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Synthesis and Characterization of Green Membranes Polyimide/Titania Bionanocomposites Containing Amino Acid and Benzimidazole Moieties for Gas Transport Properties.

INTRODUCTION

Nanocomposites (NCs) are nanostructured organic-inorganic hybrid materials resulting from the assemblage of organic and inorganic components, typically a polymer and a nanosized inorganic solid, whose characteristics determine a broad range of properties and applications [1-5], Among them, biobased NCs represent a rapidly growing field of research not only because they are ecological materials, but also due to their current and potential interest in diverse applications including biomedicine, from tissue engineering to drug delivery systems [6-8], Bionanohybrids are a new type of nanoscale organic-inorganic hybrids that involve an organic counterpart of biological origin, for instance amino acids, assembled to a nanoparticulated inorganic solid [9-13].

Aromatic polyimides (Pis) have noticeable thermal stability and excellent electrical, mechanical, and solvent resistance properties. So, they have such various applications as in automobile, aerospace parts, and microelectronic industries. The disadvantage in processing routine aromatic Pis, particularly for the most popular of them, is due to their high glass transition temperatures and insolubility in any organic medium [14-16], This behavior is due to the strong interaction between PI chains and their rigid structure [14]. Therefore, much research has been concentrated on the insertion of new functionalities in Pis without failing their otherwise excellent properties [17].

Benzimidazole is a fused heterocyclic compound having benzene and imidazole aromatic ring system. First time benzimidazole derivative (5,6-dimethylbenzimidazole) was isolated from vitamin B,2 by Brink in 1949 [18]. Some benzimidazoles have been isolated from natural marine sponges [19], Benzimidazole nucleus has pharmacological and biological activities such as anti-HIV [20], antimicrobial, antihistaminic [21], antiviral, antitumor, antidepressant antioxidant, antihypertensive, anticoagulant, antidiabetic [22], antitubercular [23], antiallergic [24], antihelmentic, antiinflammatory, and analgesic activity [25]. Benzimidazole is also a privileged scaffold of many drugs such as omeprazole, rabeprazole, lansoprazole, dexlansoprazole, pantoprazole, esomeprazole (proton pump inhibitors), mebendazole (antihelmentic), albendazole (antimicrobial), and astemizole (antihistaminic) [26-28],

Despite the favorable properties of Pis, generally they suffer low process abilities due to the limited solubility and/or high melting or softening temperatures [29], The existence of bulky pendant groups in the structure of polymer matrix is one of the useful strategies to overcome these difficulties [30-32], Besides, the more flexibility of polymer matrix improves the distribution of inorganic nanofillers through the facilitating of the polymer/ NPs interactions. Nano science and nanotechnology have inspired researchers to exploit the versatility of PI through the special properties of nano materials. Due to the presence of different functional groups, which can interact with inorganic fillers, this polymer may be a promising matrix for preparation of organic-inorganic hybrid nanocomposites.

Titanium dioxide (Ti[O.sub.2]) NPs as a very important inorganic material have emerged as an area of intense current interest motivated because of its good stability, high refractive index, and UV resistance. Ti[O.sub.2] is well known as a matter with strong redox ability, and used for water or air purification and to degrade the organic pollutants [33-36], The applications of Ti[O.sub.2] NPs are largely limited because of their high energetic hydrophilic surface, which causes the NPs to be simply aggregate. Therefore, for enhancement dispersion of the NPs, many researches have been focused upon the surface modification of NPs and new method for incorporation of inorganic nanofiller into organic matrix. Several ways have been employed for the surface modification of NPs [37-41].

Polymer/inorganic NC materials have been recently developed to improve the physical properties of polymers. The polymer/ inorganic NC constitutes of two matrices, i.e., polymer and inorganic. In these kinds of materials, the inorganic phase is dispersed at nanoscale level in the polymer phase. Due to the special structural characteristics of polymer/inorganic nanocomposites, they exhibited the high-order characteristics such as optical transparency, dielectric properties, electrical conductivity, nonlinear optical effects, quantum confinement effects, biological compatibility, and biological activity [42]. This offers new possibilities to improve the gas separation properties of pure polymers. The earliest gas permeability studies of polymer/inorganic NCsreported in the literature appeared in 1996 [43], This study on gas permeability of polymer/inorganic materials, prepared by cohydrolysis of phenyltrimethoxysilane or diphenyldimethoxysilane with tetramethoxysilane, showed encouraging selectivity for C[O.sub.2]/ [N.sub.2], and He/[N.sub.2]. The gas permeability investigations of polyimidesilica and poly (amide-imide)/Ti[O.sub.2], NCs were also reported in succession [44-47],

However, the poly (amide-imide)/Ti[O.sub.2], NCmaterial studied by Hu [48] has demonstrated slight increase of selectivity and decrease of permeability compared with pure poly(amide-imide). This might be due to the low Ti[O.sub.2], proportion in the used poly (amideimide)/Ti[O.sub.2], NCmembranes. The used poly (amideimide)/Ti[O.sub.2], NCmaterials containing high Ti[O.sub.2], content were too brittle to be applied in gas permeability characteristics. It can be supposed that the polyimide/Ti[O.sub.2], NCs with high Ti[O.sub.2] might exhibit much higher gas permeability and selectivity than that of pure polyimide.

This work concerns the fabrication of PI/titania NC containing L-isoleucine and benzimidazole aromatic ring fractions in pendant group of polymer. Structural and morphological properties of these hybrids were investigated using fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). The gas permeability of membranes with different weight ratio of titania were investigated. The gas permeation and separation performance of gas nitrogen ([N.sub.2]), oxygen ([O.sub.2]), carbon dioxide (C[O.sub.2]), and methane (C[H.sub.4]) were measured.

EXPERIMENTAL

Materials

All chemicals used were of analytical reagent grade purchased from Fluka Chemical Co. (Buchs, Switzerland), Aldrich Chemical Co. (Milwaukee, WI), Riedel-deHaen AG (Seelze, Germany), and Merck (Darmstadt, Germany) unless otherwise stated. 5-bromo-2-methylisoindoline-l,3-dione, 3,5 bis(trifluoromethyl) benzenamine, benzene-1,2-diamine, 3,5-dinitrobenzoylchloride, acetone, hydrazine hydrochloride, ferricchloride, and propylene oxide from Merck were used for the synthesis of mediators. N, N'-dimethylformamide (DMF; d = 0.94 g [cm.sup.-3] at 20[degrees]C), N-methylpyrrolidone (NMP), N, N'-dimethylacetamide (DMAc), tetrahydrofuran (THF) and dimethylsulphoxide (DMSO) was distilled over barium oxide under reduced pressure. Other reagents were used without further purification. Tetraethyl orthotitanate (Ti[(OEt).sub.4]), and acetylacetone (acac) were purchased from Neutrino Co (Tehran, Iran). [N.sub.2], [O.sub.2], C[H.sub.4], and C[O.sub.2] gases from BOC were used for the permeation study.

Equipment and Methods

Proton nuclear magnetic resonance ([sup.1]H NMR, 500 MHz) spectra were carried out using Bruker Avance 500 instrument (D-755 of the Roger-Gaudry, Germany) at room temperature (RT) in dimethylsulphoxide-[d.sub.6] (DMSO-dg). Multiplicities of proton resonance were designated as singlet (s), doublet (d), triplet (t), and multiplet (m). Fourier-transform infrared (FTIR) spectra were recorded on a spectrophotometer (Jasco-680, Japan). The spectra of solids were obtained using KBr pellets and some FTIR spectra were recorded using films of obtained materials. The vibrational transition frequencies are reported in wavenumbers ([cm.sup.-1]). Band intensities are assigned as weak (w), medium (m), strong (s), and broad (br).

Inherent viscosities were measured using a Cannon Fenske Routine Viscometer (Germany) at a concentration of 0.5 g d[L.sup.-1] at 25[degrees]C. Specific rotations were measured by a Jasco Polarimeter (Japan). Gel permeation chromatography (GPC) was performed with a Waters instrument (Waters 2414), and THF was used as an eluent (flow rate 0.5 mL [min.sup.-1]). Polystyrene was used as a standard, and a RI detector was used to record the signal in GPC. Glass transition temperatures ([T.sub.g]) were read at the middle of the transition in the heat capacity from the second heating scan.

Thermogravimetric analysis (TGA) of the polymer samples was performed on a Netzsch TG 209F1 instrument at a heating rate of 20[degrees]C [min.sup.-1] in nitrogen and air atmosphere. Differential scanning calorimetric (DSC) analysis was performed on a PE Diamond DSC instrument at a heating rate of 20[degrees]C [min.sup.-1] in nitrogen atmosphere. Ultra violet spectra of the polymer films were recorded at room temperature using Detector SD-2000 (Ocean Optics Inc.) and source lamp DH-2000. The XRD patterns were collected by using a Philips Xpert MPD X-ray diffractometer. The diffractograms were measured for 2[theta], in the range of 10-80[degrees], using a voltage of 40 kV and Cu Ka incident beam ([lambda] = 1.51418 [Angstrom]). The densities of the membranes were measured using Wallace High Precision Densimeter-X22B (UK) using water displacement method. The gas transport properties of the PI membranes were studied at 3.5 bar of applied gas pressure and at 35[degrees]C using automated Diffusion Permeameter (DP-100-A) manufactured by Porous Materials Inc., USA. The morphology of the polymers was observed using field emission scanning electron microscope (FE-SEM; Hitachi S-4160, Japan).

MONOMERS SYNTHESIS

Synthesis of New 3, 5-Diamino-N-[l-(lH-Benzoimidazol-2-Yl)-2-Methyl-Propyl]-Benzamide

A mixture of O-phenylenediamine (2) (0.1 mol) and L-isoleucine (1) (0.1 mol) was refluxed for 4 h in 4 N hydrochloric acid (50 mL) on a water bath. The reaction mixture was cooled and basified with ammonium hydroxide solution. The precipitate thus obtained was dried and recrystallized from methanol with activated charcoal treatment. The pure product obtained was a slightly white colored crystal whose melting point was 140[degrees]C -142[degrees]C and the yield was 92%. (Sch. 1) A solution of l-(lH-Benzoimidazol-2-yl)-2-methyl-propylamine (3) (0.2 mol); triethylamine (1.5 mL); and 3,5-dinitrobenzoyl chloride (4) (0.2 mol) in CH[C1.sub.3] (50 mL) was stirred at room temperature. The end of reaction was controlled by TLC (50:50 ethylacetate: n-hexane). The reaction mixture was washed with water (100 mL) and then, the organic layer was separated and dried over calcium chloride and concentrated by rotary evaporation.

The residue was purified by recrystallization from ethylacetate/n-hexane and vacuum dried to yield, (96%) of aromatic dinitro (5) as a white precipitate. In a 100 mL two necked round-bottomed flask equipped with a reflux condenser and a dropping funnel, a suspension of compound (5) (0.1mol), Pd-C 10% (0.6 g) and DMF (25 mL) was prepared. The mixture was warmed and at the same time as being stirred magnetically, hydrazine monohydrate 90% (15 mL) in ethanol (15 mL) was added drop-wise over a 2 h period through the dropping funnel, while keeping the temperature at about 60[degrees]C. The reaction mixture was then refluxed for 3 h and filtered while hot. The solvent was evaporated under reduced pressure to give light yellow precipitate, which was recrystallized from ethanol and dried under vacuum. The yield was 85% [[alpha]] [sup.25.sub.D] =-22.12[degrees] [cm.sup.3] [g.sup.-1] [dm.sup.-1] (c = 0.005 g [cm.sup.-3] in DMF).

FTIR (KBr), 3,425 (m), 3,365 (m), 3,125 (m), 1,688 (s), l,675(s), 1,625 (m), 1,658 (m), 1,530 (s), 1,422 (m), 1,321 (s), 1,214 (s), 1,125 (m) [cm.sup.-1].

[sup.1]H NMR (500 MHz, DMSO, [d.sub.6]), [delta] 1.32 (d, 3H, C[H.sub.3], J = 6.59 Hz), 2.44 (m, 1H, CH), 4.85 (s, 4H, NH), 5.75 (d, 1H, CH), 7.12 (N-H, 1H), 7.38 (Ar, 1H), 7.69 (Ar, 2H), 8.19 (Ar, 2H), 8.31 (Ar, 2H), 9.32 (N-H, 1H).

[sup.13]C-NMR (125 MHz, DMSO-[d.sub.6]), [delta] (ppm), [delta] 23.25 (C[H.sub.3]), 32.11 (CH), 59.21 (CH), 103.22 (Ar), 115.54 (Ar), 122.44 (Ar), 128.08 (Ar), 128.53 (Ar), 137.26 (Ar), 139.12 (Ar), 148.85 (Ar), 167.17 (C = 0) ppm.

Elemental analysis calculated for [C.sub.18][H.sub.21][N.sub.5]O (323.20 g/mol), C, 66.85%, H, 6.55%, N, 21.66%. Found, C, 66.54%; H, 7.01%; N, 21.32%.

Synthesis of Dianhydride Monomer Compound (11)

The dianhydride monomer compound (11) was prepared according to our published article (Sch. 2) [49],

Synthesis of Optically Active Pis (a-d)

Optically active aromatic Pis (a-d) were synthesized by the reaction of optically active diamine 6 (a-d) with aromatic dianhydride monomer (11) through the polycondensation procedure (Sch. 3). The Pis (a-d) were prepared by the following general procedure. In a 25-mL round-bottomed flask put in an ice bath, 3, 5-Diamino-N-[l-(lH-benzoimidazol-2-yl)-2-methyl-propyI]benzamide (0.5 mol) was dissolved in dry DMF; then, (0.5 mol) dry dianhydride monomer (11) was added in five portions and stirred continuously. The fresh amount of dianhydride monomer compound (11) was added after the complete dissolution of the previous one. The solution was stirred in the ice bath for further 1 h and at room temperature for 2 h to obtain poly(amic acid) (PAA) quantitatively. Thin film of PAA was fabricated by casting onto dust-free glass plate. Resulted thin film was annealed using an electric air-circulating oven at 50[degrees]C, 100[degrees]C, 150[degrees]C, 200[degrees]C, and 250[degrees]C for 1 h each and 300[degrees]C for 10 h and then were cooled and removed from glass surface using a sharp edge blade.

Synthesis of Bionanocomposites

Absolutely dried dianhydride monomer compound (11) was added to a DMF solution of diamine compound (6) (with the same molar ratios) in several portions and stirred for 1 h at 0[degrees]C. The temperature was raised up to 25[degrees]C and stirred for 2 h to form corresponding poly(amic acid) (PAA) solution. According to the desired titania percentages (assuming the complete conversion of titanate precursor to titania particles) the required quantities of Ti[(OEt).sub.4] (dissolved in acac with a molar ratios of 1:4) were added and continuously stirred for 14 h. Several thin films of mixed PAA with various ratios of titanate precursor were cast onto glass plates. Obtained thin films were annealed in an air circulating oven at 50[degrees]C, 100[degrees]C, 150[degrees]C, 200[degrees]C, and 250[degrees]C for 1 h each and 300[degrees]C for 10 h and then were cooled and removed from glass surfaces (Sch. 4).

RESULTS AND DISCUSSION

Synthesis of Monomers

Chiral diamine (6a-d) monomers were prepared by the following general procedure. For example the synthetic route of the diamine (6a) monomer, 3,5-diamino-N-[l-(lH-benzoimidazol-2-yl)-2-methyl-propyl]-benzamide (6) is outlined in Sch. 1. N-[l-(lH-Benzoimidazol-2-yl)-2-methyl-propyl]-3,5-dinitro-benzamide (5) was prepared by the condensation of l-(lH-Benzoimidazol-2-yl)-2-methyl-propylamine (3) and 3,5-dinitro-benzoyl chloride (4). The new aromatic diamine having a bulky pendent amino acid group, diamine (6a), was successfully synthesized by hydrazine Pd/C-catalytic reduction according to the synthetic route outlined in Sch. 1.

The purity of monomer 6 was checked by thin layer chromatography, which showed one spot in an ethylacetate/cyclohexane mixture (50:50) with Rf = 0.38. FTIR and [sup.1]H NMR spectroscopic techniques were applied to identify the structures of the diamine monomer 6. The transformation of nitro to amino functionality could be monitored by the change of FTIR spectra. The nitro groups of compound 5 gave two characteristic bands at around 1545 and 1352 [cm.sup.-1] (--N[O.sub.2] asymmetric and symmetric stretching).

After reduction, the characteristic absorptions of the nitro group disappeared and the amino group showed the typical N-H stretching absorption pair in the region of 3425-3365 [cm.sup.-1]. The [sup.1]H NMR the diamine monomer 6 (Fig. 1) agrees well with the proposed molecular structure of compounds 6. The [sup.1]H NMR spectrum confirms that the nitro groups have been completely transformed into amino groups by the resonance signals at 4.85 ppm corresponding to the amino protons Fig. 1.

Polymer Synthesis

At firstly, diamine monomer was dissolved in dry DMF; then, (0.5 mol) dry dianhydride monomer (11) was added in five portions and stirred continuously. The fresh amount of dianhydride monomer compound (11) was added after the complete dissolution of the previous one. The solution was stirred in the ice bath for further 1 h and at room temperature for 2 h to obtain poly(amic acid) (PAA) quantitatively. Thin film of PAA was fabricated by casting onto dust-free glass plate. Resulted thin film was annealed using an electric air-circulating oven at 50[degrees]C, 100[degrees]C, 150[degrees]C, 200[degrees]C, and 250[degrees]C for 1 h each and 300[degrees]C for 10 h and then were cooled and removed from glass surface using a sharp edge blade.

We decided to synthesize a series of new optically active Pis bearing benzimidazole pendant groups from an equimolar mixture of dianhydride monomer compound (11) and different diamine derived from amino acids, including isoleucine (lie), valine (Val), methionine (Met), phenylalanine (Phe) (Sch. 3). The polymerization of aromatic diamine (6a), 3,5-diamino-N-[l-(lH-benzoimidazol-2-yl)-2-methyl-propyl]-benzamide with dianhydride monomer compound (11) was performed. The structure of the PI was confirmed by FTIR, GPC and [sup.1]H NMR spectroscopic analyses. The FTIR spectrum of this polymer showed absorptions around 3,455 [cm.sup.-1] (N-H), 1775 [cm.sup.-1] (C = O asymmetric, imide), 1,725 [cm.sup.-1] (C = O symmetric, imide), 1,655 [cm.sup.-1] (C = 0, amide). The presence of the imide heterocycle in this polymer was revealed by absorption of 1,384 [cm.sup.-1] and 830 [cm.sup.-1] which belong to carbonyl bendings of imide.

Figure 2 shows the high-resolution [sup.1]H NMR spectrum of PI. The assignments of the *H NMR spectra agree well with the proposed polymer structure (Sch. 3). The obtained data from the polymerization reactions of dianhydride monomer with new optically active diamines 6 (a-d) are listed in Table 1. All the Pis have a chiral center in their backbone, and the results showed that these Pis are optically active with different specific rotations. The resulting Pis exhibited inherent viscosities of 0.68-0.98 dL/g in DMAc at 30[degrees]C (Table 1) and can afford transparent/self/standing films via solution casting, indicating they are high molecular weight polymers. Moreover, the number average molecular weight ([M.sub.n]), weight average molar weight ([M.sub.w]) and polydispersity index (PDI) of the synthesized polymers were further supported by GPC measurements, as shown in Table 1. GPC data show that Pis (a-d) have [M.sub.n] and PDI in the range of 66,198-66,321 and 1.87-1.93, respectively. Flexible and transparent films of Pis (a-d) could be conveniently casted from their solutions. All Pis were also characterized by elemental analysis techniques, and the results are in good agreement with the calculated ones for the proposed structures (Table 2). These results in sum confirmed the successful formation of the new Pis.

PI (a) FTIR (Film, [cm.sup.-1]). 3,455 (m, br), 3,212 (w), 3,015 (w), 2,920 (w), 1,775 (m), 1,725 (s), 1,655 (m), 1,550 (s), l,475(s), 1,387 (s), 1,355 (s), 1,319 (s), 975 (s), 952 (m), 850 (m), 735 (m), 725 (m), 660 (m), 560 (m).

PI (a) 1H NMR (500 MHz, DMSO-d6, Ppm). 0.95 (d, 3H, C[H.sub.3], J = 6.59 Hz), 2.24 (m, 1H, CH), 4.77 (d, 1H, CH), 6.36 (N-H, 1H), 7.04-7.05 (s, 2H, Ar-H), 7.14 (s, 2H, Ar-H), 7.27 (s, 1H, Ar-H), 7.33 (s, 2H, Ar-H), 7.47 (s, 2H, Ar-H), 7.57-7.59 (d, 2H, Ar-H, /=6.2), 7.69-7.71 (d, 2H, Ar-H, 7=5.4), 7.81 (s, 1H, Ar-H), 7.89-7.91 (d, 2H, Ar-H, 7=5.4), 8.46 (N-H, 1H).

Solubility of Polymers

It is well known that aromatic Pis generally show rather poor solubility in organic solvents in full imidation formation, especially for some Pis derived from rigid monomers. To enhance the solubility, bulky groups, flexible linkages or noncoplanar structures have been introduced along the polymer chains [50]. It should be noted that good solubility in low-boiling-point solvents is critical for preparing films or coatings at a relatively low processing temperature, which is desirable for advanced microelectronics manufacturing applications. Therefore, the new synthesized Pis, because of flexible linkages in the repeating unit as well as bulky trifluoromethyl groups in dianhydride monomer, are soluble in organic solvents. The presence of bulky C[F.sub.3] groups prevent the close packing of chains in the polymer backbone and inhibit close packing and lead to a decrease in the interchain interaction to improve solubility [51, 52]. The resulting Pis displayed good solubility in polar organic solvents, such as NMP, DMAc, DMF and DMSO, at room temperature. But these Pis were insoluble in general organic solvents such as xylenes, dioxane, CH[Cl.sub.3] and acetone. In THF, only PI (d) based on L-phenylalanine as a chiral moiety diamine monomer was soluble. The enhanced solubility of the resulting Pis is attributed not only to the bulkiness of the C[F.sub.3] groups but also to the benzimidazole and amino acid pendant groups in polymer backbone.

Fabrication of NC films

Scheme 4 shows the synthesis sequence for various nanocomposites. The reaction of diamine 6 and dianhydride monomer compound (11) was started at 0[degrees]C and then warmed up to room temperature due to the exothermic nature of the reactions. Resulted PAA was combined with the desired quantities, of Ti[(OEt).sub.4]/acac (1:4). The role of acac (a chelating agent) was to diminish the fast gelation of titaniate precursor via the formation of more stable titanium ion-acac complex. In all steps, the reaction was isolated from the external humidity. Thin films of the stated mixture were annealed gradually until 300CC in which the imidization process will be ended (confirmed via both FTIR spectra and TGA thermograms).

FTIR Spectra of NCfilms

The FTIR spectra of the pure PI and the NCswith different amounts of Ti[O.sub.2] NPs (5, 10 and 15 wt%) are shown in Fig. 3. It can be seen from these FTIR spectra that the characteristic peaks of pure PI and O-Ti[O.sub.2] are still maintained. It may be implied that the Ti[O.sub.2] did not react with the PI matrix. In these nanocomposites, a new broad absorption band around 450800 [cm.sup.-1] is related to the vibration of Ti-O-Ti groups; it is clear that with increasing the quantity of NPs, the intensity of absorption related to Ti-O-Ti bonds was enhanced.

X-Ray Diffraction Data

XRD patterns of PI (a), pure titania NPs (b) and PI/Ti[O.sub.2] 10 wt% (c) and PI/Ti[O.sub.2] 15 wt% are given in Fig. 4. The diffraction pattern for the pure PI shown in Fig. 5a demonstrates that the PI is completely amorphous. Presence of the non-coplanar and twisted moieties into the backbone of polymer decreased the intermolecular forces between the polymer chains and reduced the crystallinity of polymer. Figure 5b displays anatase and rutile phase for pure Ti[O.sub.2] NPs. The XRD patterns of PI/Ti[O.sub.2] NC(c) and (d) indicate characteristic peaks of anatase and rutile of Ti[O.sub.2] and PI, respectively, indicating that the morphology of Ti[O.sub.2] NPs has not been disturbed during the process.

The wide weak diffraction peak of PI still exists, but its intensity decreases. The full-width at half-maximum of the strongest characteristic reflection is used to estimate the average crystallite size by applying the Debye-Scherrer equation. The crystallite size of Ti[O.sub.2] NPs in PI is 30 nm, which is consistent with the result determined by statistical analysis of the TEM image, indicating that each individual particle is a single crystal.

TGA and DSC Analysis of NC films

The TGA curves of PI and PI/Ti[O.sub.2] NCs are shown in Fig. 5 and resulting TGA data are summarized in Table 3 that including temperatures at which 5% (T5), 10% PI/Ti[O.sub.2] (15%) ([T.sub.10]) degradation occurs and char yield at 800[degrees]C. The initial decomposition temperatures of the NCs with different Ti[O.sub.2] contents (5%, 10%, and 15%) are about 355[degrees]C. These values are related to the decomposition of pristine polymer matrix. The char yields at 800[degrees]C of the NCs with different Ti[O.sub.2] content are higher than that of pure PI. This enhancement in the char formation is ascribed to the high heat resistance exerted by the Ti[O.sub.2], because the Ti[O.sub.2] NPs have high thermal stability, so the incorporation of Ti[O.sub.2] NPs can improve the thermal stability of the nanocomposites. Pure PI and NC films were investigated using DSC analysis.

As it can be seen in Table 3, [T.sub.g] values of NC increase due to the presence of fillers within polymer matrices. It can be attributed to the inhibition of polymer chain mobility near the polymer-filler interface by their attachment to the filler or the polymer chains rigidification and/or the chain entrance to the filler particles [53]. Acceptable contact between polymer and Ti[O.sub.2] interface can be concluded from this analysis.

UV-Vis Absorption of NC films

Figure 6 shows the UV-Vis absorbance of PI and PI/Ti[O.sub.2] NCs with different levels of Ti[O.sub.2] content and the data are available in Table 4. An absorption peak appeared at 365 nm for NCs, while the peak in the bulk Ti[O.sub.2] appeared at 385 nm. The absorption peak of NCs exhibited an obvious blue shift phenomenon due to the quantum confinement effect. With increasing Ti[O.sub.2] content intensity of the absorption peaks decreased. In the UV range (280-400 nm) pure PI had high absorbance, while the absorbance value for the NCs with 15 wt% Ti[O.sub.2] is low, meaning that the transmittance of UV light was only 3.45%, and most of the UV light was shielded. The PI/Ti[O.sub.2] NCs may effectively protect against UV light, and could potentially be applied to UV-shielding materials.

Mechanical Properties of NCsfilms

The effects of Ti[O.sub.2] NPs loading on the tensile properties of PI/Ti[O.sub.2] NPs films were investigated and the results are presented in Fig. 7. Filler consisting entirely of Ti[O.sub.2] generally increases the ultimate strength, but decreases the maximum extensibility.

The mechanical properties of NCs largely depend on the external load transfer between the reinforcing nanofiller phase and the matrix [54]. According to the previous study [55], the strength should be reduced if there are no bonding sites between the organic polymer phase and the inorganic Ti[O.sub.2] phase due to the inert nature of the Pis and the weak interactions between these polymers and the Ti[O.sub.2]. In this case, the Ti[O.sub.2] acts as nonreactive and non-reinforcing filler. It is generally believed that external stress on a polymer composite is transferred from the continuous phase (polymer matrix) to the discontinuous phase (filler). Therefore, the ultimate properties of the NCs are dependent on the extent of bonding between the two phases, the surface area of the Ti[O.sub.2], and the arrangements between the Ti[O.sub.2] particles.

As can be seen in Fig. 7, the tensile strength of PI/Ti[O.sub.2] NPs films is significantly increased compared with the pure PI. The maximum stress at break (ultimate strength) was found to increase initially with increase in Ti[O.sub.2] content, and at 10 wt% Ti[O.sub.2] showed a maximum value of 114.55 MPa (relative to the 97.12 MPa of the neat PI) representing considerable improvement in tensile strength. As the Ti[O.sub.2] content increases (15 wt%), the tensile strength of PI/Ti[O.sub.2] films decreases, but still better than the pure PI film because of increasing brittleness.

According to Fig. 7, ultimate strength and initial modulus were increased with Ti[O.sub.2] contents, but ultimate elongation decreased with the increase of Ti[O.sub.2] contents, especially at higher Ti[O.sub.2] content. For elongation at break, the sample geometry (most importantly the film thickness) is an additional factor to consider and may be the determine factor why the 10% wt% Ti[O.sub.2] sample shows the best result. The above results showed that the interactions between the PI and the Ti[O.sub.2] are very important.

Electron Microscope Characterization of NCs Films

FE-SEM micrographs of bioNC cross sections are used to observe the distribution of Ti[O.sub.2] particles in polymer matrix and compatibility between the NPs and the polymer matrix. In addition, the particle size distributions of the Ti[O.sub.2] NPs in NCs are verified. The cross-sectional morphology of PI/Ti[O.sub.2] NC membranes is shown in Fig. 8a-f. As demonstrated in the FE-SEM images, there are two types of dispersion of Ti[O.sub.2] particles in the polymers. There are some particles with no aggregation, which indicates effective nanoscale mixing and homogeneous dispersion into the polymer matrix. Some of the particles, however, aggregated together to form larger particles within the polymer. As shown in the FE-SEM images, most of the aggregated particles are smaller than 50 nm in size. It is also clear from these images that all membranes have a nonporous, dense structure and there are no pinholes, connected pores, or cracks.

TEM image of NC with 10% Ti[O.sub.2] is shown in Fig. 9a and b. The central black spot and translucent parts show Ti[O.sub.2] NPs and PI polymer, respectively. TEM indicated that the Ti[O.sub.2] particles were well dispersed in the polymer matrix and remain a diameter size from 25 to 40 nm, representing that the pendant group play an important role in dispersing the NPs. From FTIR, XRD, FE-SEM, and TEM results it is clear that the Ti[O.sub.2] NPs were successfully dispersed in polymer matrix. In nanocomposites, the organic chains of pendant group of PI can fulfill steric hindrance between inorganic NPs and prevent their aggregation. The Ti[O.sub.2] NPs might be dispersed absolutely and will combine with PI via the H-bonding of C = O coupling agent with -NH, C = O, and =N- groups in PI. The -OH groups on the surface of Ti[O.sub.2] NP can bond to the amide group (C = O) of PI through interchain hydrogen bonding.

Gas Permeability and Selectivity of NCs Films

Figure lOa-d shows the effect of nanoparticles loading on [N.sub.2], [O.sub.2], C[O.sub.2] and C[H.sub.4] permeability of membranes. In order to investigate the effect of titania loading on the PI gas separation performance, PI/titania with different titania loadings were fabricated. The single gas permeation experiments showed that the permeability of all gases increased with titania NPs loading from 0 wt% to 15 wt%.

This increase might be due to the fact that the number of pinholes increased with increasing titania NPs loading ratio, and consequently diffusion mechanism increased. Furthermore, this increase may be a trend because of the interaction between polymer matrix adhered well to titania surface. Titania with a reactive surface group (OH) could be used to bond polymer chains onto the titania NPs surface, thus promoting good adhesion between the titania NPs and the bulk polymer matrix which in turn contributes to the high gas permeability [56],

Gases permeability for PI/titania membranes are higher than those without titania loading and increases with increasing titania loading ratio at fixed pressure. The C[O.sub.2] permeability of the PI/titania mixed matrix membranes initially increased with increasing titania concentrations. The C[O.sub.2] permeability of the PI/titania membrane reached to 3.55 GPU at titania loading of 5 wt%. When the titania concentration was increased to 15 wt%, the C[O.sub.2] permeability increased remarkably to 4.64 GPU. The [O.sub.2] and [N.sub.2] permeability were increased from 1.19 to 1.65 and 0.68 to 0.91, respectively, when the titania loading in the polymer matrix was increased from 5 wt% to 15 wt%.

The results suggest that the interaction between polymerchain segments and titania may disrupt the polymer-chain packing and thus enhance the gas diffusion due to more free volumes introduced among the polymer chains and defects at polymer/ NPs interface. The amine group, confirmed by [sup.1]H NMR in Fig. 2, has stronger interaction with polar gas, such as C[O.sub.2] and [O.sub.2] than non-polar gas, e.g. C[H.sub.4] and [N.sub.2]. In that case, the polar gas solubility can be enhanced and the gas permeability is increased. Although the interaction between C[H.sub.4] and amine-group can enhance the C[H.sub.4] permeability, as seen from the results, the titania NPs have more effects on the C[O.sub.2] permeability than the other gases. The strong interaction between C[O.sub.2] molecules and amine groups on the titania surface is one of the important reasons for higher C[O.sub.2] permeability. C[H.sub.4] and [N.sub.2] permeation mechanism is adsorption and diffusion in inner surface polymer matrix. Hence, the C[O.sub.2] permeation mechanism is surface diffusion, adsorption and diffusion through the polymer matrix. Another reason for increasing gas transport is that the voids created between polymer chains and titania NPs increased with increasing titania NPs loading ratio in the polymer matrix [57],

The strong interaction between C[O.sub.2] molecules and amine groups on the polymer surface is an important reason for higher C[O.sub.2] permeability. C[O.sub.2] molecules may interact with the polar groups in the mixed matrix membrane. Functional hydroxyl groups on the surface of the inorganic nanofiller in PI/titania mixed matrix membranes can also increase the gas permeability due to the interactions between polar gas and inorganic filler [58]. The separation performance of mixed matrix membranes was investigated by ideal selectivity calculation of gas pairs. Selectivity results of [O.sub.2]/[N.sub.2] and C[O.sub.2]/C[H.sub.4] are shown in Fig. 10e. As can be seen, this figure exhibits a general increase in mixed matrix membranes selectivities relative to pure PI.

When the titania loading was further increased from 5 to 15 wt%, the selectivity of C[O.sub.2]/C[H.sub.4] and [O.sub.2]/[N.sub.2] decreased. This tendency arises from the increase in free volume and voids which in turn decrease the size-selective nature of membrane. Also, the non-selective interfacial voids that are larger than the penetrating gas molecules may exist at higher loadings of titania, due to the poor adhesion of titania surface and PI matrix. An apparent reduction in ideal selectivities of gas pairs is observed in mixed matrix membranes containing 15 wt% titania NPs which can be related to unselective voids and micron-size aggregates formation in polymer matrix. Selectivity of gas pairs C[O.sub2]/C[H.sub.4] and [O.sub.2]/[N.sub.2] reached 3.56 and 1.92, respectively, at 15 wt% loading of Ti[O.sub.2] in mixed matrix membranes.

CONCLUSIONS

From the results achieved in this investigation, it can reach to the following conclusions: the synthesis of four chiral diamine, containing benzimidazoles and flexible amino acid groups and dianhydride monomer with trifluoromethyl pendant group were used for the preparation of novel optically active aromatic Pis. From the chemical point of view, the incorporation of benzimidazole and amino acids diamine into the backbone of diverse polymer systems results in versatile polymers with interesting properties such as thermal stability and good solubility. The mechanical properties of the PI and related NCswere investigated, and the results show that the ultimate strength and initial modulus were increased with Ti[O.sub.2] contents, but ultimate elongation decreased sharply with the increase in Ti[O.sub.2] contents, especially at lower Ti[O.sub.2] content. TGA results indicate that by increasing content of Ti[O.sub.2] nanofillers in polymer the thermal stability of NCsincreases and FE-SEM and TEM analysis shows homogeneous dispersion of nanofillers and compatibility of NPs with polymer matrix. Ti[O.sub.2] impregnation also improved the absorbance of UV radiation in comparison to pure PI. The effect of Ti[O.sub.2] concentration on gas permeation was studied. The gas transport in the mixed matrix membranes through the interstice between Ti[O.sub.2] and polymer chain is the major reason for the enhanced gas permeability achieved because of Ti[O.sub.2] loading. The result showed that the Ti[O.sub.2] can improve the membrane permeability for all gases. With increasing Ti[O.sub.2] concentration, permeability of bioNC membranes for C[O.sub.2] increased and selectivity of C[O.sub.2]/C[H.sub.4] decreased. It was found that as the weight ratio of Ti[O.sub.2] increases in PI composites, the permeability of C[O.sub.2] increased but the selectivity decreased slightly. These bioNCs have potential applications in medical, agricultural, drug release, packaging, and UV-shielding materials.

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Hashem Ahmadizadegan [iD], (1) Mahdi Ranjbar, (1) Sheida Esmaielzadeh (1,2)

(1) Department of Chemistry, Darab Branch, Islamic Azad University, Darab, 7481783143-196, Islamic Republic of Iran

(2) Young Researchers and Elite Club, Darab Branch, Islamic Azad University, Darab, 7481783143-196, Islamic Republic of Iran

Correspondence to: Hashem Ahmadizadegan; e-mail: h.ahmadizadegan.2005@ gmail.com

Contract grant sponsor: Iran Nanotechnology Initiative Council (INIC); contract grant sponsor: Darab branch, Islamic Azad University. Conflict of Interest. The authors declare that they have no conflict of interest. Ethical Statement. This study involved the culture of an existing cell line and did not require an animal use protocol. Informed Consent. This study did not require informed consent.

DOI 10.1002/pen.24757

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. [sup.1]H-NMR (500 MHz) spectra of chiral diamine (6a) in DMSO-[d.sub.6] at R.T. [Color figure can be viewed at wileyonIineIibrary.com]

Caption: FIG. 2. [sup.1]H-NMR (500 MHz) spectra of Pia in DMSO-[d.sub.6] at R.T. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. FTIR spectra of the (a) pure PI (a), (b) PI/Ti[O.sub.2] NC 5%, (c) PI/ Ti[O.sub.2] NC 10%, (d) PI/Ti[O.sub.2] NC 15%. [Color figure can be viewed at wileyonlineiibrary.com]

Caption: FIG. 4. XRD patterns of pure PI (a), pure Ti[O.sub.2] and NC hybrid film with different amounts of Ti[O.sub.2] nanoparticle. [Color figure can be viewed at wileyonlinelibrary.com]

Captipn: FIG. 5. TGA thermograms of pure PI (a), and NC hybrid film with different amounts of Ti[O.sub.2] nanoparticle. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. UV-Vis absorption spectra of pure PI (a), and NC hybrid film with different amounts of Ti[O.sub.2] nanoparticle. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. Tensile stress-strain curves of PI with different amount of Ti[O.sub.2] NPs. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. FE-SEM micrographs of PI/Ti[O.sub.2] 10 wt%. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. TEM micrographs of PI/Ti[O.sub.2] 10 wt%. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 10. (a-d) Effect of Nanoparticles loading on [N.sub.2], [O.sub.2], C[O.sub.2] and C[H.sub.4] permeability of membranes and (e) Effect of Ti[O.sub.2] loading on [O.sub.2]/[N.sub.2] and C[O.sub.2]/C[H.sub.4] selectivity. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: SCHEME 1. Preparation of chiral diamines (6a-d) containing amino acid and benzimidazole monomers. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: SCHEME 2. Synthesis of dianhydride with a trifluoromethyl pendent group. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: SCHEME 3. Polycondensation reactions of chiral diamines (6a-d) with dianhydride monomer. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: SCHEME 4. Fabrication pathway for PI/titania nanohybrid films. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Molecular weight characterization and some physical
properties of the PIs.

Sample     [[eta].sub.inh] (a)     [[[alpha].sup.     [M.sub.n] (c)
              (dL[g.sup.-1])       25.sub.Hg] (b)

PIa                0.90                -24.21             66225
PIb                0.86                -20.54             66279
PIc                0.68                -18.36             66198
PId                0.98                -26.32             66321

Sample      [M.sub.w] (d)     PDF (e)    Film quality

PIa             125174          1.89       Flexible
PIb            126 131          1.88       Flexible
PIc            124 154          1.87       Flexible
PId            128 144          1.93       Flexible

(a) Inherent viscosity of the Pis precursor measured at a
concentration of 0.5 g d[L.sup.-1] in DMAc at 30[degrees]C.

(b) Specific rotation at 25[degrees]C.

(c) [M.sub.n]: Number average molar mass.

(d) [M.sub.w]: Mass average molar mass.

(e) Polydispersity index: M/[M.sub.n].

TABLE 2. Elemental analysis of PIs (a-d).

Polymer    Formula (Formula weight)

PIa        [C.sub.42][H.sub.27][F.sub.6][N.sub.6][O.sub.5] (809.19)
PIb        [C.sub.42][H.sub.34][F.sub.6][N.sub.6][O.sub.5] (852.25)
PIc        [C.sub.42][H.sub.26][F.sub.6][N.sub.6][O.sub.5]S (840.16)
PId        [C.sub.42][H.sub.26][F.sub.6][N.sub.6][O.sub.5] (856.19)

Polymer          Calculated (%)             Found (%)

           C        H        N        C        H        N

PIa        62.30    3.36     10.38    63.01    3.21     10.21
PIb        63.38    4.02     9.85     62.85    4.81     9.42
PIc        60.00    3.12     10.00    61.05    3.55     9.22
PId        64.49    3.06     9.81     63.75    3.83     9.35

TABLE 3. Thermal characterization of bionanocomposites.

                                                   TGA

                                      Decomposition
                     DSC              temperature (a)

Materials            [T.sub.g] (c)      In nitrogen          Char
                                                         yield (b) (%)

PI                       142.12             255              53.54
PI/titania (5%)          155.57             390              58.21
PI/titania (10%)         160.41             392              59.12
PI/titania (15%)         164.85             395              59.79

(a) Temperature of 10% weight loss determined in nitrogen atmospheres.

(b) Residual weight (%) at 800[degrees]C in nitrogen.

(c) [T.sub.g] measured by DSC at scanning rate of 10[degrees]C
[min.sup.-1] in flowing nitrogen.

TABLE 4. The result of alternation of UV irradiation.

Materials            [[lambda].sub.max 1]   [[lambda].sub.max 2]

PI                           267                    343
PI/titania (5%)              255                    354
PI/titania (10%)             255                    356
PI/titania (15%)             258                    357
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Author:Ahmadizadegan, Hashem; Ranjbar, Mahdi; Esmaielzadeh, Sheida
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Date:Sep 1, 2018
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