Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Tuning the Molecular Weights, Chain Packing, and Gas-Transport Properties of CANAL Ladder Polymers by Short Alkyl Substitutions Holden W. H. Lai,†,∥ Francesco M. Benedetti,‡,§,∥ Zexin Jin,† Yew Chin Teo,† Albert X. Wu,‡ Maria Grazia De Angelis,§ Zachary P. Smith,*,‡ and Yan Xia*,† †
Department of Chemistry, Stanford University, Stanford, California 94305, United States Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § Department of Civil, Chemical, Environmental, and Materials Engineering, Alma Mater StudiorumUniversity of Bologna, Bologna 40131, Italy Downloaded via NOTTINGHAM TRENT UNIV on August 15, 2019 at 12:22:07 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: We used catalytic arene−norbornene annulation (CANAL) polymerization to synthesize high-molecularweight (degree of polymerization 500−800 based on Mn) rigid ladder polymers with methyl, ethyl, and isopropyl substituents that can form self-standing films. The short alkyl substitution on CANAL ladder polymers significantly impacted gastransport properties and their chain packing as revealed by variable-temperature pure-gas permeation and high-pressure sorption experiments as well as wide-angle X-ray scattering. Interestingly, a combination of methyl and isopropyl substituents enhanced both the sorption capacity and permeation of all gases tested without compromising permselectivity. Our findings suggest that varying short alkyl substitutions on ladder polymers with high fractional free volume represents an effective strategy to tune their chain packing and fractional free volume, which can enhance permeability without compromising permselectivity.
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ization.13−15 Due to their extremely rigid backbone, CANAL polymers have no detectable glass transition below their decomposition temperature.13−15 The purely nonpolar hydrocarbon structure of CANAL polymers presents a unique system to investigate structure−property relationships without other variables such as polar gas−polymer or polymer− polymer interactions. Although the norbornyl benzocyclobutene motif has been incorporated into polyimide films,16 we have been unsuccessful at obtaining intact films from our previously reported CANAL ladder polymers due to their moderate molecular weights (MWs), which are often below 50 kDa. Herein, we report a strategy for forming high-MW CANAL polymers through the introduction of short alkyl substituents, thus allowing for the formation of self-standing films. We also discovered a surprising effect of alkyl substituents on chain packing, gas sorption, and gas permeation properties. We further performed variable-temperature permeation experiments at different pressures to investigate the energetics and elucidate the mechanism of diffusion. In particular, we demonstrate that short alkyl substitution can be used to tune the size of the free volume
INTRODUCTION Ladder polymers consist of conformationally restrictive fused rings in their backbones.1,2 This unique architecture leads to many intriguing properties. Most notably, ladder polymers with rigid and densely contorted backbones experience frustrated chain packing in the bulk, leading to substantially higher fractional free volume (FFV) compared to traditional single-stranded polymers.3−5 Few types of ladder polymers have been developed due to the demanding chemistry for efficient and selective molecular ladder formation as well as relatively low solubility of rigid ladder polymers.2,3,5 The only types of high-FFV ladder polymers that have been extensively investigated for gas permeation are based on benzodioxane3,4,6−8 and Tröger’s base9−11 backbones, best known as polymers of intrinsic microporosity (PIMs).5,12 Membranes of such polymers generally combine high gas permeability and moderate permselectivity, features that make them promising materials for gas separations. To rationally design high-FFV polymer membranes for gas separations, it is essential to expand the structural diversity of PIMs and systematically investigate how subtle changes in molecular structure affect chain packing and transport properties. We recently reported the synthesis of high-FFV ladder polymers with norbornyl benzocyclobutene backbones using catalytic arene−norbornene annulation (CANAL) polymer© XXXX American Chemical Society
Received: June 4, 2019 Revised: July 18, 2019
A
DOI: 10.1021/acs.macromol.9b01155 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Synthesis of High-MW CANAL Ladder Polymers
elements and their connectivity in CANAL ladder polymer membranes and effectively increase permeability without compromising permselectivity. The understanding of structure−property relationships aids the rational design of highFFV ladder polymers with enhanced gas separation performance.
Table 1. Properties of CANAL Ladder Polymers
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polymer
Mna (kDa)
Mwa (kDa)
DPb
Đ
ρc
FFVd
CANAL-Et CANAL-Et-iPr CANAL-Me-iPr
126 188 113
634 757 381
754 795 508
5.05 4.02 3.37
1.00 0.97 0.97
0.23 0.26 0.27
a
Determined by the multiangle laser light scattering analysis. bTotal degree of polymerization reported based on the number of norbornyl units calculated from Mn. cDensity was determined using a buoyancy density kit. dCalculated using Bondi’s group contribution method.20
RESULTS Effect of Alkyl Substituents on the Synthesis of HighMW CANAL Polymers. In our previous CANAL polymerizations, we used methyl substituents ortho to the aryl bromides to direct efficient and exclusive annulation. Using 1,4-dibromo-2,5-dimethylbenzene (1-Me) and norbornadiene (NBD) as monomers, we consistently obtained CANAL-Me with weight-averaged MW (Mw) of 20−40 kDa. Attempts to synthesize higher MW polymers at extended reaction times (>24 h) and high concentrations (>1 M) resulted in insoluble materials. We hypothesized that this was due to the relatively low solubility of CANAL-Me at high MW. Short alkyl substituents, such as ethyl and isopropyl groups, may enhance the solubility of CANAL polymers. Therefore, we attempted CANAL polymerization using p-diethyl- (1-Et) or pdiisopropyl-p-dibromobenzene (1-iPr) and NBD as monomers (Scheme 1). CANAL polymerization of 1-Et with NBD in tetrahydrofuran (THF) at 150 °C in a pressure vessel for 24 h consistently gave very high-MW ladder polymers with Mw > 600 kDa, significantly higher than the achievable MW for methylsubstituted CANAL ladder polymers. Under the same conditions, CANAL polymerization of 1-iPr with NBD gave polymers with Mw < 80 kDa. We attributed the lower MW to the reduced reactivity resulting from the increased steric hindrance imposed by isopropyl groups. To circumvent the lower reactivity when directly using 1-iPr as the monomer, we synthesized a diisopropyl dinorbornene (2-iPr) via CANAL reaction in the presence of excess (5 equiv) NBD15 and then used 2-iPr to polymerize with 1-Me or 1-Et at 1:1 ratio to obtain co-polymers CANAL-Me-iPr or CANAL-Et-iPr, respectively (Scheme 1). We can routinely synthesize CANAL-Et, CANAL-Et-iPr, and CANAL-Me-iPr that are soluble in THF and chloroform with Mw > 300 kDa (Table 1). Solution casting of all of the CANAL ladder polymers with Mw > 300 kDa yielded films suitable for gas permeation experiments. Pure-Gas Permeation. Gas permeation properties of CANAL ladder polymers are reported for the first time. Following casting from chloroform, polymer films were treated in a vacuum oven at 120 °C for 24 h. Complete removal of the solvent was confirmed by thermogravimetric analysis (TGA) (Figure S8). Permeability experiments were performed on a constant-volume variable-pressure apparatus using He, H2, CH4, N2, O2, and CO2 at 35 °C with 15 psi upstream pressure.
We observed a surprisingly strong effect of the alkyl substituents of the CANAL polymers on their gas permeability but minimal effect on selectivity. For all of the gases tested, the permeability of CANAL-Me-iPr is up to 80% higher than that of CANAL-Et (Figure 1), even though the two polymers can be considered constitutional isomers. The permeability of CANAL-Et-iPr lies in between. Within the solution-diffusion framework, permeability can be expressed as the product of gas diffusivity and solubility.17 Therefore, to better understand the difference in permeability, we determined the diffusivity and solubility of O2, N2, CO2, and CH4 in the polymers (Figure 2) using the solution-diffusion model (eq 4) and the time-lag equation (eq 5). 18,19 CANAL-Et-iPr exhibited higher diffusivity than CANAL-Et and CANAL-Me-iPr, both of which exhibited similar diffusivity (Figure 2a). Conversely, CANAL-Me-iPr had the highest gas solubility (Figure 2b). The higher gas solubility in CANAL-Me-iPr overcompensates for its lower diffusivity compared to that of CANAL-Et-iPr, thus giving CANAL-Me-iPr the highest permeability for all gases tested. Overall permeability of gases in CANAL polymers increase with the polymers’ FFV (Table 1) calculated using Bondi’s group contribution method.20 Interestingly, despite the significant differences in permeability, differences in permselectivity are very small for all three CANAL polymers (Figure 1b). The similar permselectivity can be attributed to the similar diffusivity selectivity and solubility selectivity among the three CANAL polymers, as indicated by the similar slopes of the best-fit lines in Figure 2a, which correlates the diffusivity with effective diameters squared of the penetrants,21 and Figure 2b, which correlates the solubility and Lennard-Jones potential well22 of the penetrants. High-Pressure Pure-Gas Sorption. Consistent with the solubility coefficients calculated from permeability experiments, direct gas sorption experiments on polymer films up to high pressures also showed that CANAL-Me-iPr had the highest sorption capacity for N2, CH4, and CO2 (Figure 3a−c), whereas CANAL-Et and CANAL-Et-iPr had very similar sorption capacity. Interestingly, the O2 sorption isotherms are nearly identical for all three polymers (Figure 3d), indicating that the alkyl substitution has no effect on the solubility of O2. This unusual result warrants further investigation. To obtain B
DOI: 10.1021/acs.macromol.9b01155 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 1. (a) Pure-gas permeability and (b) ideal permselectivity of CANAL ladder polymers with 15 psi upstream pressure at 35 °C.
Figure 2. (a) Correlation of diffusion coefficients with gas effective diameters.21 (b) Correlation of solubility coefficients with Lennard-Jones potential wells of gases. Dashed lines are best-fit lines. Permeation experiments were performed with 15 psi upstream pressure at 35 °C. Diffusion coefficients were calculated from the time lag. Solubility coefficients were calculated from diffusion and permeability coefficients assuming the solution-diffusion model.
certain thermodynamic properties (e.g., critical temperature) of penetrants and, therefore, have no physical significance. To obtain parameters with physical significance, DMS fitting was instead performed with kD and b constrained to increase exponentially with the same slope calculated from the trend of solubility coefficient vs critical temperature at 10 bar equilibrium pressure, as described by Smith et al. 26 Simultaneously, c′H was determined as an adjustable parameter. Pressure-based parameters are listed in Table 2, whereas fugacity-based sorption isotherms and DMS parameters can be found in Figure S13 and Table S16, respectively. A sensitivity analysis of the variability of DMS parameters is presented in Table S17 and Figure S16. Consistent with their FFVs, CANAL-Me-iPr has the largest Langmuir capacity for N2, CH4, and CO2, followed by CANAL-Et-iPr and then CANALEt. Figure 3 also presents the ideal sorption selectivity for three industrially relevant gas pairs, CO2/CH4, CO2/N2, and CH4/ N2 as a function of pressure calculated using best-fit curves of
additional sorption information about the CANAL polymers, we fitted the isotherms using the dual-mode sorption (DMS) model C = kDp +
c H′ bp 1 + bp
(1)
where C is the concentration of the sorbent in the polymer, kD is the Henry coefficient, p is the equilibrium pressure, c′H is the Langmuir capacity constant, and b is the Langmuir affinity parameter. Whereas the Henry mode describes sorption of gases in the liquid-like dense phase, the Langmuir mode describes sorption of gases in the nonequilibrium free volume elements of the glassy polymer membrane.23,24 Ricci et al. demonstrated that different sets of DMS parameters can describe the same pure-gas sorption isotherm with similar goodness of fit.25 Parameters obtained through a best-fit method with no constraints for individual gas−polymer isotherms often do not follow systematic trends expected for C
DOI: 10.1021/acs.macromol.9b01155 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 3. Sorption (solid circles, left axis) and desorption (open circles, left axis) isotherms and DMS fittings (solid lines, left axis) of CANAL ladder polymers at 35 °C for (a) CO2, (b) CH4, (c) N2, and (d) O2. Ideal sorption selectivity (dashed lines, right axis) is shown for (a) CO2/CH4, (b) CH4/N2, and (c) CO2/N2.
Table 2. Pressure-Based Dual-Mode Sorption Parameters (Equation 1) and Solubility Coefficients of CANAL Polymers 1 bar polymer CANAL-Et
CANAL-Et-iPr
CANAL-Me-iPr
gas
kD (cm3(STP) cmpolymer−3 bar−1)
N2 O2 CH4 CO2 N2 O2 CH4 CO2 N2 O2 CH4 CO2
0.29 0.38 0.52 1.44 0.33 0.42 0.58 1.60 0.34 0.44 0.61 1.71
cH ′ (cm3(STP) cm−3
polymer)
20.9 30.3 43.5 50.5 17.8 28.6 38.4 46.8 24.4 28.5 49.8 57.0
b (bar−1) 0.04 0.05 0.07 0.20 0.05 0.06 0.08 0.23 0.05 0.06 0.08 0.23
(cm3(STP) cm−3 1.0 1.4 5.1 11 1.3 1.6 5.0 11 1.7 2.3 7.2 19
± ± ± ± ± ± ± ± ± ± ± ±
polymer
0.1 0.2 0.5 1 0.1 0.2 0.4 1 0.1 0.2 0.4 1
1 bar bar−1)a
(cm3(STP) cmpolymer−3 bar−1)b 1.1 1.7 4.0 10.5 1.1 1.9 3.9 10.5 1.4 2.0 5.0 12.5
± ± ± ± ± ± ± ± ± ± ± ±
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
a
Calculated from diffusion and permeability coefficients assuming the solution-diffusion model (eq 4). bCalculated by the equation = C /p, where C is the concentration of penetrant sorbed interpolated by the best-fit curve at p = 1 bar.
the sorption isotherms. As commonly observed in glassy polymers, ideal sorption selectivity decreased monotonically as pressure increased for CH4/N2. However, the ideal sorption selectivity first decreased rapidly and then increased in the case of CO2/CH4 and plateaued in the case of CO2/N2. This trend can be attributed to the plasticization of the polymer chains as the concentration of CO2 in the film increases, leading to higher uptake of CO2. Because CH4 and N2 are nonplasticizing
gases, the apparent ideal sorption selectivity for CO2/CH4 and CO2/N2 is higher than expected at a high pressure. A small extent of plasticization of the films during CO2 sorption was observed, shown by the hysteresis in the CO2 desorption isotherm. After being exposed to >50 bar of CO2, desorption points at a given pressure were slightly higher than sorption points (Figure 3a), indicating that high concentrations of CO2 led to a small extent of swelling of the polymer chains and, D
DOI: 10.1021/acs.macromol.9b01155 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules thus, increased the sorption capacity of the films. Notably, the extent of hysteresis observed in the CO2 sorption and desorption isotherms of CANAL polymers was much less than that previously reported for other glassy polymers such as polycarbonate, highlighting the strong plasticization resistance of CANAL polymers.27 Sorption experiments also provided interesting insights into how alkyl substituents can affect sorption selectivity at high pressure, which is not revealed by permeation experiments performed at a low upstream pressure. High-pressure sorption selectivity is particularly pertinent to certain high-pressure industrial gas separations applications.28 At a low pressure, ideal sorption selectivity of the three polymers was remarkably similar (Figure 3a−c). At pressure > 10 bar, the ideal sorption selectivity for CH4/N2, CO2/N2, and CO2/CH4 was higher for CANAL-Et-iPr and CANAL-Et than CANAL-Me-iPr. Given that all three polymers consist of completely nonpolar hydrocarbons, specific polymer−gas interactions are expected to be very similar among these polymers. The difference in ideal sorption selectivity at high pressure can, therefore, be attributed to the difference in chain packing and free volume structures as a result of alkyl substitutions. Indeed, Robeson has reported a small but notable correlation between sorption capacity and free volume in the polymer upper bound database.19 Our results are fully consistent when applying this interpretation to our series of CANAL polymers for their sorption capacity and sorption selectivity. Wide-Angle X-ray Scattering (WAXS). The chain packing in CANAL ladder polymer films was investigated by wide-angle X-ray scattering (WAXS). In the WAXS patterns of the CANAL polymers, one predominant and broad peak centering around q = 0.8 Å−1 was observed (Figure 4).
performed variable-temperature experiments to further elucidate chain packing and diffusion relationships in CANAL ladder polymers. Variable-Temperature Permeation. To investigate the energetics of gas transport in CANAL ladder polymers, we performed variable-temperature pure-gas permeation experiments, using CANAL-Me-iPr as a model polymer. A polymer film that had been aged for 16 days was used to ensure the stability of the polymer’s property over the course of the experiment. A positive value of the activation energy of permeation, EP, indicates that activation energy of diffusion, ED, is larger than the absolute value of the enthalpy of sorption, ΔH (i.e., E D > |ΔH|). This was indeed the case for O2, N2, and CH4 (Figure 5, Table 3). Conversely, a negative value of
Figure 5. Arrhenius plot of CANAL-Me-iPr permeability after 16 days of aging. Upstream pressure = 15 psi.
EP indicates E D < |ΔH|, as is the case for CO2 (Figure 5, Table 3). The negative value of EP = −6.1 kJ mol−1 for CO2 suggests that the transport of CO2 in the CANAL polymer may follow trends for surface diffusion.29 To further confirm this hypothesis, we performed variable-temperature CO2 permeation at different feed pressures. Subatmospheric upstream pressures were chosen to avoid plasticization of the polymer. At steady-state upstream pressures of 5, 10, and 15 psi, ΔH was determined to be −23, −21, and −20 kJ mol−1, respectively (Figure 6). The increase in ΔH with increasing pressure is consistent with the most favorable sorption sites being occupied first, leading to a lower average ΔH at a higher pressure, as the remaining less favorable sorption sites are progressively occupied. The inverse correlation between ED and ΔH (Figure 6) suggests that the desorption of CO2 from the CANAL polymer is relevant to the rate of diffusion, which is consistent with the surface diffusion model. Furthermore, we determined the E D/|ΔH| ratios to be 0.68, 0.75, and 0.79 at upstream pressures of 5, 10, and 15 psi, respectively. These E D/|ΔH| ratios are in the range of the values previously determined for surface diffusion on solid surfaces of porous carbon and glass.29,30 Similar E D/|ΔH| ratios have been observed for the transport of some gases in polymeric materials with substantially high FFV and gas permeability, such as poly[1-(trimethylsilyl)-1-propyne]31,32 and PIM-1.33−35 To further investigate the chain packing and mechanism of diffusion in CANAL ladder polymers, we used the following
Figure 4. Wide-angle X-ray scattering data of CANAL ladder polymer films.
CANAL-Et-iPr showed a scattering peak slightly shifted to the lower q corresponding to the spacing of 8.0 Å, as compared to CANAL-Et and CANAL-Me-iPr with corresponding intersegmental distances of 7.6−7.7 Å. The larger intersegmental distance of CANAL-Et-iPr is consistent with diffusion results presented in Figure 2a, which shows the largest diffusion coefficients for gases in CANAL-Et-iPr. Although WAXS reveals the average intersegmental distance between polymer chains, it does not provide information about the size and connectivity between free volume elements. Therefore, we E
DOI: 10.1021/acs.macromol.9b01155 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 3. Energetics of Permeationa in CANAL-Me-iPr EP (kJ mol−1) ED (kJ mol−1) ΔH (kJ mol−1)
N2
CH4
CO2
O2
6.5 ± 0.7 20.6 ± 0.3 −14.1 ± 0.6
8±2 24.2 ± 0.6 −16 ± 1
−6.1 ± 0.9 13.3 ± 0.4 −19.5 ± 0.8
2±2 14 ± 1 −12 ± 2
Permeation experiments were performed at 35, 45, 55, and 65 °C with upstream pressure = 15 psi.
a
Figure 7. Activation energy of diffusion for penetrants in CANALMe-iPr.
Figure 6. Correlation between activation energy of diffusion and enthalpy of sorption of CO2 in CANAL-Me-iPr. Upstream pressures are noted below each data point.
Table 4. Comparison of the Parameters of Equation 2 for Different High-FFV Polymers: Polyimide, PIM Polymers, and CANAL Polymer, Based on ED’s of CO2, O2, N2, and CH4
equation to correlate the kinetic diameter of the penetrant (dk) with ED E D = cdk 2 − f
(2)
c (kJ mol−1)
polymer
where c and f are fitting parameters. Parameter c is a measure of the size selectivity of the polymer, and f /c is the kinetic diameter of the largest hypothetical molecule that can diffuse through the polymer with zero activation energy. The term f /c can be interpreted as the smallest channel that connects free volume elements.31,36 Equation 2 is based on Brandt’s model for molecular diffusion in polymers,37 and it has been used to construct the theoretical basis for the permeabilityselectivity tradeoff of polymeric membranes for gas separation, known as the Robeson upper bound.36,38,39 The ED values of O2, CO2, N2, and CH4 in CANAL-Me-iPr showed an excellent linear correlation with dk2 of the penetrants (Figure 7). The comparison of the slope and intercept of Figure 7 indicates that CANAL-Me-iPr is very similar to rigid and glassy polymers, such as 6FDA-2,6-DAT (poly[2,6-toluene-2,2-bis(3,4-dicarboxylphenyl)hexafluoropropane diimide]) polyimide,40 the archetypal ladder polymer PIM-1,35 and other PIMs, such as PIMTMN-Trip and PIM-BTrip, which contain extended triptycene motifs (Table 4).41,42 On the other hand, f /c values around zero have been reported for polymers with flexible backbones and low FFV, such as glassy polyvinyl acetate (permeation experiments performed at
