Evaluation of Gas Transport Properties of Highly Rigid Aromatic PI

Article pubs.acs.org/IECR

Evaluation of Gas Transport Properties of Highly Rigid Aromatic PI DPPD-IMM/PBI Blends José Manuel Pérez-Francisco, José Luis Santiago-García, María Isabel Loría-Bastarrachea, and Manuel Aguilar-Vega* Unidad de Materiales, Centro de Investigación Científica de Yucatán, A.C., Calle 43, No. 130, C.P., 97205 Mérida, Yucatán, México S Supporting Information *

ABSTRACT: In this work we discuss the gas transport properties behavior of dense blend membranes of a high free volume polyimide PI DPPD-IMM with PBI using the latter polymer in an attempt to enhance gas selectivity. Thermal and mechanical properties of dense blend membranes were also evaluated. The aim is to test the use of PBI as a gate polymer for establishing a window to enhance selectivity of CO2/CH4 and O2/N2 gas pairs with a minimum loss of gas permeability. Wide-angle X-ray diffraction measurements show that PI DPPD-IMM has an open structure with a d-spacing maximum at 5.4 Å, which is not affected when PBI concentration in the blends is lower than 25 wt %. This reflects a moderate loss of gas permeability as compared to PI DPPD-IMM. Blends with PBI concentrations 50 wt % or above presented a shift in WAXD toward 4.4 Å. It is found that as PBI concentration increases in the blend, there is an enhanced selectivity for O2/N2 and CO2/CH4 gas pairs duplicating their value with respect to PI DPPD-IMM and following closely the slope of the Robeson plot. Activation energies for permeation confirm that the PI DPPD-IMM microstructure is preserved at low PBI concentration since they barely increase for blends containing PO2 > PCH4 > PN2, a fact that has been reported before for polymers with high FFV, as well as polymers with intrinsic microporosity (PIMs).13,14,17,45 On the other hand, PBI membrane shows gas permeability coefficients where PHe > PCO2 > PO2, which is the order often found in glassy polymer membranes, and they are at least 2 orders of magnitude lower than those presented by PI DPPD-IMM. Permeability coefficients for N2 and CH4 were not measured for PBI and PI/PBI(25/75) membranes because they are below the minimum measurement limit of the permeation cell used. In membranes prepared from the PI/PBI blends, as the concentration of PBI increases in the membrane, the gas permeability coefficient diminishes. When blend membranes have 12.5 and 25 wt % of PBI, gas permeability coefficients have a general trend where PCO2 > PHe > PO2 > PCH4 > PN2, which is the same order of gas permeation followed by PI DPPD-IMM membrane, an indication that microstructure of the polyimide dominates the permeability in this concentration range. Gas permeability results are in agreement with WAXD, where the maximum on d-spacing locates between 5.4 and 5.1 Å when PBI concentration is ≤25 wt %. When PBI concentration in the membrane is equal to or greater than 50 wt %, the gas permeability coefficients reach values that are below 10% of the P of pure PI DPPD-IMM for O2, N2, CH4, and CO2. The gases with the highest drop in gas permeability coefficients are N2 and CH4, which contributes to increase on ideal selectivity for the gas pairs CO2/CH4 and O2/N2. There is also a change in gas permeability coefficients order which is related to gas kinetic diameter (PHe > PCO2 > PO2 > PN2 > PCH4) as reported for glassy polymer membranes. This drastic

structures. PI DPPD-IMM repeating unit structure is formed by a planar pyrene diimide ring with pendant phenyl groups (DPPD) linked to a biphenyl group (IMM) with ortho isopropyl and methyl substituents. The combination of these two structures results in a polymer with a low packing efficiency and high FFV.14 On the other hand, PBI is an aromatic polymer with a planar structure that has four nitrogen atoms per repeating unit; two of them bear H atoms bonded, leading to H-bonding in PBI, resulting in a high packing efficiency, high density, and low FFV.30 When PBI is blended with PI DPPDIMM, the resulting PI/PBI blend membranes show a linear correlation between density and polyimide volume fraction with a positive deviation when concentrations of polymer are quite similar. In the case of FFV, it was observed that the calculated values show a positive deviation from a semilogarithmic mixing rule, although they move parallel to this model. The mixing rule used10 (ln(FFVb) = ϕ1 ln(FFV1) + ϕ2 ln(FFV2)) is based on ϕi volume fraction of each component where FFVb is the blend fractional free volume and FFV1, FFV2 are the fractional free volumes of the homopolymers in the blend. Thus, as PBI concentration increases in the blend, the combination of linear PBI and high FFV polyimide induces a better packing of polymer chains, which in turn produces a density increase and FFV decrease in the PI/PBI blend membranes. 3.3. Gas Transport Properties. Gas permeability coefficients, P, at 2 atm upstream pressure and 35 °C measured by a constant volume/variable pressure method for pure PI DPPD-IMM, PBI, and blend membranes are presented in Table 2. PI DPPD-IMM has high permeability for all tested gases (He, O2, N2, CH4, and CO2). In particular, gas E

DOI: 10.1021/acs.iecr.7b02074 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 5. (a) Gas permeability and (b) diffusion coefficients as a function of inverse FFV for PI DPPD-IMM, PBI, and PI/PBI blend membranes at 2 atm and 35 °C. Solid squares represent experimental values.

Table 3. Apparent Gas Diffusion and Solubility Coefficients at 2 atm and 35°C of PI DPPD-IMM, PBI, and PI/PBI Blend Membranes diffusion coefficient (10−8 cm2/s)

solubility coefficient (10−2 cm3(STP)/(cm3 cmHg))

membrane constituents

O2

N2

CH4

CO2

O2

N2

CH4

CO2

PI DPPD-IMM PI/PBI (87.5/12.5%) PI/PBI (75/25%) PI/PBI (50/50%) PI/PBI (25/75%) PBI

55.4 36.4 21.6 5.7 0.8 0.03

16.0 11.0 6.1 1.0

6.1 5.9 1.9 0.14

28.5 24.3 7.0 1.3 0.16 0.02

1.4 1.9 1.4 1.2 1.1 2.3

1.3 1.6 1.2 1.1

5.3 4.2 4.1 6.2

16.0 16.8 24.8 24.9 18.9 12.3

permeation given lower diffusion coefficients and therefore lower gas permeability coefficients as compared to PI DPPDIMM; see Table 2. When PBI concentration is higher than 50 wt % in the blend, P and D for all gases decrease drastically (at least 1 order of magnitude) as FFV decreases, which agrees with the behavior that was reported by Robeson et al.53 Gas selectivity for O2/N2, CO2/CH4, and CO2/N2 shows an improvement with increasing PBI concentration. This can be explained by the lowering of FFV that affects the flow through the membrane of gas molecules with the larger kinetic diameters (dk), vs CH4, more than those with the smaller kinetic diameters such as CO2. In the case of CO2/CH4 gas pair (with dk = 3.8 Å for CH4 compared to dk = 3.3 Å for CO2), the selectivity increases around 150% with increasing PBI concentration while CO2 permeability coefficients drop below 10% of the one measured from PI DPPD-IMM. This behavior is similar to the one reported by Hosseini et al.28,54 where the presence of PBI blended with Matrimid increased the selectivity for all gas pairs reported. Apparent gas diffusion, D, and apparent solubility, S, coefficients are given in Table 3 for pure PI DPPD-IMM, PBI, and PI/PBI blend membranes at 2 atm upstream pressure and 35 °C. The membranes have the general trend for the apparent diffusion coefficients DO2 > DCO2 > DN2 > DCH4 as it is usually found in dense membranes due to FFV dependence as seen in Figure 5. Apparent solubility coefficients follow the order SCO2 > SCH4 > SO2 > SN2 where CO2 is the more soluble of all gases tested. However, an uncommon behavior was observed for CO2 solubility coefficients. First, in PI DPPD-

decrease of gas permeability coefficients is in agreement with the shift to lower values of d-spacing maxima in WAXD results (see Figure 2) which is ascribed to a drastic reduction of FFV. The decrease of permeability coefficients for all gases with increasing concentration of PBI agrees with the decrease in FFV observed in the PI/PBI blends as larger amounts of PBI are present (see Figure 4). This is in agreement with the fact that as reported by Morisato et al.,46 gas diffusivity (D) depends on free volume following the equation D = AD exp[−BD /FFV]

(6)

where AD and BD are characteristic constants of the polymerpenetrant system, which are independent of the penetrant concentration. Lee et al.47 suggested that solubility would not be a strong function of free volume, and therefore, the permeability coefficient (P) is related to fractional free volume by the equation P = A exp[−B/FFV]

(7)

where A and B are also characteristic constants of each polymer-penetrant system. It has been found in several polymer families,48−50 blends and copolymers37,51,52 that the gas permeability and diffusion coefficients follow closely FFV changes as indicated by eqs 6 and 7. Figure 5 shows the change in gas permeability and diffusion coefficients with inverse FFV for PI DPPD-IMM, PBI, and PI/PBI blend membranes. It can be observed that there is a sharp correlation between P and D with 1/FFV changes. This behavior indicates that PBI in the blends acts as a barrier reducing the FFV available for gas F


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Industrial & Engineering Chemistry Research IMM and the blend with 12.5 wt %, PBI solubility coefficients are quite similar for all gases, an indication that the membrane has a minimum change in its microstructure as observed in the WAXD maximum of the amorphous halo. As the concentration of PBI in the blend increases, up to 50 wt %, the solubility coefficients increase for the more condensable gas CO2 while those for CH4, O2, an N2 decrease or remain relatively constant. This behavior could be attributed to the formation of site for specific interaction with CO2 when PI DPPD-IMM is blended with PBI. These CO2-phile sites are the result of changes in microstructure when PBI concentration increases from 12.5 wt % to 50 wt %. Similar behavior was reported by Abdollah et al.55 for PEBAX 1657/P(VAc-co-DBM) blends. P(VAc-coDBM) concentration up to 50 wt % causes P and D of CO2 to diminish with respect to PEBAX 1657, while SCO2 increases more than 3 times. The nature of changes of CO2 solubility coefficients as PBI increases in blends with PI DPPD-IMM will have to be further examined since the S values could be better measured from direct sorption isothermal experiments to corroborate the values found in here from the ratio between P/ D measurements. The correlation between permeability (P) and selectivity (α) for the gas pairs O2/N2 and CO2/CH4 related to the most permeable gas in the form of Robeson plots is shown in Figure 6. It can be observed that PBI blended with polyimide PI DPPD-IMM enhances the selectivity in all gas pairs shown. Gas pair selectivity enhancement mimics closely Robeson line. This behavior is attributed to a dependence of gas permeability coefficients on the molecular diameter of the gases of interest following closely the slope of the Robeson plot related to 1/n that depends on gas kinetic diameter only and FFV as was reported on a prior analysis.53,56 Clearly, the O2/N2 and CO2/CH4 selectivity of the PI/PBI blend membranes increases compared to the individual polymer PI DPPD-IMM. However, the separation maintains the usual trade-off between permeability and selectivity. Thus, blending of PI/PBI allows control of the relationship between permeability and selectivity to find membranes of these rigid and thermally resistant polymers that maximize permeability and selectivity, unlike other blends with rigid polymers, where they tend to fall far away from the upper bound as the lowest permeability polymer concentration increases in the blends as can be seen in Figure 6 for cPIM-1/Torlon,33 Matrimid/PBI,28 and PIM-1/Matrimid.10 3.4. Effect of Temperature on Gas Permeability Coefficients. The gas permeability coefficients temperature dependence for PI DPPD-IMM, PBI, and PI/PBI blend membranes was examined between 35 and 65 °C at 2 atm upstream pressure. The activation energy for permeation was calculated from eq 8, which reflects an Arrhenius type relationship between P and T. ⎡ −E ⎤ P = P0 exp⎢ P ⎥ ⎣ RT ⎦

Figure 6. (a) O2/N2 and (b) CO2/CH4 separation performance of PI DPPD-IMM and PI/PBI blend membranes (red square) compared to PIM-1/Matrimid10 (pink circle), cPIM-1/Torlon33 (blue triangle), and Matrimid/PBI28 (black triangle) blends.

permeation, EP, follows the general trend CH4 > N2 > He > O2 > CO2. It is also found that EP is affected by PBI concentration as it is shown in Table 4 where membranes with higher PBI concentration show higher EP. For all gases, EP presents a minimum change when polyimide concentration is high as the PI homopolymer and the blend with PI 87.5 wt %, an indication that small quantities of PBI do not affect the microstructure of the membrane, as was discussed for WAXD and mechanical properties in previous sections. When PBI concentration is 25 wt % or larger, EP increases as the concentration of PBI rises in the blend, an indication that the addition of PBI increases the barrier for the gas to permeate in the blend, a behavior often found in glassy polymers that coincides with a diminishing FFV in the blend. 3.5. Prediction of Gas Permeability in PI/PBI Blend Membranes. As preparation of polymer blends ranks among one of the most cost-effective ways of upgrading the properties of existing polymers, it is very desirable to anticipate (to be able to predict) the values of physical properties of intended blends.59 One of these properties is gas permeability

(8)

where P0 is a pre-exponential factor, EP is the activation energy for permeation, R is the molar gas constant, and T is the temperature. Figure 7 is an Arrhenius plot of gas permeability coefficients as a function of temperature for He, O2, N2, CH4, and CO2 at 2 atm upstream pressure in PI DPPD-IMM and PI/PBI blend membranes. It can be observed that for all tested gases, the gas permeability coefficients increase when the temperature rises (eq 8).57,58 For each membrane, the activation energy for G


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Figure 7. Temperature dependence of gas permeability coefficients of PI DPPD-IMM and PI/PBI blend membranes for (a) He, (b) O2, (c) N2, (d) CH4, and (e) CO2 at 2 atm.

coefficients through blend membranes. There are several models reported that could be employed to estimate P in

polymeric blends. The simplest case for miscible blends can be described by eq 9.60 H


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where ϕ1cr and ϕ2cr are critical threshold percolation values of components 1 and 2, and T1 and T2 are the critical universal exponents for the components. For discrete spherical domains,

Table 4. Activation Energy for Permeation for He, O2, N2, CH4, and CO2 in Pure PI DPPD-IMM and PI/PBI Blend Membranes

T1 = T2 = 1.833

EP (kJ/mol) membrane constituents

He

O2

N2

CH4

CO2

PI DPPD-IMM PI/PBI (87.5/12.5) PI/PBI (75/25) PI/PBI (50/50) PI/PBI (25/75)

7.55 7.27 8.29 9.52 14.07

4.20 3.76 5.91 8.37 21.67

8.20 8.20 12.65 20.38

8.79 8.79 14.37 23.04

0.94 1.08 2.83 6.66 14.48

ln Pb = ϕ1 ln P1 + ϕ2 ln P2

ϕ1cr = ϕ2cr = 0.156

In the case of low concentration, ϕ1s = ϕ1

(when 0 < ϕ1 < ϕ1cr)

ϕ2p = 0

ϕ2s = ϕ2

(when 0 < ϕ2 < ϕ2cr)

In Figure 8, a comparison of PI/PBI blend permeability values predicted by these models is shown with the

(9)

where Pb, P1, and P2 are the permeability coefficients of the blend and pure polymers 1 and 2 respectively. ϕ1 and ϕ2 are the respective volume fractions of polymers 1 and 2. Phase separated blends are more complex as the blend composition changes when component 1 is the continuous phase or component 2 is the continuous phase.61 Several models have been proposed in order to explain the behavior of permeability in heterogeneous blends. The simplest models suppose an orientation of the domains and were proposed by Kofinas et al.62 They proposed expressions for P when microdomains are oriented parallel to gas permeation direction or when microdomains are oriented in series with respect to gas permeation direction. Parallel and series models represent the extremes of continuous or discontinuous phase structure. For the case where it is not possible to suppose an orientation of the microdomains, it is not expected that these models fit experimental values. One commonly used model for heterogeneous blends is Maxwell’s model,63 which assumes that spherical domains of one component are dispersed in a matrix of the other component (continuous phase). Another well-used correlation is the Bruggeman model (eq 10), wich is used in separated phase blends and corresponds to a random packing of dispersed and isometric particles.64,65 ⎡ ⎤3 Pd /Pc − Pb/Pc ⎥ Pb = Pc⎢ ⎢⎣ (1 − ϕd)(Pd /Pc − 1) ⎥⎦

ϕ1p = 0

(10)

When one phase is entirely the continuous phase at both ends of the composition range, the parallel, series, Maxwell, and Bruggeman models offer a reasonably well-fitted prediction.61 However, for intermediate composition ranges, all these models fail. Kolarik et al.66 proposed a model (equivalent box model, EBM) that employs a combination of parallel and series contributions to the phase separated blend. This model presents the best approach in the intermediate composition range for polymer blends:61 Pb = P1ϕ1p + P2ϕ2p +

(ϕ1s + ϕ2s)2 ϕ1s P1

+

ϕ2s

Figure 8. Comparison of experimental permeability coefficients for PI/PBI blends with theoretical model predictions at different volume fractions: (a) He; (b) CO2.

(11)

P2

where ϕ1p, ϕ2p, ϕ1s, and ϕ2p are defined by the expressions ϕ1p = [(ϕ1 − ϕ1cr )/(1 − ϕ1cr)]T1

experimental data obtained for He and CO2 as a function of polyimide volume fraction. O2, N2, and CH4 results were similar to the behavior of carbon dioxide, and they can be seen in the Supporting Information in Figures S1, S2, and S3, respectively. For all tested gases, the experimental values are in the region of heterogeneous models. For He, it can be observed

ϕ1s = ϕ1 − ϕ1p (12)

ϕ2p = [(ϕ2 − ϕ2cr )/(1 − ϕ2cr )]T2

ϕ2s = ϕ2 − ϕ2p (13) I


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This behavior is ascribed to the fact that PBI acts as a barrier to diffusion. Several blend models for gas permeability coefficients prediction in polymer blends were evaluated. Results showed that O2, N2, CH4, and CO2 experimental gas permeability coefficients for PI/PBI blends with

Evaluation of Gas Transport Properties of Highly Rigid Aromatic PI

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