Membranes from rigid block hexafluoro copolyaramides: Effect of block lengths on gas permeation and ideal separation factors

Journal of Membrane Science 443 (2013) 36–44

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Membranes from rigid block hexafluoro copolyaramides: Effect of block lengths on gas permeation and ideal separation factors Maria I. Loría-Bastarrachea, Manuel Aguilar-Vega n Materials Unit, Centro de Investigación Científica de Yucatán A.C., Calle 43 No. 130, Col. Chuburna de Hidalgo, C.P. 97200, Mérida, Yuc., Mexico

art ic l e i nf o

a b s t r a c t

Article history: Received 26 February 2013 Received in revised form 16 April 2013 Accepted 20 April 2013 Available online 29 April 2013

Dense membranes were prepared from three different rigid block copolyaramides, one block bearing two bulky hexafluoro (–CF3) groups and a lateral tert-butyl group (–C–(CH3)3), and the second block without the lateral tert-butyl group. The effect of block length, at constant comonomer concentration, on thermal properties as well as gas permeability coefficients and separation factors is analyzed. The results indicate that block copolyaramide membranes present a density that is quite similar but slightly lower, as the length of the blocks that form the copolymer increase, that falls in between the density of the homopolyamides. The fractional free volume, FFV, increases in the block copolymers as the block length increases. This result is attributed to an inefficient packing of the copolymer molecules as the block length gets larger. As a result, the permeability and diffusion coefficients in the block copolymers are larger than those in the parent homopolymers. The gas separation factors remain with a minimum change even though there is a gain in gas permeability; therefore, block copolymerization using highly rigid blocks, due to differences in packing, presents the advantage of a higher gas permeability coefficient with a minimum loss in selectivity. The rigidity of these copolymers presents advantages for high temperature applications. & 2013 Elsevier B.V. All rights reserved.

Keywords: Gas permeability Aramides Gas separation factors Block copolymers Fractional free volume

1. Introduction Gas separation by membranes has been the focus of attention for some time and it has been clearly identified that the ideal membrane should have high gas permeability and a high separation factor. However, it is well known that for membranes prepared by polymeric materials there exists a trade off since those membranes that present high permeability present a low separation factor while those presenting low permeability possess high separation factors. Therefore, there is still interest in developing materials that will present high selectivity for a specific gas or gas pair with a high permeability above the upper bond value as discussed by Robeson and others [1–3]. The relationship between gas transport properties and the structure of different polymers has been studied in a systematical way for several rigid aromatic polymer families such as polysulfones [4–6], polycarbonates [6,7] polyesters [8–10], polyimides [11], polynorbornene dicarboxyimides [12,13] and polyamides [14–17] among others. In these highly aromatic polymers, it has been established that bulky pendent groups from the main chain will increase gas permeability while maintaining the selectivity for several particular gas pairs by hindering structural rotations in amorphous rigid

n

Corresponding author. Tel.: +52 999 942 8330; fax: +52 999 9813900. E-mail addresses: [email protected], [email protected] (M. Aguilar-Vega).

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.04.042

polymers [18,19]. For some time now copolymerization has been recognized as an important tool for the development of materials with specific characteristics [20,21]. Copolymerization offers the opportunity to prepare polymeric materials with properties tailored in the desired direction. Thus, the introduction of an appropriate second monomer in the polymer, or sometimes a third one, has been used to tailor material properties such as degree of crystallinity, thermal stability, elastic modulus, glass transition temperature and gas transport and gas separation properties [14,15,20–25]. In particular for gas transport and separation properties there have been reports on the effect of random copolymerization on gas transport properties of aromatic rigid copolyamides [14,15] and copolyesters [8,9]. Random copolymerization of rigid aromatic copolymers with two different monomers results in properties such as glass transition temperature, Tg, elastic modulus, E, permeability coefficients and gas separation factors that can be predicted with a simple mixing rule involving the comonomers concentration present in the particular copolymer. Gas permeability coefficients and ideal gas separation factors, in particular, follow the additive rule that depends on comonomer concentration in the particular copolymer. Thus, using the additive rule it is possible to determine gas permeability and separation factors in random copolymers. On the other hand, it is not possible to enhance selectivity for a given gas pair, since the ideal separation factors fall in between those of the homopolymers. It is also seen

M.I. Loría-Bastarrachea, M. Aguilar-Vega / Journal of Membrane Science 443 (2013) 36–44

4,4′-(hexafluroisopropylidene) dianiline and 5-tert-butylisophthalic acid, HFA/TERT, while the second block is built from 4,4′(hexafluoroisopropylindene) dianiline and isophthalic acid, HFA/ ISO. The resulting block copolyamides named HFA/TERT-b-HFA/ISO were prepared by solution polycondensation in two steps in a 50% molar ratio of HFA/TERT to HFA/ISO with three different block lengths (9:9, 12:12 and 18:18). The block composition was regulated by a stoichiometric imbalance based on the Carothers equation [36].

that the trade off, which implies that an increase of gas permeability gives place to a decrease in selectivity, is maintained. A possible option to achieve higher permeability coefficients with higher separation factors is the use of block copolymers since they present differences in the microstructure that could favor the permeation of one gas over the other, opening the opportunity to tune up gas permeation and gas separation by controlling the microstructure of the copolymer phases. Gas permeability measurements in block copolymers have been reported in polystyrene–polybutadiene block copolymers with an oriented lamellar structure by Csernica et al. [26] who found that gas permeability coefficients depend strongly on the direction of soft to rigid lamellar microstructure. In this arrangement permeation was well described by a three phase model that incorporates the interfacial regions of the block copolymer [27]. A similar behavior in poly (styrene-b-ethylene oxide-b-styrene) triblock copolymers was observed by Patel and Spontak [28]; they observed a high affinity to CO2 attributed to the ethylene oxide block presence. The affinity to CO2 was also found in some polymers with ethylene oxide blocks by Okamoto et al. in segmented polyether imides with soft and rigid microphases [29]. Kim et al. [30] and Bondar et al. [31] also determine the affinity of polyether segments in polyamide-bethylene oxide block copolymers PEBAX towards CO2. Recently, Reijerkerk et al. [32] reported the use of blends and copolymers containing PEO and its strong ability to interact with CO2 to increase the separation of this gas from other gases. It also appears that poly(ether-b-amide) thermoplastic elastomers decrease their gas permeability significantly, up to 3.5 times, upon uniaxial orientation due to phase orientation as reported by Armstong et al. [33]. In all cases the reported gas transport properties involve the presence of a soft phase in the block copolymer. For applications such as oxygen enriched air for combustion engines, or separation of combustion gases there are a couple of studies: one from cardocopolybenzoxazol membranes [34], and one from rigid block copolymers [35]. The large rigid blocks and cardo moieties help to increase permeability with little or no loss in selectivity for gas separation. This could be an interesting route to increase the productivity of high temperature membranes without losing the permeability. In this work we describe the preparation of three different rigid block copolyaramides bearing two bulky hexafluoro (–CF3) groups and a lateral tert-butyl group, (–C-(CH3)3), and the second block without the lateral tert-butyl group. The effect of block length, keeping the comonomer concentration constant, on thermal properties as well as gas permeability coefficients and gas separation factors is analyzed. The polyamide blocks are built from two similar aramides, the first one that is obtained from the combinations of

2. Experimental 2.1. Reagents The monomers 4,4′-(hexafluoroisopylidene) dianiline (HFA), the aromatic dicarboxylic acids: isophthalic acid (ISO), and 5tert-butylisophthalic acid (TERT) were purchased from Aldrich Chemical Co. and were used without further purification. The chemical structures of these monomers are shown in Table 1. Anhydrous calcium chloride (Baker) was dried under vacuum at 180 1C for 24 h before use. The reagents N-methyl-2-pyrrolidinone (NMP, 99.5% Aldrich), pyridine (Py, 99% Aldrich), triphenyl phosphite (TPP, 97% Aldrich), N,N-dimethylacetamide (DMAc, 99.8% Aldrich) and methanol (MeOH, 99% Baker) were used as received. 2.2. Synthesis of polyaramides Polyaramides HFA/TERT and HFA/ISO were synthesized by direct polycondensation of the 4,4′-(hexafluoroisopylidene) dianiline (HFA) with the appropriated aromatic dicarboxylic acid, using the method reported by Yamazaki that was described in previous works [14,15]. The reaction was carried out at 1:1 mol ratio between the diamine and the diacid as described in Table 2. 2.3. Preparation of HFA/TERT block segment and HFA/ISO block segment The acid terminated HFA/TERT blocks were obtained by the reaction of 1.25 mmol of diamine HFA with different amounts of diacid TERT. The exact feed ratios are given in Table 2. In a typical block preparation, a three neck 50 mL round bottom flask equipped with mechanical stirrer and nitrogen inlet was charged with HFA and TERT monomers and CaCl2 (15% w/w), 3.2 mL of NMP, 0.8 mL of triphenyl phosphite and 0.8 mL of pyridine. Then, this reaction mixture was heated to 100 1C for 3 h under nitrogen (N2) atmosphere. At the same time, the diamine terminated HFA/ISO blocks were obtained by the reaction of 1.25 mmol of isophthalic acid (ISO)

Table 1 Monomersa used in the synthesis of polyaramides and HFA/TERT-b-HFA/ISO block copolyaramides. Diamines

Structure

4,4′-(hexafluroisopropylidene) dianiline (HFA)

Purity 98%

CF3 H2N

C

NH2

CF3 Diacids 5-tert-butylisophthalic acid (TERT)

Isophthalic acid (ISO)

37

Structure

HOOC

HOOC

Purity

COOH

COOH

98%

99%

38

M.I. Loría-Bastarrachea, M. Aguilar-Vega / Journal of Membrane Science 443 (2013) 36–44

Table 2 Feed ratio of monomers for preparation of polymers and prepolymers for block copolymers, r values and calculated degree of polymerization (Xn) of the blocks. Polymer

HFA/TERT HFA/ISO HFA/TERT-b-HFA/ISO 9 HFA/TERT-b-HFA/ISO 12 HFA/TERT-b-HFA/ISO 18

HFA/TERT block

HFA/ISO block a

HFA (mmol)

TERT (mmol)

r /Xn (m)

HFA (mmol)

ISO (mmol)

ra/Xn (n)

1.25 – 1.25 1.25 1.25

1.25 – 1.5062 1.4227 1.3440

1/– 0.8299/9 0.8786/12 0.9299/18

– 1.25 1.5062 1.4227 1.3440

– 1.25 1.25 1.25 1.25

– 1/0.83/9 0.88/12 0.93/18

HFA/TERT block carboxylic acid terminal end groups and HFA/ISO block amine terminal end groups. a

The stoichiometric imbalance, r, was calculated using p ¼ 0.98 and the Carothers equation: X n ¼ ð1 þ rÞ=ð1 þ r−2rpÞ [36].

with different amounts of 4,4'-(hexafluoroisopropylidene) dianiline (HFA). The exact feed ratios are given in Table 2. The reaction was carried out in a three neck 50 mL round bottom flask equipped with a mechanical stirrer and nitrogen inlet that was charged with monomers, HFA and ISO in the desired ratio, and CaCl2 (18% w/w), 2.8 mL of NMP, 0.7 mL of triphenyl phosphite and 0.7 mL of pyridine. The reaction system was heated to 100 1C for 3 h under N2 atmosphere. 2.4. Synthesis of block copolyaramides After 3 h of reaction for the preparation of HFA/TERT and HFA/ ISO block segments separately, the HFA/TERT block was transferred quickly to the reaction vessel of HFA/ISO. An additional amount of hot NMP (1 mL) was used for complete transfer of HFA/ TERT block segment to the HFA/TERT block containing flask to avoid losses. At this point, an additional amount of anhydrous CaCl2 (5% w/w) was added in the new reaction system. The reaction was continued for another 20 h at 100 1C under N2 atmosphere. Finally, the copolymer solution was precipitated in 400 mL of methanol. The fibrous product obtained was collected by filtration and then washed several times with methanol. The block copolyaramide as obtained was dried in a vacuum oven at 100 1C for 24 h. 2.5. Film preparation Films of the polyaramides and block copolyaramides were prepared by solvent casting from solutions containing 0.5 g of the polymer in 6 mL of dimethyl acetamide. These solutions were filtered and poured into aluminum rings on the surface of an aluminum foil and the solvent was eliminated slowly at 70 1C. The membranes obtained were vacuum dried at 180 1C for 24 h and an additional drying step was performed at 220 1C for 48 h to eliminate completely the solvent. 2.6. Characterization The solubility of each polyaramide and copolyaramide was tested in different organic solvents at 25 1C, using 0.1 g of the polymer in 5 mL of solvent; chloroform (CHCl3), 1,1,2,2-tetrachloroethane (TCE), tetrahydrofuran (THF), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc) and N-methyl-pyrrolidinone (NMP) were used. The density of each polyamide and block copolyamide films was measured using the density gradient column method, where the gradient was established by aqueous calcium nitrate solutions in the range between 1.25 and 1.35 g/cm3 at 23 1C. X-ray diffraction (XRD) measurements were performed on a Siemens D-5000 X-ray diffractometer equipment using Cu-Kα radiation (λ ¼1.54 Å), at 40 kV and 15 mA. The measurements

were between 01 and 601 2θ at a scanning rate of 0.5 deg/min in samples of the polymer films. Thermogravimetric analysis of polyaramides and HFA/TERT-bHFA/ISO block copolyaramides was carried out using a thermogravimetric analyzer TGA-7 (Perkin-Elmer), at a heating rate of 10 1C/min, between 50 and 800 1C under nitrogen atmosphere. Dynamic mechanical properties of the as cast polymer and block copolymer membranes were studied, in particular comparison of differences in damping factor, Tan δ¼E″/E′, where E″ is the loss modulus and E′ is the storage modulus, which is a measure of the ability of the polymer chains to relax; it is related to the ability of the polymer molecules to dissipate or store energy and acquire mobility. These molecular changes are related to the same processes that give rise to the glass transition temperature and mark the characteristic onset between the glass and rubber transition of the polymer. The dynamic mechanical properties of the polyaramides and HFA/TERT-b-HFA/ISO block copolyaramides as a function of temperature were determined in film strips of 15 mm  2.2 mm (length  width), and thickness in the range 0.15–0.19 mm. The analysis was carried out in a dynamic mechanical analyzer, DMA-7 (Perkin-Elmer), using the extension mode between 50 and 320 1C, at a heating rate of 5 1C/min and at a frequency of 1 Hz under nitrogen atmosphere. Gas permeability coefficients, P, for five different pure gases, helium (He), oxygen (O2), nitrogen (N2), methane (CH4), and carbon dioxide (CO2), were determined in a gas permeation cell using the constant volume method as described elsewhere [37]. The gases used in the study were obtained from Praxair Corp. with purities 499.99%. Gas permeability coefficients were determined under steady-state conditions at different upstream pressures (2, 5, 7.5 and 10 atm). The measurements were made at 35 1C for each pure gas. For safety reasons, O2 permeability coefficients were not determined at pressures higher than 5 atm. The transport of gases in glassy polymers is known to occur through a solution-diffusion mechanism. Under steady-state conditions, the mechanism for gas permeation and diffusion is that the gas molecules dissolve at the high pressure side, diffuse through the membrane due to a concentration gradient, and reemerge at the low pressure side. Following Fick's law and relating the gas-phase pressure to the concentration inside the polymer through a thermodynamic relationship, such as Henry's law, one can easily derive Eq. (1), which describes the flux of a gas through a membrane, resulting from a pressure difference over the membrane: J¼

P Δp l

ð1Þ

where J is the gas flux through membrane ðcm3 ðSTPÞ=cm2 sÞ, l the membrane thickness (cm), P the permeability coefficient (Barrer) and Δp the pressure difference over the membrane (cm Hg).

M.I. Loría-Bastarrachea, M. Aguilar-Vega / Journal of Membrane Science 443 (2013) 36–44

The gas permeability coefficient, P, is known to be the product of diffusion (D) and solubility (S) coefficients: P ¼ DS

ð2Þ

The apparent diffusion coefficient, D, was estimated by the time-lag (θ) method under steady-state conditions, at a pressure differential of 2 atm, applying the following equation: 2

D¼

l 6θ

ð3Þ

where l is the membrane thickness and θ the time-lag. Apparent diffusion coefficients for helium were not estimated because the permeation of this gas is too fast to be measured by the time lag method under the testing conditions (membrane area 0.785 cm2 and 95 μm thickness). The apparent solubility coefficient, Sapp ððSTPÞcm3 =cm3 cmHgÞ; is obtained from the ratio of permeability coefficient to the apparent diffusion coefficient at 2 atm pressure difference: Sapp ¼

P D

39

length of the block increases, a properly differentiated shoulder of the HFA/ISO block at 250 1C is increasingly present, as well as the α-relaxation peak near that of HFA/TERT. For the n ¼m block containing 9 repeating units the maximum in the α-relaxation peak appears at 303 1C corresponding to the HFA/TERT block. The number of repeating units in the block increases the shoulder for the β-relaxation in HFA/ISO and the maximum tends to be more characteristic and appears near the ones of each corresponding aramide. Thus the copolymer with 18 repeating units in each block presents a well defined tan δ at 307 1C, the same temperature of HFA/TERT aramide, and a noticeable shoulder at 250 1C corresponding to HFA/ISO aramid block, an indication that two blocks of the copolymers are present in the expected structure. It is also clear that the aramides HFA/TERT and HFA/ISO show a well defined α-transition with a maximum at 307 1C and 293 1C respectively.

ð4Þ

The ideal separation factors (selectivity) of polyaramides and copolyaramide block copolymers membranes were calculated from the ratio of pure gas permeability coefficients applying the following equation: αA=B ¼

PA PB

ð5Þ

where PA and PB are the permeability coefficients of the pure gases A and B respectively.

3. Results and discussion 3.1. Rigid block copolyaramides characterization The block copolyaramides and their homopolymer aramide membrane films were cast by evaporation of DMAc solutions. The resulting transparent slightly yellow films were tough and rigid. Fig. 1 shows the structure of the block copolyaramides HFA/ TERT-b-HFA/ISO. In all block copolymers, the actual molar concentration of the blocks HFA/TERT and HFA/ISO was kept constant, n ¼m, while each block was prepared with 9, 12 and 18 aramide repeating units and combined with the other block with the same number of repeating units. In the prepolymer preparation and just before the HFA/TERT and HFA/ISO blocks were combined the only difference was a small increase in viscosity as the length of the blocks increased since the 9 units block was more fluid than the 18 unit aramide block prepolymer. Dynamic mechanical analysis thermograms showing tan δ as a function of temperature are given in Fig. 2 for the block copolyaramides, and HFA/TERT and HFA/ISO aramides. In order to make the comparison easier, the thermograms have been shifted by 10 units with respect to the one preceding starting from HFA/TERT. As observed in this figure, tan δ for HFA/ISO has a shoulder at 250 1C labeled as β-relaxation and presents a maximum attributed to its α-relaxation at 290 1C. For HFA/TERT tan δ presents a α-relaxation peak with a maximum at 307 1C that is well defined without shoulders. For the block copolymers HFA/TERT-b-HFA/ISO as the

Fig. 2. Tan δ as a function of temperature from dynamic mechanical analysis for HFA/TERT, HFA/ISO and HFA/TERT-b-HFA/ISO block copolymers with different block lengths.

Fig. 3. X-ray diffraction patterns for HFA/TERT, HFA/ISO and HFA/TERT-b-HFA/ISO block copolymers, with increasing block length between 9 and 18 repeating units.

Fig. 1. Structure of HFA/TERT-b-HFA/ISO block copolyaramides; m and n are the block lengths. For these copolymers n¼ m.

40

M.I. Loría-Bastarrachea, M. Aguilar-Vega / Journal of Membrane Science 443 (2013) 36–44

HFA/ISO presents a sub-Tg transition, Tβ, at 250 1C. The block copolymers show clearly the sub-Tg transition at 250 1C attributed to the HFA/ISO block and the α-transitions between 303 and 307 1C attributed to the HFA/TERT block. Fig. 3 shows the X-ray diffraction patterns for the homopolymers, HFA/TERT and HFA/ISO, as well as those from the block copolymers with different block lengths. All of them show an amorphous halo with a maximum at 171 2θ for HFA/TERT and a shoulder around 231 2θ, while HFA/ISO shows a similar maximum at 17 and 241 2θ. For the block copolyaramides as the size of the blocks increases between 9 and 18 blocks the amorphous pattern tends to differentiate to show the maximum at 171 and a shoulder

at 241 2θ. This is an indication of the blocky nature of the copolymers. Fig. 4 shows the thermograms for thermal decomposition of HFA/ISO, HFA/TERT and block copolymers HFA/TERT-b-HFA/ISO, that have been displaced in order to make it easier to see the differences among them. As observed, in this figure HFA/TERT presents an onset of decomposition at 450 1C while HFA/ISO shows an initial loss around 2% at 400 1C and the onset of decomposition at 450 1C. HFA/TERT-b-HFA/ISO block copolymers fall in between the values of the homopolymers. Table 3 summarizes the thermal and the physical properties of HFA/TERT, HFA/ISO and the block copolymers with a 50 mol% content of each block. As can be

Fig. 5. Ideal separation factors for HFA/TERT, HFA/ISO and HFA/TERT-b-HFA/ISO block copolymers at 2 atm and 35 1C.

Fig. 4. Thermal decomposition under nitrogen atmosphere of HFA/TERT, HFA/ISO and 50 mol% HFA/TERT-b-HFA/ISO block copolymers.

Table 3 Thermal transitions, thermal stability and density of HFA/TERT, HFA/ISO and HFA/ TERT-b-HFA/ISO block copolyamides. Polymer

Weight loss at 500 1C (%)

HFA/TERT HFA/TERT-b-HFA/ISO 18 HFA/TERT-b-HFA/ISO 12 HFA/TERT-b-HFA/ISO 9 HFA/ISO HFA/TERT-co-HFA/ISO randoma

8.4 13.5 12.5 12.6 14.5 10.0

a b

Tβ (1C) – 245.8 256.0 260.8 240.6 –

Tα (1C)

307 307 305 303 293 300

ρ (g/cm3) FFVb

1.305 1.346 1.348 1.349 1.422 1.360

0.156 0.1619 0.1606 0.1600 0.144 0.150

Fig. 6. Pure gas permeability coefficients as a function of 1/FFV at 35 1C and 2 atm for HFA/TERT, HFA/ISO and HFA/TERT-b-HFAISO block copolymers.

Values reported in Ref. [14]. FFV calculated from group contribution of Van Krevelen [37].

Table 4 Pure gas permeability coefficientsa and ideal separation factors of HFA/TERT, HFA/ISO and HFA/TERT-b-HFA/ISO block copolyamides at 2 atm upstream pressure and 35 1C. Membrane

HFA/TERT HFA/TERT-b-HFA/ISO 18 HFA/TERT-b-HFA/ISO 12 HFA/TERT-b-HFA/ISO 9 HFAISO HFA/TERT-co-HFA/ISO randomb a b

Permeability, P (Barrer)a

Selectivity αA/B ¼PA/PB

He

CO2

O2

CH4

N2

66.43 154.82 86.93 90.68 32.46 45.70

26.82 70.93 37.94 40.09 7.60 11.97

5.88 15.43 8.59 8.27 1.68 2.86

0.86 2.13 1.09 1.01 0.18 0.33

1.16 3.06 1.61 1.62 0.28 0.51

Permeability values are expressed in Barrer ¼ 1  10−10 ⌊cm3 ðSTPÞcm=cm2 s cm Hg⌋ [20,21]. From Ref. [14].

CO2/O2

CO2/CH4

CO2/N2

O2/N2

4.56 4.59 4.41 4.84 4.52 4.18

31.18 33.30 34.80 39.69 42.22 36.27

23.12 23.17 23.56 24.74 27.14 23.47

5.07 5.04 5.33 5.10 6.00 5.60

M.I. Loría-Bastarrachea, M. Aguilar-Vega / Journal of Membrane Science 443 (2013) 36–44

seen, the aromatic polyamides and block copolyamides present high thermal resistance with weight losses at around 12 wt% at 500 1C under nitrogen atmosphere for all of them, with the block copolymers showing values intermediate between the

41

homopolymers and closer to those presented by HFA/ISO. Since the block copolymers have the same molar composition, differences in thermal stability between them are minimal. As a reference, Table 3 shows the values reported before for a random

Table 5 Pure gas diffusion coefficients and ideal separation factors of HFA/TERT, HFA/ISO and HFA/TERT-b-HFA/ISO block copolyamides at 2 atm upstream pressure and 35 1C. Membrane

HFA/TERT HFA/TERT-b-HFA/ISO 18 HFA/TERT-b-HFA/ISO 12 HFA/TERT-b-HFA/ISO 9 HFAISO HFA/TERT-co-HFA/ISO randoma a

D  108 (cm2/s)

Diffusivity selectivity αDA/B ¼DA/DB

CO2

O2

CH4

N2

CO2/CH4

CO2/N2

O2/N2

3.35 52.71 31.95 16.90 1.43

8.10 268.2 65.97 46.15 3.20

0.39 8.29 3.88 2.51 0.13

1.61 30.14 16.04 13.44 0.77

8.59 6.35 8.23 6.73 11.00

2.08 1.74 1.99 1.25 1.85

5.03 8.89 4.11 3.43 4.15

2.00

5.70

0.17

1.31

11.76

1.52

4.35

From Ref. [14].

Fig. 7. Pure gas permeability coefficients for He, CO2, CH4, O2, and N2 at pressures up to 10 atm in membranes prepared from HFA/TERT, HFA/ISO and HFA/TERT-b-HFA/ISO block copolymers.

42

M.I. Loría-Bastarrachea, M. Aguilar-Vega / Journal of Membrane Science 443 (2013) 36–44

copolymer with the same molar composition [14], and the results for the copolymers are quite similar. Therefore, the results of the TGA analysis indicate that no significant effect on decomposition with respect to the homopolymer is associated with the length of the block in the copolymer. It was also found that the density of HFA/ISO, 1.422 g/cm3, is higher than that presented by HFA/TERT, 1.305 g/cm3. Densities of the block copolyamides fall around 1.34 g/cm3 in between those of the aramides with minimal differences. The block copolymers present density differences centered in the third decimal, being slightly higher for the copolymer whose block lengths are 9 repeating units of each block, and lower for the one where the block length is 18 units. Using the measured experimental density values, the fractional free volume, FFV, for the homopolyamides and the block copolymers was obtained using Bondi's group contribution method and the following equation: FFV ¼

ðV−V o Þ ; V

ð6Þ

where V is the specific volume obtained from the experimental density of the homopolymer or block copolymer, and Vo is the occupied volume calculated as Vo ¼1.3 Vw where Vw is the van der Waals volume, calculated from Bondi's group contribution method as reported by Van Krevelen [38]. For the copolymers the occupied volume was assumed to follow a simple additive mixing rule as proposed for the random copolymers [14]: Vo

cop

¼ ω1 V o1 þ ω2 V o2

ð7Þ

where Vo cop is the occupied volume of the copolymer, ω1 and ω2 are the weight fractions of the homopolymers in the copolymer and Vo1 and Vo2 are the occupied volumes of the homopolymers that form the copolymer. The resulting fractional free volumes, FFV, for the homopolyamides and the block copolyamides are listed in the last column of Table 3. The lowest FFV is found for HFA/ISO followed by that of HFA/TERT; all block copolyamides show FFV higher than those of the homopolyamides that increase as the length of the block increases. This result indicates that as there is a chain size increase in the rigid block copolymer, the packing of the chains becomes increasingly difficult due to interference of the blocks length and distribution in the copolymer. It is also observed that this kind of interference does not occur in the random copolymer with the same composition because the FFV is 0.150 that falls in between that of the homopolymers. This result will affect the gas transport properties of the block copolymers as they will have a larger FFV available for gas permeation and diffusion.

problem and adjusts to a weighted average P value of the two homopolymers [14]. It is also seen that ideal separation factors, αAB, are similar for the block copolymers as compared to those of HFA/TERT while they are slightly below those of HFAISO which has a lower P. This implies that for the block copolymers there is a gain in total permeability coefficients without losing selectivity as illustrated by Fig. 5 although the trade off is maintained since the block copolymer with the lower P has the largest selectivity, αA/B, of all copolymers. The increase in FFV impacts the permeability coefficients, P, by increasing the permeability of all gases thorough the dense polymer membranes obtained from the block copolyamides. Permeability coefficients for different polymers as a function of the inverse of fractional free volume have been successfully correlated by the following equation [6,14,39,40]:   B P ¼ P o exp − FFV

ð8Þ

where P is the permeability coefficient (Barrer), Po is a preexponential factor (Barrer) and B is a constant characteristic for each polymer and gas. The correlation of Eq. (8) with fractional free volume, FFV, for HFA/TERT, HFA/ISO and the block copolymers HFA/TERT-b-HFA/ISO with different chain lengths is shown in Fig. 6. The experimental P values for all gases show good agreement with Eq. (8), indicating that differences in chain packing, as the length of the block copolymer chains increases, induce a larger fractional free volume, FFV, and produce a higher gas permeability. As a further test, experimental values of P for the 50 mol% random copolymer HFA/TERT-co-HFA/ISO reported before [14] are presented in Fig. 6; the experimental values of the random copolymer also show good agreement with Eq. (8). In Table 5 experimentally measured apparent diffusion coefficients, D, as obtained from time lag measurements for HFA/TERT, HFA/ISO and HFA/TERT-b-HFA/ISO block copolymers with different block sizes are reported. The results indicate that there is an increase in pure gas diffusion, D, as the block size in the copolymer increases. For the block copolymer with 18 units blocks, apparent diffusion coefficients, D, are the largest and they decrease as the block length decreases, and in all cases, they are above those of the homopolymers and the random copolymer. It is also seen in Table 5 that the diffusivity selectivity, αDA/ B ¼DA/DB, is lower for the membranes that present higher diffusion coefficients. Thus an increase in FFV produces an increase of P and

3.2. Gas transport properties Gas permeability coefficients for the homopolyamides and block copolyamides were measured at 35 1C and a pressure gradient of 2 atm; the results of the measurements are given in Table 4. It is observed that in all cases the permeability coefficients, P, of the block copolymers are larger than those presented by the homopolymers. It is also seen that an increase in chain length from 9 to 12 units per each block has little effect on the gas permeability coefficients, P, particularly for large diameter gases such as CH4 and N2. On the other hand, the P of the block copolymer with 18 units is almost twice that of the other two copolymers, and around 3 times the one found for HFA/TERT which is the homopolymer with the largest P. The result is attributed to a lower packing of the polymer chains that is hindered by the rigidity of the blocks, since a random copolymer with the same molar composition does not show the same packing

Fig. 8. Upper bound plot of CO2 permeability and ideal selectivity of gas pair CO2/ CH4 with HFA situating HFA polyaramides (●) and HFA/TERT-b-HFA/ISO block copolyaramides (★) of this work.

M.I. Loría-Bastarrachea, M. Aguilar-Vega / Journal of Membrane Science 443 (2013) 36–44

43

Table 6 Pure gas permeability coefficientsa and ideal separation factors of HFA/TERT-b-HFA/ISO and HFA/TERT-b-DBF/ISO block copolyamides at 2 atm upstream pressure and 35 1C. Permeability, P (Barrer)a

Membrane

b

HFA/TERT-b-HFA/ISO18 HFA/TERT-b-HFA/ISO 12b HFA/TERT-b-HFA/ISO 9b HFA/TERT-b-DBF/ISO 18c,d HFA/TERT-b-DBF/ISO 12c,d HFA/TERT-b-DBF/ISO 9c,d a b

Selectivity αA/B ¼ PA/PB

He

CO2

O2

CH4

N2

154.8 86.93 90.68 71.3 44.4 48.7

70.93 37.94 40.09 20.9 16.6 18.5

15.43 8.59 8.27 4.7 3.3 4.1

2.13 1.09 1.01 0.56 0.53 0.64

3.06 1.61 1.62 0.72 0.44 0.75

CO2/O2

CO2/CH4

CO2/N2

O2/N2

4.59 4.41 4.84 4.44 4.96 4.45

33.30 34.80 39.69 37.42 31.18 28.68

23.17 23.56 24.74 28.75 37.09 24.60

5.04 5.33 5.10 6.4 7.4 5.5

Permeability values are expressed in Barrer ¼ 1  10−10 ⌊cm3 ðSTPÞcm=cm2 s cmHg⌋ [20,21]. Isophthalic block, HFA/ISO, prepolymer prepared with HFA diamine

H2N NH2 .

F F

c

F F F

F

From Ref. [35].

H2N

d

Isophthalic block, DBF/ISO, prepolymer prepared with DBF diamine

NH2

.

O

D with a similar or slightly lower selectivity for some of the gas pairs of interest. The behavior of permeability coefficients, P, with an increase in pressure gradient from 2 to 10 atm across the membrane is shown for each pure gas in Fig. 7. For He and O2, there is no noticeable change in permeability coefficients, P, with increasing pressure. On the other hand, in the case of CH4 and N2 there is a slight decrease in P with increasing pressure. CO2 shows a slightly larger decrease around 15% in P as the pressure increases in the interval measured. The decrease in P for the latter gases is ascribed to an increase in the sorption coefficient, S, as pressure increases, in particular for CH4 and CO2 which are the more soluble gases, as described by the dual sorption mode mechanism, see Ref. [2]. The behavior is similar to that reported for the same type of glassy rigid polymers [7–11,14–17,39]. It also shows that these block copolyamide membranes do not show a tendency for plasticization in the pressure range tested. Fig. 8 shows a plot of CO2 permeability coefficients and ideal selectivity, α, for the gas pair CO2/CH4 for the homopolyamides and block copolyaramides using the upper bound values reported by Robeson in 2008 [1]. It is seen that the preparation of membranes using the block copolyaramides presents the advantage of moving toward the upper limit since permeability coefficients are 2.3–2.6 times higher than those of HFA/TERT aramide and ideal selectivities are the same or slightly larger than the ones presented by HFA/TERT for the gas pairs presented, see Table 4. It is also seen in the same table that block copolyaramides, depending on the block length and gas, present permeability coefficients, P, that are 1.8–3.1 times larger than those of the random copolymer with the same composition with similar separation factors. The latest result indicates that block copolymerization should be a better option for performing gas separation with these types of membranes. Moreover, in Table 6 a comparison of permeability coefficients, P, and ideal separation factors, αAB of the block copolymers

HFA/TERT-b-HFA/ISO reported here with those block copolymers reported before for HFA/TER-b-DBF/ISO is presented [35]. From the comparison of these block copolymers, it is clear that the presence of the two bulky (–CF3) groups in the isophthalic blocks (HFA/ISO), as compared to the (–CQO) group on DBF/ISO, gives rise to permeability coefficients, P, that are at least 2–3 times larger for the former, a result that is attributed to the bulkiness of (–CF3) groups that has the effect of increasing hindrance of rotation of the adjacent phenyls and inhibition of polymer chain packing. In contrast ideal separation factors, αAB, present similar values for both types of block copolymers with the same block lengths. The influence of the bulky structure reflects positively in gas permeability, P, with a minimum or no loss in ideal separation factors which in turn moves the HFA/TERT-b-HFA/ISO block copolymers closer to the upper bond limit.

4. Conclusions The preparation of polymeric membranes formed by blocks of rigid HFA copolyaramides, one of them bearing a bulky lateral tertbutyl group, with increasing block length was shown to be possible. The results from dynamic mechanical and thermal properties indicate that the desired blocky structure was obtained in the final membrane materials. It was also found that block copolyaramide membranes present a density that is quite similar but slightly lower, as the length of the blocks that form the copolymer increase. The calculated fractional free volume, FFV, indicates that there is an increase in FFV in the block copolymers as the block length increases. This result is ascribed to an inefficient packing of the copolymer molecules as the block length gets larger. As a result, the permeability and diffusion coefficients in the block copolymers are larger than those in the parent homopolymers. The gas separation factors remain with a

44

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