hard PA segments provide mechanical stability. Different types of the PE and PA segments and their relative ratio in the copolymers result in diverse properties. Studies on their sorption and permeation properties suggested strong interactions between the polar CO2 gas and the PE blocks in the copolymers.4,5 Pebax2533 is a hydrophobic grade with a good gas permeability owing to the large amount of poly(tetramethylene oxide) (PTMO, 80 wt %).4 Pebax © 2019 American Chemical Society

biocompatibility favors its application for medical uses (e.g., short-term implantation in humans and virus-proof surgical sheeting)6 and different studies focused on these polymers as membrane materials.7−10 Polyvinyl butyral (PVB) is the product of partially butyric aldehyde acetylated polyvinyl alcohol. As a result, PVB is a random copolymer of vinyl butyral and vinyl alcohol units. PVB is low-cost, nontoxic and tasteless polymeric material. PVB is recognized as an ecofriendly substitute for polyvinyl chloride11 and some grades have received food contact approval.12 Due to a high tensile strength, impact resistance, and transparency, PVB sheets are applied to produce windshields. PVB could have a great potential to be used as an antifouling membrane owing to the polar and hydrophilic vinyl alcohol unit. However, PVB is brittle and not suitable enough in the membrane forming property and permeability.13 Therefore, PVB is investigated in composites and composite membranes with the addition of selected nanoparticles.14 The alternative approach adopted in this work to improve the properties of PVB is represented by polymer blending. The present study focuses on blending Pebax2533 and PVB polymers in order to modulate mechanical and gas transport properties. Attractive aspects of these blends are represented by the use of an eco-friendly low-cost material as PVB and a Received: February 14, 2019 Revised: April 3, 2019 Published: April 19, 2019 11264

DOI: 10.1021/acs.jpcc.9b01459 J. Phys. Chem. C 2019, 123, 11264−11272

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recorded from 4000 to 650 cm−1, with a resolution of 4 cm−1 and averaged for each sample. Tensile tests were carried out at room temperature with a single column testing machine (Zwick/Roell, model Z2.5), having a 50 N load cell and pneumatic clamps. The crosshead speed was 10 mm/min and the grip separation distance was 20 mm. For samples with a very high maximum deformation, usually at low PVB content, one of the flat clamp surfaces was replaced by a rounded surface to avoid slipping of the sample. The average value and the standard deviation of the Young’s modulus, the break strength and the maximum deformation were determined on a series of at least 5 samples having a length of 10 cm and a width of 0.5 cm. The viscoelastic properties of the prepared films were studied by DMA on a Metravib DMA/25 instrument equipped with a shear jaw for film clamping. Rectangular-shaped samples (30 mm × 5 mm) were cut from the films. The experiment was carried out in a ramp temperature mode by applying a periodic sinusoidal strain to the sample and measuring the resultant force. In details, a dynamic strain of amplitude 10−4 at 1 Hz was applied from −100 to 150 °C with a heating rate of 2 °C/ min. The ratio between the loss (E″) and the storage (E′) moduli defines the damping factor (tan δ). The thermal properties of the membranes were determined by differential scanning calorimetry (DSC) on a Setaram DSC131 instrument. Prior to the measurements, the samples were hermetically sealed and cooled from room temperature to −100 °C using liquid nitrogen. The explored temperature range was −100 to 150 °C, at a scan rate of 10 °C min−1 and purging N2 gas. Melting and glass-transition temperature values were obtained from second scan thermograms. 2.4. Gas Permeation Tests. Gas permeation tests on the dense membranes were carried out at a feed pressure of 1 bar and 25 °C with single gases (H2, He, N2, O2, CH4, and CO2). A fixed volume/pressure increase instrument (Elektro & Elektronik Service Reuter, Germany) was used as reported elsewhere.16 Before each experiment, the membrane samples were carefully evacuated by a turbo molecular pump included in the apparatus. The membrane was exposed to the feed gas, and a pressure transducer in a calibrated volume at the permeate side monitored the pressure increase over time. The gas permeability (P) was obtained from the slope of the pressure curve at steady state conditions. It is expressed in Barrer [1 Barrer = 10−10 cm3 (STP) cm cm−2 cmHg−1 s−1]. The extrapolation of the linear portion of the pressure on the abscissa provides the gas time lag(θ). The diffusion coefficient, D, of each gas through the membrane can be evaluated as follows17

biocompatible thermoplastic poly(ether-b-amide) copolymer. In addition, they are both soluble in ethanol, a convenient green solvent.15 Dense films based on the novel Pebax/PVB blend system were prepared by solution casting. Their compatibility was investigated by Fourier transform infrared (FT-IR) spectroscopy and in a wide temperature range by thermal analysis (DSC) and dynamic mechanical analysis (DMA). The mechanical properties of the films were also evaluated in tensile testing. The prepared self-supported membranes were tested in gas permeation. Different macroscopic theoretical models were applied to interpret the observed performance of the blends in relation to the properties of the starting neat polymers.

2. EXPERIMENTAL SECTION 2.1. Materials. Polyvinyl butyral (PVB), Mowital 30 HH, was kindly provided by Kuraray Europe GmbH. This polymer grade has a high degree of acetalization, a content of polyvinyl alcohol of 11−14% and a content of polyvinyl acetate of 1−4%. Pebax2533, a block copolymer consisting of 80 wt % PTMO and 20 wt % PA (Nylon 12), was received in pellets from Arkema, Italy. The chemical structure of both polymers is reported in Figure 1. Pebax2533 is alcohol-soluble as well as PVB. Ethanol (absolute, VWR) was selected as solvent for both polymers.

Figure 1. Chemical structures for PVB and Pebax2533.

Gases for permeation tests were supplied by SAPIO, Italy at a minimum purity of 99.999%. 2.2. Membrane Preparation. The polymers were separately dissolved in ethanol at a concentration of 3 wt % and then mixed in different amounts. Ethanol is a convenient solvent for Pebax2533.16 The dissolution of Pebax2533 required the heating under reflux conditions for ca. 2 h, while PVB instantaneously dissolved in ethanol. The investigated PVB/Pebax ratios of the blends were 10/90, 20/ 80, 33/66, 50/50, 66/33, and 80/20 wt/wt. Films of the two neat polymers were prepared as well and used as reference. The isotropic membranes were prepared in flat sheet configuration by controlled solvent evaporation, casting the dope solution within a stainless steel ring placed onto a Teflon support. The casting ring was left partially covered at room conditions overnight to promote a slow solvent evaporation. Finally, the films were heated at 50 °C for a few hours. 2.3. Membrane Characterization. Scanning electron microscopy (SEM) was adopted to investigate the morphology of the prepared films. Before the analysis, the samples were cryo-fractured in order to observe the cross-section and coated with a thin film of gold by sputtering. Images were acquired by an EVO|MA 10 (Zeiss, Italy) microscope, working in highvacuum mode. Structural characteristics of pure PVB, Pebax, and their blends were investigated by using the FT-IR analysis in a spectrophotometer (Spectrum One, PerkinElmer), equipped with the attenuated total reflection device. Four scans were

D=

l2 6θ

(1)

The “solution-diffusion” transport model (P = DS), that is assumed in dense polymeric films, was applied for discussing the gas transport properties of membranes.18 Accordingly, the solubility coefficient (S) can be indirectly obtained. The ideal selectivity was calculated as the ratio of the individual permeability values for two gases and can be decoupled into solubility−selectivity and diffusivity−selectivity αA/B = PA /PB = SA /SB × DA /DB

(2)

Specimens having a circular effective area of 11.3 or 2.14 cm2 were used. As for the tensile testing, the film thickness was 11265

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Figure 2. SEM images of the cross-section of Pebax2533/PVB films.

where δD, δP, and δH are the dispersive (van der Waals), polar (related to the dipole moment) and hydrogen-bonding terms of the solubility parameter, respectively. The mutual affinity between polymer and solvent can be quantified by evaluating the “interaction distance” (Δδ), which is the difference of the solubility parameters (δ)19

measured with a digital micrometer (IP65, Mitutoyo), taking the average of multiple point measurements.

3. RESULTS AND DISCUSSION The prepared films were transparent and with thickness in the range of 20−60 micron. The main macroscopic feature of the blends containing PVB is their easy handling, differently from pure Pebax2533 that is very sticky at a thickness lower than 50 μm. This character is particularly interesting in applications where very thin films are required. On the other hand, PVBbased films are brittle, therefore limiting their processing and application. 3.1. SEM Analysis. SEM images of the cross-sectional morphology of the prepared membranes are presented in Figure 2. The morphological analysis evidences a quite homogeneous microstructure, without any phase separation, as macroscopically observed by the transparency of all films obtained blending Pebax2533 and PVB at different proportions. The cross-section created during membrane fracturing is smoother for PVB and increasingly rough as the Pebax concentration increases in the membranes. The absence of anisotropy through the cross-section of the films guarantees that the mechanical and gas permeation tests, which will be discussed in the following, are correctly representative of the intrinsic behavior of the materials. Table 1 reports the Hansen solubility parameters (HSPs) (δ) for the polymers and their solvent, as well as for two gases (CO2 and N2) δ 2 = δ D2 + δ P 2 + δ H 2

Δδ = [4(δ Dp − δ Ds)2 + (δ Pp − δ Ps)2 + (δ Hp − δ Hs)2 ]0.5 (4)

where subscripts p and s refer to the polymer and solvent, respectively. Solvents with small differences in the solubility parameters could dissolve the selected polymer easily. The same evaluation can be done considering the interaction distance for two polymers. The values reported for Pebax2533 were calculated with the group contribution method.20 Since Pebax are phase-separated block copolymers, PVB can selectively interact with one of the Pebax domains (PTMO or PA12). Therefore, Table 1 includes the solubility parameters for the individual Pebax blocks, showing a likely preferential interaction of PVB and the hard PA12 block. 3.2. FT-IR Analysis. The molecular interactions between PVB and Pebax were examined by using the FT-IR (Figure 3).

(3)

Table 1. HSPs of Pebax2533, PVB, Ethanol, and Some Gases solubility parameter (MPa)0.5 material

δD

δP

δH

δt

refs

Pebax2533 PA12 PTMO PVB ethanol water CO2 N2

17.6 18.5 16.2 18.6 15.8 15.5 15.7 11.9

7.6 8.1 3.3 4.4 8.8 16.0 6.3 0

6.8 9.1 2.2 13.0 19.4 42.3 5.7 0

20.3 22.2 16.7 23.1 26.5 47.8 17.9 11.9

21 19 21 22 19 9 9 9

Figure 3. FT-IR spectra of PVB, Pebax2533, and representative Pebax2533/PVB blend membranes. Transmittance vs wavenumber (cm−1). 11266

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The Journal of Physical Chemistry C The PA block in Pebax shows relatively sharp peaks at around 3300, 1640 and 1734 cm−1 that are attributed to the −N−H−, H−N−CO, and O−CO groups, respectively.23 The distinct peak at around 1100 cm−1 is assigned to the stretching vibration of C−O−C group within the PTMO segment in the neat Pebax. PVB presents several structural units: residual polyvinylalcohol and acetate group due to incomplete acetalization reactions. Vibrational bands at 1130 and 997 cm−1 can be assigned to the C−O−C stretching vibrations of the cyclic acetal of PVB. In both cases, there is a double peak indicating a free and an H-bond fraction for the same group. In addition, the PVB spectra present absorptions arising from acetalic functions (several peaks in the region between 1050 and 1150 cm−1). The band at ca. 1740 cm−1 relates to CO stretching vibration of acetate group. The peaks at ca. 1000 and 1245 cm−1 are attributed to C−O−C stretching vibrations of acetate group. The broad peak at around 3500 cm−1 in PVB (−OH asymmetric stretching vibration) indicated a significant amount of hydroxyl groups due to residual PVA. This peak is present at a lower wave number than usual (3600 cm−1), showing self H-bonding. These noncondensed OH groups provide the connection (hydrogen bonding) between PVB and Pebax. Indeed, the −OH peak is no more evident in the blends. The PTMO peaks are relatively unchanged for the blend membranes, indicating weak interactions between PVB and PTMO sections. On the other hand, the peak (1540 cm−1) due to the N−H deformation and C−N stretching of the PA block in Pebax24 changes its shape in the blends. A considerable amount of free amide contributing to the spectrum of Pebax disappears upon the addition of PVB, indicating hydrogen bonding with the PA segments in Pebax. At the same time, the peaks representative of the cyclic acetal group in neat PVB are of two types (peaks at ca. 1100 cm−1), while in the blends the free portion at larger wave number is lost. In the same region, there is the overlapping of the PTMO peak at 1100 cm−1 (C−O−C symmetric and asymmetric stretching). The band at ca. 1350 cm−1 (assigned to the amorphous content of PEO25) in pure Pebax is shifted to greater wave numbers in the blends. These results, together with a reduced signal for the −OH asymmetric stretching, clearly indicate distinct interactions between the two polymers. 3.3. Mechanical Properties. 3.3.1. Tensile Testing. The Young’s modulus (E), tensile strength, and elongation at break measured by tensile testing on the neat polymers and their blends at room temperature are shown in Figure 4. By increasing the PVB loading in the blends, the Young’s modulus continuously increases (Figure 4a), while the maximum deformation decreases (Figure 4b). The modulus shows a sharper increase when PVB becomes the predominant component. The experimental data for the Young’s modulus were compared to the predictions of different models. In particular, the parallel (eq 5) and series (eq 6) models correspond to the upper and lower limit for the elastic modulus, respectively, regardless of morphology E blend = ϕ1E1 + ϕ2E2

(5)

1/E blend = ϕ1/E1 + ϕ2 /E2

(6)

Figure 4. Mechanical properties of PVB, Pebax2533, and Pebax2533/ PVB blend membranes plotted in a semilog plot vs the PVB content. (a) Young’s modulus. (b) Break strength and maximum deformation.

where Eblend, E1, and E2 are the Young’s modulus of the blend and of neat components (1 and 2) and ϕ1 and ϕ2 are the respective volume fractions of the two polymers in the blend. The Barentsen approach26 combines the series and parallel models with the introduction of the concept of percolation, defining an equation for series model of parallel parts (eq 7) and another for a parallel model of serial linked parts (eq 8) Ea = Em[λ 2Ed + (1 − λ 2)Em] /[(1 − λ)λ 2Ed + (1 − λ 2 + λ 3)Em]

(7)

E b = (1 − λ 2)Em + (λ 2EdEm)/[λEm + (1 − λ)Ed ]

(8)

in which λ = ϕd, the volume fraction of the dispersed phase, while Em and Ed are the Young’s modulus of the continuous matrix and of the dispersed phase, respectively. The Maxwell model considers randomly distributed and non-interacting homogeneous spheres in a homogeneous medium27 3

E blend = Em ·

Ed + 2Em − 2ϕd(Em − Ed) Ed + 2Em + ϕd(Em − Ed)

(9)

The Davies model (eq 10), instead, represents macroscopically homogeneous and isotropic blends.28 It reproduces very well also situations in which the two phases present moduli that differ by a factor of 1000,29 as in the present case. E blend1/5 = ϕ1E11/5 + ϕ2E21/5

(10)

The Young’s modulus of the blends is comprised between the parallel and series model previsions (Figure 4a). However, 11267

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Figure 5. DMA graphs of PVB, Pebax2533, and representative Pebax2533/PVB blend membranes. (a) Storage modulus vs temperature; (b) tan δ vs temperature.

On the other side, the storage modulus of Pebax2533 is almost 1 order of magnitude lower than that of PVB and shows a net decreasing upon heating, already from −80 °C. This implies a rubbery state of the film, which progressively softens by heating due to a large number of structural transitions involving different polymer domains in the Pebax copolymer. In addition, above 25 °C, the excessive softening produces a very sticky membrane to the point that E′ was not more detectable and thus poorly suited for practical applications under mechanical stress. By controlling the blending ratio between the two polymers, however, the stiffness of the resulting film can be easily tuned: increasing PVB content, both storage modulus and thermal resistance of the blend increase, managing to ensure a good compromise between mechanical resistance and membranes flexibility. The temperature evolution of the tan δ reported in Figure 5b provides more insight about the thermophysical properties of the different membranes.32,33 Pebax2533 is characterized by an intense asymmetrical signal in the temperature region between −75 and −25 °C. As will be confirmed by the DSC data, in fact, in this range a large series of separated transitions occur in both PTMO and PA phases.34 Among these, the most significant are both amenable to the PTMO component, which is the predominant phase in the Pebax copolymer.35 The first one emerged at circa −65 °C can be ascribed to the glass transition of the PTMO domains, as also demonstrated by Boulares et al.36 Furthermore, at higher temperatures, the PTMO segments undergo to a partial cold crystallization, leading to the shoulder peak at −45 °C. Finally, just above −25 °C, the dumping capacity increases again, resulting from the melting of the PTMO segments. Concerning the blended membranes, subtle transitions emerged in the loss tangent of the 33/66 and 50/50 films. In both cases, in fact, the dumping capacity only slightly increases above −75 °C and again above 0 °C because of the polymer softening during heating. This provides further support about the absence of microphase separation between the components. The 80/20 blend film shows a tan δ profile very similar to that of pure PVB. In fact, the symmetrical peak at 57 °C clearly indicates an inward shift of the Tg for the PVB phase. Therefore, it can be concluded that, at low Pebax content, the interaction with the PA segments partially improves the chain mobility of PVB domains, leading to a lower Tg with respect to pure PVB (65 °C). 3.4. Thermal Properties. The blend miscibility was further analyzed by DSC analysis. Figure 6 shows the DSC patterns of the polymeric membranes under study, performed

the experimental data cannot be modeled with one single relation. This indicates a change in the morphology and structure depending on the blend composition.30 In particular, at low PVB loadings, the Young’s modulus was close to that predicted by a Barentsen series model, considering Pebax as the continuous matrix. Only at high concentrations of PVB, the moduli approached the Davies model prediction for cocontinuous polymer blends. The break strength is slightly decreasing at low PVB concentrations (Figure 4b), while at above 50 wt % PVB becomes the main component in the blend membranes and mainly contributes to the effective mechanical strength. The introduction of PVB into the Pebax matrix would destroy Pebax chain arrangement, leading to a slight decrease in the mechanical strength. The blending approach improves the processability of PVB, increasing the elongation at break. The Nielsen equation can be used to model the elongation at break for polymer composites and blends in the case of a good adhesion between the phases31 εb1/5 = ε1(1 − ϕ1)1/3

(11)

where εb is the elongation at break of the blend and ε1 is the elongation at break of the polymer constituting the matrix and ϕ1 is its volume fraction. Accordingly, the introduction of a dispersed phase into a matrix would cause a dramatic decrease in elongation at break. The Nielsen model, considering Pebax as the polymer constituting the matrix, well predicts this trend, indicating a good adhesion of the two polymers. 3.3.2. Dynamic Mechanical Analysis. DMA was employed to assess the miscibility of the prepared blends, following the transitions occurring as a function of temperature in the prepared films. The temperature dependence for the dynamic storage modulus (E′) and the loss tangent (tan δ) obtained from DMA is reported in Figure 5a,b, respectively. PVB film shows the highest modulus, reaching about 1.8 × 109 Pa, which remains almost invariable up to 65 °C, revealing high stiffness and good thermal resistance of this polymer. The peak at 70 °C displayed by tan δ (Figure 5b) is the glass transition of the PVB chains, which leads to a softening of the polymer segments, therefore resulting in a sudden fall down of E′. These values are in agreement with the elastic modulus evaluated at room temperature by uniaxial tensile testing for PVB (Figure 4a). Based on these findings, one aspect is worthy of being emphasized: the PVB chains keep their glassy state in a wide temperature range, leading to an excessive brittleness of the resulting membranes. 11268

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in Figures 7 and 8, respectively. The permselectivity of selected gas pairs that are relevant to many industrial processes is

Figure 7. Permeability of different gases through Pebax2533, PVB, and Pebax2533/PVB blend membranes. Figure 6. DSC thermograms of PVB, Pebax2533, and representative Pebax2533/PVB blend membranes. Second heating.

in the temperature range −100/150 °C, obtained during the second heating thermal scans. The thermogram profiles of the unblended PVB and Pebax2533 based-films are included as well. The PTMO and PA units in Pebax are both semicrystalline, while PVB is amorphous. Indeed, PVB is characterized by a single glass transition temperature (Tg) at circa 61 °C, while the Pebax2533 displays a more complex pattern. In the −80/ 40 °C temperature region, two features are detectable, likely amenable to the Tg of PTMO and PA domains. Strong support for this interpretation was provided in the discussion of the DMA analysis. At higher temperatures, two net endothermic peaks are recorded at about 10 and 50 °C, respectively, indicating the coexistence of soft blocks and strong crystals PTMO in the investigated Pebax2533 film; the first melting at 7 °C (TmI) and the second at 43 °C (TmII).37 The third minor endothermic peak observed at 118 °C is associated to the melting temperature of the rigid PA domains. All these findings indicate that the Pebax structure in the film state is characterized by a microphase separation.38 The solution blending of PVB and Pebax produces a strong change in the DSC profiles and thus in the structure, depending on the blend’s composition. The thermograms of the 33:66 and 50:50 blend ratios show only one small and sharp peak related to the melting of the PTMO soft blocks, at 15 and 20 °C, respectively. Noteworthy is the absence of the Tm(PA) signal. This indicates that the PVB chains preferentially interact with the PA12 hard segments of Pebax2533, corroborating what emerged from the FTIR investigation. At the same time, in the DSC pattern of both blends, a single glass transition temperature was detected, with a remarkable drop respect to the Tg of the neat PVB. The evidence confirms that Pebax2533 and PVB domains do not separate into segregated phase, pointing out the thermodynamic miscibility of the system. By further increasing the amount of PVB content until 80/20 blend ratio, a DSC thermogram similar to that of pure PVB was obtained. In this case, in agreement with the DMA results, a slight shift of the Tg of this blend emerged also from the DSC investigation: the PVB segments experience higher mobility upon interaction with the low PA fraction in the blend. 3.5. Gas Transport Properties. The gas permeation tests provided the permeability and diffusion coefficients as reported

Figure 8. Diffusion coefficient of different gases through Pebax2533, PVB, and Pebax2533/PVB blend membranes.

Table 2. Permselectivity of Pebax2533, PVB, and Pebax2533/PVB Blend Membranes PVB/Pebax

permselectivity (−)

wt/wt

CO2/N2

H2/N2

O2/N2

CO2/H2

0/100 10/90 20/80 33/66 50/50 66/33 80/20 100/0

27.8 27.2 26.6 25.2 24.9 23.3 19.9 15.1

5.03 5.63 6.11 6.99 10.2 14.6 18.5 30.2

2.62 2.64 2.70 2.80 2.90 3.20 4.00 4.10

5.52 4.84 4.35 3.60 2.44 1.60 1.07 0.50

summarized in Table 2. Permanent gases are molecular probes providing information on the membrane microstructure. Their permeation properties confirm the dense, poreless nature for all solution-cast films. The addition of Pebax2533 to the less permeable PVB progressively increased the gas permeability and the ideal selectivity for polar/non polar gas pairs as in the CO2/N2 case; instead, the selectivity for fast/slow gases (e.g., H2/N2) is better in PVB-rich membranes. Figure 7 shows a general reduction of the permeability for the different gases when the amount of PVB in the blends increases. This trend is analogous to that of the diffusion coefficients (Figure 8). Typically, N2 and CO2 present close 11269

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The Journal of Physical Chemistry C diffusion coefficients, while methane has the lowest values. The smaller gases, such as H2, were the more mobile in all the membranes. CO2 was the most soluble species among the gases tested in all films. Indeed, the HSP values listed in Table 1 demonstrate the preferential affinity of the selected polymers for CO2 versus N2 owing to the presence of polar groups into their backbone.39 Figure 7 clearly shows a predominant Pebax contribution in the majority of the blends that behave as “reverse-selective membranes”, preferentially separating larger molecules that are more soluble than smaller species.40 Indeed, the solubility contribution is prevalent in the Pebax films. As a result, the highly soluble CO2 molecule is the most permeable in Pebax, leading to an interesting CO2/H2 selectivity. On the other hand, the CO2/O2 permeation ratio of the prepared films meets the requirement for the creation of an ideal atmosphere to prolong the shelf life of fruits and vegetables in the case of food packaging.41 The main role of the diffusion, according to the gases molecular size, dictates the gas permeability order through the neat PVB membrane, as in amorphous glassy polymers. A closer analysis of the gas permeation properties, decoupling the contribution of diffusion and solubility to permeability, reveals a transition from a solubility-related behavior to a diffusioncontrolled one only at a very high PVB concentration. Analogously to the mechanical properties, the gas permeability through the blended membranes was interpreted by using different theoretical models. The parallel (eq 12) and the series models (eq 13) reproduce phase-separate systems Pblend = ϕ1P1 + ϕ2P2

(12)

1/Pblend = ϕ1/P1 + ϕ2 /P2

(13)

Figure 9. CO2 permeability plotted as a function of the PVB concentration in the PVB/Pebax2533 membranes. Experimental data (at 25 °C and 1 bar) and predictions of different theoretical models.

models are not valid to describe the Pebax/PVB system, particularly at low concentration of the PVB phase. Instead, the homogeneous model well describes the observed trend on the whole concentration range.1,42 Indeed, plotting the permeability data of the blend membranes on a logarithmic scale, the values aligned on a linear trend, according to the relationship proposed for homogeneous polymer blends (eq 13). Departures from the logarithmic form were observed for CO2 permeability in other miscible polymer blends. Blends of polysulfone and polyimide (PSF/PI) evidenced strong interactions of CO2 with the PI, resulting in a penalized gas permeation.43 No deviations for CO2 were observed in the PVB/Pebax samples (Figure 9). Therefore, gas permeation indisputably indicates a miscibility of the two materials agreeing with morphological, thermal and mechanical analyses.

where Pblend, P1, and P2 are the permeability coefficients of the blend and of neat components (1 and 2) and ϕ1 and ϕ2 are the volume fractions of the two polymers, respectively. For heterogeneous systems, considering one polymer as “dispersed phase” in a matrix of another polymer, the Maxwell model27 could be used to predict the effective permeability of the blend Pblend = Pm·

4. CONCLUSIONS Blends of PVB, a low cost material, with the Pebax2533 copolymer were successfully prepared by solvent mixing using ethanol, a green solvent. Defect-free dense films were obtained by means of solution casting, followed by controlled solvent evaporation. FT-IR spectroscopy indicates interactions of PVB with the PA12 domains of Pebax, which are confirmed by DSC thermal analysis in view of an inward shift of the glass transition temperature of PVB. Macroscopically, significant improvements in the flexibility and toughness were achieved for PVB by solution mixing it with a biocompatible thermoplastic poly(ether-b-amide) copolymer containing a high PE amount. The addition of Pebax elastomer acts as a plasticizer of the PVB matrix, resulting in a larger processability window of the stiff PVB, increasing the elongation at break and gas permeability. On the other hand, the presence of PVB in the blend significantly limits the sticky effect of Pebax, also in very thin films, enhancing the thermal resistance under mechanical stress, as evidenced by DMA. The results of the experiments and macroscopic modeling in the case of gas transport properties show a good agreement with the miscible blend permeability model in the whole concentration range. The tunable mechanical and gas permeation properties displayed by the novel Pebax/PVB blends are particularly interesting to produce high-performance multifunctional nanostructured polymer blends for different applications including flexible packaging.

Pd + 2Pm − 2ϕd(Pm − Pd) Pd + 2Pm + ϕd(Pm − Pd)

(14)

where Pm and Pd are the permeability of the continuous matrix and of the dispersed phase, respectively, while ϕd represents the volume fraction of the dispersed phase. Instead, the gas permeability of binary miscible blends can be predicted by the logarithmic additive model as follows1,42 ln Pblend = ϕ1 ln P1 + ϕ2 ln P2

(15)

This analysis was carried out for CO2, plotting its permeability as a function of the PVB content in the membranes (Figure 9). The CO2 permeability data in the prepared blends are upper than those estimated by a series model. On the other hand, the parallel model predicted a greater permeability than that measured. The Maxwell models restrict the existence area for the blends. The experimental data are located between the equation that considers PVB as continuous phase and the same model based on Pebax as continuous phase. At high PVB concentration, the data became closer to those estimated by the Maxwell model with PVB as continuous phase. These results show that “separate phase” 11270

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Paola Bernardo: 0000-0003-0370-6319 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Arkema Italy is gratefully acknowledged for providing the Pebax2533 pellets. Kuraray Europe GmbH is gratefully acknowledged for providing the PVB 30HH powder. The authors thank Dr. G. Chiappetta, ITM-CNR, Italy, for the SEM analyses.

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