Synthesis, Characterization and Gas Transport Properties of

Article pubs.acs.org/IECR

Synthesis, Characterization and Gas Transport Properties of Polyamide-Tethered Polyhedral Oligomeric Silsesquioxane (POSS) Nanocomposites Parthasarathi Bandyopadhyay and Susanta Banerjee* Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India S Supporting Information *

ABSTRACT: Polyamide-tethered polyhedral oligosilsesquioxane (POSS) nanocomposites (PAMIP) with well-defined architectures have been synthesized through Michael addition reaction between the maleimide-containing fluorinated new copolyamides (PAMI) with amino-functionalized POSS (POSS-NH2) nanofiller. The chemical structures of the polyamide− POSS nanocomposites were characterized by 1H NMR and Fourier transform infrared spectroscopy. The effects of POSS-NH2 on the morphology and properties of the PAMIP membranes were examined by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction and transmission electron microscopy (TEM) analysis. POSS molecules tethered to polyamide form sphere shaped POSS aggregate cages (phase separated morphology) were confirmed by TEM analysis. All the PAMIP membranes showed very high permeability for different gases compared to PAMI membranes. The increase in permeability is accompanied by increase in diffusivity coefficient of the PAMIP membranes for different gases.

1. INTRODUCTION Polyhedral oligomeric silsesquioxane (POSS) is a nanoscale inorganic cage structure having a silicon−oxygen−silicon framework attached by organic substituents connected to silicon atoms.1,2 The commercially available POSS compounds are mainly octamers with the general formula (RSiO1.5)8 consisted of a rigid and cubic silica core, where R is a hydrogen atom or an organic functional group such as an alkyl, alkylene, acrylate, hydroxyl, or epoxide unit.1,3 There are two main kinds of octamers or octa-silsesquioxanes (R8Si8O12, T8) molecules used in the preparation of POSS-containing hybrid polymers: T8 POSS molecules having eight reactive or unreactive eight corner organic groups of the same kind, and those with one reactive group and seven unreactive groups (monofunctional POSS molecules).4 It is possible to prepare POSS containing hybrid polymers with POSS units attached as a dangling block to the polymer backbone by using monofunctional POSS molecules. The corner organic groups provide the POSS molecules with desired reactivity and solubility and help to disperse POSS molecules in hybrid polymers by physical/ chemical interaction with the polymer matrix.4−6 Polyhedral oligomeric silsesquioxanes can be used as monofunctional or graftable monomers, difunctional comonomers, surface modifiers, or polyfunctional cross-linkers for the preparation of POSS modified polymers.7 POSS molecules can be incorporated into the polymeric matrixes easily using chemical coupling, copolymerization, cross-linking or physical blending.1−7 The macrophase separation that usually occurs through the aggregation of POSS units can be avoided by the covalent bond formation between the POSS units and the polymers.3,4 The unique combination of an inorganic core and a organic periphery in POSS molecules has offered the opportunity to prepare high performance hybrid polymeric materials that unite many desirable properties of conventional organic and inorganic components such as high thermal and mechanical © 2014 American Chemical Society

properties, nonflammability, solubility and oxidative resistances with excellent dielectric properties.1−7 POSS containing hybrid polymers have several unique applications, such as in the field of biomedicine, electronic, optical, magnetic nanodevices, sensors and catalysts.3,8 POSS is a building block of zeolite; therefore, it can produce same sieving ability as that of zeolites.9 Moreover, smaller size and tailorable organic groups make POSS molecules more attractive and promising materials for gas separation study.6,9,10 Hybrid polymeric membranes synthesized from organic polymers and inorganic nanostructured POSS are attractive because it is reported that when small concentrations of hybrid polymers based on POSS structures are physically incorporated into organic polymer membranes (i.e., those based on the vinyl structure), the gas permeability coefficients increase several orders of magnitude, but an increase in permeability is accompanied by dramatic losses in the selectivity for different gas pairs.11,12 Until now, the majority of the research activity in the gas permeation area of polymer modified with POSS nanocomposites has been focused to study the effect of POSS nanoparticles on the permeability and selectivity of different gases through these nanocomposites, such as polyethylene, polypropylene, polystyrene and polyimides.6,11−14 Aromatic polyamides (aramids) are known for their high thermal, mechanical and solvent resistance properties and also known as high performance polymers.15−19 The reaction between monofunctionalized POSS with a reactive polymer provides another approach to building up polyamide-tethered POSS nanocomposites. Liu and Lee reported aromatic polyamide-tethered POSS nanocomposites via Michael addiReceived: Revised: Accepted: Published: 18273

June 13, 2014 October 13, 2014 October 29, 2014 October 29, 2014 dx.doi.org/10.1021/ie503475k | Ind. Eng. Chem. Res. 2014, 53, 18273−18282

Industrial & Engineering Chemistry Research

Article

Scheme 1. Reaction Scheme and Structures of PAMIs (PAMI-a to PAMI-d)

(99%) and PSS-(3-(2-aminoethyl)-amino)propylheptaisobutyl substituted polyhedral oligomeric silsesquioxane (POSS-NH2) were purchased from Sigma-Aldrich (USA) and triphenyl phosphite (TPP), N,N-dimethylacetamide (DMAc), acetic anhydride, sodium acetate and glacial acetic acid were purchased from E. Merck, India. Toluene (E. Merck, India) was dried by refluxing over sodium metal. Pyridine (E. Merck, India) was purified by stirring with NaOH and distilled under reduced pressure. CaCl2 (E. Merck, India) and anhydrous K2CO3 (E. Merck, India) were dried for 12 h at 120 °C prior to use. Methanol (Rankem, India) was used for precipitation of polymers. 1-Methyl-2-pyrrolidinone (NMP) (E. Merck, India) was purified by stirring with NaOH and distilled from P2O5 prior to use. The compound 4-fluoro-4′-nitro-3-trifluoromethylbiphenyl was prepared according to the procedure reported earlier.20 Four different bis(ether amine), 1,4-bis-[{2′-trifluoromethyl 4′-(4″-aminophenyl)phenoxy}]phenyl, 4,4-bis-[2′-trifluoromethyl 4′-(4″-aminophenyl)phenoxy]biphenyl, 1,4-bis[{2′-trifluoromethyl 4′-(4″-aminophenyl)phenoxy}]2,5-di-tertbutylbenzene and 9,9-bis-[3-phenyl-4-{2′-trifluoromethyl 4′(4″-aminophenyl)phenoxy}phenyl]fluorene were synthesized according to procedures reported in our earlier publications.16−19 The 5-maleimidoisophthalic acid (MIPA) was synthesized according to a procedure reported in the literature.21 2.2. Measurements and Characterization. 1H NMR spectra was recorded on a Bruker 400 and 200 MHz instrument (Switzerland) using DMSO-d6 as the solvent. FTIR spectra of the copolyamide (PAMI) and nanocomposite (PAMIP) were recorded from a NEXUS 870 FTIR (Thermo Nicolet) spectrophotometer at room temperature. Gel permeation chromatography (GPC) was performed with a Waters GPC instrument (Waters 2414). Tetrahydrofuran (THF) was used as an eluent (flow rate 0.5 mL/min, polystyrene was used as a standard and an RI detector was used to record the signal). Tensile strength and elongation at break of thin polyamide membranes were evaluated with the help of an UTMINSTRON, PLUS, Model No. 8800 instrument. Test samples with dimensions of 10 mm × 25 mm and a thickness of around

tion reaction of maleimide-containing polyamides with 1aminopropylheptaisobutyl polyhedral oligosilsesquioxane.15 The present work reports the synthesis of a series of new maleimide containing copolyamides (PAMIs) and their nanocomposites using PSS-(3-(2-aminoethyl)amino)heptaisobutyl substituted polyhedral oligomeric silsesquioxane (POSS-NH2) through Michael addition reaction as reported by Liu and Lee.15 Besides, this work also reports the gas transport properties of the prepared PAMIs and PAMIPs. In our previous reports, we described the synthesis, characterization and gas transport properties of four structurally different fluorinated polyamide (PA) membranes starting from 4,4′(hexafluoroisopropylidene)bis(benzoic acid) with different fluorinated diamines namely, 1,4-bis-[{2′-trifluoromethyl 4′(4″-aminophenyl)phenoxy}]phenyl, 4,4-bis-[2′-trifluoromethyl 4′-(4″-aminophenyl)phenoxy]biphenyl, 1,4-bis-[{2′-trifluoromethyl 4′-(4″-aminophenyl)phenoxy}]2,5-di-tert-butylbenzene and 9,9-bis-[3-phenyl-4-{2′-trifluoromethyl-4′-(4″aminophenyl)phenoxy}phenyl]fluorene, respectively. 16−19 Four PAMIs were prepared using the 4,4′(hexafluoroisopropylidene)bis(benzoic acid) (HFA) and 5maleimidoisophthalic (MIPA) with above four fluorinted diamines, repectively. Finally, PAMIs reacted with a fixed amount POSS-NH2 via Michael addition reaction to prepare PAMIPs. Different instrumental techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and 1H NMR were used to study the formation of the polyamide-tethered POSS nanocomposites. Thermal and mechanical properties of these hybrid membranes were also studied. Besides, the gas transport properties of these PAMIPs membranes were systematically investigated and compared with their parent PAMIs membranes to study the effect of addition of POSS toward the gas transport properties.

2. EXPERIMENTAL SECTION 2.1. Starting Materials. Maleic anhydride, 5-aminoisophthalic acid, 4,4′-(hexafluoroisopropylidene)bis(benzoic acid) (HFA), tetrakis(triphenylphosphine)palladium(0) 18274

dx.doi.org/10.1021/ie503475k | Ind. Eng. Chem. Res. 2014, 53, 18273−18282


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Scheme 2. Reaction Scheme and Structures of PAMIPs (PAMIP-a′ to PAMIP-d′)

80−85 μm were used for the measurement of tensile strength and percentage of elongation at break. Tests were done using a cross-head speed of 5% min−1 of the specimen length. Differential scanning calorimetry (DSC, NETZSCH DSC 200PC instrument) was used (heating rate of 20 °C min−1) to determine the glass transition temperature of the copolymers and nanocomposite under nitrogen. Glass transition temperatures (“Tg”) were taken at the midpoint of the step transition in the second heating run. A Pyris-Diamond TG/DTA (USA) thermal analyzer instrument was used (heating rate of 10 °C min−1) to measure the thermal stability of the polymers under synthetic air (N2:O2 = 80:20). Wide angle X-ray diffraction (WAXD) information was recorded by a Rigaku, Ultema III Xray diffractometer using a Cu Kα (0.154 nm) source. The Cu Kα source operated at 40 kV and 40 mA and the range of 2θ for XRD measurement was 10−40°. The average d-spacing (dsp) for the amorphous peak maxima was calculated using Bragg’s equation. Transmission electron microscopy (TEM) was undertaken for the ultramicrotome membranes using a TEM instrument (FEI-TECNAI G2 20S -TWIN) at an operating voltage of 100 kV. The hybrid membranes were ultramicrotomed under cryogenic conditions with a thickness of 100 nm and were embedded in carbon-coated copper grids for TEM analysis. The gas transport properties of the PAMI and PAMIP membranes were studied at 3.5 bar of applied gas pressure and at 35 °C using an automated diffusion permeameter (DP-100-A) manufactured by Porous Materials Inc., USA. Ultrahigh pure (99.99%), methane, nitrogen, oxygen and carbon dioxide gases from BOC Gases, India were used for the permeation study. The permeability coefficient, ideal perm selectivity (α), diffusion coefficient (D), ideal diffusivity selectivity (αD), solubility coefficient (S) and ideal solubility selectivity (αS) were determined according to reported

procedures.16−19 The reproducibility of the measurements was checked from three independent measurements using different effective membranes from different area of same membrane. It was observed that the reproducibility of the measurements was better than ±5% for different gases. 2.3. Synthesis of Copolyamides with Maleimide Pendent Groups (PAMIs). The reaction scheme and structures of PAMIs (PAMI-a, PAMI-b, PAMI-c and PAMId) have been depicted in Scheme 1. The copolymers were synthesized by reacting an equimolar amount of diamine with equimolar amounts of the mixture of two diacids (MIPA and HFA). The molar ratios of MIPA and 4,4′(hexafluoroisopropylidene)bis(benzoic acid) were 0.1 and 0.9, respectively. All these copolyamides were synthesized by phosphorylation polycondesation. A representative polymerization procedure of PAMI-a in detail is given in the Supporting Information. 2.4. Preparation of Polyamide-POSS Nanocomposites (PAMIPs). The reaction scheme and structures of the Polyamide-POSS nanocomposites (PAMIP-a′, PAMIP-b′, PAMIP-c′ and PAMIP-d′) are shown in Scheme 2. A detailed preparation procedure of PAMIPs is given in the Supporting Information. 2.5. Membrane Preparation. The polyamide−POSS nanocomposite (PAMIP) and pure copolyamide (PAMI) membranes were prepared by casting 10−15% (w/v) homogeneous polymer solutions in DMAc solvent onto clean glass Petri dishes. Petri dishes were placed in an oven at 80 °C and left for overnight and then heated at 120 °C for 24 h. Finally, the membranes were kept for 48 h in a vacuum oven (2 mbar) at 120 °C for complete removal of any residual solvent. The dense polymer membranes were removed by immersing the Petri dishes in boiling water and the membranes were kept 18275



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again under vacuum at 120 °C for 4 h. All membranes were between 80 and 85 μm thick. The membranes were used to record solubility tests, thermal analyses, tensile tests, X-ray diffraction, TEM and gas transport properties measurements.

addition reaction was confirmed by FTIR and 1H NMR spectroscopy. FTIR spectra of POSS-NH2, PAMI-a, and PAMIP-a′ are shown in Figure 1. FTIR spectra of all these

3. RESULTS AND DISCUSSION 3.1. Copolymer Synthesis and Characterization. Four copolyamides (PAMI-a, PAMI-b, PAMI-c and PAMI-d) were prepared by the direct polycondensation of different fluorinated diamines with a mixture of two diacids, HFA and MIPA (diacid:diamine = 1:1 mole ratio) in NMP as solvent and in the presence of TPP and pyridine as condensing agents. The molar ratio of diacids, i.e., MIPA/MIPA+HFA, was 0.1. The diamines used were namely, 1,4-bis-[{2′-trifluoromethyl 4′-(4″aminophenyl)phenoxy}]phenyl, 4,4-bis-[2′-trifluoromethyl 4′(4″-aminophenyl)phenoxy]biphenyl, 1,4-bis-[{2′-trifluoromethyl 4′-(4″-aminophenyl)phenoxy}]2,5-di-tert-butylbenzene and 9,9-bis-[3-phenyl-4-{2′-trifluoromethyl 4′-(4″aminophenyl)phenoxy}phenyl]fluorene. All these PAMIs were soluble [10% (w/v)] at room temperature in THF, DMAc, NMP, DMF and pyridine. All copolymers were also soluble in DMSO except PAMI-c. The number-average molecular weight (Mn), polydispersity index (PDI) and the inherent viscosity (ηinh) values of the PAMIs are reported in Table 1. Inherent viscosities of these synthesized

Figure 1. FTIR spectra of POSS-NH2, PAMI-a and PAMIP-a′.

nanocomposites showed characteristic absorption bands for Si−O−Si at 1109 cm−1 along with −CH3 (isobutyl group) absorptions appeared at 2954 and 2871 cm−1. The above fact supports the successful attachment of POSS into the polyamide chain.15 All these PAMIPs showed two signals (both are singlet) above 10.45 ppm corresponding to two different amide protons. These signals are assigned to be corresponding to amide protons of 4,4′-(hexafluoroisopropylidene)bis(benzoic acid) containing part and 5-maleimidoisophthalic acid containing part, respectively. The aromatic protons and the maleimide group protons exhibited signals from 8.1 to 7.1 ppm [Figure 2]. Protons of isobutyl moieties of the POSS containing part in all PAMIPs exhibited are identified at δ ∼ 0.57 ppm (-Si−CH2-), ∼ 0.88 ppm [-CH−(CH3)2], and ∼1.78 ppm (-Si−CH2−CH).2,15 Protons of the aminopropyl group of the POSS containing part in the PAMIPs are identified at ∼0.57 ppm (-SiCH2CH2CH2NH-) and at δ ∼ 1.78 ppm (-SiCH2CH2CH2NH-).15 Signals at ∼2.88 and at 2.73 ppm are assigned to the -HN−CH2−CH2−NH- and -SiCH2CH2CH2NH- protons. Although, the signals correspond to similar protons of POSS molecules in different PAMIPs slightly change their position and intensity. The above NMR peaks confirmed the Michael addition reaction between POSSNH2 and copolymer. Total percentage of POSS incorporation in PAMIPs was evaluated from their 1H NMR spectra. Total percentage of POSS incorporation into PAMIP-a′, PAMIP-b′ and PAMIP-d′ was 46.86, 55.16 and 69.55%, respectively (Table 2). Highest percentage of POSS incorporation has been found in PAMIPd′. This might be due to the extreme bulky nature of the diamine moiety in PAMI-d (parent polymer of PAMIP-d′). As a result, interchain distance between polymer repeat units in PAMI-d (parent polymer of PAMIP-d′) is higher compared to in PAMI-a (parent polymer of PAMIP-a′) and PAMI-b (parent polymer of PAMIP-b′). Therefore, bulky POSS molecules get enough space to react with PAMI-d (parent polymer of PAMIP-d′). Percentage of POSS incorporation in PAMIP-c′ was not evaluated due to its insolubility in DMSO-d6. 3.2.2. Polyamide-POSS Nanocomposite Morphology. Xray diffractograms of the PAMI, PAMIP membranes and POSSNH2 are displayed in Figure 3. There are three distinct

Table 1. Inherent Viscosity and Molecular Weight of the Copoly(ether amide)s (PAMIs) polymer

ηinh (dL g−1)

Mna (g mol−1)

PDIb

PAMI-a PAMI-b PAMI-c PAMI-d

0.68 0.74 0.63 0.66

77000 80500 63000 75000

2.6 2.3 2.2 2.0

ηinh, inherent viscosity 31 °C. aNumber-average molecular weight; polystyrene calibration. bPolydispersity index.

copolyamides were in the range of 0.63−0.74 dL/g. The number-average molecular weights of these copolymers were in the range of 63 000−80 500 with polydispersity index values between 2.0 and 2.6, which signifies high conversion of the monomers into polymer. Analytical details (FTIR, NMR) of these PAMIs have been reported in the Supporting Information. 3.2. Polyamide-POSS Nanocomposite Preparation and Characterization. Polyamide-POSS nanocomposites (PAMIP-a′, PAMIP-b′, PAMIP-c′ and PAMIP-d′) were synthesized by the reaction of four different maleimidecontaining copolyamides (PAMIs) with a fixed amount of POSS-NH2 via Michael addition reaction in THF (solvent) and using p-toluenesulfonic acid (pTSA) as a catalyst. Michael addition reaction between polymers having maleimide group with bulky POSS-NH2 was relatively slow and the yield of the product was not very high compared to the Michael addition reactions employing small molecules.15 It was attributed to the steric hindrance imparted by the bulky POSS group attached to polymer chain.15 POSS molecule inhibited additional POSS groups from reacting with polymer chains.15 Alike, the parent copolymers, the polyamide−POSS nanocomposites were also soluble in NMP, THF, DMF, DMAC and pyridine including DMSO (except for PAMIP-c′). 3.2.1. FTIR and 1H NMR Spectroscopy of PAMIPs. Polyamide-POSS nanocomposite formation through Michael 18276



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Figure 2. 1H NMR spectra of PAMIP-a′, PAMIP-b′ and PAMIP-d′, in DMSO-d6 (*1 and *2 signals for H2O and DMSO, respectively).

Table 2. Physical Properties of Copoly(ether amide)s and Copoly(ether amide)s-POSS Nanocomposite Membranes polymer PAMI-a PAMIP-a′ PAMI-b PAMIP-b′ PAMI-c PAMIP-c′ PAMI-d PAMIP-d′

% of POSS incorporationa 46.86 55.16 n.d. 69.55

Tgb (°C)

Td10c (°C)

TSd (Mpa)

Mode (GPa)

EB (%)f

263 269 256 270 280 272 271 272

361 402 362 373 365 387 434 438

82 74 71 64 83 56 86 64

1.86 1.62 1.77 1.45 1.78 1.32 1.87 1.48

21 20 8 12 8.0 9.3 7.0 6.5

a

Determined from 1H NMR spectra. bGlass transition temperature determined by DSC, heating rate at 20 °C/min. c10% degradation temperature measured by TGA in air, heating rate at 10 °C/min. d Tensile strength. eYoung’s modulus. fPercent of elongation at break. n.d. not determined due to insolubility in DMSO-d6.

Figure 3. X-ray diffraction curves of PAMI and PAMIP nanocomposite membranes.

structure of POSS-NH2 in the resulting nanocomposite membranes may be ascribed to the covalent bond formation between copolyamide and POSS-NH2.6,22 The diffraction peak (halo maxima) showed by PAMIPs membranes shifted to the smaller angle in comparison to their respective PAMI membranes. This was attributed to the polyamide chain structure in the composite material was slightly modified by

diffraction peaks at 2θ = 8.39°, 11.17° and 19.38° by POSSNH2. These diffraction peaks indicate the presence of crystalline structure of POSS-NH2. There was no such diffraction peak ascribed to the crystal of POSS-NH2 molecule in the WAXS patterns of the POSS modified copolymer membranes. Therefore, the disappearance of the crystalline 18277



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composite membrane are attributed to the silicon migration to nanocomposite membrane surfaces during membrane formation. Totally free movement of the POSS cages was not possible in polymer because POSS molecules were covalently bonded to the polyamide. Therefore, silicon migration could be performed only by means of dangling POSS group rotation around the polymer chain toward surfaces and the polymer chain-packing trend toward the direction of silicon groups facing surfaces.15 3.2.3. Thermal and Mechanical Properties. Thermal properties were investigated by DSC and TGA. TGA thermograms of PAMI and PAMIP membranes have been depicted in the Supporting Information (Figure S2). The 10% weight loss temperatures for these PAMI and PAMIP membranes were in the range of 361−434 °C and 373−438 °C, respectively, under synthetic air [Table 2]. The 10% weight loss temperatures for PAMIPs were higher than the PAMIs. It was reported in the literature that the POSS moiety improves the thermal stability of the polyimides.1,24,25 Therefore, the presence of POSS particles resulted in higher thermal stability of POSS modified polyamides (PAMIPs) compared to the parent polyamides (PAMIs).25 DSC plots of these PAMIs and PAMIPs are given in the Supporting Information (Figure S3). The POSS modified hybrid polymers PAMIP-a′ and PAMIP-b′ showed higher glass transition temperatures than their parent copolymers PAMI-a and PAMI-b (Table 2). Higher Tg values of PAMIP-a′ and PAMIP-b′ were attributed to the isobutyl groups of the POSS cage inhibit the local segmental motion of the polymer chains and increase the Tg values of the POSS modified polymers.15 The presence of POSS cages reduces the interchain interaction and therefore reduces the interchain H-bonding.15 So, the Tg value might also decrease. This was observed in case of PAMIPc′ in comparison to its parent copolymer PAMI-c. Wu et al. reported that, the variation of glass transition temperature in the random copolymers is the net result of all three effects: free volume, steric barrier and POSS with polymeric segment interaction.22 The reduction or increment in Tg values upon POSS modification might be explained by a combination of these structural characteristics. The reduction in Tg value is the addition of free volume effect by the POSS group. PAMIP-d′

the introduction of POSS and intermolecular main-chain spacing of PAMIPs expands compared to their parent PAMIs.22 The PAMI-POSS nanocomposites bulk morphology was also investigated with TEM analysis [Figure 4]. The spherical dark

Figure 4. TEM images of PAMIP membranes.

region represents the sphere shaped POSS aggregate cages. POSS-rich-domain formation in the polyamides was caused by POSS-cage phase separation from polyamides. This phase separation was attributed to the difference between hydrogenbonding interactions between the polar amide units of the polymer chain and van der Waals interactions between isobutyl groups of POSS molecule.7,15 The TEM micrographs provide direct evidence of polyamide−POSS nanocomposite formation. The TEM micrograph shows an isotropic structure with more or less spherical domains in case of PAMIP-a′.23 Generally, POSS fractions in the nanocomposites surfaces were higher than those in the nanocomposite bulk. The nonhomogeneous silicon (low surface energy) distributions in PAMIP nano-

Table 3. Gas Permeability Coefficients (P) measured at 35 °C (3.5 bar) and Permselectivities (α) Values of Copoly(ether amide)s and Copoly(ether amide)s-POSS Nanocomposite and Their Comparison with Other Reported Polyamides and Polyimide-POSS Nanocomposites polymer

P (CO2)

P (O2)

P (N2)

P (CH4)

α (CO2/CH4)

α (O2/N2)

PA “6b” PAMI-a PAMIP-a′ PA “II” PAMI-b PAMIP-b′ PA “Ib” PAMI-c PAMIP-c′ PA “B” PAMI-d PAMIP-d′ PI-POSS-I PI-POSS-II

21.4 28.4 51.0 28.7 19.6 49.4 157.0 115.0 137.6 67.4 52.2 56.0 43.6 42.5

5.2 8.3 11.3 8.5 4.9 10.4 40.0 26.0 32.0 15.0 11.9 15.4 20.8 18.3

0.65 1.16 1.74 1.20 0.79 1.42 5.20 3.48 4.70 1.70 1.60 2.1 3.93 3.65

0.58 0.47 1.44 1.10 0.34 1.14 5.10 2.89 5.10 1.33 0.97 1.23 1.81 1.54

36.89 60.40 35.42 26.09 57.65 43.33 30.78 39.79 26.98 50.67 53.81 45.53 24.09 27.60

8.00 7.15 6.49 7.08 6.20 7.32 7.69 7.47 6.80 8.82 7.43 7.33 5.29 5.01

ref 16 from do 17 from do 18 from do 19 from do 6 6

this study

this study

this study

this study

P = gas permeability coefficient in barrer. 1 barrer = 10−10 cm3 (STP) cm/cm2 s cm Hg. 18278



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Table 4. Gas Diffusion Coefficients (D) in 10−8 cm2/s, Solubility Coefficients (S) in 10−2 cm3 (STP)/cm3 cm Hg, Diffusivity selectivity (αD) Values and Solubility Selectivity (αS) Values of the Copoly(ether amide)s and Copoly(ether amide)s-POSS Nanocomposite at 35 °C and 3.5 bar CO2

O2

N2

CH4

(CO2/CH4)

(O2/N2)

polymer

D

S

D

S

D

S

D

S

αD

αS

αD

αS

PAMI-a PAMIP-a′ PAMI-b PAMIP-b′ PAMI-c PAMIP-c′ PAMI-d PAMIP-d′

4.90 8.63 2.71 4.51 13.27 24.24 5.10 7.35

5.80 5.91 7.23 10.95 8.67 5.68 10.24 7.62

6.07 11.76 4.19 4.93 15.00 35.70 6.63 10.75

1.37 0.96 1.17 2.11 1.73 0.90 1.79 1.43

1.05 1.41 2.06 2.47 3.67 7.32 2.14 2.94

1.10 1.23 0.38 0.57 0.95 0.64 0.75 0.71

0.75 0.99 1.39 1.42 2.04 4.36 1.61 2.81

0.63 1.45 0.24 0.80 1.42 1.17 0.60 0.44

6.53 8.72 1.95 3.18 6.50 5.56 3.17 2.62

9.21 4.08 30.13 13.69 6.11 4.85 17.07 17.32

5.78 8.34 2.03 2.00 4.09 6.10 3.10 3.66

1.25 0.78 3.08 3.70 1.82 1.41 2.39 2.01

S: gas solubility coefficients (S = P/D).

the polyamide−POSS nanocomposites (PAMIP) membranes compared to the PAMI membranes also supported by their Xray diffractograms (Figure 3). The diffraction maxima showed by PAMIPs membranes shifted to the smaller angle in comparison to their respective parent PAMI membranes. The POSS molecule creates free volume element larger enough to allow the diffusion of the larger gas molecules (CH4 and N2). Therefore, the PAMIP membranes exhibited comparatively lower selectivity (CO2 and CH4 gas pair) for the size based separation of the penetrant molecules due to presence of voluminous POSS molecules in comparison to the PAMI membranes. The decreasing order of diffusion coefficients of all gases through these PAMI and PAMIP membranes was observed to be as D (O2) > D (CO2) > D (N2) > D (CH4). For gases like O2, N2 and CH4; the diffusivity coefficient decreases with the increase of kinetic diameter of gas molecule. The diffusivity coefficient of CO2 was smaller than that of O2 although CO2 has a smaller kinetic diameter. This is attributed to the quadrupolar interactions of CO2 with the amide linkages of the PA backbone, which reduce its diffusivity coefficient value.16−19 Further, CO2 molecule has a “kinetic diameter” of 3.3 Å and its collision diameter of 3.94 Å. Thus, CO2 molecule might have a larger effective size than O2 and as a result D (O2) > D (CO2).28 The CO2 molecule showed higher solubility coefficient compared to other gases. This is attributed to the high critical temperature of CO2 and the gas molecule induces some sort of interaction with the carbonyl or amide linkage of the polyamides. The very high permeability of CO2 for individual polymeric membranes resulted from its enhanced solubility coefficient values compared to the other gas solubility coefficient values.18,19,29 Gas transport through PAMIP membrane matrix can take place by the following four pathways:30 (i) through the Si−O cage of POSS molecule; (ii) through the aggregates of POSS formed within the polymer matrix; (iii) through the interfacial region between POSS and bulk polymer matrix; (iv) only through the polymer matrix and molecular sieves dispersed in the polymeric membrane might not be accessible to the gas molecules. The gas transport by first three ways would lead to an increase in the gas permeabilities of the gas molecules with either an increase or a decrease in permselectivity values for a pair of gas. In the present study, in general, the gas permeability

showed no significant change in glass transition temperature in comparison to the PAMI-d. Dasgupta et al. also reported similar Tg values for POSS modified composite membranes in comparison to the pure polyimide membranes.6 DSC thermograms of these PAMIP composite membranes also did not exhibit any endothermic peak corresponding to the melting of POSS-NH2 (Tm = 108 °C), which indicates that the POSS particles were well attached to their parent copolymers. The tensile strength values and Young’s modulus of PAMIP membranes were lower than those of the pure PAMI membranes [Table 2]. The reduction in the tensile strength and Young’s modulus of these POSS-polyamide membranes as compared to that of their parent copolyamide membranes was attributed to the rigidity imparted by the POSS molecule and weak interaction between polymer chains in the nanocomposite.7,26 3.3. Gas Transport Properties. 3.3.1. Effect of POSS on Gas Transport Properties. Gas permeability coefficient and permselectivity values of the PAMIs and PAMIPs are depicited in Table 3. The diffusion coefficient, solubility coefficient, solubility and diffusivity selectivity values have been presented in Table 4. The gas permeability of four different gases through these PA membranes follow the order as P (CO2) > P(O2) > P (N2) > P (CH4). This decreasing order of permeability for different gases was reverse order with the respective kinetic diameter of gas molecules, CO2 (3.3 Å) < O2 (3.46 Å) < N2 (3.64 Å) < CH4 (3.8 Å). The gas permeability coefficient value increases and permselectivity value for CO2/CH4 and O2/N2 gas pairs decreases of PAMIP membranes in comparison to their respective parent PAMI membrane except PAMIP-b′ (permselectivity value of O2/N2 increases from PAMI-b to PAMIP-b′) (Table 3). Therefore, from parent PAMI to PAMIP, permeability increases and selectivity decreases, i.e., obeying general trade-off relation. It is also general trend that permeability increases and selectivity decreases from parent polymer to polymer-POSS nanocomposite.6,12 PAMIP membranes showed higher diffusivity coefficient values for all gases in comparison to their parent copolymers (PAMI). Increase in permeability coefficient values comes from higher diffusivity coefficient values of PAMIPs compared to PAMIs [Table 4]. Because it is well-known that increases in the diffusivity coefficient values are explained by increases in free volume. 27 Therefore, the increase in the permeability coefficients of the PAMIP membranes is attributed to the increases in the fractional free volume caused by the relatively low concentration of the quite more bulky POSS molecules.12 The POSS cage increases the fractional free volume (FFV) of 18279



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comparison of the permselectivity for CO2/CH4 gas pair versus permeability coefficient of CO2 of these PAMIs and PAMIPs with other reported PAs and polyimide−POSS nanocomposite is presented as a Robeson plot31 in Figure 6.

coefficient values increase and permselectivity values for CO2/ CH4 and O2/N2 gas pairs decrease in POSS modified PAMIP membranes in comparison to the their parent PAMI membranes. Therefore, the observed permeability behavior of PAMIP mixed matrix membranes can be attributed to the transport of penetrants through the above-mentioned first three ways. Increase in permeability from PAMI-a to PAMIP-a′ and PAMIP-b to PAMIP-b′ is much higher compared to increase in permeability from PAMI-c to PAMIP-c′ and PAMI-d to PAMIP-d′. This phenomenon may be related to the much higher FFV in PAMI-c and PAMI-d. This is due to the presence of two bulky tert-butyl groups in diamine moiety of PAMI-c (parent polymer of PAMIP-c′) and extreme bulky nature of diamine moiety of PAMI-d (parent polymer of PAMIP-d′). Therefore, introduction of bulky POSS moiety into a high FFV environment, e.g., PAMI-c (parent polymer of PAMIP-c′) and PAMI-d (parent polymer of PAMIP-d′) may inhibit chain packing and increases permeability to a lesser extent compared to the introduction of POSS moiety into a low free volume matrix, e.g., PAMI-a (parent polymer of PAMIP-a′) and PAMIb (parent polymer of PAMIP-a′) where POSS moiety could disrupt chain packing more strongly. 3.3.2. Comparison of Gas Transport Properties with Other Reported Fluorinated Poly(ether amide)s and Polyimide− POSS Nanocomposite Membranes. To the best our knowledge, so far, gas transport properties of the polyamide-tethered POSS nanocomposite mebrane are not reported in the literature. For comparison of the gas transport properties, we choose permeability and permselectivity values of some reported polyimide−POSS nanocomposite membranes (PIPOSS-I and PI-POSS-II).6 We also took permeability and permselectivity values of some reported fluorinated poly(ether amide)s (PAs) having no maleimide containing part, which are structurally analogous to PAMIs, e.g., PA “6b”, PA “II”, PA “Ib” and PA “B”.16−19 All these permeability and permselectivity values are reported in Table 3. The comparison of permselectivity for the O2/N2 gas pair versus permeability coefficient of O2 of these PAMIs and PAMIPs with other reported PAs and polyimide−POSS nanocomposite is presented as a Robeson plot31 in Figure 5. Similarly, the

Figure 6. Robeson plot for a comparison of CO2/CH4 selectivity vs CO2 permeability coefficients of PAMIs and PAMIPs with earlier reported polyamides and polyimide−POSS nanocomposites.6,16−19,31

The gas permeability coefficient values slightly decreased (except PAMI-a versus analogous PA “6b”) and permselectivity values (CO2/CH4) increased in maleimide-containing copolyamides (PAMIs) membranes in comparison to the analogous polyamide (PA “6b”, PA “II”, PA “Ib” and PA “B”) having no maleimide containing part.16−19 The lower permeability values of the PAMI membrane might be attributed to the increase of hydrogen bonding due to more carbonyl groups as well as decrease in fluorine content of PAMIs. The higher permselectivity of CO2/CH4 gas pair of PAMI membrane was due to the higher permeability of CO2 gas in comparison to other gases. The presence of more carbonyl groups increase the interaction with CO2 and makes the gas molecule selectively more permeable compared to other gases. PAMIP membranes have higher permeability and permselectivity values for CO2/ CH4 and O2/N2 gas pairs in comparison to the earlier reported polyimide−POSS nanocomposite membranes [except permeability of O2]. In polyamide−POSS nanocomposite (PAMIP) membranes the permeability increases and permselectivity decreases (except for O2 and N2 gas pair) in comparison to the PAMI membranes. This is attributed to the POSS molecules increase the interchain distance (FFV) between the polymer chains, therefore increase the permeability values of PAMIP membranes.

4. CONCLUSIONS Four POSS-tethered aromatic polyamide nanocomposites (PAMIPs) with a fixed POSS fraction were prepared through Michael addition between four new maleimide-containing copolyamides (PAMIs) and PSS-(3-(2-aminoethyl)-amino)propylheptaisobutyl substituted polyhedral oligomeric silsesquioxane (POSS-NH2). Phase separated morphology formation of PAMIP membrane was confirmed from TEM analysis. POSS-rich-domain formation in the polyamide−POSS composite was caused by POSS-cage phase separation from polyamides. This phase separation is attributed to the difference between hydrogen-bonding interactions between the polar

Figure 5. Robeson plot for a comparison of O2/N2 selectivity vs O2 permeability coefficients of PAMIs and PAMIPs with earlier reported polyamides and polyimide−POSS nanocomposites.6,16−19,31 18280



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(6) Dasgupta, B.; Sen, S. K.; Banerjee, S. Aminoethylaminopropylisobutyl POSS-polyimide nanocomposite membranes and their gas transport properties. Mater. Sci. Eng., B 2010, 168, 30−35. (7) Leu, C. M.; Chang, Y. T.; Wei, K. H. Synthesis and dielectric properties of polyimide-tethered polyhedral oligomeric silsesquioxane (POSS) nanocomposites via POSS-diamine. Macromolecules 2003, 36, 9122−9127. (8) Kawakami, Y.; Kakihana, Y.; Miyazato, A.; Tateyama, S.; Hoque, M. A. Polyhedral oligomeric silsesquioxanes with controlled structure: Formation and application in new Si-based polymer systems. Adv. Polym. Sci. 2011, 235, 185−228. (9) Morrison, J. J.; Love, C. J.; Manson, B. W.; Shannon, I. J.; Morris, R. E. Synthesis of functionalised porous network silsesquioxane polymers. J. Mater. Chem. 2002, 12, 3208−3212. (10) Leu, C. M.; Reddy, G. M.; Wei, K. H.; Shu, C. F. Synthesis and dielectric properties of polyimide-chain-end tethered polyhedral oligomeric silsesquioxane nanocomposites. Chem. Mater. 2003, 15, 2261−2265. (11) Sammons, J. Gas permeability and gas separation using POSS® materials. In: Proceedings of the POSS Nanotechnology Conference, Huntington Beach, CA, September 25−27, 2002. (12) Ríos-Dominguez, H.; Ruiz-Treviño, F. A.; Contreras-Reyes, R.; González-Montiel, A. Syntheses and evaluation of gas transport properties in polystyrene−POSS membranes. J. Membr. Sci. 2006, 271, 94−100. (13) Iyer, P.; Iyer, G.; Coleman, M. Gas transport properties of polyimide-POSS nanocomposites. J. Membr. Sci. 2010, 358, 26−32. (14) Dasgupta, B.; Banerjee, S. Preparation and evaluation of gas transport properties of poly(ether imide)-polyhedral oligomeric silsesquioxane nanocomposite membranes. Adv. Sci. Lett. 2012, 10, 14−23. (15) Liu, Y. L.; Lee, H. C. Preparation and properties of polyhedral oligosilsequioxane tethered aromatic polyamide nanocomposites through Michael addition between maleimide-containing polyamides and an amino-functionalized polyhedral oligosilsequioxane. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4632−4643. (16) Bandyopadhyay, P.; Bera, D.; Banerjee, S. Semifluorinated, organo-soluble new aromatic poly(ether amide)s: Synthesis, characterization and gas transport properties. J. Membr. Sci. 2011, 382, 20−29. (17) Bandyopadhyay, P.; Bera, D.; Banerjee, S. Synthesis, characterization and gas transport properties of semifluorinated new aromatic polyamides. Sep. Purif. Technol. 2013, 104, 138−149. (18) Bandyopadhyay, P.; Bera, D.; Ghosh, S.; Banerjee, S. Di-tertbutyl containing semifluorinated poly(ether amide)s: Synthesis, characterization and gas transport properties. J. Membr. Sci. 2013, 447, 413−423. (19) Bandyopadhyay, P.; Bera, D.; Ghosh, S.; Banerjee, S. Synthesis, characterization and gas transport properties of cardo bis(phenylphenyl)fluorene based semifluorinated poly(ether amide)s. RSC Adv. 2014, 4, 28078−28092. (20) Kute, V.; Banerjee, S. Novel semi-fluorinated poly(ether imide)s derived from 4-(p-aminophenoxy)-3-trifluoromethyl-4′-aminobiphenyl. Macromol. Chem. Phys. 2003, 204, 2105−2112. (21) Kalgutkar, A. S.; Crews, B. C.; Marnett, L. J. Design, synthesis, and biochemical evaluation of N-substituted maleimides as inhibitors of prostaglandin endoperoxide synthases. J. Med. Chem. 1996, 39, 1692−1703. (22) Wu, J.; Haddad, T. S.; Kim, G. M.; Mather, P. T. Rheological behavior of entangled polystyrene-polyhedral oligosilsesquioxane (POSS) copolymers. Macromolecules 2007, 40, 544−554. (23) Matějka, L.; Strachota, A.; Pleštil, J.; Whelan, P.; Steinhart, M.; Šlouf, M. Epoxy networks reinforced with polyhedral oligomeric silsesquioxanes (POSS). Structure and morphology. Macromolecules 2004, 37, 9449−9456. (24) Seçkin, T.; Köytepe, S.; Adıgüzel, H. I.̇ Molecular design of POSS core star polyimides as a route to low-k dielectric materials. Mater. Chem. Phys. 2008, 112, 1040−1046. (25) Wahab, M. A.; Mya, K. Y.; He, C. Synthesis, morphology, and properties of hydroxyl terminated-POSS/polyimide low-k nano-

amide units of the polymer chain and van der Waals interactions between isobutyl groups of POSS molecule. Incorporation of POSS molecules in PAMIP increases the thermal stability in comparison to their parent copolyamides. There was no significant change in glass transition temperatures (269−272 °C) in between different PAMIPs. The presence of bulky POSS groups increases the interchain distance between the polymer chains of nanocomposites as it is confirmed from WAXD analysis compared to their parent polymers. The increase in permeability coefficients accompanied by increase in diffusivity coefficient values of the PAMIP membranes (PCO2 = 49.40 to 137.60 and PO2 = 10.40 to 32 barrer) compared to the PAMI membranes (PCO2 = 19.60 to 115.00 and PO2 = 4.90 to 26 barrer). This was attributed to the increase in the fractional free volume caused by the relatively low concentration of the bulky POSS unit in PAMIPs. In terms of gas transport properties, these hybrid membranes are very attractive because they support the mechanical stresses imposed in the permeation cells for their good combination of thermal and mechanical properties.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of copolyamides and copolyamide−POSS nanocomposites. 1H NMR spectrum (Figure S1) of PAMI-a, TGA thermograms (Figure S2) and DSC curves (Figure S3) of PAMIs and PAMIPs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*S. Banerjee. E-mail: [email protected]. Tel.: +913222283972. Fax: +91-3222255303. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P. Bandyopadhyay acknowledges CSIR, New Delhi, for providing him a research fellowship to carry out this work. The authors thank AvH Foundation for donation of the GPC instrument used in this work and Department of Science and Technology (DST), India for financial support as project sponsor (Grant No. SR/S3/ME/0008/2010) for this work.

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REFERENCES

(1) Wu, S.; Hayakawa, T.; Kikuchi, R.; Grunzinger, S. J.; Kakimoto, M. A. Synthesis and Characterization of Semiaromatic Polyimides Containing POSS in Main Chain Derived from Double-DeckerShaped Silsesquioxane. Macromolecules 2007, 40, 5698−5705. (2) Li, Z.; Tan, B. H.; Jin, G.; Li, K.; He, C. Design of polyhedral oligomeric silsesquioxane (POSS) based thermo-responsive amphiphilic hybrid copolymers for thermally denatured protein protection applications. Polym. Chem. 2014, 5, 6740−6753. (3) Kuo, S. W.; Chang, F. C. POSS related polymer nanocomposites. Prog. Polym. Sci. 2011, 36, 1649−1696. (4) Zhang, W.; Müller, A. H. E. Architecture, self-assembly and properties of well-defined hybrid polymers based on polyhedral oligomeric silsequioxane (POSS). Prog. Polym. Sci. 2013, 38, 1121− 1162. (5) Wright, M. E.; Petteys, B. J.; Guenthner, A. J.; Fallis, S.; Yandek, G. R.; Tomczak, S. J.; Minton, T. K.; Brunsvold, A. Chemical modification of fluorinated polyimides: New thermally curing hybrid polymers with POSS. Macromolecules 2006, 39, 4710−4718. 18281



Article

composite films. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 5887− 5896. (26) Leu, C. M.; Chang, Y. T.; Wei, K. H. Polyimide-side-chain tethered polyhedral oligomeric silsesquioxane nanocomposites for lowdielectric film applications. Chem. Mater. 2003, 15, 3721−3727. (27) Higuchi, A.; Agatsuma, T.; Uemiya, S.; Kojima, T.; Mizoguchi, K.; Pinnau, I.; Nagai, K.; Freeman, B. D. Preparation and gas permeation of immobilized fullerene membranes. J. Appl. Polym. Sci. 2000, 77, 529−537. (28) Stern, S. A.; Liu, Y.; Feld, W. A. Structure/permeability relationships of polyimides with branched or extended diamine moieties. J. Polym. Sci. B: Polym. Phys. 1993, 31, 939−951. (29) Wang, Z.; Chen, T.; Xu, J. Gas and water vapor transport through a series of novel poly(aryl ether sulfone) membranes. Macromolecules 2001, 34, 9015−9022. (30) Kulkarni, P. P. Effect of Polyhedral Oligomeric Silsesquioxane on Gas Transport Properties of Polyimide. Ph.D. Thesis, The University of Toledo, Toledo, OH, 2007. (31) Robeson, L. M. The upper bound revisited. J. Membr. Sci. 2008, 320, 390−400.

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