Hygromorphic Polymers: Synthesis, Retro-Michael Reaction, and

Mechanical properties of polyimide films were investigated by a strain control dynamic mechanical analyzer (DMA, TA Instruments RSA III) in tension. ...
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Hygromorphic Polymers: Synthesis, Retro-Michael Reaction, and Humidity-Driven Actuation of Ester−Sulfonyl Polyimides and Thermally Derived Copolyimides David H. Wang, Ruel N. McKenzie, Philip R. Buskohl, Richard A. Vaia, and Loon-Seng Tan* Functional Materials Division AFRL/RXA, Materials & Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433-7750, United States S Supporting Information *

ABSTRACT: With a view toward broadening the adaptive capability of polyimide-based systems that have been shown to be mechanically responsive to light, heat, and thermal-electrical stimuli, a simple diamine containing a highly polar ester− sulfonyl (ES) pendant was synthesized via a two-step route. It was polymerized with five common dianhydrides in Nmethylpyrrolidinone to afford poly(amic acid), PAA, solutions, which were subsequently converted to a series of amorphous polyimides containing ester−sulfonyl (−CH2CH2SO2Me) pendants, generically designated as PI-ES, by either chemical imidization at room temperature in the same pot or heat treatment of PAA cast film at 175 °C. The chemically imidized polyimide films are tough and creasable, but the thermally imidized ones are brittle because of much lower molecular weights (GPC results). In addition, a series of thermally derived copolymers designated as PI-ES:A, which contains ES and carboxylic acid (A) pendants, were prepared from PI-ES via a retro-Michael reaction at 250 °C, in which A was formed from ES pendant with the concomitant expulsion of vinyl methyl sulfone molecule. For various comparison purposes, the homopolymers, PI-A containing 100% A pendant and nonfunctional PI-N (i.e., without any stimuli-responders), were also prepared from their respective dianhydrido and diamino monomers. In addition to physical/mechanical characterization by FTIR, thermal analysis, WAXD, and DMA, the thin films of PI-ES, PI-A, and PI-ES:A have shown remarkable locomotion and beam-like oscillation under gradient (nonequilibrium) conditions created by humidity (or methanol vapor) while the PI-N, Ultem, and Nafion films were nonresponsive under the same conditions. While the state-of-the-art humidity-driven actuators have illustrated the innovative bilayer designs and clever utilization of responsive polymeric and nanocomposite systems, in which ionic moieties play the critical role in hosting the water molecules, this work shows that a simple, wholly covalent, and amorphous polymer in monolithic form can be hygromorphic and motile, and specifically this newly found humidity-gradient responsivity would enhance the functional versatility of polyimides.

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cycles. For instance, the doped PPy film containing perchlorate counterions would undergo rapid bending upon asymmetrical water-vapor sorption8 and would crawl on a wet filter paper, culminating in the designs of a soft motor capable of directly transducing chemical potential of water sorption into a continuous circular motion and origami (folded PPy film) actuators.9 Furthermore, the doped PPy film would contract in air under an applied voltage, thus generating Joule heating to desorb water molecules and providing an electromechanical control of the device.10,11 In an alternative design in which semisolid polyelectrolye had been used, a polymer composite film based on polypyrrole−polyolborate (PPy-POB) was shown to spontaneously and reversibly capture and release the ambient water vapor to induce film expansion and contraction, resulting in rapid and continuous locomotion on

INTRODUCTION An actuator is a mechanical device that is powered by a certain source of energy, such as electric current, pressure, and chemical energy, and can transform that energy into motion.1 According to the energy source utilized for actuation, responsive polymeric materials generally can be divided into three classes: electroactive polymers;2 light- or heat-responsive elastomers;3 and pH- or solvent-responsive gels.4 Recently, the actuation and power-generation systems that can harvest ambient energy from water gradients have attracted a great deal of attention, especially in the development of “dry” or liquid-free polymer actuators5,6 based on conducting polymers (CP) such as polypyrroles (PPy), polythiophenes,7 and polyanilines (PANI) as well as their composite systems. Thus, high hydrophilicity of the doped state and the stress− strain generated from movement of water molecules directed by humidity variation are being exploited instead of volume change driven by movement of ions between CP and electrolyte solution during electrochemical doping−dedoping © XXXX American Chemical Society

Received: February 2, 2016 Revised: March 25, 2016

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DOI: 10.1021/acs.macromol.6b00250 Macromolecules XXXX, XXX, XXX−XXX

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a wet surface. The PPy-POB machine was strong and powerful enough to lift objects 380 times heavier than itself and transport cargo 10 times heavier than itself.12 A similar version of dry CP actuators based on a polythiophene, viz. poly(3,4-ethylenedioxythiophene)−polystyrenesulfonate or commonly known as PEDOT:PSS, has also been developed.5,7 In a nontraditional bilayer design with a collective capillarity feature to create the effect of asymmetric water diffusion, a composite actuator, composed of doped polyaniline (PANI) nanotubes chemically synthesized in situ and embedded in a polycarbonate membrane, with one end of the nanotubes attached to a subsequently surface-deposited PANI layer, was fabricated. The so-called “nanotubes embedded membrane (NEM)” showed water diffusion behavior quite similar to those observed for biological ion channels and pumps and displayed excellent moisture-propelled oscillatory motion and artificial-muscle capability.13 From the standpoint of dielectric materials, innovative approaches to humidity-driven actuation take advantage of the stress−strain generated from the orientational change in liquid-crystalline polymers (LCP) and networks (LCN) containing moieties (e.g., OH and COOH) that are sensitive to polar solvent vapors. As lyotropic or thermotropic LCP, cellulose derivatives, such as partially hydroxypropylated cellulose (HPC)14 and partially modified cellulose stearoyl ester (CSE),15 may have networks with solid state properties similar to LCE and LCN.16 Their solution-cast films are hygroscopic and have been shown to be promising materials in the development of humidity-powered, soft motors. Building on their first hygroscopic and mechanically robust LCN polymers containing COOH groups that are capable of reversible hydrogen-bonding and become hydrophilic after alkaline treatment,17 Broer and Schenning et al. have developed a family of humidity actuators in monolithic form18 or in bilayer configuration19 with a uniaxially oriented polyamide-6 substrate and having large responses to humidity change as manifested in bending, folding, and curling motions.20 Bilayer actuators based on alternating layer-by-layer deposition of poly(cation)/poly(anion) films on hydrophobic polymer substrates and engineered to power devices capable of unidirectional and humidity-controllable locomotion on a ratchet track have also been described.21,22 While the above-mentioned examples for humidity-driven actuators have illustrated the innovative bilayer designs and clever utilization of responsive polymeric and nanocomposite systems, in which ionic moieties play the critical role in hosting the water molecules, it appears that no simple, wholly covalent polymer in monolithic form with hygromorphic and motile properties has been reported. To develop fundamental understanding in this materials gap and as part of our continuing research on adaptive polyimide-based systems that have been shown to be mechanically responsive to light,23 heat,24 and thermal-electrical25 stimuli, here we report the results of synthesis and characterization of a series of simple but new ester−sulfone (ES)-containing polyimides and related thermally derived copolymers, assessing their mechanical responsivity relative to structurally similar polyimides containing CO2H pendants in a humidity gradient as a step toward developing adaptive structures functional in both dry and wet environments.

Article

EXPERIMENTAL SECTION

Materials. 3,3′,4,4′-Diphenyl sulfone tetracarboxylic dianhydride (DSDA) was purchased from TCI America and recrystallized from acetic anhydride before use. 4,4′-Bisphenol A dianhydride (BPADA) was purchased from Aldrich and recrystallized from toluene/acetic anhydride before use. 3,3′,4,4′-Benzophenone tetracarboxylic dianhydride (BTDA) and 1,1,1,3,3,3-hexafluoro-2,2-bis(4-phthalic anhydride)propane (6FDA) were purchased from Chriskev Company, Inc., and sublimed before use. Ultem 1000 was obtained from GE Plastics (now SABIC Innovative Plastics). All other reagents, reference compound (methyl vinyl sulfone), and solvents were purchased from Aldrich Chemical Inc. and used as received, unless otherwise noted. Instrumentation. Mechanical properties of polyimide films were investigated by a strain control dynamic mechanical analyzer (DMA, TA Instruments RSA III) in tension. The glass transition temperatures of the polyimides were determined at the maximum tan δ (loss modulus/storage modulus) by a stress control DMA (TA Instruments DMA Q400EM) with a heating rate of 4 °C/min in a nitrogen atmosphere. Attenuated total reflection IR (ATR-IR) was measured on a Bruker Alpha-R spectrometer. Proton and carbon nuclear magnetic resonance spectra were measured at 300 MHz on a Bruker AVANCE 300 spectrometer. Differential scanning calorimetry (DSC) analyses were performed in nitrogen or air atmosphere at a heating rate of 10 °C/min using a TA Instruments Q1000 differential scanning calorimeter. Thermogravimetric analysis (TGA) was conducted in either nitrogen (N2) or air atmosphere at a heating rate of 10 °C/min using a TA Hi-Res TGA 2950 thermogravimetric analyzer. Mass spectral data were obtained via sample insertion directly to the mass spectrometer port of a Varian 1200 Series gas chromatograph/mass spectrometer. Elemental analyses were performed at the Systems Support Branch, Materials & Manufacturing Directorate, Air Force Research Lab, Dayton, OH. Melting points were obtained from Buchi Melting Point Apparatus B-545 with a heating rate of 2 °C/min. Wideangle X-ray experiments were carried out on a Statton box camera at 53 mm sample to image plate distances in transmission mode using Cu Kα generated by a Rigaku Ultrax18 system. Water Absorption Testing. Each piece (2 cm × 2 cm × 20 μm; ∼50 mg) of PI film was dried in a vacuum oven under at 120 °C for 2 h. It was allowed to cool to room temperature and weighed (W0). Then the film was submerged in a distilled water bath for 2 days. After it had been taken out of the bath, its surfaces were first wiped dry with paper towel before it was weighed again (Wf). The percentage of water absorption (W%) was calculated based on the following equation: W% = (Wf − W0)/W0 × 100%. Density Measurements. A small piece of polyimide film was added into a mixture of CCl4 and methanol in a cylinder column, and the ratio of two solvents was adjusted until it was suspended in the solution. Then a 5 mL sample of the film-suspension solution was taken out by a pipet and weighed. The density was calculated by dividing the weight of mixture with its volume (5 mL). An average value was taken from three measurements. 2-(Methylsulfonyl)ethyl 3,5-Dinitrobenzoate (3). Into a 250 mL three-necked, round-bottomed flask equipped with a magnetic stir bar and nitrogen inlet and outlet were placed 3,5-dinitrobenzoyl chloride (1; 8.48 g, 40.0 mmol), 2-(methylsulfonyl)ethanol (2; 4.96 g, 40 mmol), pyridine (10.0 g), and CH2Cl2 (100 mL). The homogeneous mixture was stirred at room temperature for 24 h. The resulting white precipitates were collected by filtration and recrystallized from ethanol/toluene (1:1) to yield 10.6 g (61%) of white crystals; mp 138.5−140.3 °C. NMR (DMSO-d6, δ in ppm): 3.11 (s, 3H, CH3), 3.73−3.76 (t, 2H, SO2CH2), 4.76−4.79 (t, 2H, CO2CH2), 8.95−8.96 (d, 2H, Ar−H), 9.04−9.06 (t, 1H, Ar−H). MS (m/z): 318 (M+). Anal. Calcd for C10H10N2O8S: C, 37.74%; H, 3.17%; N, 8.80%. Found: C, 37.74%; H, 3.12%; N, 8.90%. ATR-IR (bulk powder; cm−1): 3103, 3027, 3012, 2930, 1730 (CO), 1632, 1539 (asym NO2), 1464, 1349 (sym NO2), 1298 (asym SO2), 1281, 1195, 1171, 1145, 1131 (sym SO2), 1079, 1005, 984, 949, 919, 761, 719, 665, 548, 486, 410. B

DOI: 10.1021/acs.macromol.6b00250 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules 2-(Methylsulfonyl)ethyl 3,5-Diaminobenzoate (ES-Diamine, 4). 2-(Methylsulfonyl)ethyl 3,5-dinitrobenzoate (3; 3.18 g, 10.0 mmol) dissolved in THF (50 mL) and palladium on activated carbon (0.20 g) was placed in a hydrogenation bottle. The bottle was tightly secured on a Parr hydrogenation apparatus, flushed four times with hydrogen gas, and pressurized to 55 psi. After the mixture had been agitated at room temperature for 6 h under the hydrogen pressure of 55 psi, it was filtered through Celite. The filter cake was washed with THF, and then the filtrate was concentrated on a rotavap to a volume of ∼25 mL. The resulting mixture was heated to refluxing, until all the solid dissolved, and allowed to cool to room temperature to afford, after filtration and drying, 2.10 g (81.4%) of white needle crystals; mp 113.2−115.5 °C. 1H NMR (DMSO-d6, δ in ppm): 3.05 (s, 3H, CH3), 3.56−3.59 (t, 2H, SO2CH2), 4.50−4.53 (t, 2H, CO2CH2), 5.00 (s, 4H, NH2), 6.02−6.03 (t, 1H, Ar−H), 6.41−6.42 (d, 2H, Ar−H). MS (m/ z): 258 (M+). Anal. Calcd for C10H14N2O4S: C, 46.50%, H, 5.46%, N, 10.85%. Found: C, 46.65%, H, 5.39%, N, 10.89%. ATR-IR (bulk powder, cm−1): 3438, 3416, 3345 (NH2), 3217, 3009, 2995, 2920, 1771 (CO), 1626, 1596, 1493, 1387, 1355, 1300 (asym SO2), 1281, 1237, 1194, 1128 (sym SO2), 1102, 1010, 967, 939, 854, 766, 715, 606. 4,4′-(4,4′-Hexafluoroisopropylidenediphenoxy)bis(phthalic anhydride) (6F-BPADA, 5b). Compound 5b was prepared according to previously reported procedure.26,27 The experimental and characterization details (Figures SI-5 and SI-6) are described in the Supporting Information. Representative Procedures for Preparation of PI-ES, PI-A, and PI-N Are Provided under “SC”, “ST”, “T1”, and “T2” Corresponding to the Imidization Conditions Indicated in Schemes 2 and 3. SC: Representative Procedure for Preparation of PEI-ES via Solution Chemical Imidization. (7a-SC). 2(Methylsulfonyl)ethyl 3,5-diaminobenzoate (4; 0.5166 g, 2.000 mmol) and NMP (8.0 mL) were added to a 25 mL three-necked flask equipped with a magnetic stirrer, nitrogen inlet, and outlet and stirred under dry nitrogen at room temperature for 30 min. BPADA (5a; 1.041 g, 2.000 mmol) was then charged. The light yellow solution was agitated at room temperature for 24 h to afford a viscous poly(amic acid) solution. A mixture of pyridine (0.5 mL) and acetic anhydride (0.5 mL) was added to the solution. Stirring was continued for an additional 24 h, and the solution was poured into ethanol to precipitate the polymer product. Fibrous polyimide was collected by filtration, followed by Soxhlet extraction with ethanol for 48 h. The polyimide was finally dried overnight in vacuum oven at 100 °C. ATRIR (fibers, cm−1): 3065, 2965, 2930, 1777, 1716, 1620, 1598, 1504, 1477, 1456, 1395, 1350 (asym SO2), 1265, 1232, 1125 (sym SO2), 1077, 1013, 846, 764, 742, 626. 6FPEI-ES (7b-SC). 2-(Methylsulfonyl)ethyl 3,5-diaminobenzoate (4; 0.5166 g, 2.000 mmol), 6F-BPADA (5b; 1.257 g, 2.000 mmol), and NMP (8.0 mL) were used. ATR-IR (fibers, cm−1): 3070, 2934, 1780, 1724, 1603, 1510, 1478, 1459, 1398, 1355 (asym SO2), 1261, 1236, 1208, 1175, 1136 (sym SO2), 1067, 968, 956, 929, 849, 784, 744, 628. 6FDI-ES (7c-SC). 2-(Methylsulfonyl)ethyl 3,5-diaminobenzoate (4; 0.5166 g, 2.000 mmol), 6FDA (5c; 0.889 g, 2.000 mmol), and NMP (8.0 mL) were used. 1H NMR (fibers, DMSO-d6, δ in ppm): 3.06 (s, 3H, CH3), 3.31 (s, 2H, SO2CH2), 4.70 (s, 2H, COOCH2), 7.78 (s, 2H, Ar−H), 7.92 (s, 1H, Ar−H), 7.97 (s, 2H, Ar−H), 8.20 (s, 4H, Ar−H). ATR-IR (fibers, cm−1): 3082, 2932, 1785, 1720, 1600, 1459, 1398, 1353 (asym SO2), 1297, 1241, 1207, 1190, 1127 (sym SO2), 1095, 990, 962, 847. DSDI-ES (7d-SC). 2-(Methylsulfonyl)ethyl 3,5-diaminobenzoate (4; 0.5166 g, 2.000 mmol), DSDA (5d; 0.7165 g, 2.000 mmol), and NMP (8.0 mL) were used. ATR-IR (fibers, cm−1): 3099, 2930, 1784, 1722, 1600, 1554, 1458, 1399, 1360 (conjugated asym SO2), 1313 (aliphatic asym SO2), 1287, 1223, 1178, 1147 (conjugated asym SO2), 1127 (aliphatic asym SO2), 1100, 1060, 965, 917, 762, 739, 671, 638, 562. BTDI-ES (7e-SC). 2-(Methylsulfonyl)ethyl 3,5-diaminobenzoate (4; 0.5166 g, 2.000 mmol), BTDA (5c; 0.6444 g, 2.000 mmol), and NMP (8.0 mL) were used. The polymer precipitated from solution in 2 h after addition of acetic anhydride and trimethylamine due to poor solubility.

ST: Representative Procedure for Preparation of PEI-A via Solution Thermal Imidization. (15a-ST). 3,5-Diaminobenzoic acid (10; 0.761 g, 5.000 mmol) and NMP (12.6 mL) and toluene (5 mL) were added to a 50 mL three-necked flask equipped with a magnetic stirrer, Dean−Stark trap, and nitrogen inlet and outlet and stirred under dry nitrogen at room temperature for 30 min. BPADA (5a; 2.602 g, 5.000 mmol) was then charged. The light yellow solution was agitated at room temperature for 24 h to afford a viscous poly(amic acid) solution. The light yellow solution was agitated and heated to 150 °C/1 h, 160 °C/1 h, 170 °C/1 h, 180 °C/1 h, and 190 °C/1 h to afford a very viscous, gel-like solution. It was diluted by adding NMP (5 mL), allowed to cool to room temperature. The final mixture was poured into ethanol to precipitate a white fibrous solid, which was collected and dried in the oven at 50 °C overnight. The film samples were prepared by dissolving the dried polymer in DMAc with 10 wt % solid contents, cast onto glass slides followed by vacuum evaporation of DMAc at 50 °C, and heat-treated at 100 °C/2 h, 150 °C/2 h, 175 °C/1 h, 200 °C/1 h, and 250 °C/1 h. The film thickness was approximately 20−50 μm. ATR-IR (film, cm−1): 3067, 2966, 2927, 2500−3500 (br, COOH), 1778, 1715, 1597, 1503, 1476, 1444, 1397, 1348, 1266, 1230, 1172, 1013, 930, 837, 744, 625, 541. 6FDI-A (15c-ST). 3,5-Diaminobenzoic acid (10; 0.6087 g, 4.000 mmol), 6FDA (5c; 1.777 g, 4.000 mmol), NMP (12.0 mL), and toluene (5 mL) were used. 1H NMR (fibers, DMSO-d6, δ in ppm): 7.77 (s, 2H, Ar−H), 7.84 (s, 1H, Ar−H), 7.93−7.95 (d, 2H, Ar−H), 8.08 (s, 2H, Ar−H), 8.18−8.20 (d, 2H, Ar−H), 13.43 (br s, 1H, COOH). ATR-IR (fibers, cm−1): 3091, 2500−3500 (br, COOH), 1784, 1718, 1596, 1452, 1399, 1350, 1298, 1240, 1206, 1188, 1086, 990, 964, 846, 717, 631. T1: Representative Procedure for Preparation of PEI-ES. (7a-T1). 2-(Methylsulfonyl)ethyl 3,5-diaminobenzoate (4; 0.5166 g, 2.000 mmol) and NMP (8.0 mL) were added to a 25 mL three-necked round-bottomed flask equipped with a magnetic stirrer and nitrogen inlet and outlet and stirred under dry nitrogen at room temperature for 30 min. BPADA (5a; 1.041 g, 2.000 mmol) was then charged. The light yellow solution was agitated at room temperature for 24 h to afford a viscous poly(amic acid) solution. This solution was poured into a glass dish, followed by vacuum evaporation of NMP at 50 °C, and heat-treated at 100 °C/2 h, 150 °C/2 h, and 175 °C/1 h to form imidized polymers. The film thickness was approximately 20−50 μm. ATR-IR (film, cm−1): 3065, 2967, 2930, 1777 νsym(imide CO), 1715 νsym(imide CO), 1597, 1503, 1477, 1444, 1396, 1350, 1265, 1231, 1119, 1076, 1013, 838, 741. 6FPEI-ES (7b-T1). 2-(Methylsulfonyl)ethyl 3,5-diaminobenzoate (4; 0.5166 g, 2.000 mmol), 6F-BPADA (5b; 1.257 g, 2.000 mmol), and NMP (8.0 mL) were used. ATR-IR (film, cm−1): 3070, 2934, 1780, 1724, 1603, 1510, 1478, 1459, 1398, 1355, 1261, 1236, 1208, 1175, 1136, 1067, 968, 956, 929, 849, 784, 744, 628. 6FDI-ES (7c-T1). 2-(Methylsulfonyl)ethyl 3,5-diaminobenzoate (4; 0.5166 g, 2.000 mmol), 6FDA (5c; 0.889 g, 2.000 mmol), and NMP (8.0 mL) were used. 1H NMR (fibers, DMSO-d6, δ in ppm): 3.06 (s, 3H, CH3), 3.64 (s, 2H, SO2CH2), 4.69 (s, 2H, COOCH2), 7.28−7.31 (d, 4H, Ar−H), 7.48−7.59 (m, 8H, Ar−H), 7.90 (s, 1H, Ar−H), 8.02−8.04 (d, 2H, Ar−H), 8.17 (s, 2H, Ar−H). ATR-IR (film, cm−1): 3082, 2932, 1785, 1720, 1600, 1459, 1398, 1353, 1297, 1241, 1207, 1190, 1127, 1095, 990, 962, 847. DSDI-ES (7d-T1). 2-(Methylsulfonyl)ethyl 3,5-diaminobenzoate (4; 0.5166 g, 2.000 mmol), DSDA (5d; 0.7165 g, 2.000 mmol), and NMP (8.0 mL) were used. ATR-IR (film, cm−1): 3094, 2930, 1783, 1716, 1597, 1456, 1396, 1366, 1311, 1283, 1221, 1176, 1176, 1124, 1095, 1057, 1006, 963, 914, 855, 761, 737, 669, 635, 557. BTDI-ES (7e-T1). 2-(Methylsulfonyl)ethyl 3,5-diaminobenzoate (4; 0.5166 g, 2.000 mmol), BTDA (5e; 0.6444 g, 2.000 mmol), and NMP (8.0 mL) were used. ATR-IR (film, cm−1): 3093, 2930, 1779, 1713, 1595, 1455, 1396, 1354, 1288, 1248, 1195, 1125, 1090, 959, 921, 853, 768, 714, 631. PEI-ES:A Copolymers via Solid-State Thermal Imidization and Retro-Michael Reaction. 2-(Methylsulfonyl)ethyl 3,5-diaminobenzoate (4; 0.5166 g, 2.000 mmol) and NMP (8.0 mL) were added to a 25 mL three-necked round-bottomed flask equipped with a magnetic C

DOI: 10.1021/acs.macromol.6b00250 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Synthesis of Ester−Sulfonyl (ES) Diamine Monomer (4)

polymers, and inorganic salts33 Aliphatic sulfone and sulfoxide have been incorporated into polymer backbones or side chains to generate polymers with functionality that can promote multiphase-transfer catalysis,34 dipole−dipole-induced liquidcrystalline phases,35 and very high dielectric constant and relatively low permittivity loss values.36 However, the high water affinity of these polar groups in polymeric systems, especially sulfone which is more resistant to oxidation than sulfoxide, has yet to be exploited for active-type utility such as humidity-driven morphing and actuation. Monomer Synthesis. It is interesting to note that while 2methylsulfonylethyl-3,5-diaminobenzoate (4, Scheme 1) is a known compound,37 its synthesis and utility as a monomer in polymerization have not been reported in the literature. Thus, we have devised a simple, two-step synthesis of this ester− sulfonyl diamine (ES-diamine; 4) as depicted in Scheme 1. Briefly, esterification of 3,5-dinitrobenzoyl chloride (1) and 2(methylsulfonyl)ethanol (2) afforded 2-(methylsulfonyl)ethyl3,5-dinitrobenzoate (3), which was subsequently converted to the desired diamine (4) by catalytic hydrogenation. The structure of 4 was confirmed by the 1H and 13C NMR spectra (Figure SI-1) as well as the elemental analysis and IR results. The identities and purity of all other intermediates were also confirmed by NMR, IR results, and elemental analysis. Polyimide Synthesis. The ES-diamine 4 was polymerized with sublimed dianhydrides, BPADA, 6F-BPADA, 6FDA, DSDA, and BTDA, in NMP to yield a series of poly(amic acid) (PAA) precursors at room temperature. Then, PAA’s were either thermally imidized at 175 °C (conditions labeled as “T1” in Scheme 2) or chemically imidized by acetic anhydride and pyridine (conditions labeled as “SC”) to afford polyimides containing ester−sulfonyl (ES) groups, PI-ES’s (7a−e). The IR spectra (Figure SI-2) of polyimides prepared by both methods are almost identical. Typically, PAA cast films are thermally imidized at temperatures above 200 °C. However, during the early stage of this work, we found that when the PAA 6a cast film was heat-treated at temperatures above 200 °C, the corresponding copolymer containing ester−sulfonyl and carboxylic acid pendants (PEI-ES:A, 8a; Scheme 2) were obtained cleanly from the retro-Michael reaction of the ES pendant (i.e., −COO−CH2CH2SO2Me), resulting in the formation of COOH-pendant and methyl vinyl sulfone molecule. These retro-Michael products were easily detected by 1H NMR experiments after a piece of PEI-ES film had been heated at 250 °C for 2 h in an NMR tube, followed by being dissolved in DMSO-d6 at room temperature (Figure 1). Based on the NMR analysis and area-integration results, about 42 mol % of ES groups were found to have converted into carboxylic acids. The new byproduct peak in spectrum 2b is identical to that of the authentic methyl vinyl sulfone sample (spectrum 1c). Although ∼50% of methyl vinyl sulfone generated had escaped into air during the heat treatment at 250 °C, the other half remained in the polymer film because of the relatively

stirrer and nitrogen inlet and outlet and stirred under dry nitrogen at room temperature for 30 min. BPADA (5a; 1.041 g, 2.000 mmol) was then charged. The light yellow solution was agitated at room temperature for 24 h to afford a viscous poly(amic acid) solution. This solution was poured into a glass dish, followed by vacuum evaporation of NMP at 50 °C, and heat-treated at 100 °C/2 h, 150 °C/2 h, 175 °C/1 h, 200 °C/1 h, and 250 °C/1−16 h to form a series of imidized copolymers of PEI-ES and PEI-A. Retro-Michael reaction of the methyl sulfonyl ethyl ester pendant and late stage of imidization of amic acid moiety occurred concurrently at temperatures ca. 250 °C. The resulting copolyimides are designated as PEI-ES:A-x hr, where x corresponds to number of hours at 250 °C and qualitatively correlated to the amount of CO2H pendants generated. The film thickness was approximately 20−50 μm. ATR-IR (film, cm−1) for PEI-ES:A-4 hr: 3086, 2500−3500 (br, COOH), 1785, 1718, 1600, 1458, 1399, 1361 (asym SO2), 1298, 1240, 1206, 1189 (sym SO2), 1148, 1090, 989, 963, 846, 741, 718, 631, 568. PEI-A (15a-T2). 3,5-Diaminobenzoic acid (10; 0.761 g, 5.000 mmol), BPADA (5a; 2.602 g, 5.000 mmol), and NMP (12.6 mL) were used in the polymerization procedure and imidization conditions described above for PEI-ES:A. ATR-IR (film, cm−1): 3067, 2966, 2927, 2500−3500 (br, COOH), 1778, 1715, 1597, 1503, 1476, 1444, 1397, 1348, 1266, 1230, 1172, 1013, 930, 837, 744, 625, 541. 6FPI-A (15c-T2). 3,5-Diaminobenzoic acid (10; 0.6087 g, 4.000 mmol), 6FDA (5c; 1.777 g, 4.000 mmol), and NMP (12.0 mL) were used in the polymerization procedure and imidization conditions described above for PEI-ES:A. ATR-IR (fibers, cm−1): 3091, 2500− 3500 (br, COOH), 1784, 1718, 1596, 1498, 1399, 1350, 1298, 1240, 1206, 1188, 1140, 1086, 990, 964, 846, 744, 717, 645. PEI-N (Ultem-1000, 14a-T2). 3,5-Diaminobenzene (9; 0.2162 g, 2.000 mmol), BPADA (5a; 0.889 g, 2.000 mmol), and NMP (10.0 mL) were used were used in the polymerization procedure and imidization conditions described above for PEI-ES:A. ATR-IR (film, cm−1): 3081, 1784, 1719, 1625, 1603, 1495, 1456, 1437, 1353, 1297, 1240, 1206, 1189, 1140, 1100, 1005, 985, 891, 846, 786, 755, 717, 679, 629, 569, 545. 6FDI-N (14c-T2).28 3,5-Diaminobenzene (9; 0.2162 g, 2.000 mmol), 6FDA (5c; 1.041 g, 2.000 mmol), and NMP (10.0 mL) were used were used in the polymerization procedure and imidization conditions described above for PEI-ES:A. ATR-IR (film, cm−1): 3066, 2966, 2873, 1777, 1716, 1619, 1599, 1495, 1476, 1444, 1350, 1265, 1233, 1172, 1100, 1072, 1013, 920, 837, 776, 741, 682, 624, 543. Cast films of commercial Ultem-1000 were prepared from either chloroform or DMAc solution (10% w/v) and similarly dried prior to characterization experiments.

■

RESULTS AND DISCUSSION Simple organic sulfones and sulfoxides such as dimethyl sulfone (DMSO2) and dimethyl sulfoxide (DMSO) are interesting compounds because of their high polarity and being freely soluble or miscible with water. While their molecular polarity is quite similar, DMSO (dipole moment, 3.96 D, gas phase;29 4.3 D,30 liquid) is a hygroscopic liquid at room temperature whereas DMSO2 (dipole moment, 4.44 ± 0.1 D gas phase;31 4.25 D32) is a white crystal, which melts at 109 °C into a liquid thermally stable up to ∼248 °C, and thus both are good solvents for a wide variety of compounds including organics, D

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Scheme 2. Syntheses of Ester−Sulfonyl Polyimides (PI-ES’s, 7) and Thermally Converted Copolymer Series (PI-ES:A, 8) via Retro-Michael Reaction of the Methylsulfonyl Ethyl Ester Pendanta

a Two sets of imidization conditions denoted as (T1) and (SC) were used for 7a−e, and a selected PI-ES:A copolymer series was prepared via retroMichael reaction of the methylsulfonyl ethyl ester pendant in the PEI-ES (7a) dilms. Inset depicts the structures of the bisphthalimide compounds (VI and VII; see also Scheme SI-2) utilized to model thermal retro-Michael reaction in PI-ES polyimides (see Supporting Information for experimental details).

E

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Figure 1. 1H NMR spectra and peak assignments of PEI-ES (a), PEI-ES after heated at 250 °C/N2 for 2 h (b), and commercial sample of methyl vinyl sulfone (c) for authentication. All spectra were taken in DMSO-d6, which has protio residues with signals at 3.38 ppm (H2O) and 2.50 ppm (DMSO). Based on area integration (A) of the pertinent peaks numerically or alphabetically labeled in spectrum 2(b), ∼42 mol % of PEI-ES was found to have decomposed into PEI-A as calculated from the equation [Ae/(Ae + A8)] × 100% and ∼50 mol % loss of methyl vinyl sulfone formed as calculated from the equation (1 − Aa,b/Ae) × 100%, where Ai is the area of the corresponding peak i.

nonvolatile nature of methyl vinyl sulfone (bp 115−120 °C/19 Torr s38). To collaborate the occurrence of retro-Michael reaction by 1 H NMR spectroscopic experiments, two model compounds (VI and VII in Schemes 2 and Figure SI-9 for retro-Michael reactant and product, respectively; experimental details provided in the Supporting Information) were synthesized from the condensation of the diamine monomers 4 and 10 with 2 equiv of phthalic anhydride, respectively. As depicted in Figure SI-9, the 1H NMR results of the thermal reaction of the model compound VI (Figure SI-9b) in solid state (heating in a Teflon-tape-sealed NMR tube at 250 °C for 2 h followed by addition of DMSO-d6) confirm the occurrence of retro-Michael reaction, in which about 45 mol % of VI was decomposed into VII and methyl vinyl sulfone (Figure SI-9c). Interestingly, the reaction was fully reversible (i.e., to Michael addition reaction) in solution at room temperature, as evidenced by the complete disappearance of methyl vinyl sulfone peaks and substantial decrease of VII peaks (Figure SI-9d). There was still a small amount of VII residue (∼10 mol %) after Michael addition reaction, which would roughly correspond to the amount of methyl vinyl sulfone lost into the air during the 250 °C−2 h treatment. For comparison purposes, nonfunctional (pendant = H) and COOH-pendant polyimides, namely PI−N’s (14a,c) and PI-A’s (15a,c) respectively, were also prepared by polycondensating m-phenylenediamine (9, m-PDA) and 3,5-diaminobenzoic acid (10, DABA39) with BPADA and 6FDA, respectively, to form

the respective poly(amic acid)s. Two sets of imidization conditions were used for these less temperature-sensitive polyimides, namely, (ST) solution imidization at 190−200 °C and (T2) imidization of cast PAA film up to 250 °C (Scheme 3). Their structures were confirmed by ATR-IR spectroscopy, as evidenced by expected the symmetrical and asymmetrical stretches assignable to the imide-carbonyls around 1777−1785 and 1714−1720 cm−1, respectively. Both thermally derived PEI-ES:A copolymers (8a, Scheme 2, and vide inf ra) and PEI-A (15a) exhibited a broad absorption at 2500−3500 cm−1 due to the vibration of carboxylic O−H. In comparing the IR spectra (Figure SI-2) of PEI-ES, PEI-ES:A-4 hr, PEI-A, and PEI-N (Ultem), we found that the νasymSO2 (∼1350 cm−1) and νsymSO2 (∼1130 cm−1) stretches that are present in PEI-ES are obscured by nearby strong bands in PEI-ES:A-4 hr and are absent in both PEI-A and PEI-N (Ultem) samples. Thermal and Mechanical Properties. The glass transition temperatures (Tg’s) were determined by both DSC and DMA techniques. Tg’s were measured from inflection in baseline on DSC thermograms and from the peak of tan δ (DMA), respectively. Generally, the Tg’s values from DMA are higher than the DSC values (Table 1 and Figures SI-3a to SI-3i). Of the three types of polyimides and given the same dianhydride, the ester−sulfone or “ES” series exhibit the lower Tg’s (192− 254 °C by DSC; 221−231 °C by DMA), most probably due to the plasticizing effect of ES groups, and PI-A’s (249−278 °C by DSC and 273−296 °C by DMA) show the highest Tg’s because of the interchain hydrogen bonding of carboxylic acids. Derived F

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Scheme 3. Syntheses of Reference Polyimides: Nonfunctional PI-N’s (14a,c) and PI-A’s with Carboxylic Acid Pendants (15a,c) via Either Thermal (T) or Chemical (C) Imidization Conditionsa

a

For polyimides isolated, their cast films were prepared by DMAc solutions.

The thermomechanical properties of the polyimide films were characterized by DMA to confirm the expected trends (Table 1). For “ES” polyimide series, the films of the most rigid BTDI-ES have the largest Young’s modulus (2.15 GPa) and the lowest modulus belongs to the most flexible member, 6F-PEIES (1.07 GPa), in agreement with the observed Tg trend. However, 6FDI-A and PEI-A have even higher moduli, 2.13 and 3.08 GPa, respectively, apparently stemming from effective cross-linking and close packing driven by the interchain hydrogen bonding of the COOH pendants. Comparing the moduli of 6FDI-ES, 6FDI-N, and 6FDI-A (all having the same polymer backbone), the polyimides with ester−sulfonyl groups exhibited the smallest modulus. This result is rather interesting as it implicates that the ES pendants in our case would significantly frustrate chain packing (presumably as a polymeranchored DMSO2 solvent), instead of engaging in strong chain-stiffening, dipole−dipole interactions to induce the formation of liquid crystalline phases or crystallization in a number of previously reported aliphatic poly(sulfone)s and poly(oxyethylene)s because of the sulfone’s high polarity.41,42 The opposite effect appears to be owing to the length of the two alkyl groups, methyl and −CH2CH2− in our case, which

from the more rigid dianhydrides, DSDI-ES and BTDI-ES show no detectable glass transition up to 250 °C; their Tg’s could not be determined beyond this temperature because their thermal degradation started just slightly above 300 °C (Figures SI-3a to SI-3e). Nevertheless, the Tg trend of the “ES” series seems to be parallel with increasing rigidity of dianhydride monomers, roughly, BTDI ∼ DSDI > 6FDI > 6F-PEI > PEI, and in agreement with the trend observed for other structurally similar polyimides.39,40 The thermal stability was evaluated by TGA. As expected, the PI-ES samples showed the lowest thermal stability due to the aliphatic ester−sulfone side chains. They all exhibited a twostage degradation process (Figure SI-4). Their 5 wt % degradation temperatures are in the range of 282−321 °C in air and 292−335 °C in N2. The nonfunctional PEI-N (Ultem) and 6FDI-N exhibited the best thermal and thermo-oxidative stabilities. Their 5 wt % degradation temperatures are in the range of 492−504 °C in air and 497−509 °C in N2 (Figure SI4). The PI-A samples showed lower thermal stability than PI− N’s since the carboxylic groups underwent decarboxylation above 300 °C. G

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Macromolecules Table 1. Thermal, Mechanical Properties, and Water Uptakes of Polyimide Films sample

diamine

dianhydride

Tga (DSC, °C)

Tgb (DMA, °C)

Td5%c (°C) in air

Td5%c (°C) in N2

PEI-ES (7a) 6F-PEI-ES (7b) 6FDI-ES (7c) DSDI-ES (7d) BTDI-ES (7e) PEI-N (14a) ultem (14a) 6FDI-N (14c) PEI-A (15a) 6FDI-A (15c) CP2g

ES-diamine ES-diamine ES-diamine ES-diamine ES-diamine m-PDA m-PDA m-PDA m-DABA m-DABA APB

BPADA 6F-BPADA 6FDA DSDA BTDA BPADA BPADA 6FDA BPADA 6FDA 6FDA

192 221 254 UDf UDf 217 217 280 249 278 199

221 231 UDf UDf UDf 247 248 307 273 296 219

282 321 313 297 284 502 504 492 441 459 526

300 335 322 302 292 513 509 497 443 464 530

E′d (GPa) 1.38 1.07 1.27 1.75 2.15 2.68 2.74 1.67 3.08 2.13 1.9

± ± ± ± ± ± ± ± ± ± ±

0.14 0.10 0.24 0.16 0.25 0.18 0.19 0.23 0.28 0.13 0.15

water absorptione (wt %) 2.24 2.39 2.31 4.81 4.86 1.23 1.22 1.31 2.86 4.81 0.93

± ± ± ± ± ± ± ± ± ± ±

0.05 0.08 0.11 0.07 0.21 0.06 0.04 0.03 0.12 0.10 0.12

Tg measured from inflection in baseline on DSC thermogram obtained in N2 with a heating rate of 10 °C/min. For samples 7a−7e, the first scan was run to 200 °C, cooling to room temperature followed by rescanning to 300 °C. For all other samples, both initial scan and rescan were run to 350 °C. bTg measured from the peak of tan delta (DMA) as an average value taken from three measurements. cTemperature at which 5% weight loss recorded on TGA thermogram obtained with a heating rate of 10 °C/min. dModulus determined in tension mode at 25 °C as an average value taken from three specimens per sample. eWeight percentage increase after films were immersed in distilled water for 2 days. fUD = Tg undetected up to ca. 250 °C and not determined because retro-Michael and decarboxylation reactions are likely to have occurred above 300 °C for these polyimides. g Reference 23a, polymer structure of CP2: a

the CO2H-containing PI-A’s,39,43 with higher polarity and hydrogen-bonding capability than other polyimides in this work, show excellent solubility in both THF and acetone,44 especially the latter which is an uncommon solvent for polyimides. Film Fabrication. For ester−sulfone-containing polyimides (PI-ES), we found that the choice of imidization methods, i.e., chemical and thermal imidization, and conditions had direct impact on the film quality, likely because of the equilibrium nature of poly(amic acid) solution that is sensitive to the imidization conditions and pathways and influences the outcome of polyimide molecular weight. For example, the cast films of PI-ES obtained from chemical imidization in solution at room temperature are creasable while those obtained from thermal imidization at 175 °C are brittle (Table 2; T1 conditions). In the case of structurally similar and COOH-containing polyimides, mechanically robust films could not be cast from similarly chemical imidized polymer but could be cast from polymer after thermal imidization in solution (ST: 190−200 °C); for example, comparing 6FDI-A-T1 with 6FDIA-ST in Table 2. As subsequently determined, the molecular weight of the COOH-containing polyimide, 6FDI-A (15c), was much higher when it had been generated under ST conditions (6FDI-A-ST sample; Mn ∼ 17 kDa; Mw ∼ 83 kDa) than under T1 conditions (6FDI-A-T1 sample; Mn ∼ 9 kDa; Mw ∼ 27 kDa). Therefore, in order to further substantiate the molecularweight effect in these fabrication methods/imidization conditions, the number-average molecular weights (Mn), weightaverage molecular weights (Mw), and polydispersity index (PDI) of two ester−sulfone-containing polyimides (6F-PEI-ES, 7b, and 6FDI-ES, 7c; Table 2) were determined by gel permeation chromatography (GPC) and compared. As expected, the molecular weights of an ester−sulfone-containing polyimide is much higher when obtained from the room temperature, chemical imidization in solution (SC route Table 2) than from the thermally imidization at 175 °C in solid state

are much too short in comparison to liquid crystalline aliphatic polysulfones, which have n-octyl or n-hexadecyl side chain or a long alkylene backbone.41,42 WAXD. The morphology of the materials was characterized with wide-angle X-ray diffraction (WAXD). As depicted in Figure 2, all the polyimides in this work were completely

Figure 2. WAXD diffraction curves of PI-ES, PI-A, and PI-N samples.

amorphous as evidenced by the featureless diffraction patterns of these materials. The influence of any crystallinity on the humidity actuating response of these materials is deemed to be negligible. Solubility. Eight organic solvents, i.e., ethanol, acetone, CH2Cl2, CHCl3, THF, DMSO, DMAc, and NMP, were used to evaluate the solubilities of polyimides, and the results are summarized in Table SI-1. All the polymers are insoluble in ethanol, which was used as a precipitating solvent after chemical imidization. With the exception of BTDI-ES, they are all soluble in polar aprotic solvents such as DMSO, DMAc, and NMP. PEI-ES, 6F-PEI-ES, 6FDI-ES, and PEI-N (Ultem) are soluble in chlorinated solvents (CH2Cl2, CHCl3). It is noteworthy that H

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Macromolecules Table 2. Imidization Conditions, Polyimide Molecular Weights, Polydispersity, and Film Quality sample 6F-PEI-ES (7b) 6F-PEI-ES (7b) 6FDI-ES (7c) 6FDI-ES (7c) 6FDI-ES (7c) 6FDI-ES (7c) 6FDI-A (15c) 6FDI-A (15c)

polymerization solvent NMP NMP NMP NMP DMAc DMAc NMP NMP

imidization conditions b

SC T1c SCb T1c SCb T1c T1c STc

Mna

Mwa

PDIa

15450 7070 14300 5410 19900 6240 8820 17900

41500 29100 58500 17800 59400 18900 26600 83300

2.67 4.12 4.09 3.30 2.98 3.03 3.02 4.66

film quality tough, brittle tough, brittle tough, brittle brittle tough,

creasable creasable creasable

creasable

Number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity determined using GPC in THF at 30.0 °C with polystyrene standard. bSC: solution chemical imidization at room temperature. cT1: thermal imidization of cast PAA films at 175 °C. dST: one-pot, solution thermal imidization at 190−200 °C. a

(T1 route, Table 2). For example, the Mn and Mw of 6F-PEI-ES (7b) and 6FDI-ES (7c) are SC (Mn ∼ 15−20 kDa, Mw ∼ 42− 59 kDa) versus T1 (Mn ∼ 5−7 kDa; Mw ∼ 18−29 kDa). It is generally known that the mechanical properties of polymer films underwent dramatic changes when poly(amic acid) (more coil-like and easily plasticized residual solvent and water) were converted into the corresponding polyimides (more stiff and compact), with brittle behavior between 150 and 200 °C and a more ductile response above 275 °C.45a−c The fluctuation in mechanical properties has been attributed to the reduction in molecular weight, most probably caused by a small but significant amount of depolymerized amic acid repeat units.45c Similarly, molecular weights of COOH-pendant polyimides (e.g., 6FDI-A, 15c) produced from one-step thermal imidization (ST conditions) are much higher than the ones obtained from two-step thermal imidization (T1 conditions), thus resulting in better mechanical properties of the films. Thermally Derived PI-ES:A Copolyimides by RetroMichael Reaction. To further understand the structure− property relationship in hydromorphic performance, we found that the thermal, solid-state, retro-Michael reaction of ester− sulfone-containing polyimides (vide supra and Scheme 2) with an excellent combination of temperature and time dependence (i.e., allowing control of the initiation and extent of reaction) and regiospecificity (i.e., no side reactions) to be a simple method to generate a series of copolyimides having the same polymer backbone and containing variable ratio of ES and COOH (A) pendants. Thus, PEI-ES (7a) was selected, and its film samples were heated in an oven under N2 at 250 °C at a set of durations (0−16 h). The resulting copolyimides are designated as PEI-ES:A-xhr, where xhr corresponds to number of heat-treatment hours at 250 °C. In TGA (air) experiments, the samples first started to degrade at 270 °C due to the sidechain cleavage and reached a plateau at 350−440 °C (Figure 3). Aromatic components degraded above 440 °C. The percentage of ester−sulfone to carboxylic acid (ES → A) conversion was calculated based on the weight loss at 400 °C, and the results are plotted in Figure 4. There are two linear degradation processes (0−4 and 4−16 h). The conversion rate is much faster between 0 and 4 h than between 4 and 16 h. About 64% of ester sulfonyl was converted into carboxylic acid in the first 4 h heating at 250 °C (Figure 4). The rate is lower is probably because of higher concentration of COOH formed to slow down the escape of methyl vinyl sulfone molecule via the transition state of Michael adduct. Unfortunately, these films became insoluble due to thermal cross-linking (vide inf ra), precluding their usefulness in NMR experiment to assess the methyl vinyl sulfone contents.

Figure 3. TGA thermograms of PEI-ES and PEI-ES:A-xhr samples in air. The extent of retro-Michael reaction that varies with heating time at 250 °C is determined by is determined by calculating the percentage of weight increase at 400 °C, i.e., ES → A conversion % = (weight %PEI‑A − weight %PEI‑ES:A)/(weight %PEI‑A − weight %PEI‑ES).

Figure 4. TGA monitoring of retro-Michael reaction by following the ester−sulfonyl → carboxylic acid conversion after the respective PEIES samples had been heated at 250 °C for the predetermined durations.

In further probing, the films of a COOH-containing polyimide, PEI-A (15a) were subject to the same heat treatment as the PEI-ES at 250 °C for 1−16 h and designated as PEI-A-xhr. Their Tg’s increased from 273 to 277 °C after PEI-A films had been heated for predetermined periods up to 16 h (Table 3). Their moduli increased slightly in first 2 h and I

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Macromolecules Table 3. Thermal and Mechanical Properties of Thermally Derived Copolyimide Films sample

conva (%)

Tgb (DMA, °C)

Td5%c (°C) in air

Td5%c (°C) in nitrogen

PEI-ES (7a) PEI-ES:A-1hrf PEI-ES:A-2hrf PEI-ES:A-4hrf PEI-ES:A-8hrf PEI-ES:A-16hrf PEI-A (12a) PEI-A-1hr PEI-A-2hr PEI-A-4hr PEI-A-16hr PEI-N (15a)

0 13.0 32.1 64.1 83.5 100 100 100 100 100 100

221 247 267 279 286 287 273 274 274 275 277 247

282 334 337 345 417 463 441 438 428 422 432 502

300 342 343 361 429 472 443 442 432 428 438 513

E′d (GPa) 1.38 1.86 1.30 1.28 1.27 1.31 3.08 3.21 3.28 2.97 3.11 2.68

± ± ± ± ± ± ± ± ± ± ± ±

0.14 0.24 0.12 0.17 0.15 0.22 0.28 0.31 0.21 0.18 0.25 0.19

densitye (g/cm3) 1.332 1.326 NDg NDg NDg 1.323 1.320 1.320 NDg NDg 1.322 1.271

± 0.003 ± 0.004

± 0.005 ± 0.003 ± 0.004

± 0.005 ± 0.007

a

Percentage of carboxylic acid converted from ester−sulfonyl groups or present in polymer. bTg measured from the peak of tan delta (DMA) as an average value taken from three measurements. cTemperature at which 5% weight loss recorded on TGA thermogram obtained with a heating rate of 10 °C/min. dModulus determined in tension mode at 25 °C as an average value taken from three specimens per sample. eDensity was measured by floating the samples in CCl4/methanol mixture. A 5.00 mL of the mixture was drawn by a pipet and weighed. An average value was taken from three measurements. fPEI-EA films were heated at 250 °C for 1−16 h under N2 to yield PEI-ES:A copolymers via retro-Michael reaction. gNot determined (ND).

Figure 5. A series of snapshots to illustrate the water-gradient actuation and locomotion of a 6FDI-A PI film on a piece of wet paper towel.

decreased slightly from 3.08 GPa thereafter, which indicated that such thermal treatment did not induce any thermal degradation (e.g., thermal decarboxylation reaction) and adverse effect to the mechanical properties. TGA results confirm that the PEI-A and PEI-A-xhr samples are similar in thermal tolerance. In comparing PEI-ES:16hr and PEI-A-16hr results, we suspect that some structural defect induced during the heat treatment and retro-Michael reaction of PEI-ES is probably the cause of the generally poorer mechanical properties of thermally derived copolyimides PEI-ES:A-xhr. To qualify such a defect, the solubility of PEI-ES:A-xhr and PEI-A-xhr samples were tested in DMAc. While the PEI-ES:A1hr film was still soluble in DMAc, all the other PEI-ES:A-xhr ones became insoluble (Figure-SI-10a), which indicates the possibility of cross-linking reaction in addition to retro-Michael reaction. PEI-A-1hr and -2hr films were soluble in DMAc, but PEI-A-4hr and −16hr films were not. In the latter case, thermal annealing might have increased amount of stable, interchain carboxylic acid dimers, i.e., H-bonding cross-links (Figure-SI10b). This rationale is supported by the fact that PEI-N (Ultem) film, which had been heated at 250 °C for 16 h, was still soluble in DMAc.

The WAXD curves of PEI-ES:As exhibit the featureless diffraction patterns (Figure-SI-11), indicating they are same as PEI-ES, which is amorphous. Humidity-Driven Actuation. Water uptake testing was conducted on the all the polymers in an attempt to find the correlation between the water sorption and hygromorphic properties of the films (Table 1). Generally speaking, polar groups such as sulfonyl (−SO2−) and carboxylic acid (−CO2H) groups increase the polymer’s ability to absorb the moisture. BTDI-ES, DSDI-ES, and 6FDI-A show the highest water uptake (∼4.8%). Nonfunctional polymers (PEI-N and 6FDI-N) absorb the least amount of water in the range of 1.22 and 1.31% while the more polar counterparts in Table 1 have uptake values between 2.39 and 2.86%. Overall, these polyimides have much lower water affinity than perchloratedoped, electrochemically polymerized polypyrrole (PPy·ClO4; 9.9% uptake at 94% RH9) and Nafion (15−25% uptake46). Okuzaki et al. previously compared the mechanochemical behavior of PPy·ClO4 films made of nonionic polymers such as polyolefin, polyester, nylon, and polystyrene and reported no noticeable humidity-induced deformation for them.9a,b However, we have found that a thin polymer film (∼3 cm × 3 cm and 30 μm thick) of PEI-ES (a nonionic polymer) has an ability to be self-actuating and locomotive on a wet surface (see Figure J

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No actuation was observed in the absence of a humidity gradient, as tested by using deionized water as the source and sink. However, under a constant flux of water vapor from the source, the films actuated and maintained a constant conformation. The relative actuation was observed to be dependent on the molecular configuration. Overall, the presence of hydrophilic groups in the polymer structure had a positive effect on actuation when compared to films containing no hydrophilic polymer backbone or side chain, e.g., PEI-ES vs PEI-N (Figure 6b). While both PEI-N and 6FDI-N contain no hydrophilic pendants, the 6FDI-N backbone has been reported to be rather hydrophilic28 and would account for the observed curvature angle (vs zero curvature for PEI-N), which is still smaller than those polyimides containing hydrophilic pendants and being hygromorphic/motile. When the rigidity of the backbone increased, with the pendant groups remaining unchanged, a general increase in the actuation was observed (Figure 6b). 6FDI-ES somewhat falls out of line likely because of the added effect of its relatively more hydrophilic backbone as noted for 6FDI-N. There were slight changes in the film curvature when the pendant group was changed from ester−sulfone to carboxylic acid. The effect of pendant groups and backbone architecture on film curvature can be visualized in Figure 6b. Together, these results highlight that humidity-driven actuation can be generated from a nonionic polymer by simply grafting a sufficient amount of highly polar methylsulfonyl ethyl ester groups to the PEI backbone. Advantages of our approach include the convenient solubility of PEI polymer and the monolithic nature of the product, which eliminates phase separation or other interfacial issues of a multicomponent system.

SI-12 and Video 1 in the Supporting Information and TOC graphic). Briefly, upon being laid flat on a piece of water-wet paper towel, two opposite parts of the film were able to curl up like a pair of wings while standing still and then moved to another spot by flipping over when the “wings” are in close contact, following by a quick roll-over and flattening action. The apparently self-propelling movement continued across the wet surface until when the film reached the edge of the humidity field. In addition, we found that similarly fabricated film from an unmodified poly(ether−imide) (Ultem) or a highly hygroscopic polymer, namely Nafion, were practically unresponsive under the same testing conditions. Apparently, the former lacks suitable functional group with high affinity for polar molecules (e.g., H2O and MeOH), and the water molecules are known to tenaciously bind to the sulfonic acid groups in the latter at room temperature. During the course of this work, it became apparent that amorphous polymers containing other simple and highly polar moieties such as COOH pendants can be hygromorphic and motile as well under nonequilibrating humidity conditions. A series of representative snapshots of the actuation and locomotion sequence of the 6FDI-A film are depicted in Figure 5. A video that shows the continuous flapping of a piece of 6FDI-A (15c) film with one of its end weighted down on a piece of wet paper towel is provided in Supporting Information (Video 2). Humidity Gradient Actuation Assessment. A steady state humidity response of circular films with diameter of 2.7 cm was quantified using a custom-built humidity gradient chamber. The humidity gradient was generated by using deionized water as the source and a saturated aqueous solution of lithium chloride (LiCl(aq)) as the sink. The separation between the source and the sink was maintained at 4 cm. Deionized water generates an equilibrium relative humidity of 100% while LiCl(aq) generates an equilibrium relative humidity of 11% (see Figure 6a). Therefore, a linear estimate of the steady state humidity gradient is ∼22%/cm. The films were placed in the chamber and the equilibrium curvature of the discs was imaged.

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CONCLUSIONS We report to our knowledge the first case of nonoriented and nonionic polymers, whose thin films are hygromorphic and motile on wet surfaces (neutral pH), by attaching highly polar pendants such as ester−sulfone (ES) and carboxylic acid (A) to a relatively stiff polyimide backbone. In addition, a series of random copolyimides containing ES and A pendants were developed by time-controlled heat treatment of PI-ES samples at 250 °C with the aid of thermally induced, retro-Michael reaction in solid state, which was found to be rather clean and regiospecific. The reversibility of this retro-Michael reaction has been revealed by 1 H NMR monitoring the thermal decomposition of a model compound under the same conditions. Depending on imidization conditions, the polyimide films that have been chemically imidized are tough and creasable whereas the similarly prepared but thermally imidized films are brittle because of lower molecular weights. The thin films of imide polymers and copolymers containing ES, A, and their combinations showed remarkable vapor-gradient actuation (water or MeOH vapor) by demonstrating numerous oscillatory cycles and locomotion on moist surfaces without performance degradation during storage, whereas similar films of Ultem and Nafion (ionized form) were practically nonresponsive under the same conditions. While this discovery has provided us with an attractive opportunity to expand the utility of high performance polymers in energy-related (transduction and harvesting) applications and adaptive devices, understandably there are still many issues to be discerned, especially those related to the molecular factors and mechanics that collectively drive the actuation and locomotion of the film. In

Figure 6. Humidity gradient drives film curvature. (a) Schematic of the steady state humidity gradient cell where dC/dz is the humidity gradient in the cell. Arrows indicate direction of flux. (b) Effect of side chain and backbone on film curvature; trace is added for clarity. K

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future, the results of our ongoing work on the combination of copolymer synthesis, local patterning, and mechanical design to provide a novel platform to integrate mechanical responsivity and feedback control within a single material system will be reported.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00250. Preparative procedures for 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]hexafluoropropane dianhydride (6FBPADA) and model compounds, 2-(methylsulfonyl)ethyl 3,5-diphthalimidobenzoate (VI) and 3,5diphthalimidobenzoic acid (VII); Figures SI-1 to SI-12, Schemes SI-1 to SI-2, Table SI-1 (PDF) Movie file showing tumbling movement of 6FDI-ES (7c) film on a piece of wet paper towel (AVI) Movie file showing flapping movement of 6FDI-A (15c) film with one end restrained by a weight on a piece of wet paper towel (AVI)

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AUTHOR INFORMATION

Corresponding Author

*(L.-S.T.) E-mail [email protected]; phone 937-255-9153. Present Address

R.N.M.: National Research Council Fellowship Program. Notes

The authors declare no competing financial interest. D.W.H.: Also affiliated with UES Inc., Dayton, OH.

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ACKNOWLEDGMENTS This work was completed at Air Force Research Laboratory (AFRL) at Wright-Patterson Air Force Base with funding support from Materials and Manufacturing Directorate (RX) and Air Force Office of Scientific Research (AFOSR). We are grateful to Prof. Lei Zhu (Case Western Reserve University) for helpful discussions on the chemistry and dielectric properties of methyl sulfone-containing polymers.

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