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50) polyphosphazene
Journal of Membrane Science 172 (2000) 167–176
Gas permeability of poly(bis-trifluoroethoxyphosphazene) and blends with adamantane amino/trifluoroethoxy (50/50) polyphosphazene Kazukiyo Nagai a , Benny D. Freeman a,∗ , Angela Cannon b , Harry R. Allcock b,1 a
Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7265, USA b Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA Received 16 August 1999; received in revised form 21 January 2000; accepted 24 January 2000
Abstract Gas permeability coefficients of poly(bis-trifluoroethoxyphosphazene), (PTFEP) and its blends with adamantane amino/ trifluoroethoxy polyphosphazene (PAdATFEP) containing adamantane groups on 50 mol% of the side chains were determined at 35◦ C. The PTFEP films prepared by both spin casting and solvent casting had the same permeability coefficients, within the precision of the measurements. PTFEP/PAdATFEP blends were phase-separated. Relative to pure PTFEP, gas permeabilities were reduced from 45 to 70% (depending on the penetrant) by blending 18.8 mol% of PAdATFEP with PTFEP. The permeability coefficients of low-sorbing, permanent gases (i.e. H2 , O2 , and N2 ) decreased as penetrant size increased, suggesting that such polymers sieve these permanent gas molecules primarily based on penetrant size rather than penetrant solubility. Gas permeation properties of CO2 and hydrocarbon vapors did not change monotonically with penetrant size, suggesting an interplay of gas diffusivity and gas solubility on overall gas permeability. Blending PAdATFEP with PTFEP increased gas selectivity. Ideal separation factors of these PTFEP/PAdATFEP blends were significantly below upper bound values. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Poly(bis-trifluoroethoxyphosphazene); Adamantane amino/trifluoroethoxy polyphosphazene; Gas permeabilities; Hydrocarbon vapors
1. Introduction Polyphosphazenes have a backbone of alternating phosphorus and nitrogen atoms [1–4]. The chemical properties of these polymers can be easily tailored by alteration of side group structure [1–4]. Permeabilities of oxygen (under wet and dry conditions) and gases such as carbon dioxide, hydrogen, ∗ Corresponding author. Tel.: +1-919-515-2460; fax: +1-919-513 3435. E-mail address: benny [email protected] (B.D. Freeman) 1 Co-corresponding author.
and hydrocarbons in various polyphosphazenes have been reported [3–12]. Microcrystalline, film-forming poly[bis-(trifluoroethoxy)phosphazene] (PTFEP) (cf. Fig. 1) is the most permeable polyphosphazene known [12]. Its permeation properties lie between those of polydimethylsiloxane and low density polyethylene [8]. Considering that this polymer is semicrystalline, gas transport in amorphous regions of PTFEP is expected to be very high. High CO2 solubility in PTFEP is an interesting feature which is ascribed to interactions between fluorinated units in the polymer and CO2 . In this regard, high CO2 solubility (relative to other gases) has also been reported for
0376-7388/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 3 3 3 - 1
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organopolysiloxanes bearing fluorinated side chains [13]. PTFEP has a glass transition (Tg ) and two first order transitions [a transition from three-dimensional crystallites to a two-dimensional or mesomorphic state (T(1)) and a transition from the mesomorphic state to the isotropic melt state (Tm )] [3,8,9,14–19]. The ranges for these transitions for as-cast PTFEP are rather wide [e.g., Tg =−82 to −66◦ C, T(1)=66–83◦ C, and Tm =218–245◦ C] [3,8,9,14–19]. Thermal, mechanical, and gas permeation properties change as the temperature increases from below to above T(1) [9,15–19]. For example, heating through this transition caused CO2 , O2 , and N2 permeability coefficients of a PTFEP sample with a T(1) transition at 69.9◦ C to gradually deviate from the usual Arrhenius behavior above ca. 45◦ C, and a sharp increase in permeability was observed around 65◦ C [9]. Above 71◦ C, permeability was again well-described by an Arrhenius model. Adamantyl units are bulky, hydrophobic groups that can impart unusual properties to polymers. Allcock and Krause recently reported the synthesis and characterization of polyphosphazenes bearing adamantyl side groups [20]. Sequential addition of the adamantyl nucleophiles and sodium trifluoroethoxide to the macromolecular intermediate led to co-substituted polyphosphazenes (cf. Fig. 1). The co-substitution of adamantyl units with trifluoroethoxy increased the glass transition temperature from −66◦ C for PTFEP to between −44 and +180◦ C, depending on the type and ratio of adamantyl substituents in the polymer [20].
The objective of this study was to determine the effect of adamantane content on gas permeability in blends of PTFEP and adamantane amino/ trifluoroethoxy (50/50) polyphosphazene. Adamantane groups were substituted randomly on 50 mol% of the side chains of each phosphazene repeat unit. This brittle polymer was then blended with PTFEP to produce free standing films with varying concentrations of adamantyl groups.
2. Experimental 2.1. Materials and film preparation protocols PTFEP and adamantane amino/trifluoroethoxy (50/50) polyphosphazene (PAdATFEP) were synthesized as described previously [3,20]. The repeat units of these polymers are presented in Fig. 1. Blends of PTFEP containing 30 and 40 wt.% PAdATFEP were prepared by a solution processing method described below. These blends contained 13.8 and 18.8 mol% adamantane units, respectively. PTFEP homopolymer films were prepared by both solvent casting and spin casting. The solvent-cast PTFEP and blend films were prepared on a Teflon coated plate using a 10 wt.% solution of the polymer in tetrahydrofuran. The solvent was evaporated at ambient conditions for a week. Spin cast PTFEP films were prepared from a 20 wt.% tetrahydrofuran solution of the polymer on a glass plate using an EC 101DT-CB15REM Photo Resist Spinner (Headway Research) at 130 rpm for 22 min. No residual solvent could be detected in the films by differential scanning calorimetry (DSC). Film thicknesses were uniform and ranged from 49 to 106 m, depending on the sample. 2.2. Gas permeation measurements
Fig. 1. Repeat units of poly(bis-trifluoroethoxyphosphazene) [PTFEP] and poly(adamantane amino/trifluoroethoxy (50/50) phosphazene) [PAdATFEP].
Pure gas permeabilities of polyphosphazene films were determined at 35◦ C using the constant pressure/variable volume method [21]. The gases were nitrogen, oxygen, hydrogen, methane, ethane, propane, and carbon dioxide. The feed pressure was 200 psig for all gases except C3 H8 (C3 H8 : 40 psig). Permeate pressure was maintained at 0 psig. The permeability
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coefficients are reported in Barrers, where 1 Barrer is 10−10 cm3 (STP)cm/(cm2 s cm Hg). 2.3. Characterization Film density was determined based on film weight and volume at ambient conditions. The polymer cohesive energy density (CED) and solubility parameter (SP) were estimated using the group contribution method of Fedors [22]. Other group contribution approaches (e.g. Small, Hoy, and van Krevelen) could not be used since they did not include a value for phosphorus [23,24]. Fedors’ data do not include a contribution specifically for the –N=P– bond, so values for phosphorus and nitrogen atoms were used for these estimates. A Perkin–Elmer DSC 7 differential scanning calorimeter (DSC) was used to determine the polymer thermal properties at a heating rate of 10◦ C/min from −120 to 100◦ C and from 25 to 300◦ C. Tg was determined using a first derivative analysis. T(1) and Tm are reported as the peak values of these transitions. Wide angle X-ray diffraction (WAXD) spectra were recorded using a Philips diffractometer (Goniometer Type: PW3050/q-q). Cu K␣ radiation (1.54 Å) was employed, and the instrument was operated at 40 kV and 40 mA. The weight percent crystallinity was estimated by calculating the areas under the amorphous halo and crystalline peaks after establishing a baseline for the spectrum. The crystalline content was then estimated as AC /(AA +AC ) where AC is the area associated with the crystalline peaks and AA is the area associated with the amorphous halo.
3. Results and discussion 3.1. Characterization of polyorganophosphazene films Table 1 summarizes the physical and thermal properties of an as-cast PTFEP homopolymer film and as-cast blend films containing 13.8 and 18.8 mol% adamantane. The adamantane amino/trifluoroethoxy (50/50) polyphosphazene (PAdATFEP) homopolymer was brittle and powder-like. Therefore, blends con-
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taining more than 40 wt.% PAdATFEP could not be prepared as strong, defect-free films for permeation studies. PTFEP films were hazy, but blend films containing 13.8 and 18.8 mol% adamantane were opaque/white. Film density was higher than that of conventional non-fluorinated polymers, but lower than that of poly(tetrafluoroethylene) (2.0–2.3 g/cm3 at 23◦ C) [25]. Hirose et al. reported the density of PTFEP to be 1.707 g/cm3 for a sample molded under compression at 15 MPa and 150◦ C [8]. The molding apparatus was left hot for 30 min and then cooled with water. Their density value was lower than ours (1.82 g/cm3 ) due to lower crystallinity in our sample, as discussed later in more detail. The blend density was lower than that of the homopolymer. However, within the precision of the density data, blends containing 13.8 and 18.8 mol% adamantane had the same density. As adamantane content increased, the calculated cohesive energy density (CED) and solubility parameter (SP) also increased, consistent with the increase in polymer chain packing. As adamantane amino content increases, the concentration of low cohesive energy density trifluoroethoxy groups in the blends is diluted, which would tend to increase the CED [24]. Fig. 2 presents WAXD spectra of PTFEP and blends films. The WAXD spectrum of PTFEP has a strong peak at 2θ =20.6◦ (d=4.3 Å) with two small peaks at 2θ =8–10◦ (d=8.8–11 Å) and a broad halo between approximately 12 and 25◦ . The WAXD spectrum of PAdATFEP has two broad peaks at 2θ =6.9◦ (d=12.8 Å) and 17.6◦ (d=5.0 Å). Both blend films have peaks in the same location as the PTFEP homopolymer film, with a small, broad shoulder at 2θ=15–19◦ attributed to PAdATFEP. The peak at 2θ =20.6◦ is ascribed to homopolymer crystals in PTFEP [14,15]. Crystallinities of blend films were very similar to those of PTFEP films. Hirose et al. report 60% crystallinity in a molded PTFEP film [8]. In contrast, our PTFEP film had approximately 35 wt.% crystallinity. Sun and Magill reported initial crystallinity values of less than 40% for solvent-cast PTFEP films [19]. This difference in crystallinity level is the source of the density difference between our sample and that of Hirose et al. While the crystalline regions of most polymers are more dense than the amorphous regions, PTFEP is similar to poly(4-methyl-1-pentene) in this respect, since
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K. Nagai et al. / Journal of Membrane Science 172 (2000) 167–176
Fig. 2. WAXD spectra of a PTFEP homopolymer film and PTFEP/PAdATFEP blend films (adamantane content: 13.8 and 18.8 mol%). The spectra for the blends have been shifted vertically for easier viewing.
it has a crystal density that is lower than the density of amorphous regions of the polymer [26]. Therefore, since our sample has a lower amount of crystallites than that of Hirose et al., it has a higher density. Many poly(organophosphazenes) have a glass transition (Tg ), one or more mesophase transitions (T(1)), and a clearing temperature (Tm ) where an isotropic melt forms [3]. As previously described, these temperatures in as-cast PTFEP vary widely [e.g. −82
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and the blend films. If the blends exhibited significant mixing of the components, then the crystallinity of the blend would normally be much lower than that of the parent homopolymer, PTFEP. However, as there is practically no reduction in crystallinity in the blends relative to that of the homopolymer and since the PAdAFTEP mixed-substituent polymer is noncrystalline, the blends are probably rather strongly phase-separated. The mesophase transition T(1) of PTFEP is reported to be sensitive to film preparation methods and thermal treatments [17,19]. In Table 1, T(1)2 is the main peak which is generally reported as T(1) of PTFEP. A small peak preceding T(1)2 was observed, and its value is recorded as T(1)1 . Sun and Magill reported a main T(1) value of 66◦ C and a small peak around 47◦ C for an as-cast PTFEP sample [19]. This small peak shifted to higher temperature while the main peak stayed constant as the annealing temperature increased from 40 to 50◦ C. At an annealing temperature of 60◦ C, both peaks seemed to merge and appear as a single peak at 68◦ C. The single T(1) peak increased to 74◦ C at an annealing temperature of 125◦ C. PAdAFTEP homopolymer has a Tg of 180◦ C, a T(1) transition temperature of 200◦ C, and does not exhibit a melting point since it is a noncrystalline material [20]. Masuko et al. reported that, for a PTFEP film cast from solution in tetrahydrofuran, T(1) and the enthalpy change associated with this transition (1H(T(1))) were 82.4◦ C and 1.8 [J/g], respectively [17]. When the sample film was cooled from 120 to 27◦ C at 5◦ C/min after annealing above T(1), T(1) and 1H shifted to 88.4◦ C and 1.6 [J/g], respectively. In our results, the T(1) values attributed to PTFEP were not changed by blending, but 1H was lower in the blends than in homopolymer PTFEP. In this regard, PAdATFEP appears to disorder noncrystalline regions of PTFEP participating in the mesophase. Blend films showed a single high temperature thermal transition at 222–223◦ C. In the homopolymers comprising the blend, one peak appears at 200◦ C (T(1) of PAdATFEP), and another appears at 241◦ C (Tm of PTFEP). The peak observed at 222–223◦ C is midway between these two thermal transitions and may represent the superposition of these peaks. We report the value of this transition in brackets in Table 2 to indicate that it could not be definitively assigned to either the T(1) of PAdATFEP or the Tm of PTFEP.
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Table 2 Gas permeability coefficients of PTFEP Film preparationa
Spin cast Solvent cast Solvent cast Solvent cast and moldedb
Thickness (m) 49 80 –c 118
Crystallinity (wt.%)
Temperature (◦ C)
Permeability [Barrers]
Reference
CO2
H2
O2
C2 H 6
CH4
N2
C3 H 8
35 35 –c 60
35 35 –c 25
430 470 580 200
140 130 –c 63
73 77 120 35
52 50 –c 20
41 41 58 19
31 35 52 15
30 23 –c 17
This work This work Nelson et al. [10,11] Hirose et al. [8]
a
All films were cast from solution in tetrahydrofuran. The films molded under compression at about 15 MPa and 150◦ C for 30 min. c No information. b
3.2. Effects of film preparation method on gas permeability of PTFEP PTFEP films were prepared by both solvent casting and spin casting. The WAXD spectra of films prepared by these two methods were essentially identical. As shown in Table 2, the film preparation method did not affect gas permeabilities. However, the permeability coefficients of gases in solvent-cast PTFEP films reported by Nelson et al. [10] were higher than those obtained in this study. Even though gas permeabilities of PTFEP films prepared by both solvent casting and spin casting were the same in this study, it is probable that the gas permeation properties of PTFEP may be influenced by film preparation methods since the mesophase structure of this polymer is reported to depend on processing history [17,19]. Also, the crystallinity in the sample studied by Nelson et al. was not reported, and this would impact gas permeability coefficients. Our permeability coefficients are significantly higher than those of Hirose et al. [8], consistent with much lower crystallinity in our samples. The O2 permeability coefficient of PTFEP determined in this study was one order of magnitude lower than that of poly(dimethylsiloxane) (600 Barrers at 25◦ C) [27], which is the most permeable rubbery polymer. However, PTFEP was more permeable to O2 than other rubbery polymers [e.g. natural rubber (18 Barrers) and butyl rubber (1 Barrer) at 25◦ C] [27]. 3.3. Effects of adamantane content on gas permeability Fig. 3a presents the effect of adamantane content on gas permeabilities of PTFEP and its blends with PA-
dATFEP at 35◦ C. The maximum experimental error of the gas permeation measurements was ±5%. The ratio of the blend permeability to the permeability of pure PTFEP is presented in Fig. 3b. Gas permeabilities were reduced by 45–70% (depending on the penetrant) by adding 18.8 mol% of the adamantane amino structure to PTFEP. Except for propane, the permeability coefficients of these penetrants decrease monotonically with increasing adamantane content. Several theories are reported for gas transport in phase-separated, two-component blends [28]. They consider transport in the continuous and discontinuous phases of blends. These theoretical approaches require the experimental permeability data of both polymers in the blends. However, for PTFEP/PAdATFEP blends, this approach was limited. Fig. 4 presents gas permeabilities as a function of critical volume, a convenient measure of penetrant size important for gas diffusivity in polymers [29]. For the light supercritical gases (i.e., H2 , O2 , and N2 ), the order of gas permeability and permeability ratio [Permeability(blend)/Permeability(PTFEP)] is: H2 >O2 >N2 , which is in the order of increasing penetrant size. These results are rationalized based on the strong dependence of polymer diffusion coefficient on penetrant size and free volume. The following model, based on the Cohen and Turnbull theory, is often used to describe the effect of penetrant size and free volume on diffusion coefficients for a single phase system (i.e. homopolymer) [30,31]: γ v∗ (1) D = A exp − FFV where A is a pre-exponential factor which depends weakly on temperature, FFV is fractional free volume,
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Fig. 3. (a) Gas permeabilities and (b) gas permeability ratios of PTFEP and blends at 35◦ C as a function of adamantane content in the blend. Gas permeability ratio is defined as P(Blend)/P(PTFEP). 1 Barrer=10−10 cm3 (STP)cm/(cm2 s cm Hg).
and γ is a free volume overlap parameter, which is between 0.5 and 1. Penetrant size is strongly correlated with v ∗ , the minimum free volume element size required by the penetrant to execute a diffusion step. From Eq. (1), penetrant diffusion coefficients decrease exponentially with increasing penetrant size (v ∗ ), and the effect of penetrant size on diffusion coefficients is stronger in lower free volume matrices. In Fig. 4a, for the light gases, permeability of PTFEP homopolymer decreases with increasing penetrant size, which is qualitatively consistent with Eq. (1) and suggests that the order of permeability coefficients for these gases is governed by their diffusion
coefficients. The blends show the same trend as PTFEP. The blends are phase-separated. If the PAdATFEP domains were impermeable, causing penetrants to detour around these regions, then the permeability ratio should be similar for all penetrants, because the penetrants would diffuse only in the continuous PTFEP matrix. However, in Fig. 4b, the permeability ratio decreases with increasing penetrant size for the light gases, suggesting that penetrants diffuse not only in the PTFEP matrix but also through regions of PAdATFEP. The permeability of CO2 and the hydrocarbons do not exhibit such simple behavior, suggesting that
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Fig. 4. (a) Gas permeabilities and (b) gas permeability ratios of PTFEP and blends (adamantane content: 13.8 and 18.8 mol%) at 35◦ C as a function of critical volume of the penetrant, Vc . Gas permeability ratio is defined as P(Blend)/P(PTFEP). 1 Barrer=10−10 cm3 (STP)cm/(cm2 s cm Hg).
solubility plays a more important role in the transport properties of these more condensable penetrants. The permeability coefficients of all polyphosphazene films to CO2 are higher than those to H2 . H2 is smaller than CO2 and should, therefore, have larger diffusion coefficients than CO2 , but CO2 is expected to be much more soluble than H2 [29]. The permeabilities of C2 H6 , CH4 , and N2 were in the following order: C2 H6 >CH4 >N2 . This is not in the order of decreasing gas size but is in the order of decreasing expected gas solubility. Therefore, the permeability coefficients of these more strongly sorbing hydrocarbon penetrants depend on a subtle interplay between gas diffusivity and gas solubility. These results suggest that the poly-
mer has a rather weak size sieving ability. As shown in Fig. 4b, the permeability ratio for the hydrocarbons was larger than that of N2 for all cases except propane in the sample containing 18.8 mol% adamantane. In the blend film containing 18.8 mol% adamantane, the order of the permeability ratios for the hydrocarbons was: CH4
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Fig. 5. Gas permeabilities of PTFEP and blends (adamantane content: 13.8 and 18.8 mol%) at 35◦ C as a function of critical temperature of the penetrant, Tc . 1 Barrer=10−10 cm3 (STP)cm/(cm2 s cm Hg).
The relationship between the logarithm of solubility coefficients of various gases in PTFEP and penetrant Lennard–Jones force constant, which is proportional to critical temperature, was linear except for CO2 and N2 O [8]. Solubilities for CO2 and N2 O in PTFEP were higher than expected based on the relationship between solubility coefficients for nonpolar gases in PTFEP and Lennard–Jones force constant. CO2 was the most permeable penetrant tested for all of the polymers, which is consistent with the rubbery nature of the polymer and possibly favorable interactions between CO2 and the fluorinated polymer matrix. In Fig. 5, there is no systematic trend in permeability with critical temperature, suggesting that a subtle interplay between both solubility and diffusivity is important in determining the transport properties in PTFEP and its blends, particularly for the more condensable penetrants (i.e. CO2 and the hydrocarbons).
3.4. Separation factors Robeson analyzed separation factor versus permeability data for many gas pairs using the following equation [32]: αij = k −1/n Pi 1/n
(2)
where α ij is the separation factor, Pi /Pj , and Pi and Pj (Pi >Pj ) are permeability coefficients of penetrants i and j, respectively. The values of k and n were calculated from the upper bound linear relationship between separation factor and gas permeability for the best polymers for a particular separation. Table 3 summarizes experimental separation factors together with the upper bound selectivities calculated using Eq. (2) and the experimental permeability coefficients. As adamantane content increased, the
Table 3 Separation factors of PTFEP and PAdATFEP/PTFEP blends Gas pairs
H2 /N2 H2 /O2 H2 /CH4 O2 /N2 CO2 /CH4 a b
Robeson’s parametersa
0 mol%b
13.8 mol%b
18.8 mol%b
k
n
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
52, 918 35, 760 18, 500 389, 224 1, 073, 700
−1.5275 −2.2770 −1.2112 −5.8000 −2.6264
3.7 1.7 3.2 2.2 12
51 12 59 4.4 19
5.0 2.0 3.9 2.6 11
78 16 100 5.0 26
6.0 2.1 4.9 2.8 12
76 15 98 5.0 27
Data from ref. [32]. Adamantane content in PTFEP/PAdATFEP blends.
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experimentally-determined separation factors gradually increased. However, the polyphosphazene samples used in this study had low separation factors relative to the upper bound values. This result is consistent with the experimental permeability results since the permeability coefficients do not exhibit a strong monotonic decrease with increasing penetrant size, and polymers that define the upper bound curves typically show rather sharp decreases in permeability with increasing penetrant size [29]. 4. Conclusions Gas permeabilities of PTFEP and its blends with adamantane amino/trifluoroethoxy (50/50) polyphosphazene (PAdATFEP) were determined. PTFEP/PAdATFEP blends were phase-separated. Gas permeabilities were reduced by 45–70% by adding 18.8 mol% of the adamantane structure to PTFEP. For light, supercritical gases (i.e. H2 , O2 , and N2 ), gas permeability decreased with increasing penetrant size, suggesting that the permeability of these penetrants is strongly dependent on gas diffusivity. Gas permeation properties of CO2 and hydrocarbons did not exhibit simple monotonic trends with either penetrant critical temperature or critical volume. In this case, both gas solubility and diffusivity were important in determining the changes in permeability from one penetrant to another. The addition of adamantane produced a modest increase in selectivity. However, ideal separation factors for these materials were very low relative to selectivities of upper bound materials calculated using Robeson’s equation. Acknowledgements We thank the Army Research Office for partial funding of this project (H.R.A. and A.C.) and Wendy E. Krause for the synthesis of the PAdATFEP polymer. References [1] [2] [3] [4]
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Gas permeability of poly(bis-trifluoroethoxyphosphazene) and blends with adamantane amino/trifluoroethoxy (50/50) polyphosphazene Kazukiyo Nagai a , Benny D. Freeman a,∗ , Angela Cannon b , Harry R. Allcock b,1 a
Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7265, USA b Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA Received 16 August 1999; received in revised form 21 January 2000; accepted 24 January 2000
Abstract Gas permeability coefficients of poly(bis-trifluoroethoxyphosphazene), (PTFEP) and its blends with adamantane amino/ trifluoroethoxy polyphosphazene (PAdATFEP) containing adamantane groups on 50 mol% of the side chains were determined at 35◦ C. The PTFEP films prepared by both spin casting and solvent casting had the same permeability coefficients, within the precision of the measurements. PTFEP/PAdATFEP blends were phase-separated. Relative to pure PTFEP, gas permeabilities were reduced from 45 to 70% (depending on the penetrant) by blending 18.8 mol% of PAdATFEP with PTFEP. The permeability coefficients of low-sorbing, permanent gases (i.e. H2 , O2 , and N2 ) decreased as penetrant size increased, suggesting that such polymers sieve these permanent gas molecules primarily based on penetrant size rather than penetrant solubility. Gas permeation properties of CO2 and hydrocarbon vapors did not change monotonically with penetrant size, suggesting an interplay of gas diffusivity and gas solubility on overall gas permeability. Blending PAdATFEP with PTFEP increased gas selectivity. Ideal separation factors of these PTFEP/PAdATFEP blends were significantly below upper bound values. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Poly(bis-trifluoroethoxyphosphazene); Adamantane amino/trifluoroethoxy polyphosphazene; Gas permeabilities; Hydrocarbon vapors
1. Introduction Polyphosphazenes have a backbone of alternating phosphorus and nitrogen atoms [1–4]. The chemical properties of these polymers can be easily tailored by alteration of side group structure [1–4]. Permeabilities of oxygen (under wet and dry conditions) and gases such as carbon dioxide, hydrogen, ∗ Corresponding author. Tel.: +1-919-515-2460; fax: +1-919-513 3435. E-mail address: benny [email protected] (B.D. Freeman) 1 Co-corresponding author.
and hydrocarbons in various polyphosphazenes have been reported [3–12]. Microcrystalline, film-forming poly[bis-(trifluoroethoxy)phosphazene] (PTFEP) (cf. Fig. 1) is the most permeable polyphosphazene known [12]. Its permeation properties lie between those of polydimethylsiloxane and low density polyethylene [8]. Considering that this polymer is semicrystalline, gas transport in amorphous regions of PTFEP is expected to be very high. High CO2 solubility in PTFEP is an interesting feature which is ascribed to interactions between fluorinated units in the polymer and CO2 . In this regard, high CO2 solubility (relative to other gases) has also been reported for
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organopolysiloxanes bearing fluorinated side chains [13]. PTFEP has a glass transition (Tg ) and two first order transitions [a transition from three-dimensional crystallites to a two-dimensional or mesomorphic state (T(1)) and a transition from the mesomorphic state to the isotropic melt state (Tm )] [3,8,9,14–19]. The ranges for these transitions for as-cast PTFEP are rather wide [e.g., Tg =−82 to −66◦ C, T(1)=66–83◦ C, and Tm =218–245◦ C] [3,8,9,14–19]. Thermal, mechanical, and gas permeation properties change as the temperature increases from below to above T(1) [9,15–19]. For example, heating through this transition caused CO2 , O2 , and N2 permeability coefficients of a PTFEP sample with a T(1) transition at 69.9◦ C to gradually deviate from the usual Arrhenius behavior above ca. 45◦ C, and a sharp increase in permeability was observed around 65◦ C [9]. Above 71◦ C, permeability was again well-described by an Arrhenius model. Adamantyl units are bulky, hydrophobic groups that can impart unusual properties to polymers. Allcock and Krause recently reported the synthesis and characterization of polyphosphazenes bearing adamantyl side groups [20]. Sequential addition of the adamantyl nucleophiles and sodium trifluoroethoxide to the macromolecular intermediate led to co-substituted polyphosphazenes (cf. Fig. 1). The co-substitution of adamantyl units with trifluoroethoxy increased the glass transition temperature from −66◦ C for PTFEP to between −44 and +180◦ C, depending on the type and ratio of adamantyl substituents in the polymer [20].
The objective of this study was to determine the effect of adamantane content on gas permeability in blends of PTFEP and adamantane amino/ trifluoroethoxy (50/50) polyphosphazene. Adamantane groups were substituted randomly on 50 mol% of the side chains of each phosphazene repeat unit. This brittle polymer was then blended with PTFEP to produce free standing films with varying concentrations of adamantyl groups.
2. Experimental 2.1. Materials and film preparation protocols PTFEP and adamantane amino/trifluoroethoxy (50/50) polyphosphazene (PAdATFEP) were synthesized as described previously [3,20]. The repeat units of these polymers are presented in Fig. 1. Blends of PTFEP containing 30 and 40 wt.% PAdATFEP were prepared by a solution processing method described below. These blends contained 13.8 and 18.8 mol% adamantane units, respectively. PTFEP homopolymer films were prepared by both solvent casting and spin casting. The solvent-cast PTFEP and blend films were prepared on a Teflon coated plate using a 10 wt.% solution of the polymer in tetrahydrofuran. The solvent was evaporated at ambient conditions for a week. Spin cast PTFEP films were prepared from a 20 wt.% tetrahydrofuran solution of the polymer on a glass plate using an EC 101DT-CB15REM Photo Resist Spinner (Headway Research) at 130 rpm for 22 min. No residual solvent could be detected in the films by differential scanning calorimetry (DSC). Film thicknesses were uniform and ranged from 49 to 106 m, depending on the sample. 2.2. Gas permeation measurements
Fig. 1. Repeat units of poly(bis-trifluoroethoxyphosphazene) [PTFEP] and poly(adamantane amino/trifluoroethoxy (50/50) phosphazene) [PAdATFEP].
Pure gas permeabilities of polyphosphazene films were determined at 35◦ C using the constant pressure/variable volume method [21]. The gases were nitrogen, oxygen, hydrogen, methane, ethane, propane, and carbon dioxide. The feed pressure was 200 psig for all gases except C3 H8 (C3 H8 : 40 psig). Permeate pressure was maintained at 0 psig. The permeability
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coefficients are reported in Barrers, where 1 Barrer is 10−10 cm3 (STP)cm/(cm2 s cm Hg). 2.3. Characterization Film density was determined based on film weight and volume at ambient conditions. The polymer cohesive energy density (CED) and solubility parameter (SP) were estimated using the group contribution method of Fedors [22]. Other group contribution approaches (e.g. Small, Hoy, and van Krevelen) could not be used since they did not include a value for phosphorus [23,24]. Fedors’ data do not include a contribution specifically for the –N=P– bond, so values for phosphorus and nitrogen atoms were used for these estimates. A Perkin–Elmer DSC 7 differential scanning calorimeter (DSC) was used to determine the polymer thermal properties at a heating rate of 10◦ C/min from −120 to 100◦ C and from 25 to 300◦ C. Tg was determined using a first derivative analysis. T(1) and Tm are reported as the peak values of these transitions. Wide angle X-ray diffraction (WAXD) spectra were recorded using a Philips diffractometer (Goniometer Type: PW3050/q-q). Cu K␣ radiation (1.54 Å) was employed, and the instrument was operated at 40 kV and 40 mA. The weight percent crystallinity was estimated by calculating the areas under the amorphous halo and crystalline peaks after establishing a baseline for the spectrum. The crystalline content was then estimated as AC /(AA +AC ) where AC is the area associated with the crystalline peaks and AA is the area associated with the amorphous halo.
3. Results and discussion 3.1. Characterization of polyorganophosphazene films Table 1 summarizes the physical and thermal properties of an as-cast PTFEP homopolymer film and as-cast blend films containing 13.8 and 18.8 mol% adamantane. The adamantane amino/trifluoroethoxy (50/50) polyphosphazene (PAdATFEP) homopolymer was brittle and powder-like. Therefore, blends con-
169
taining more than 40 wt.% PAdATFEP could not be prepared as strong, defect-free films for permeation studies. PTFEP films were hazy, but blend films containing 13.8 and 18.8 mol% adamantane were opaque/white. Film density was higher than that of conventional non-fluorinated polymers, but lower than that of poly(tetrafluoroethylene) (2.0–2.3 g/cm3 at 23◦ C) [25]. Hirose et al. reported the density of PTFEP to be 1.707 g/cm3 for a sample molded under compression at 15 MPa and 150◦ C [8]. The molding apparatus was left hot for 30 min and then cooled with water. Their density value was lower than ours (1.82 g/cm3 ) due to lower crystallinity in our sample, as discussed later in more detail. The blend density was lower than that of the homopolymer. However, within the precision of the density data, blends containing 13.8 and 18.8 mol% adamantane had the same density. As adamantane content increased, the calculated cohesive energy density (CED) and solubility parameter (SP) also increased, consistent with the increase in polymer chain packing. As adamantane amino content increases, the concentration of low cohesive energy density trifluoroethoxy groups in the blends is diluted, which would tend to increase the CED [24]. Fig. 2 presents WAXD spectra of PTFEP and blends films. The WAXD spectrum of PTFEP has a strong peak at 2θ =20.6◦ (d=4.3 Å) with two small peaks at 2θ =8–10◦ (d=8.8–11 Å) and a broad halo between approximately 12 and 25◦ . The WAXD spectrum of PAdATFEP has two broad peaks at 2θ =6.9◦ (d=12.8 Å) and 17.6◦ (d=5.0 Å). Both blend films have peaks in the same location as the PTFEP homopolymer film, with a small, broad shoulder at 2θ=15–19◦ attributed to PAdATFEP. The peak at 2θ =20.6◦ is ascribed to homopolymer crystals in PTFEP [14,15]. Crystallinities of blend films were very similar to those of PTFEP films. Hirose et al. report 60% crystallinity in a molded PTFEP film [8]. In contrast, our PTFEP film had approximately 35 wt.% crystallinity. Sun and Magill reported initial crystallinity values of less than 40% for solvent-cast PTFEP films [19]. This difference in crystallinity level is the source of the density difference between our sample and that of Hirose et al. While the crystalline regions of most polymers are more dense than the amorphous regions, PTFEP is similar to poly(4-methyl-1-pentene) in this respect, since
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Fig. 2. WAXD spectra of a PTFEP homopolymer film and PTFEP/PAdATFEP blend films (adamantane content: 13.8 and 18.8 mol%). The spectra for the blends have been shifted vertically for easier viewing.
it has a crystal density that is lower than the density of amorphous regions of the polymer [26]. Therefore, since our sample has a lower amount of crystallites than that of Hirose et al., it has a higher density. Many poly(organophosphazenes) have a glass transition (Tg ), one or more mesophase transitions (T(1)), and a clearing temperature (Tm ) where an isotropic melt forms [3]. As previously described, these temperatures in as-cast PTFEP vary widely [e.g. −82
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and the blend films. If the blends exhibited significant mixing of the components, then the crystallinity of the blend would normally be much lower than that of the parent homopolymer, PTFEP. However, as there is practically no reduction in crystallinity in the blends relative to that of the homopolymer and since the PAdAFTEP mixed-substituent polymer is noncrystalline, the blends are probably rather strongly phase-separated. The mesophase transition T(1) of PTFEP is reported to be sensitive to film preparation methods and thermal treatments [17,19]. In Table 1, T(1)2 is the main peak which is generally reported as T(1) of PTFEP. A small peak preceding T(1)2 was observed, and its value is recorded as T(1)1 . Sun and Magill reported a main T(1) value of 66◦ C and a small peak around 47◦ C for an as-cast PTFEP sample [19]. This small peak shifted to higher temperature while the main peak stayed constant as the annealing temperature increased from 40 to 50◦ C. At an annealing temperature of 60◦ C, both peaks seemed to merge and appear as a single peak at 68◦ C. The single T(1) peak increased to 74◦ C at an annealing temperature of 125◦ C. PAdAFTEP homopolymer has a Tg of 180◦ C, a T(1) transition temperature of 200◦ C, and does not exhibit a melting point since it is a noncrystalline material [20]. Masuko et al. reported that, for a PTFEP film cast from solution in tetrahydrofuran, T(1) and the enthalpy change associated with this transition (1H(T(1))) were 82.4◦ C and 1.8 [J/g], respectively [17]. When the sample film was cooled from 120 to 27◦ C at 5◦ C/min after annealing above T(1), T(1) and 1H shifted to 88.4◦ C and 1.6 [J/g], respectively. In our results, the T(1) values attributed to PTFEP were not changed by blending, but 1H was lower in the blends than in homopolymer PTFEP. In this regard, PAdATFEP appears to disorder noncrystalline regions of PTFEP participating in the mesophase. Blend films showed a single high temperature thermal transition at 222–223◦ C. In the homopolymers comprising the blend, one peak appears at 200◦ C (T(1) of PAdATFEP), and another appears at 241◦ C (Tm of PTFEP). The peak observed at 222–223◦ C is midway between these two thermal transitions and may represent the superposition of these peaks. We report the value of this transition in brackets in Table 2 to indicate that it could not be definitively assigned to either the T(1) of PAdATFEP or the Tm of PTFEP.
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Table 2 Gas permeability coefficients of PTFEP Film preparationa
Spin cast Solvent cast Solvent cast Solvent cast and moldedb
Thickness (m) 49 80 –c 118
Crystallinity (wt.%)
Temperature (◦ C)
Permeability [Barrers]
Reference
CO2
H2
O2
C2 H 6
CH4
N2
C3 H 8
35 35 –c 60
35 35 –c 25
430 470 580 200
140 130 –c 63
73 77 120 35
52 50 –c 20
41 41 58 19
31 35 52 15
30 23 –c 17
This work This work Nelson et al. [10,11] Hirose et al. [8]
a
All films were cast from solution in tetrahydrofuran. The films molded under compression at about 15 MPa and 150◦ C for 30 min. c No information. b
3.2. Effects of film preparation method on gas permeability of PTFEP PTFEP films were prepared by both solvent casting and spin casting. The WAXD spectra of films prepared by these two methods were essentially identical. As shown in Table 2, the film preparation method did not affect gas permeabilities. However, the permeability coefficients of gases in solvent-cast PTFEP films reported by Nelson et al. [10] were higher than those obtained in this study. Even though gas permeabilities of PTFEP films prepared by both solvent casting and spin casting were the same in this study, it is probable that the gas permeation properties of PTFEP may be influenced by film preparation methods since the mesophase structure of this polymer is reported to depend on processing history [17,19]. Also, the crystallinity in the sample studied by Nelson et al. was not reported, and this would impact gas permeability coefficients. Our permeability coefficients are significantly higher than those of Hirose et al. [8], consistent with much lower crystallinity in our samples. The O2 permeability coefficient of PTFEP determined in this study was one order of magnitude lower than that of poly(dimethylsiloxane) (600 Barrers at 25◦ C) [27], which is the most permeable rubbery polymer. However, PTFEP was more permeable to O2 than other rubbery polymers [e.g. natural rubber (18 Barrers) and butyl rubber (1 Barrer) at 25◦ C] [27]. 3.3. Effects of adamantane content on gas permeability Fig. 3a presents the effect of adamantane content on gas permeabilities of PTFEP and its blends with PA-
dATFEP at 35◦ C. The maximum experimental error of the gas permeation measurements was ±5%. The ratio of the blend permeability to the permeability of pure PTFEP is presented in Fig. 3b. Gas permeabilities were reduced by 45–70% (depending on the penetrant) by adding 18.8 mol% of the adamantane amino structure to PTFEP. Except for propane, the permeability coefficients of these penetrants decrease monotonically with increasing adamantane content. Several theories are reported for gas transport in phase-separated, two-component blends [28]. They consider transport in the continuous and discontinuous phases of blends. These theoretical approaches require the experimental permeability data of both polymers in the blends. However, for PTFEP/PAdATFEP blends, this approach was limited. Fig. 4 presents gas permeabilities as a function of critical volume, a convenient measure of penetrant size important for gas diffusivity in polymers [29]. For the light supercritical gases (i.e., H2 , O2 , and N2 ), the order of gas permeability and permeability ratio [Permeability(blend)/Permeability(PTFEP)] is: H2 >O2 >N2 , which is in the order of increasing penetrant size. These results are rationalized based on the strong dependence of polymer diffusion coefficient on penetrant size and free volume. The following model, based on the Cohen and Turnbull theory, is often used to describe the effect of penetrant size and free volume on diffusion coefficients for a single phase system (i.e. homopolymer) [30,31]: γ v∗ (1) D = A exp − FFV where A is a pre-exponential factor which depends weakly on temperature, FFV is fractional free volume,
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Fig. 3. (a) Gas permeabilities and (b) gas permeability ratios of PTFEP and blends at 35◦ C as a function of adamantane content in the blend. Gas permeability ratio is defined as P(Blend)/P(PTFEP). 1 Barrer=10−10 cm3 (STP)cm/(cm2 s cm Hg).
and γ is a free volume overlap parameter, which is between 0.5 and 1. Penetrant size is strongly correlated with v ∗ , the minimum free volume element size required by the penetrant to execute a diffusion step. From Eq. (1), penetrant diffusion coefficients decrease exponentially with increasing penetrant size (v ∗ ), and the effect of penetrant size on diffusion coefficients is stronger in lower free volume matrices. In Fig. 4a, for the light gases, permeability of PTFEP homopolymer decreases with increasing penetrant size, which is qualitatively consistent with Eq. (1) and suggests that the order of permeability coefficients for these gases is governed by their diffusion
coefficients. The blends show the same trend as PTFEP. The blends are phase-separated. If the PAdATFEP domains were impermeable, causing penetrants to detour around these regions, then the permeability ratio should be similar for all penetrants, because the penetrants would diffuse only in the continuous PTFEP matrix. However, in Fig. 4b, the permeability ratio decreases with increasing penetrant size for the light gases, suggesting that penetrants diffuse not only in the PTFEP matrix but also through regions of PAdATFEP. The permeability of CO2 and the hydrocarbons do not exhibit such simple behavior, suggesting that
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Fig. 4. (a) Gas permeabilities and (b) gas permeability ratios of PTFEP and blends (adamantane content: 13.8 and 18.8 mol%) at 35◦ C as a function of critical volume of the penetrant, Vc . Gas permeability ratio is defined as P(Blend)/P(PTFEP). 1 Barrer=10−10 cm3 (STP)cm/(cm2 s cm Hg).
solubility plays a more important role in the transport properties of these more condensable penetrants. The permeability coefficients of all polyphosphazene films to CO2 are higher than those to H2 . H2 is smaller than CO2 and should, therefore, have larger diffusion coefficients than CO2 , but CO2 is expected to be much more soluble than H2 [29]. The permeabilities of C2 H6 , CH4 , and N2 were in the following order: C2 H6 >CH4 >N2 . This is not in the order of decreasing gas size but is in the order of decreasing expected gas solubility. Therefore, the permeability coefficients of these more strongly sorbing hydrocarbon penetrants depend on a subtle interplay between gas diffusivity and gas solubility. These results suggest that the poly-
mer has a rather weak size sieving ability. As shown in Fig. 4b, the permeability ratio for the hydrocarbons was larger than that of N2 for all cases except propane in the sample containing 18.8 mol% adamantane. In the blend film containing 18.8 mol% adamantane, the order of the permeability ratios for the hydrocarbons was: CH4
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Fig. 5. Gas permeabilities of PTFEP and blends (adamantane content: 13.8 and 18.8 mol%) at 35◦ C as a function of critical temperature of the penetrant, Tc . 1 Barrer=10−10 cm3 (STP)cm/(cm2 s cm Hg).
The relationship between the logarithm of solubility coefficients of various gases in PTFEP and penetrant Lennard–Jones force constant, which is proportional to critical temperature, was linear except for CO2 and N2 O [8]. Solubilities for CO2 and N2 O in PTFEP were higher than expected based on the relationship between solubility coefficients for nonpolar gases in PTFEP and Lennard–Jones force constant. CO2 was the most permeable penetrant tested for all of the polymers, which is consistent with the rubbery nature of the polymer and possibly favorable interactions between CO2 and the fluorinated polymer matrix. In Fig. 5, there is no systematic trend in permeability with critical temperature, suggesting that a subtle interplay between both solubility and diffusivity is important in determining the transport properties in PTFEP and its blends, particularly for the more condensable penetrants (i.e. CO2 and the hydrocarbons).
3.4. Separation factors Robeson analyzed separation factor versus permeability data for many gas pairs using the following equation [32]: αij = k −1/n Pi 1/n
(2)
where α ij is the separation factor, Pi /Pj , and Pi and Pj (Pi >Pj ) are permeability coefficients of penetrants i and j, respectively. The values of k and n were calculated from the upper bound linear relationship between separation factor and gas permeability for the best polymers for a particular separation. Table 3 summarizes experimental separation factors together with the upper bound selectivities calculated using Eq. (2) and the experimental permeability coefficients. As adamantane content increased, the
Table 3 Separation factors of PTFEP and PAdATFEP/PTFEP blends Gas pairs
H2 /N2 H2 /O2 H2 /CH4 O2 /N2 CO2 /CH4 a b
Robeson’s parametersa
0 mol%b
13.8 mol%b
18.8 mol%b
k
n
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
52, 918 35, 760 18, 500 389, 224 1, 073, 700
−1.5275 −2.2770 −1.2112 −5.8000 −2.6264
3.7 1.7 3.2 2.2 12
51 12 59 4.4 19
5.0 2.0 3.9 2.6 11
78 16 100 5.0 26
6.0 2.1 4.9 2.8 12
76 15 98 5.0 27
Data from ref. [32]. Adamantane content in PTFEP/PAdATFEP blends.
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experimentally-determined separation factors gradually increased. However, the polyphosphazene samples used in this study had low separation factors relative to the upper bound values. This result is consistent with the experimental permeability results since the permeability coefficients do not exhibit a strong monotonic decrease with increasing penetrant size, and polymers that define the upper bound curves typically show rather sharp decreases in permeability with increasing penetrant size [29]. 4. Conclusions Gas permeabilities of PTFEP and its blends with adamantane amino/trifluoroethoxy (50/50) polyphosphazene (PAdATFEP) were determined. PTFEP/PAdATFEP blends were phase-separated. Gas permeabilities were reduced by 45–70% by adding 18.8 mol% of the adamantane structure to PTFEP. For light, supercritical gases (i.e. H2 , O2 , and N2 ), gas permeability decreased with increasing penetrant size, suggesting that the permeability of these penetrants is strongly dependent on gas diffusivity. Gas permeation properties of CO2 and hydrocarbons did not exhibit simple monotonic trends with either penetrant critical temperature or critical volume. In this case, both gas solubility and diffusivity were important in determining the changes in permeability from one penetrant to another. The addition of adamantane produced a modest increase in selectivity. However, ideal separation factors for these materials were very low relative to selectivities of upper bound materials calculated using Robeson’s equation. Acknowledgements We thank the Army Research Office for partial funding of this project (H.R.A. and A.C.) and Wendy E. Krause for the synthesis of the PAdATFEP polymer. References [1] [2] [3] [4]
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