Structure Manipulation in Triptycene-Based Polyimides through Main

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Structure Manipulation in Triptycene-Based Polyimides through Main Chain Geometry Variation and its Effect on Gas Transport Properties Jennifer R Weidman, Shuangjiang Luo, Qinnan Zhang, and Ruilan Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04946 • Publication Date (Web): 30 Jan 2017 Downloaded from http://pubs.acs.org on February 7, 2017

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Industrial & Engineering Chemistry Research

Structure Manipulation in Triptycene-Based Polyimides through Main Chain Geometry Variation and its Effect on Gas Transport Properties

Jennifer R. Weidman, Shuangjiang Luo, Qinnan Zhang, Ruilan Guo*

University of Notre Dame, Department of Chemical and Biomolecular Engineering, Notre Dame, IN 46556

* Corresponding author, +1-574-631-3453 (tel), +1-574-631-0317 (fax), [email protected]

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ABSTRACT Two new triptycene-based polyimides, 6FDA-1,4-trip_ortho and 6FDA-2,6-trip_para, were synthesized to investigate the effect of varying polymer backbone geometry on chain packing and gas transport properties. Changing the imide linkage geometry from para to ortho reduced gas permeabilities by ~48% due to more efficient chain packing of the asymmetric ortho structure, which is demonstrated by decreased d-spacing and fractional free volume. Varying the triptycene orientation from the 1,4- to 2,6-connection also caused a decrease in permeability (e.g., 29% decrease for PCO2). This is likely the result of reduced chain mobility, as evidenced by increased Tg, and a shift in free volume distribution towards smaller cavities, as supported by smaller d-spacing. Physical aging studies show that equilibrium specific volume of these isomeric polymers is similar, as evidenced by nearly identical gas transport properties exhibited by all aged samples.

Keywords: triptycene, polyimide, linkage geometry, gas separation membrane

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1. INTRODUCTION The use of polymeric membranes in gas separations as a viable alternative to traditional industrial separation techniques (e.g. cryogenic distillation, pressure-swing adsorption) has been subject to widespread research efforts in the past several decades. While membrane separation offers advantages such as reduced energy consumption and lowered costs, and has been successfully implemented in applications including hydrogen recovery from ammonia purge gas and air separation for nitrogen enrichment, it still faces many challenges.1–4 The greatest obstacle for the gas separation membrane field is the sacrifice of high selectivity for high gas permeability, and vice versa.5,6 In order to reduce the membrane area required for a given separation and produce a high purity product, it is desired to create new polymer materials with simultaneously enhanced permeability and selectivity. This problem can be addressed through the careful manipulation of macromolecular structure. Enhancements in gas permeability can be obtained through increasing fractional free volume (FFV) by introducing structural elements that disrupt chain packing, such as bulky moieties, sterically-hindered linkage geometries, and rigid, kinked main chain segments.7 In order to achieve desired selectivities, it is critical to create a narrow size distribution of these free volume elements. A macromolecular design motif that has gained a lot of attention in recent years to tackle both of these criteria simultaneously is the use of triptycene, a shape-persistent structural unit that introduces intrinsic and tailorable microcavities in resulting polymers.8–20 Triptycene is a bulky, rigid, 3-dimensional molecule comprised of three benzene “blades” connected by a central hinge, resulting in a pinwheel shape.21–25 Polymers containing triptycene in their backbone structures usually have high fractional free volume due to the highly disrupted chain packing by the bulky triptycene units; additionally, the unique internal free volume (IFV) elements delineated by the clefts between each pair of benzene blades tend to add extra amount of intrinsic free volume facilitating high gas permeabilities.25 More importantly, the IFV of the triptycene clefts is similarly sized to common gas penetrants and, therefore, can aid in improving sieving capability.10,11,19,20,25 Another attractive aspect of triptycene is the wide array of options for chemical synthesis, including variations to the connection points for polymer backbone linkage and introduction of substituent groups.



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We previously reported on several series of triptycene-based polymers with the 1,4triptycene connection,10,19,20 which links the triptycene moiety on either side of a single benzene blade, as shown in Figure 1. This backbone orientation generates effects similar to those found from incorporating a bulky substituent group, since the triptycene central hinge and two benzene blades are left protruding from the main chain. We have demonstrated in these studies that triptycene structure is very useful and versatile in constructing desired free volume architecture to provide tailorable gas transport properties that meet various separation needs. Other studies have made use of the 2,6-triptycene connection (Figure 1), which builds the contortion center into the polymer backbone, resulting in a rigid, kinked main chain structure that has shown promising gas transport properties.8,9 Additionally, the KAUST-PI series, which technically connects the triptycene unit at four points (2,3,6,7), but still utilizes the same orientation and idea of integrating the triptycene contortion site into the polymer backbone, produced combinations of high permeabilities and high selectivities residing well above the upper bound.12,13 These studies provide motivation for further investigation of the effect of varying triptycene orientation and points of connection. Another design strategy that has been employed in the literature to increase steric hindrance in polymer chain packing is the utilization of ortho geometry, either through bulky substituents ortho to the imide ring or through ortho linked moieties in the main chain.12,26–30 The effects of ortho linkages include restricted rotation around the imide bond, nonplanar structures reducing the effects of charge transfer complex (CTC) interactions, and looser polymer chain packing.

Figure 1. (a) The triptycene molecule and (b) the depiction of two common orientations utilizing the 1,4- and 2,6-connection in triptycene-based polymers This study provides further insight into the effects of varying the polymer backbone geometry on gas transport properties, specifically for triptycene-based polyimides. Two new isomeric triptycene-based diamine monomers were synthesized and used along with a 3 ACS Paragon Plus Environment

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commercial dianhydride, 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), to produce polyimides. The two new triptycene-based polyimides were compared to our previously reported 6FDA-1,4-trip_para polyimide 10,11 to form a series of isomeric polyimides that systematically investigate the effects of varying the backbone orientation from the 1,4- to the 2,6-triptycene connection and the linkage geometry from para to ortho. The different chain packing induced by varying linkage geometry or chain tortuosity is expected to alter the overall fractional free volume, as well as free volume size distribution, in the corresponding polyimides, leading to finely tailored gas transport properties. To investigate these effects, the polymers were evaluated for their microstructures, including d-spacing and fractional free volume, to correlate with their gas transport properties (i.e., permeability/selectivity, diffusivity and solubility coefficients). 2. EXPERIMENTAL 2.1. Materials The aromatic dianhydride, 2,2’-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) was purchased from Akron Polymer Systems. To cyclize any possible o-diacid impurities, the 6FDA was dried under vacuum at 180 °C overnight before use. From Fisher Scientific, 1-methyl-2-pyrrolidinone (NMP) was purchased and used as received. Ethyl acetate, anhydrous sodium sulfate, and anhydrous dichloromethane were purchased from EMD and used as received. Ethyl ether was purchased from BDH and used as received. Sodium carbonate and 1-fluoro-2-nitrobenzene were purchased from Alfa Aeser and used as received. Potassium carbonate was also purchased from Alfa Aeser and was stored in a convection oven at 90 °C until use. All other materials were purchased from Sigma Aldrich and used as received. 2.2. Synthesis of Triptycene-based Diamine Monomers Following the reaction schemes shown in Scheme 1, two new triptycene-based diamine monomers (2 and 9) were synthesized for use in the polycondensation reactions to produce triptycene-based polyimides with varying backbone geometries. To investigate the ortho configuration between the imide and ether bonds, 1,4-bis(aminophenoxy)triptycene (2, referred to as 1,4-ortho diamine in following discussions) was synthesized using an adaptation of our synthesis protocol from a previous study,10 which is shown in Scheme 1(a). Synthesis details are 4 ACS Paragon Plus Environment


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given in the supporting information (SI). The second diamine explores the effect of integrating the contortion site of the triptycene unit by utilizing the 2,6 connection (see Figure 1). The synthesis procedure for 2,6-bis(aminophenoxy)triptycene (9, referred to as 2,6-para diamine in following discussions), shown in Scheme 1(b), is based on the procedures reported in the literature. 12,31,32 Scheme 1. (a) Synthesis of triptycene-1,4-diamine_ortho monomer (b) Synthesis of triptycene-2,6-diamine_para monomer

2,6-Dihydroxyanthracene (3): In a 500 mL 2-neck round bottom flask equipped with a stir bar and nitrogen protection, sodium borohydride (7.08 g, 0.19 mol) was added to 155 mL of 1M sodium carbonate solution. Next, anthraflavic acid (3 g, 0.012 mol) was added in batches to the solution, and the mixture was allowed to stir at room temperature overnight. In an ice bath, the reaction mixture was poured into 13 mL 6M hydrochloric acid and covered with 13 mL ethyl acetate. The aqueous layer was extracted with ethyl acetate three times. The combined organic layers were then washed with saturated sodium bicarbonate solution, dried with anhydrous


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Na2SO4, and concentrated under reduced pressure to obtain 3 as a light brown powder, which was dried under vacuum at 60 °C overnight (yield = 88%). 1H spectrum available in SI. 2,6-Dimethoxyanthracene (4): 2,6-dihydroxyanthracene (3) (2.31 g, 0.011 g), dried potassium carbonate (19.74 g, 0.14 mol), and 120 mL acetone were added to a 250 mL 2-neck round bottom flask equipped with a stir bar and nitrogen protection. Dimethyl sulfate (27.72 g, 0.220 mol) was added drop-wise while the reaction mixture was stirred. Next, the reaction mixture was heated to 54 °C and allowed to reflux for 30 h. After cooling, the mixture was precipitated in 125 mL water, the solid was collected via vacuum filtration, and washed in water. Dimethoxyanthracene (4) was collected as a light yellow/tan powder and dried under vacuum at 70 °C overnight (yield = 78%). 1H spectrum available in SI. 2,6-Dimethoxytriptycene (6): A dry 500 mL 2-neck flask was connected to a nitrogen purge, and 2,6-dimethoxyanthracene (4) (1.57 g, 0.0066 mol), 1,2-epoxypropane (16.1 g, 0.28 mol), and dichloroethane (150 mL) were added. This mixture was set aside while the diazonium salt, 2carboxybenzenediazonium (5), was synthesized. Compound 5 is a heat- and shock-sensitive material, so caution should be exercised during this synthesis. In particular, the filter cake should not become completely dry, a wet towel should be kept within reach, the powder should not be scraped, and a safety shield should be employed.33–35 To make 5, anthranilic acid (7.23 g, 0.053 mol) and 80 mL ethanol were added to a 250 mL 2-neck round bottom flask equipped with a stir bar. The solution was submerged in an ice bath, and 5.3 mL concentrated HCl and 13.2 mL iospentyl nitrate were added. The mixture was allowed to stir for 10 minutes. Next, 80 mL of ethyl ether was added, followed by an additional 5 minutes of stirring. The resulting salt (5) was carefully filtered, washed on the filter paper with additional ether, and lightly dried for a few minutes under low vacuum. The salt was then directly transferred from the filter paper to the 500 mL flask containing 4 and 1,2-epoxypropane in dichloroethane. The reaction mixture was heated to 40 °C, followed by step-wise heating to a final temperature of 83 °C where it refluxed for 4 h. The solvent was then evaporated and the product was purified using column chromatography over silica gel using an eluent of dichloromethane:hexanes = 3:2 (yield = 76%). 1H spectrum available in SI. 2,6-Dihydroxytriptycene (7): A 250 mL 2-neck flask equipped with a magnetic stir bar was flame-dried and attached to a nitrogen purge to cool. Once cool, 2,6-dimethoxytriptycene (6) 6 ACS Paragon Plus Environment


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(1.58 g, 0.005 mol) and anhydrous dichloromethane (50 mL) were added to the flask and cooled to 0 °C. Then, boron tribromide (7.55 g, 0.03 mol) was added drop-wise. The mixture was allowed to stir and gradually come to room temperature. After 2.5 h, the reaction was quenched with water, extracted three times with a mixture of ethyl acetate:dichloromethane = 1:1, dried, and concentrated. The product (7) was collected and dried under vacuum at 60 °C overnight (yield = 89%). 1H spectrum available in SI. 2,6-bis(nitrophenoxy)triptycene (8): 2,6-dihydroxytriptycene (7) (1.76 g, 0.006 mol), dried potassium carbonate (1.87 g, 0.014 mol), and dimethylformamide (18 mL) were added to a 100 mL 2-neck round bottom flask equipped with a magnetic stir bar. The mixture was allowed to stir at room temperature for 40 min and then 1-fluoro-4-nitrobenzene (1.73 g, 0.012 mol) was added. The reaction mixture was heated to reflux under nitrogen protection at 151 °C for 8 h. After cooling, the mixture was precipitated in a mixture of methanol and water (1:1 v/v, 100 mL). The solid was then washed in methanol, collected, and dried under vacuum at 100 °C overnight. (yield = 75%). 1H spectrum available in SI. 2,6-bis(aminophenoxy)triptycene (9): 2,6-bis(nitrophenoxy)triptycene (8) (2.45 g, 0.005 mol), 10% Pd/C catalyst (0.12 g), and ethanol (100 mL) were added to a 250 mL 2-neck round bottom flask equipped with a magnetic stir bar and brought to reflux at 78 °C. Hydrazine monohydrate (4.04 g, 0.081 mol) was then added dropwise. The reaction mixture was allowed to reflux overnight. The ethanol was then removed under vacuum. The product was dissolved in a minimal amount of hot DMF and the catalyst was removed via vacuum filtration through packed Celite®. The DMF solution as then precipitated in methanol. The diamine monomer (9) was collected and dried under vacuum at 100 °C overnight (yield = 72%). 2.3. Synthesis of Triptycene-based Polyimides Three triptycene-based polyimides were synthesized via polycondensation reactions using chemical imidization between the triptycene diamine monomers and the commercial 6FDA dianhydride, as shown in Scheme 2. In a typical case of 6FDA-1,4-trip_ortho polyimide synthesis, a 125 mL three-neck round bottom flask equipped with a mechanical stirrer was flame-dried and connected to a nitrogen purge to cool. Once cool, the 1,4-ortho diamine (2) (1.000 g, 0.002 mol) was added to the flask and dissolved in anhydrous N,N-Dimethylacetamide (DMAc) (8.9

mL) at room temperature. The flask was then cooled to 0 °C and 6FDA (0.948 g, 7 ACS Paragon Plus Environment

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0.002 mol) was added. The mixture was allowed to stir for 6 h, while coming to room temperature, which resulted in a viscous poly(amic acid) solution. Imidization reagents, acetic anhydride (1.306 g, 0.013 mol) and pyridine (1.013 g, 0.013 mol), were then added and the solution was allowed to stir for ~20 h at room temperature. The polyimide, 6FDA-1,4-trip_ortho, was then precipitated in methanol (800 mL), collected, and dried under vacuum at 180 °C for 24 h. The 6FDA-2,6-trip_para polyimide was synthesized following the same procedures using 6FDA and triptycene-2,6-diamine_para (9). Detailed synthesis of 6FDA-1,4-trip_para polyimide is reported in our previous study.10 Scheme 2. Synthesis of 6FDA-triptycene polyimides

2.4. Film Casting Thin polymer films were fabricated using the solution casting method. The polymer fibers were dissolved in NMP (~7%, w/v) at room temperature, filtered through a 0.45 µm PTFE syringe filter, and spread as a thin layer onto a leveled glass plate. Film formation was facilitated through the use of an IR lamp to evaporate the solvent slowly overnight. The film was then removed from the glass plate and dried under vacuum at 180 °C for 24 h. To remove any residual NMP, non-solvent exchange was carried out by soaking the films in methanol for 24 h, followed by an additional drying under vacuum at 180 °C for 24 h. Film thicknesses ranged from 78 to 103 microns. 2.5. Characterization



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H nuclear magnetic resonance (1H NMR) experiments were performed on a Bruker 400

MHz or 500 MHz NMR spectrometer at room temperature using DMSO-d6 or CDCl3 as a solvent. Size exclusion chromatography (SEC) was used to determine molecular weights of the polymers in tetrahydrofuran with a polystyrene standard at 35 °C. Differential scanning calorimetry (DSC) was performed with a TA Instruments DSC 2000 from 25 – 400 °C at a heating rate of 10 °C/min in a nitrogen atmosphere. The midpoint of the change in slope on the second heating cycle was used to evaluate the glass transition temperature. Thermal gravimetric analysis (TGA) was performed on a TA Instruments TGA 500. The polymer films were tested in a nitrogen atmosphere from 50 – 800 °C at a heating rate of 10 °C/min. The polymer chain packing was characterized by analyzing interchain d-spacing obtained from wide-angle x-ray diffraction (WAXD). A D8 Advance Davinci diffractometer with Cu Kα radiation was used with operation at 40 mA and 40 kV and a scan speed of 5 seconds/step and 0.02° per step. Values of 2θ ranged from 5° to 45°. Bragg’s law was used to calculate the dspacing, as follows:36

=

(1)

where λ is the wavelength of 1.5418 Å for CuKα radiation and θ is the scattering angle corresponding to the peak, as read directly from the graph. Polymer film densities were determined experimentally using the buoyancy method with an analytical balance (ML204, Mettler Toledo) coupled with a density kit. Deionized water was chosen as the buoyant liquid because there is negligible uptake in all film samples after being submerged for 24 h. The densities were then used in the following equation to determine the polymers’ fractional free volume (FFV):

=

(2)

where VO is the occupied volume of the polymer and V is the specific volume of the polymer. To calculate VO, Bondi’s group contribution method was used: = 1.3 ∑ 9 ACS Paragon Plus Environment

(3)

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where VVDW is the van der Waals volume of each structural group in the polymer.37–39 2.6. Pure Gas Permeability Measurements The triptycene-based polyimides were evaluated for their pure gas permeabilities using a constant-volume, variable-pressure system operated at 35 °C. The polymers were tested for their fresh and aged gas transport properties. Separate samples were cut from the film for the fresh and aged samples. The samples were stored at atmospheric conditions during the aging process. The polyimide films were tested with selected gases H2, CH4, N2, O2, and CO2 (tested in this order). Prior to permeability measurement, the system was degassed overnight, the downstream was isolated, and the pressure rise in the permeate volume was monitored over time. This rate of

pressure increase was determined to be the system leak rate,

. The permeability was

then determined by introducing the test gas upstream of the sample at a fixed pressure (ranging from 30 up to 230 psig), waiting for steady state (at least 6 times of the lag time, θ), and

monitoring the pressure increase in the permeate volume over time, . Pure gas permeability was calculated as follows: "# =

$

%

( − #&'

*

(4)

where Vd is downstream volume (cm3), l is membrane thickness (cm), p2 is upstream absolute pressure (cm Hg), A is the area of the film accessible to gas transport (cm2), R is the gas constant (cm Hg · cm3/ (cm3 STP · K)), T is temperature (K), and (dp1/dt)ss and (dp1/dt)leak are steady-state rates of pressure rise (cm Hg/s) in the downstream volume at fixed upstream pressure and when the system is sealed under vacuum, respectively.40 The pure gas permeabilities were used to determine the ideal selectivities of the polyimide films using the following equation: .

+#/- = ./

(5)

0

where αA/B is the ideal selectivity of the gas pair in which A represents the more permeable gas.41 The diffusion coefficients (D) of these polyimide films were determined for N2, CH4, and CO2 by measuring lag times, using Equation 6. 40,42 Generally, lag times were determined at 30

psig and 35 °C; however, in the cases of low permeation rates, i.e.,

higher pressure of 130 psig was used. These instances are noted in the data.


/

> 0.15, a


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%

4 = 5

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(6)

where l is the membrane thickness (cm) and θ is the time lag (s). Then using the solutiondiffusion model of gas transport in dense films, the solubility coefficients (S) could be determined by: " =4×7

or 7 = "/4

(7)

Additionally, the respective diffusivity selectivity (+,#/- ) and solubility selectivity (+9,#/- ) can be determined as follows:

9

+,#/- = / and +9,#/- = 9/ 0

0

(8)

3. RESULTS AND DISCUSSION 3.1. Synthesis and Polymer Thermal Properties The triptycene-based diamine monomers were designed by taking advantage of the unique paddlewheel-like molecular configuration of the triptycene unit to investigate the effect of linkage geometry variations in the backbone. Three isomeric triptycene-based diamine monomers with different linkage geometry are considered in this study, i.e., 1,4-ortho diamine (2), 2,6-para diamine (9), and 1,4-para diamine reported in our previous study.10 As such, comprehensive comparisons of para vs. ortho and 1,4- vs. 2,6-linkage can be made to elucidate the fundamental structure-property relationships of the corresponding new polyimides. The two new diamines (2 and 9) were synthesized in high purity via the routes shown in Scheme 1, and their structures were confirmed using 1H NMR with unambiguous peak assignment (Figure 2).


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Figure 2. 1H NMR spectra of (a) 1,4-ortho diamine 2 and (b) 2,6-para diamine 9 in DMSO-d6 All triptycene-based diamine monomers were combined with 6FDA in polycondensation reactions (Scheme 2) to produce corresponding polyimides with varying backbone linkage geometry, namely, 6FDA-1,4-trip_ortho, 6FDA-2,6-trip_para, and 6FDA-1,4-trip_para.10 The 1

H NMR spectra of the two new triptycene-based polyimides are shown in Figure 3, which

confirm their fully imidized structures. These isomeric polyimides are comprehensively compared to investigate the dependence of physical and transport properties on the backbone linkage geometry. In particular, comparing the new 6FDA-1,4-trip_ortho polyimide with 6FDA1,4-trip_para from our original study10 examines the effect of backbone symmetry by switching from a symmetric para linkage to an asymmetric structure, where the bulky phenoxytriptycene moiety is ortho-positioned with respect to the imide ring. The 6FDA-2,6-trip_para polyimide integrates the intrinsic contortion site of the triptycene unit into the polymer backbone by utilizing two separate benzene blades of triptycene as connection points. Comparisons between 1,4- and 2,6-linked structures provide further insights into the role of the triptycene structural unit in regulating the overall chain packing in triptycene-containing polymers. 12 ACS Paragon Plus Environment


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Figure 3. 1H NMR spectra of (a) 6FDA-1,4-trip_ortho and (b) 6FDA-2,6-trip_para polyimides in DMSO-d6 It is important for polymer membranes to have excellent thermal stability and high glass transition temperatures (Tg) for practical separation processes, such as hydrogen purification and natural gas sweetening.2 As shown in Table 1, the triptycene-based polyimides adhere well to these requirements with thermal decomposition temperatures at 5% weight loss of 508 °C and 478 °C and glass transition temperatures of 290 °C and 320 °C for 6FDA-1,4-trip_ortho and 6FDA-2,6-trip_para, respectively. Moving from the 1,4-trip_para to the 1,4-trip_ortho geometry caused a slight decrease in Tg, which is probably a result of two competing effects of the ortho linkage. First, ortho linkages have been shown to cause greater steric hindrance, limiting rotation around the imide bond, which can cause an increase in Tg. However, the steric hindrance introduced by the ortho linkage can result in a nonplanar conformation between the imide ring and diamine moiety, reducing the charge-transfer complex (CTC) formation between the acid dianhydride moieties and the diamine moieties typical for polyimides.30,43,44 This reduction in chain interactions can consequently lower Tg. The observed Tg reduction seems to suggest that 13 ACS Paragon Plus Environment

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the CTC effect is more significant in determining the chain rigidity. Conversely, given the same para geometry, switching from the 1,4-triptycene orientation to the 2,6-connection causes an increase in Tg from 300 to 320 °C. This can likely be attributed to the incorporation of configuration-based kinked, yet rigid, structure of the 2,6 connection into the polymer backbone. Additionally, the interchain interactions in the 2,6-linked polyimide are likely enhanced due to the interlocking effect of the triptycene blades, leading to increased Tg. This supramolecular effect will be discussed further in the following sections. Table 1. Molecular Weights and Thermal Properties of the 6FDA-Triptycene Polyimides Mn (g mol-1)

PDI

Td, 5% (°C)

Td, 10% (°C)

Tg (°C)

6FDA-1,4-trip_ortho

7.27 × 104

1.7

508

532

290

6FDA-1,4-trip_para10

4.18 × 104

3.0

528

546

300

6FDA-2,6-trip_para

1.50 × 104

2.2

478

525

320

3.2. Chain Packing and Fractional Free Volume (FFV) High fractional free volume polymers, particularly those with a narrow size distribution of free volume elements, are highly desired for gas separation membranes to facilitate fast diffusion and enable superior sieving capability. Our recent studies have demonstrated that incorporating the rigid, bulky iptycene units in polyimide backbones is very useful in constructing tailorable free volume architectures that feature bimodal size distribution of free volume microcavities.11,19,45 The small size microcavities are from the internal free volume defined by the triptycene clefts, which are similar in size to many gas molecules, enhancing selectivity. The large microcavities are brought about from disrupted chain packing due to the steric hindrance introduced by the bulky triptycene moiety, increasing free volume and improving permeability. In this study, the chain packing and free volume trends of the two new triptycene-based polyimides were investigated using wide-angle X-ray diffraction, density measurements, and the group contribution method estimation of FFV. Wide-angle X-ray diffraction patterns for fresh 6DFA-1,4-trip_ortho and 6FDA-2,6trip_para polyimides are shown in Figure 4 and the calculated d-spacing values are listed in Table 2. For the 1,4-ortho polymer, three distinct peaks (labeled A, B, and C) are identified and 14 ACS Paragon Plus Environment


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in the 2,6-para pattern, there is a left shoulder (A), two main peaks (B and C), and a right shoulder (D). Peak A is positioned around 15° and corresponds to d-spacing values of 5.8 Å and 6.2 Å for 1,4-ortho and 2,6-para polyimides, respectively. These large interchain distances can be attributed to the main amorphous packing of the chain segments containing the bulky triptycene moieties. A domain of more compact packing, most likely due to chain segments lacking the triptycene moieties, is seen at peak position B (d-spacing of 4.5 Å for 1,4-ortho and 5.2 Å for 2,6-para). Peak C corresponds to a d-spacing of ~4.0 Å, which is characteristic of π-π stacking.46–48 Lastly, the 2,6-para polymer shows a domain of high packing efficiency with a dspacing value of 3.3 Å, which is likely due to the close partial contact of the benzene blades perpendicular to the polymer backbone as illustrated in Figure 5 (indicated by the blue circle). This phenomenon is also noted in a similar 2,6-linked triptycene polyimide reported earlier.8

Figure 4. Wide-angle X-ray diffraction patterns for the fresh films of triptycene-based polyimides Table 2. Wide-angle X-ray diffraction (WAXD) 2θ and d-spacing, density, and fractional free volume (FFV) of the fresh films of 6FDA-triptycene polyimides.

1,4-ortho 1,4-para 10,11 2,6-para

A 15.3 13.7 14.4

2θ (°) B C 19.8 22.6 16.5 17.1 22.4

D 26.8

d-spacing (Å) A B C D 5.8 4.5 3.9 6.5 5.4 6.2 5.2 4.0 3.3


Density (g cm-3)

FFV (%)

1.402 ± 0.005 1.349 ± 0.006 1.347 ± 0.001

12.5 15.6 15.6

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Comparing the d-spacing for the three triptycene-based polyimides can shed light on the effects of linkage geometry (ortho vs. para) and triptycene connection (1,4 vs. 2,6). Both new triptycene-based polyimides showed smaller interchain distances for domains at peak A and B, compared to the previous 6FDA-1,4-trip_para polyimide. Changing the linkage geometry from 1,4-para to 1,4-ortho causes a significant decrease in d-spacing from 6.5 to 5.8 Å and from 5.4 to 4.5 Å for peaks A and B, respectively. This is reasonable since 6FDA-1,4-trip_ortho has a lower Tg, suggesting reduced charge transfer (CT) interactions and higher chain mobility that allows for more efficient chain packing relative to its para counterpart. In a similar look at geometric structure/property relationships, previous studies have shown that switching to the asymmetric meta linkage from the para linkages in aromatic polymers, such as polysulfones, causes more efficient packing and effectively lowers FFV, Tg, and permeabilities and increases selectivities.42,49,50 It seems that in the case of the 6FDA-triptycene polyimide series, the asymmetric ortho-linkage geometry likewise results in more compact chain packing. Varying the triptycene orientation from the 1,4-para to 2,6-para connection causes a slight reduction in the dspacing as well (6.5 to 6.2 Å for peak A and 5.4 to 5.2 Å for peak B). As depicted in Figure 5, when the triptycene moiety is integrated into the polymer backbone via the 2,6-position, only one benzene blade is left free to protrude and facilitate in chain packing disruption, whereas the 1,4 connection leaves two blades extended away from the main chain, which can account for this difference in interchain distances.

Figure 5. Visual interpretation of different packing phenomena suggested by experimental data for the triptycene-based polyimides with 2,6- and 1,4-connections. The blue circle portrays the close partial contact of the triptycene blades perpendicular to the polymer backbone, i.e. “interlocking” of the blades. The yellow highlighting represents greater potential for partial filling by neighboring polymer segments, particularly in the 2,6-orientation as all three IFV clefts are positioned closely to the main chain; green highlighting represents an open triptycene cleft that has been farther removed from the main chain due to the 1,4-orientation, making it less susceptible to being filled by the neighboring polymer chain.



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Polymer densities were measured using the buoyancy method and then used in the group contribution method to determine the FFV of the polymers. Because the repeat units are isomeric structures and have the same occupied volume, the FFV and density trends are simply inverse of one another. Therefore, only the FFV trends will be discussed in depth. As shown in Table 2, the triptycene connection (1,4 vs. 2,6) did not play a significant role in FFV, as 6FDA-1,4-trip_para and 6FDA-2,6-trip_para were nearly identical. Conversely, given the same 1,4-triptycene connection, utilizing the ortho linkage geometry causes a marked decrease in FFV when compared with the para geometry. This result is consistent with the trend seen in the WAXD data; the reduced interchain distance in the 1,4-ortho polymer backbone is reflected in an overall reduction in free volume of the para analog. 3.3. Analysis of Gas Permeation Properties Fresh and aged polymer films were tested for their gas transport properties using a constant-volume, variable-pressure system. Like our previous 6FDA-1,4-triptycene series,10,11 these triptycene-based polyimides operate primarily on the size-sieving mechanism, as indicated by decreasing permeability consistent with increasing gas kinetic diameter (i.e., PH2 > PCO2 > PO2 > PN2 > PCH4). Figure 6 shows a representative plot of permeability vs. feed pressure for a fresh 6FDA-1,4-trip_ortho film illustrating this trend. Additionally, for all samples considered in this study, the feed pressure has a very little effect on the permeability. In particular, all films did not show CO2-induced plasticization up to a feed pressure of 17 atm, which would have been characterized by an upturn in permeability at a given pressure (i.e. plasticization pressure point). Instead, CO2 permeability decreased with increasing feed pressure, which is consistent with dualmode sorption with no plasticization.42


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Figure 6. The dependence of the gas permeabilities of the fresh 6FDA-1,4-trip_ortho film on feed pressure. Pure gas permeabilities, diffusivity coefficients, and solubility coefficients for CH4, N2, and CO2 are shown in Table 3 for 6FDA-1,4-trip_ortho and 6FDA-2,6-trip_para, with 6FDA1,4-trip_para data11 for comparison. Gas transport data for H2 and O2 are available for select films in the Supporting Information. As previously discussed, subtle variations to the polymer backbone geometry (i.e., ortho vs. para; 1,4- vs. 2,6-triptycene connection) generate differences in chain packing and free volume in the polymer films, which carry through to the gas transport properties. This is expected because the free volume of the material is closely related to the amount of space available for diffusion; specifically, the gas permeability can be correlated to FFV in the following way:51,52

Structure Manipulation in Triptycene-Based Polyimides through Main

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