Preparation of a Ductile Biopolyimide Film by Copolymerization

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Preparation of a Ductile Biopolyimide Film by Copolymerization Hojoon Shin,†,‡ Siqian Wang,§ Seiji Tateyama,†,‡ Daisaku Kaneko,§ and Tatsuo Kaneko*,†,‡ †

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Frontier Research Academy for Young Researchers, Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, Sensuicho, Tobata-ku, Kitakyushu-city, Fukuoka 804-8550, Japan ‡ Advanced Low Carbon Technology Research and Development Program (ALCA), Japan Science and Technology (JST), Chiyoda-ku, Tokyo 102-0076, Japan §

S Supporting Information *

ABSTRACT: We developed ductile biopolyimide (BPI) films derived from a renewable aromatic diamine, 4,4′-diaminotruxillic acid, with a mixture of two different dianhydrides. The BPI copolymers show high thermal resistance with T10 values of over 406 °C and Tg values of over 208 °C, improved tensile strength and elongation at break, and a Young’s modulus maintained at around 4 GPa. In addition, the copolymers showed an adhesion strength to the carbon plate of 0.22−4.47 MPa, which is similar to that of cyanoacrylate superglue. In summary, we develop tough films composed of the copolymers derived from benzophenonetetracarboxylic dianhydride and oxydiphthalic dianhydride. The film shows a strain energy as high as that of a commercial Kapton film. We confirm the ductility of the film by microscopy and tensile testing after it was completely folded.

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show low values of the strain at break of 1.8−4.5%,17 indicating brittleness that restricts their use in the development of industrial materials. Therefore, improvements in the strain at break and strain energy are required for industrial applications; the compatibility of the films with hard materials is also important. Here, we polymerized the bio-based photodimer of a microorganism-derived 4-aminocinnamic acid (4ACA) with various tetracarboxylic dianhydrides to improve the ductility of the BPI films by copolymerization. The combination of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and 4,4′oxydiphthalic anhydride created the toughest film of all of the prepared BPI films; it was unbreakable when folded completely.

INTRODUCTION The development of high-performance bio-based polymers is indispensable in the establishment of a green sustainable lowcarbon society because commonly developed aliphatic biobased polymers with relatively low softening temperatures such as poly(lactic acid),1 poly(hydroxyalkanoate),2 poly(butylene succinate),3 and polyamides (polyamide 11 and polyamide 66) have limited industrial applications. Because of this need, we developed aromatic bio-based polymers from hydroxycinnamate derivatives such as p-coumaric acid (4-hydroxycinnamic acid),4,5 and caffeic acid (3,4-dihydroxycinnamic acid)6,7 for polyarylate design. On the other hand, outstanding development of high thermal and mechanical performance resins can be seen using polysaccharides composed of heterocyclic chains such as cellulose8−11 and chitin,12 which were dissociated into nanosized assemblies by some special methods. Several of them were focused on transparent resins, and uniquely wood/ poly(methyl methacrylate) composites have been reported as a transparent wood with a high mechanical strength.13,14 Our approach to develop a bio-based transparent resin is to utilize 4,4′-diaminotruxillic acid (4ATA), which is the first bio-based aromatic diamine prepared by photodimerization of microbial 4-aminocinnamate. 4ATA-derived bioplastics such as polyurea,15 polyamide,16 and polyimide (PI)17,18 show ultrahigh thermal and mechanical performances. Of these polymers, biopolyimide (BPI) shows extremely high performance: a 10% weight loss temperature (T10) of over 425 °C and a glass transition temperature (Tg) of over 350 °C. The BPI films also show high tensile strength and a high Young’s modulus, with maximum values of 98 MPa and 13 GPa, respectively, and good cell compatibility. The high mechanical strength and good transparency of the PI films make them useful in the development of electronic devices. However, the BPI films © 2016 American Chemical Society

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EXPERIMENTAL SECTION

Materials. Dianhydrides such as 1,2,3,4-tetracarboxycyclobutane dianhydride (CBDA; Aldrich), pyromellitic dianhydride (PMDA; TCI), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA; Aldrich), 3,4,3′,4′-biphenyltetracarboxylic dianhydride (BPDA; Aldrich), 4,4′-oxydiphthalic anhydride (ODPA; Aldrich), and 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride (DSDA; Aldrich) were recrystallized in acetic anhydride by refluxing for 5 h and then cooling to 0−5 °C. The crystals were collected carefully by filtration, washed in hot dioxane, and dried under vacuum. 4-Aminocinnamic acid (4ACA; TCI), trimethylsilyl chloride (TMSCl; Aldrich), and N,N-dimethylacetamide (DMAc; 99.8% anhydrous from Kanto Chemical) were used as received. 4,4′-Diaminotruxillic acid Received: Revised: Accepted: Published: 8761

June 8, 2016 July 22, 2016 July 26, 2016 July 26, 2016 DOI: 10.1021/acs.iecr.6b02221 Ind. Eng. Chem. Res. 2016, 55, 8761−8766

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Industrial & Engineering Chemistry Research

Scheme 1. Synthetic Route and Chemical Structures of BPI Copolymers Derived from 4,4′-Diamino-a-truxillate with Two Dianhydrides

tensiometer at room temperature. The number-average molecular weight (Mn), weight-average molecular weight (M w ), and molecular weight distribution (PDI) were determined by gel permeation chromatography [GPC; concentration of 5 g/L, N,N-dimethylformamide (DMF) eluant] after calibration with Pullulan Standards. The transmittance was recorded on a PerkinElmer Lambda 25 UV/vis spectrophotometer at room temperature around 450 nm. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on Seiko Instruments SII SSC/5200 and X-DSC7000T calorimeters, respectively, at a heating rate of 10 °C/min under a nitrogen atmosphere. The polymer specimens were dried at 100 °C for 1 h to remove any absorbed moisture before both TGA and DSC. Adhesion testing of the PI films in shear mode was carried out with a tensile testing machine (Instron 3366, Canton, MA). The trifluoroacetic acid solutions of BPI were sandwiched between two rectangular carbon substrates (10 × 50 × 10 mm3) and then completely dried in the oven for 2 h at 40 °C to bond, where the adhesion area was 10 × 10 mm2. The bonding samples were held by two fixtures near the midpoint of the sample and pulled in opposite directions. The force and displacement of the fixture were simultaneously monitored, and the stress was defined as the force divided by the bonding area.

dimethyl (4ATA), which was used as an aromatic diamine monomer, was prepared by the previously reported method of photodimerization.17 Polymer Synthesis. A typical procedure for polymerization of poly(amic acid) (PAA) is shown inScheme 1. A diamine of 4ATA (0.20 g, 0.56 mmol) was dissolved in DMAc (0.56 mL, 0.6 M) in a 10 mL test tube under a nitrogen atmosphere. A mixture of two dianhydrides (0.56 mmol in total) out of CBDA, PMDA, BTDA, ODPA, BPDA, and DSDA was added into the diamine solution. Then the reaction mixture was vigorously agitated by an overhead stirrer at room temperature to create a pale-yellow solution and further agitated for 48 h to yield a viscous solution of PAA. The solution was diluted using DMAc and then added dropwise into ethanol to precipitate small fibrils of PAA, which were collected by filtration, thoroughly washed with water, and dried under vacuum for 12 h. The film of PAA was obtained by casting a DMAc solution onto a silicon wafer plate and heating at 60−70 °C. The specimen of PAA for UV/vis measurements was prepared by spin-casting (spin rate = 1000 rpm, MS-A100 Spin-coater, Misaka Co., Ltd.) to adjust to the 9−15 μm thickness. The PI films were obtained by thermal imidization of the PAAs under vacuum by stepwise heating at 100, 150, 200, and 250 °C for 1 h at each step in an oven. Measurements. NMR measurements of PAAs were performed by a Bruker Biospin AVANCE III 400 MHz, 5 mm spectrometer using dimethyl sulfoxide (DMSO)-d6 as the solvent. The Fourier transform infrared spectra were recorded with a PerkinElmer Spectrum One spectrometer between 4000 and 600 cm−1 using a Diamond attenuated-total-reflectance accessory. The tensile measurements were carried out at an elongation speed of 0.5 mm/min on an Instron 3365

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RESULTS AND DISCUSSION Polymer Syntheses. PAA copolymers were prepared by polycondensation of the aromatic diamine 4ATA with stoichiometric amounts of dianhydride monomer mixtures of two of the following: CBDA, PMDA, BTDA, ODPA, BPDA, and DSDA (Scheme 1). The copolymerization procedure was analogous to that of the homopolymer preparation reported in

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DOI: 10.1021/acs.iecr.6b02221 Ind. Eng. Chem. Res. 2016, 55, 8761−8766

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Industrial & Engineering Chemistry Research Table 1. Thermal Properties and Transparency of Copolyamide Films polyimide

Mna (×105)

Mwa (×105)

PDIa

T5b (°C)

T10b (°C)

P-CBDA/PMDA P-CBDA/BTDA P-CBDA/ODPA P-CBDA/BPDA P-CBDA/DSDA P-PMDA/BTDA P-PMDA/ODPA P-PMDA/BPDA P-PMDA/DSDA P-BTDA/ODPA P-BTDA/BPDA P-BTDA/DSDA P-ODPA/BPDA P-ODPA/DSDA P-BPDA/DSDA P-CBDA P-PMDA P-BTDA P-ODPA P-BPDA P-DSDA

7.31 6.65 4.88 7.07 7.86 8.46 6.93 7.44 7.78 8.90 8.13 7.12 8.30 5.27 6.54 2.78 4.61 2.25 2.20 1.70 1.97

10.8 10.0 6.78 12.5 9.88 12.0 10.6 11.4 11.4 15.3 12.2 12.5 13.1 8.78 9.94 3.99 3.19 3.06 3.15 2.56 2.78

1.48 1.50 1.38 1.77 1.25 1.42 1.53 1.47 1.47 1.72 1.51 1.76 1.58 1.66 1.51 1.43 1.45 1.36 1.44 1.51 1.41

397 389 397 399 388 408 402 407 398 407 406 397 409 399 401 365 410 400 395 395 410

415 406 408 413 399 424 418 423 412 424 425 411 425 412 414 390 425 420 410 410 425

Tgc (°C) 243 243 250 245

270 225 225 236 208 227 243

258 248 254 275

% Td 89 80 86 85 85 82 83 79 74 81 86 85 82 80 83 88 80 79 68 65 82

a

The weight-average molecular weight, Mw, number-average molecular weight, Mn, and distribution of the polymer molecular weight, PDI, were measured using PAA precursors by GPC. b5% and 10% weight loss temperatures, T5 and T10, respectively, were recorded by picking up the data from the TGA curve scanned at a heating rate of 10 °C/min under a nitrogen atmosphere, cThe glass transition temperature, Tg, was measured by a DSC thermogram scanned at a heating rate of 10 °C/min under a nitrogen atmosphere. dThe transmittance at 450 nm, % T, was measured by a UV/vis spectrophotometer. Homopolymer data were transferred from the previous paper.17

a previous paper.17 All of the PAA copolymers were soluble in DMAc solvent, and yellowish films with high transparency were easily prepared by casting at 60−70 °C. The films were subsequently imidized to anneal stepwise at 100, 150, 200, and 250 °C for 1 h at each step in an oven, and imide ring formation was confirmed by IR spectroscopy. The resulting PI films were abbreviated as P-CBDA, P-PMDA, P-BTDA, PODPA, P-BPDA, and P-DSDA, as shown in Scheme 1. The PI films had a deeper yellow color than the PAAs, which corresponded to a cutoff wavelength shift toward higher value by transmittance spectra, as shown in the previous report17 (Figure S1 in the Supporting Information). Mw, Mn, and PDI were determined using the PAA copolymers and are summarized in Table 1. PAAs had high Mw and Mn values in the ranges of (6.78−15.3) × 105 and (4.88−8.90) × 105, respectively, and PDI ranged from 1.25 to 1.77 (Table 1). The molecular weights of the copolymers were higher than those of the homopolymers,17 presumably because of the increased solubility based on the copolymer effects on increasing solution entropy, which means that the mobility in solution should be high enough to increase the polymerization reactivity. The solubility of the copolymers was tested; the PAAs were soluble in polar solvents such as concentrated sulfuric acid, N-methyl-2-pyrrolidone, DMAc, DMF, and DMSO at room temperature. On the other hand, all of the BPI copolymers were soluble only in trifluoroacetic acid and concentrated sulfuric acid. We can clearly deny the crosslinking effects upon an increase in the molecular weight because the PDIs of BPI copolymers show no significant difference from those of BPI homopolymers. Thermal Properties and Transparency. Thermal degradation of the BPI copolymers was investigated by TGA in a nitrogen atmosphere at a heating rate of 10 °C/min, and the

5% and 10% weight-loss temperatures (T 5 and T 10 , respectively) were determined (Table 1). All of the BPIs exhibited good resistance against thermal decomposition; the T5 values ranged from 398 to 409 °C and the T10 values ranged from 399 to 425 °C, comparable to the respective values for homopolymers.17 The Tg values of the BPI copolymers were measured by DSC under a nitrogen atmosphere and are summarized in Table 1. The copolymers show Tg values over 200 °C, which were higher than those of conventional bio-based polymers11 but lower than those of BPI homopolymers. 17 Some BPI copolymers containing CBDA and/or PMDA had Tg values that were so high (above 350 °C) that they could not be determined using the DSC machine without thermal degradation. The BPI homopolymers from CBDA or PMDA also showed Tg values too high to measure, meaning that the copolymer containing these components had too high softening temperature. The copolymer BPI films were pale yellow with a transparency at 450 nm of more than 70%, while Kapton had a dark-brown color with almost 0% transparency. The opaqueness of the Kapton film was due to the characteristic absorption spanning from the UV to visible regions, caused by strong charge-transfer (CT) interactions in the electron-rich oxydianiline component with dianhydride moieties. Most of the copolymers containing CBDA units showed higher transparency than the other copolymers. CBDA is one of the simplest alicyclic dianhydride monomers; nonconjugated aromatic structures in the dianhydride component lead to a weak electron-accepting ability of the imide moieties.19 Figure S2 shows that the transmittance spectra of BPI copolymer films illustrating cutoff wavelengths of almost BPI copolymers were shorter than those of BPI homopolymers except for P-CBDA 8763

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Industrial & Engineering Chemistry Research Table 2. Mechanical Properties of Copolyimide Films Estimated by Tensile Tests polyimide P-CBDA/PMDA P-CBDA/BTDA P-CBDA/ODPA P-CBDA/BPDA P-CBDA/DSDA P-PMDA/BTDA P-PMDA/ODPA P-PMDA/BPDA P-PMDA/DSDA P-BTDA/ODPA P-BTDA/BPDA P-BTDA/DSDA P-ODPA/BPDA P-ODPA/DSDA P-BPDA/DSDA P-CBDA P-PMDA P-BTDA P-ODPA P-BPDA P-DSDA Kapton

Ea (GPa) 4.2 4.3 3.5 3.7 3.1 4.1 2.9 3.7 3.8 4.1 4.4 3.1 3.9 3.6 3.9 10.01 8.02 4.24 13.39 4.36 4.77 2.8

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.46 0.34 0.23 0.06 0.15 0.49 1.01 0.20 0.46 0.08 0.38 0.69 0.67 0.28 0.39 3.68 1.19 0.18 3.03 0.55 0.75

σa (MPa) 79 98 63 61 67 105 65 56 82 113 110 80 28 96 66 75 89 48 98 71 90 63

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

21.8 24.7 1.8 37.4 9.3 17.7 24.1 5.3 14.5 5.0 11.9 16.3 6.9 10.4 1.1 6.62 9.24 0.75 5.71 2.14 5.30

εa (%) 2.8 3.2 2.1 2.1 2.5 4.9 3.5 1.9 2.9 9.4 3.0 4.0 0.9 5.1 2.2 1.82 2.48 1.72 4.49 2.42 3.31 12.8

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

strain energya (J/mm3)

adhesion strengthb (MPa)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.93 0.78 2.54 0.43 1.78 0.22 2.10 0.39 0.67 2.89 1.32 0.94 1.14 0.39 0.26 1.29 0.48 0.95 3.66 1.53 4.47 0.93

0.65 1.36 0.14 1.33 0.38 0.72 0.60 0.10 0.38 2.88 0.81 0.16 0.14 0.95 0.31 0.28 0.12 0.33 0.43 0.43 0.32

0.58 1.67 0.66 0.76 0.84 2.28 1.14 0.54 1.21 4.95 1.45 1.59 0.12 2.47 0.71 ND ND ND ND ND ND 4.03

0.08 1.14 0.02 0.72 0.24 0.80 0.46 0.08 0.39 1.93 0.63 0.31 0.05 0.68 0.14

a The values of Young’s modulus E, tensile strength σ, elongation at break ε, and strain energy were obtained from a tensiometer at room temperature. Homopolymer data were transferred from the previous paper.17 ND means not determined. Underlined values of the copolymers were higher than those of both corresponding homopolymers. bThe adhesion strength was determined by tensile testing in shear mode as shown in the Experimental Section.

and P-DSDA, meaning the yellow color of copolymers is thin. This might be attributed to the simple dilution by thin colorinducing units. Even in color-inducing components such as BPDA and ODPA, their mixing made the color thinner, presumably because of the mutual hindrance of interchain stacking. Mechanical Properties. The mechanical properties of the BPI films were measured by a tensile test (Table 2). The BPI films had Young’s modulus (E) values in the range of 2.9−4.4 GPa, tensile strength (σ) ranging from 28 to 113 MPa, and strain at break (ε) in the range of 0.9−9.4%. While E values were almost constant regardless of the copolymer composition, σ and ε were dependent on the BPI structure. Several of the BPI copolymers showed higher ε values than those of the corresponding homopolymers (underlined in Table 3), indicating the improved ductility of the BPI film by copolymerization. All of the BPI copolymers containing BTDA components were in this group (stress−strain curves in Figure S3A of the Supporting Information). The homopolyimide derived from BTDA did not show very much elongation, presumably because of the long rigid structure of benzophenone. On the other hand, copolymerization induced structural disordering, making interchain stacking difficult, even when the additional component was also rigid. In this case, the polymer chain can be flexible based on the improved ability of the carbonyl connecting groups to rotate the benzene rings in the tetraacid. The effects of copolymerization on improving the flexibility were confirmed in BPI films composed of tetraacid combinations with connecting groups such as BTDA/ODPA, BTDA/DSDA, and ODPA/DSDA. In particular, the incorporation of the ODPA component into P-BTDA produced a film with the highest ε value of all, presumably due to the facile rotation of the ether linkage of the ODPA structure because the

Table 3. Thermal and Mechanical Performances of Copolyimides from ODPA and BTDA with Various Compositionsa composition (ODPA/ BTDA)

T5 (°C)

T10 (°C)

Tg (°C)

E (GPa)

σ (MPa)

ε (%)

strain energy (J/mm3)

100/0 80/20 60/40 50/50 40/60 20/80 0/100

395 392 393 407 389 385 400

410 408 409 424 406 408 420

248 229 230 225 233 237 258

13.40 3.07 3.04 4.07 2.96 3.29 4.20

98 88 90 113 77 88 48

4.5 7.6 6.6 9.4 4.4 5.5 1.7

2.21 3.32 2.60 4.95 1.69 2.40 0.41

Values of Young’s moduli, E, tensile strength,σ, elongation at break, ε, and strain energy were obtained from tensiometer at room temperature. Homopolymer data were transferred from the previous paper.17 a

ODPA homopolymer showed a high ε value. In BPI copolymers containing the BTDA component, the σ values were also improved compared to those of the corresponding homopolymers. As a consequence, the BPI copolymers derived from the combination of BTDA and ODPA showed the highest strain energy, comparable to that of commercial Kapton. The compatibility of the BPI films with graphite is also important in the application of the films to the development of electronic devices. Therefore, we measured the adhesion strength of the BPI polymers to a carbon substrate. The results are summarized in Table 2. The shear adhesive of P-ODPA was 3.66 MPa and the shear adhesive of P-DSDA was 4.47 MPa, which is equivalent to that of conventional instant superglue made from α-cyanoacrylate polymers.20 When the shear adhesive strength of the three component copolymers was 8764

DOI: 10.1021/acs.iecr.6b02221 Ind. Eng. Chem. Res. 2016, 55, 8761−8766

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Figure 1. Tentative mechanism for toughening by copolymerization, based on the interchain CT interaction (dotted lines) between benzophenonetetracarboxylate and oxydiphthalate components.

measured, P-BTDA/OPDA copolyimide exhibited the highest performance (2.89 MPa) of all of the copolyimides. However, the adhesion strength values showed no clear correlation with any other mechanical tensile data, presumably because of the complexity of factors impacting adhesion such as interfacial interaction and PI cohesion failure. We adjusted the fractions of ODPA and BTDA to change the composition of the BPI copolyimides and measured the thermal and mechanical properties, in order to optimize the structures. The data are summarized in Table 3. The values of E, T5, T10, and Tg were almost completely independent of the copolymer composition, while the σ and ε values were highest at a composition of 50/50 (ODPA/BTDA). The strain energy was highest in the BPI copolymers with 50/50 composition as well. One reason for the improved strain energy may be appropriate interchain interactions. While the benzene rings in BTDA connected using three electron-withdrawing carbonyl groups, the benzene rings in ODPA connected using an electron-donating ether group. The difference in the π-electron density can induce some amount of CT interaction, as shown in Figure 1, which can increase the mechanical strength. In order to determine the ductility of the BPI film (P-BTDA/ ODPA), the BPI film created by casting over a DMF solution following imidization by stepwise heating was folded over. The film was not cracked or broken even partially, as shown in Figure 2A. We folded the film as many as four times, and it never ruptured, which was also confirmed by microscopy. Figure 2B shows microscopic images with no crack part even around the fold center, and a cross-polarized microscopic image (top of Figure 2B) shows a bright cross along the folded lines, which reveals the orientation of the films. Additionally, clear blue and orange contrast is seen in the middle image of Figure 2B taken under a retardation plate with a wavelength of 530 nm, indicating that the positive birefringence occurred as a result of polymer chain elongation by the folding treatment. Birefringence demonstrated that strain remained around the folded portions, which can affect the mechanical properties. We performed tensile testing of the folded film, resulting in little change in the stress−strain curves by folding. The film was fractured at the portion apart from the folded line, as shown in Figure S3B in the Supporting Information, indicating that the folded treatment does not reduce the mechanical strength. The microscopic and mechanical analyses proved that the BPI film can be applied to foldable devices.

Figure 2. Bendability of the biocopolyimide film derived from 4,4′diamino-a-truxillate dimethyl with oxydiphthalic dianhydride and benzophenonetetracarboxylic dianhydride. (A) Real images. The film size was 15 × 15 × 0.03 mm3. (B) Microscopic images (scale bar 100 mm) taken under cross-nicol polarimetry (top), under cross-nicol polarimetry with a retardation plate (middle), and without any polarimetry (bottom). The polarimetry direction is to the right of the images. The film was bendable without any crack generation confirmed in the microscale.

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CONCLUSION We prepared various BPI films derived from a renewable aromatic diamine, 4,4′-diamino-α-truxilic acid (4ATA), which was prepared as a photodimer of 4ACA bioavailable from genetically manipulated Escherichia coli. 4ATA was copolymerized with two of various dianhydrides to produce BPI copolymers by polycondensation. The BPI copolymers show high thermal resistance with T10 values of over 406 °C and Tg values of over 208 °C, improved tensile strength and elongation at break, and a Young’s modulus maintained around 4 GPa. In particular, the BPI copolymers derived from ODPA and BTDA gave the highest ε values and strain energy of any of the copolymers prepared here, presumably due to an appropriate interchain CT interaction between BTDA benzene rings connecting with three electron-withdrawing carbonyl groups and ODPA benzene rings connecting with an electron-donating ether group. In addition, BPI polymers showed strong adhesion behavior to the carbon plate, with a maximum strength of 4.47 MPa (as high as cyanoacrylate superglue). As a consequence, the BPI copolymers derived from a combination of BTDA and ODPA showed a high strain energy, comparable to that of commercial Kapton. The strong thermal and mechanical performances of BPI polymers suggest that they may become valuable industrial materials. The films were unbreakable when folded completely, and then the folded film was fractured 8765

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Acrylic Coatings with Improved Properties and Unreduced Transparency. ACS Sustainable Chem. Eng. 2016, 4, 3766. (12) Jin, J.; Lee, D.; Im, H.-G.; Han, Y. C.; Jeong, E. G.; Rolandi, M.; Choi, K. C.; Bae, B.-S. Chitin Nanofiber Transparent Paper for Flexible Green Electronics. Adv. Mater. 2016, 28, 5169. (13) Li, Y.; Fu, Q.; Yu, S.; Yan, M.; Berglund, L. Optically Transparent Wood from a Nanoporous Cellulosic Template: Combining Functional and Structural Performance. Biomacromolecules 2016, 17, 1358. (14) Zhu, M.; Song, J.; Li, T.; Gong, A.; Wang, Y.; Dai, J.; Yao, Y.; Luo, W.; Henderson, D.; Hu, L. Highly Anisotropic, Highly Transparent Wood Composites. Adv. Mater. 2016, 28, 5181. (15) Jin, X.; Tateyama, S.; Kaneko, T. Salt-induced reinforcement of anionic bio-polyureas with high transparency. Polym. J. 2015, 47, 727. (16) Tateyama, S.; Masuo, S.; Suvannasara, P.; Oka, Y.; Miyazato, A.; Yasaki, K.; Teerawatananond, T.; Muangsin, N.; Zhou, S.; Kawasaki, Y.; Zhu, L.; Zhou, Z.; Takaya, N.; Kaneko, T. Ultra-strong, transparent polytruxillamides derived from microbial photodimers. Macromolecules 2016, 49, 3336. (17) Suvannasara, P.; Tateyama, S.; Miyasato, A.; Matsumura, K.; Shimoda, T.; Ito, T.; Yamagata, Y.; Fujita, T.; Takaya, N.; Kaneko, T. Biobased Polyimides from 4Aminocinnamic Acid Photodimer. Macromolecules 2014, 47, 1586. (18) Kumar, A.; Tateyama, S.; Yasaki, K.; Ali, Md. A.; Takaya, N.; Singh, R.; Kaneko, T. Ultrahigh performance biobased polyimides from 4,4′diaminostilbene. Polymer 2016, 83, 182. (19) Lu, Y.; Hu, Z.; Wang, Y.; Fang, Q.-X. Organosoluble and lightcolored fluorinated semialicyclic polyimide derived from 1,2,3,4cyclobutanetetracarboxylic dianhydrides. J. Appl. Polym. Sci. 2012, 125, 1371. (20) Hiraishi, N.; Kaneko, D.; Taira, S.; Wang, S.; Otsuki, M.; Tagami, J. Mussel-mimetic, bioadhesive polymers from plant-derived materials. J. Invest. Clin. Dent. 2015, 6, 59.

following tensile testing not at the folded portion but at the normal portion. The ductility of the transparent biopolyimide can lead to industrial applications as inorganic glass alternatives.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02221. Details of characterization, transmittance curves of BPI homopolymer and copolymer films, representative stress−strain curves, and a specimen picture broken by elongation after folding (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-761-51-1633. Fax: +81761-51-1635. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by ALCA, JST, Tokyo, Japan (Grant 5100270). This contribution was identified by Dr. Don Wardius (Covestro LLC) as the Best Presentation in the session “Sustainable Polymers, Processes & Applications” of the 2016 ACS Spring National Meeting in San Diego, CA.

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REFERENCES

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DOI: 10.1021/acs.iecr.6b02221 Ind. Eng. Chem. Res. 2016, 55, 8761−8766