Extremely High Thermal Resistive Poly(p-phenylene benzobisoxazole

May 8, 2012 - Extremely High Thermal Resistive Poly(p-phenylene benzobisoxazole) with Desired Shape and Form from a Newly Synthesized Soluble Precurso...
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Extremely High Thermal Resistive Poly(p-phenylene benzobisoxazole) with Desired Shape and Form from a Newly Synthesized Soluble Precursor Takahiro Fukumaru,† Tsuyohiko Fujigaya,*,†,‡ and Naotoshi Nakashima*,†,‡,§ †

Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka Nishi-ku, Fukuoka 819-0395, Japan ‡ International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka Nishi-ku, Fukuoka 819-0395, Japan § JST-CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan S Supporting Information *

ABSTRACT: The poly(p-phenylene benzobisoxazole) (PPBO) fiber, known as Zylon, has a very high thermal stability as well as mechanical strength when compared to any other polymers due to its ladder-like rigid structure. However, one of the critical drawbacks of its stiff structure is its insolubility in organic solvents, and only strong acids can be used use for fiber spinning of this polymer. To overcome the poor processability caused by this insolubility in organic solvents, a soluble PPBO precursor was designed and synthesized by the reaction of tert-butyldimethylsilyl (TBS) group-functionalized 4,6-diaminoresoisinol with terephthaloyl chloride for polycondensation. The obtained TBS-functionalized PPBO precursor (TBS-prePBO) shows an excellent solubility in common organic solvents, such as N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO). Transparent TBS-prePBO films made by the solution-cast method provided PPBO films after thermal treatment at 500 °C for 1 h. The structure of the obtained PPBO films was characterized by IR and XRD techniques and found that the films exhibited extremely high thermal stabilities, namely, the synthesized PPBO polymer decomposition temperature reached 670 °C in flowing N2, which is the highest temperature among the organic polymers reported so far.

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°C17) due to the introduction of a single bond linkage in the main chain structure of the PBOs. Therefore, in order to realize the fabrication of PBO films with the ultimate thermal stability, the development of a processable PPBO is strongly required. Typically, PPBOs are prepared by condensation polymerization using a strong acid such as poly(phosphoric acid) (PPA) as the solvent and a condensation reagent26 (Scheme 1)3 since only such an acid

INTRODUCTION Aromatic polymer films have attracted much attention in the fields of microelectronics and in aerospace and defense applications due to their excellent mechanical, thermal, and electrical properties and light weight.1 Among these the polymers, polybenzoxazoles (PBOs) have superior thermal stabilities compared to any other heat-resistant polymers, such as polyimides (decomposition temperature, ∼ 600 °C)1 and polybenzimidzoles (PBI: decomposition temperature, ∼620 °C)2 due to their inert ladder-like rigid structure. Especially, PPBO3 fibers are known to have the highest thermal stability (decomposition temperature, 650 °C).4 However, the major drawback of the rigid structure is the insolubility of the PPBO in organic solvents as well as the absence of a glass transition temperature, which hinders the fabrication of PPBO with the desired shape and form such as films. To overcome the poor processability, various soluble PBOs have been developed including fluorinated PBOs,5−11 PBOs containing an aliphatic main chain,12−15 ester-linked PBOs,16−21 and others.22−25 However, unfortunately, improvement of the solubility caused a lower thermal stability (decomposition temperature, ∼620 © 2012 American Chemical Society

Scheme 1. Synthetic Scheme of Zylon

Received: March 30, 2012 Revised: April 28, 2012 Published: May 8, 2012 4247

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can be used as the solvent for PPBO. Fabrication of PPBO films cast from a PPA solution after condensation was reported.27,28 However, the difficulty in the removal of the residual PPA and deterioration of the mechanical properties have been pointed out.29 Another strategy for the fabrication of PPBO films is the fabrication of precursor films followed by conversion to PPBO films. In this approach, only the PPBO precursor is required to be soluble in an organic solvent, and in principle, the polymerization solution is directly used for the fabrication similar to the fabrication of polyimide films from a polyamic acid solution. Indeed, poly(hydroxyamide)5,17,30−33 and poly(hydroxyimide)34−36 were reported to provide the PBO structure by a thermal cyclodehydration reaction. In this study, we describe the design and synthesis of a soluble PPBO precursor followed by the fabrication of PPBO films as shown in Scheme 2. One difficulty in the design and

according to previous methods.37 Zylon was kindly provided from Toyobo. Synthesis of 4,6-Di(tert-butyldimethylsilylamino)-1,3-di(tertbutyldimethylsiloxy)benzene (TBS-DAR). Under a nitrogen atmosphere, 4,6-diaminoresorcinol dihydrochloride (0.213 g, 1 mmol) was dissolved in DMF (10 mL), to which tert-butyldimethylsilyl chloride (1.5 g, 10 mmol) and triethylamine (2.0 mL, 27 mmol) were added, and then stirred at room temperature for 24 h to produce a precipitate, which was filtered and then rinsed several times with water. The obtained solid was dried at 80 °C for 12 h in a vacuum to give a product as a white powder (0.55 g, yield 93%). 1H NMR (300 MHz, CDCl3, δ ppm): 6.27 (s, 1H, Ar−H), 6.24 (s, 1H, Ar−H), 3.63 (s, 2H, N−H), 1.00−0.95 (m, 36H, (CH3)3), 0.23−0.19 (m, 24H, Si(CH3)2). IR (ATR, ν, cm−1): 3400 (ν N−H), 2955 (νAr−H), 2860 (ν CH3), 1260 (δ CH3). MALDI MS m/z 596.20 [M]+·, calcd 596.18. Anal. Calcd for C30H64N2O2Si4: C, 60.34; H, 10.80; N, 4.69. Found: C, 60.35; H, 10.73; N, 4.79. Synthesis of PPBO Precursor (TBS-prePBO). Under a nitrogen atmosphere, TBS-DAR (0.597 g, 1.0 mmol) was dissolved in an NMP solution (2 mL) and then cooled to 0 °C, to which terephthaloyl chloride (0.203 g, 1.0 mmol) was added, and then the mixture was stirred at room temperature for 48 h. The solution was poured into 200 mL of methanol to produce a precipitate, which was filtered and rinsed several times with water. The obtained solid was dried at 80 °C for 24 h in a vacuum that produced a yellowish powder (0.42 g, yield 82%). The inherent viscosity of the obtained polymer was 0.75 dL g−1 (measured at a concentration of 0.5 g dL−1 in NMP at 30 °C). 1H NMR (300 MHz, DMSO-d6, δ ppm): 9.70−9.59 (m, 2H, N−H), 8.13−8.10 (s, 4H, Ar−H), 7.79−7.69 (m, 1H Ar−H), 6.64−6.60 (s, 1H, Ar−H), 1.00−0.87 (m, 18H, (CH3)3), 0.26−0.21 (m, 12H, Si(CH3)2). IR (ATR, ν, cm−1): 3425 (ν N−H), 2955 (ν Ar−H), 2860 (ν CH3), 1660 (ν CO), 1530 (ν N−H) 1365 (δ CH3). Fabrication of Free-Standing PPBO Films. TBS-prePBO (100 mg) in DMAc (2 mL) filtered through a 0.2 μm PTFE membrane filter was cast on a glass substrate. The obtained film was dried at 80 °C for 1 h and then at 100 °C for 1 h. The film was peeled off the glass substrate by immersing the substrate in water. The obtained film was dried at 80 °C for 4 h under vacuum and subsequently heated in a vacuum at 200 °C for 1 h, 250 °C for 1 h, 300 °C for 1 h, and finally 350 °C for 1 h. IR (ATR, ν, cm−1): 1625 (CN). Anal. Calcd for C14H6N2O2 + 0.375H2O: C, 69.78; H, 2.51; N, 11.63. Found: C, 69.78; H, 2.72; N, 11.30. Measurements. The inherent viscosity measurements were performed using an Ostwald viscometer. The 1H NMR spectra were measured using an AV300 M spectrometer (Bruker Biospin). The FTIR spectra were measured using a Spectrum 65 FT-IR (Perkin-Elmer) spectrometer equipped with an ATR apparatus. The MALDI-MS spectrograph was obtained using an Autoflex (Bruker). A thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurement were carried out using an EXSTAR TG/DTA 6300 (SII Nanotechnology) at the heating rate of 10 °C min−1 under flowing N2 (200 mL min−1) and DSC 6220 (SII Nanotechnology) at the rate of 10 °C min−1, respectively. The X-ray diffraction patterns were collected using an X-ray diffractometer, SmartLab (Rigaku), in the 2θ scan range of 3°−50° using Cu Kα (λ = 0.154 01 nm) radiation operated at 30 mA and 40 kV. The scanning electron microscope images of the cross-section morphology of the films were measured at 2.0 kV using a JSM6701F (JEOL). The mechanical properties of the films were measured at 25 °C using an EZ-S (Shimadzu) at a displacement rate of 1.0 mm min−1. The size of the specimen was 20 mm, 10 mm, and 20 μm in length, width, and thickness, respectively. The density of the PPBO film was determined by dividing the weight of the film with the film volume calculated using the film thickness and the area.

Scheme 2. Synthesis of PPBO via TBS-prePBO

synthesis of such soluble PPBO precursor is the poor solubility of the monomer of PPBO, 4,6-diaminoresorcinol (DAR). We have already reported that the functionalization of the DAR dramatically improved the solubility and allowed an easy polycondensation reaction by mixing with terephthaloyl chloride at room temperature.37 However, the functionalization with the trimethylsilyl (TMS) group in place of DAR caused deprotection due to high reactivity toward nucleophiles, such as moisture water, and provided an insoluble poly(hydroxyamide) after isolation from the reaction solution probably due to the strong hydrogen bonding of the phenolic OH groups. We have chosen the tert-butyldimethylsilyl (TBS) group and used it as a functional unit of DAR in place of TMS because the TBS group was reported to be 104 times more stable than the TMS group.38 To the best of our knowledge, this is the first report describing the fabrication of PPBO films with extremely high thermal stability (decomposition temperature, 670 °C) without using a strong acid solvent.

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EXPERIMENTAL SECTION

Materials. 4,6-Diaminoresorcinol dihydrochloride, terephthaloyl chloride, and tert-butyldimethylsilyl chloride were purchased from TCI. The terephthaloyl chloride was purified by recrystallization from n-hexane, and the other reagents were used as received. Triethylamine was purchased from Kishida Reagents Chemicals and purified by distillation over potassium hydroxide. N-Methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), methanesulfonic acid, methanol, ethanol, and toluene were purchased from Wako Pure Chemical and used as received. 4,6-Di(trimethylsilylamino)-1,3-di(trimethylsiloxy)benzene (TMS-DAR) and poly(hydroxyamide) (prePBO) were synthesized

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RESULTS AND DISCUSSION Polymerization and Characterization of PPBO Precursor (TBS-prePBO). The precursor of poly(p-phenylene benzobisoxazole) (PPBO) was synthesized by the condensation 4248

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1). At TBS-DAR = 1.0 M, which is close to the saturated concentration, the polycondensation at room temperature for 24 and 48 h was found to provide a high molecular weight polymer with the inherent viscosity of 0.86 and 1.45 dL g−1 in NMP at 30 °C, respectively (see entries 7 and 8 in Table 1). The obtained polymer powders were characterized by IR spectroscopy. As shown in Figure 1a, characteristic peaks of the amide bonding (1640 and 1550 cm−1, CO stretching mode and N−H bending mode, respectively) due to the TBSprePBO were observed, which clearly indicated the progress of the polycondensation reaction. Figure 1b shows the 1H NMR spectrum of the TBS-prePBO. In the polymer, the TBS group and the aromatic group were determined by 1H NMR spectroscopy to be TBS:aromatic = 30:6, which indicates that the hydroxyl groups of the TBS-prePBO were fully protected by the TBS group. This fact manifests the superior stability of the TBS group compared to the TMS group as the protecting group of the prePBO. Successful protection of the hydroxyl group by the TBS group realized an excellent solubility in the organic solvents, such as NMP, N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO). Thermal treatment of the TBS-prePBO was monitored by TGA together with the IR technique. Figure 2a (solid line) shows the TGA curve of the TBS-prePBO measured at 10 °C min−1 under flowing N2. The TGA curve of a nonprotected PPBO precursor (prePBO) prepared by the condensation of TMS-DAR with terephthaloyl chloride in NMP at 0 °C for 6 h is also shown in Figure 2a (dotted line) for comparison. It was found that a greater weight loss (45.4 wt %) was observed for the TBS-prePBO at around 350−400 °C. Judging from the weight loss of the prePBO in a similar temperature range (17.3 wt %) that is assignable to the ring-closing dehydration,17,39,40 the weight loss for the TBSprePBO at this temperature range was attributed to the combination of the deprotection of the TBS (calcd = 45.8 wt

reaction of TBS-functionalized DAR (denoted TBS-DAR, see Supporting Information, Figure S1) with terephthaloyl chloride as shown in Scheme 1b. A series of polymerization conditions were carried out, and the results are summarized in Table 1. It Table 1. Polymerization Conditions of TBS-prePBO entry

solvent

concn of TBSDAR [M]

1 2 3 4 5 6 7 8 9

NMP NMP NMP NMP NMP NMP NMP NMP DMAc

0.5 0.5 0.5 0.5 0.25 0.5 1.0 1.0 0.5

temp [°C]

time [h]

yield [%]

inherent viscosity [dL g−1]a

r.t. r.t. r.t. r.t. r.t. 80 r.t. r.t. r.t.

6 12 24 48 24 24 24 48 48

86 72 82 90 70 86 82 94

0.45 0.69 0.75 0.35 0.63 0.86 1.45 0.72

Measured at a concentration of polymer = 0.5 g dL−1 in NMP at 30 °C.

a

was found that no polymer solid was obtained as a precipitate after the reaction for 6 h in N-methylpyrrolidone (NMP) at room temperature (entry 1 in Table 1), which is somewhat different from the polymerization reaction of trimethylsilylprotected DAR (TMS-DAR), in which the polymerization for 6 h provided a high molecular weight material.17 However, it was found that longer reaction time produced polymer solids, and under such conditions, the viscosities of the solutions increased with the increasing reaction time (entries 2−4 in Table 1). The bulky TBS group would be considered to act as a steric hindrance for the formation of the amide bond. As shown in Table 1, higher monomer concentrations produced polymers with higher molecular weights (see entries 3, 5, and 7 in Table

Figure 1. (a) FT-IR spectrum of TBS-prePBO. (b) 1H NMR spectrum of TBS-prePBO. 4249

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Figure 2. (a) TGA curves of prePBO (dotted line) and TBS-prePBO (solid line) measured at the heating rate of 10 °C min−1 under flowing N2. (b) FT-IR spectra of PPBO prepared from TBS-prePBO (solid line) and Zylon (dotted line).

ends is dominant for the thermal stability. The IR spectra measured after annealing at 500 °C for different annealing times gave almost identical spectra (see Supporting Information, Figure S3), suggesting that the concentration of the defect sites in the PPBO is lower than the detection limit of the IR method. To examine the presence of Tg, we carried out DSC study and found that no Tg of the polymer was observed in the temperature range of 30−350 °C (see Supporting Information, Figure S4). Fabrication and Characterization of PPBO Film. The excellent solubility of the TBS-prePBO allows us to fabricate its films from the solution-cast method. A DMAc solution of the TBS-prePBO was cast on a glass substrate, and then the obtained film was peeled off from the substrate by immersing it in water as described in the Experimental Section. As shown in Figure 4a, the obtained free-standing film was yellowish with a film thickness of 18.1 μm. The film was heated at every 100 °C for 1 h from 100 to 500 °C under vacuum. The formation of the PPBO from the TBS-prePBO film upon heating was monitored by IR at each heating temperature (Figure 4b). The peak intensity of the amide vibration mode at around 1650 cm−1 was found to decrease after heating at 250 °C. After heating at 350 °C, the peak at around 1625 cm−1 which is assignable to the CN vibration of the newly appeared benzoxazole ring as indicated by the arrow, which is almost identical to that of the Zylon fibers. The observed spectral change obtained by the 350 °C heating well agrees with the TGA result shown in Figure 2a, in which a greater weight loss by the polymer is recognized. Figure 4c shows a photograph of the PPBO film prepared from the TBS-prePBO after heating at 500 °C for 1 h in vacuum. The film is 11.0 μm thick. The observed 42.7%

%) together with the dehydration due to the ring-closing reaction (calcd weight loss = 7.2 wt %). Indeed, the FT-IR spectrum of the TBS-prePBO after heating to 500 °C (red line in Figure 2b (solid line)) shows the characteristic peak at around 1625 cm−1 to the assignable to the CN vibration from the benzoxazole ring, supporting the formation of the PPBO structure. It is notable that the spectrum is in good agreement with that of the Zylon fibers shown in Figure 2b (dotted line). We examined the effect of the annealing time on the thermal stability of the PPBO. Upon heating to 500 °C, an additional 8.7 wt % weight loss, which corresponded to the 4.8 wt % weight loss calculated on the basis of the initial TBS-prePBO weight, was observed after a 15 h heating as shown in Figure 3a, probably due to the dehydration caused by the ring closure reaction. Considering the original 45.4 wt % weight loss, the total weight loss (50.2 wt %) agreed well with the calculated value of the TBS group and dehydrated water (53.0 wt %). Figure 3b shows the TGA curves of the PPBO prepared from the TBS-prePBO annealed at 500 °C at different annealing times. It is obvious that the onset temperatures of the decomposition increased with the increasing annealing time, and the PPBO obtained highest thermal resistivity after a 24 h annealing. We assumed that the thermal stability increased as a consequence of the decreasing number of unclosed oxazole bonds that remained even after the annealing. Of interest, the PPBO prepared from the lower molecular weight also exhibited identical onset temperatures similar to those of the highest molecular weight polymer at each annealing time (see Supporting Information, Figure S2). This fact suggests that the effect of the defect sites on the main chain, such as the amide linkage, rather than that at the chain 4250

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Figure 3. (a) Monitoring the weight reduction upon annealing at 500 °C under flowing N2. (b) TGA curves of PPBO annealed at 500 °C with given annealing time measured at the heating rate of 10 °C min−1 under flowing N2.

Figure 4. (a) Photograph of the TBS-prePBO film. (b) FT-IR spectra of TBS-prePBO films obtained after given heating treatments. FT-IR spectra of TBS-prePBO (black line) and Zylon (red line) are shown for comparison. (c) Photograph of the PPBO film.

decrease is probably caused by the deprotection of the TBS group as well as by dehydration. Scanning electron micrograph (SEM) measurements of the cross section of a PPBO film showed a densely filled structure without any voids in the film (see Supporting Information, Figure S5). This is a clear contrast to that of the PPBO film prepared from a PPBO precursor protected by the tert-butoxycarbonyl (t-Boc) group, forming a porous structure generated by the t-Boc decomposition leading to a low density (1.02 g cm−3).37 The density range of the PPBO film prepared from the TBS-prePBO was 1.55−1.68 g cm−3, which is comparable to that of the Zylon fibers (1.54−1.56 g cm−3).4 The mechanical properties of the TBS-prePBO film and the PPBO film were measured. The tensile strength and tensile modulus were 50.0 MPa and 1.9 GPa, respectively, for the TBSprePBO film and 42.0 MPa and 2.5 GPa, respectively, for the PPBO film, which were not much different from those of the PPBO films prepared from a methanesulfonic acid solution (tensile strength and tensile modulus are 68.0 MPa and 3.6 GPa, respectively41). It is concluded that a PPBO film with good mechanical properties was fabricated without using any strong acid as a solvent. However, the mechanical strength of the PPBO film was much weaker than that of the Zylon fibers. A lower anisotropic alignment of the chains in the PPBO film may explain the result. Zylon fibers of PPBO are fabricated from its lyotropic liquid crystal solution and have a high anisotropy along the fiber axis direction, resulting in the extremely high mechanical strength. On the other hand, in the amorphous film, the polymer chains are randomly entangled and the stiffness of the main-chain structure is not effectively

reflected in the film. To compare the packing structure of the polymer chain in the PPBO film together with the Zylon fibers, X-ray diffraction (XRD) measurements were conducted. The Zylon fibers showed intense peaks at around 2θ = 16° (d = 0.56 nm) and 26° (d = 0.34 nm), attributed to the “side-by-side” distance on the (200) plane and “face-to-face” distance on the (010) plane between two neighboring polymer chains, respectively (Figure 5a, blue line).42 On the other hand, PPBO film exhibited a broad weak peak, which clearly indicates a weaker chain alignment in the PPBO film (Figure 5a, red line). The intrinsic viscosity measured in methanesulfonic acid at 30 °C for the PPBO was 5.3 dL g−1, while the PPBO fiber synthesized using methansulfonic acid as the solvent having the tensile strength and tensile modulus were 2.6 and 138 GPa, respectively, is reported to be 14.0 dL g−1.43 The lower molecular weight of PPBO is expected to affect the mechanical strength. In order to confirm the thermal stability of the PPBO films, the TGA measurements of the PPBO film prepared at the optimized annealing condition (500 °C for 24 h) were carried out under flowing N2, and the result is shown in Figure 5b. The TGA curves of Zylon fibers and the PPBO powder are also shown for comparison. The PPBO film shows a 5% weight loss temperature at 671 °C, which is almost identical to that of the PPBO powder and close to that of the Zylon fibers (690 °C). The slight weight reduction of the Zylon fibers observed in the temperature range of 150−650 °C is probably due to the impurity such as residual solvent or the chemicals used for the surface treatment since after washing the film with solvent 4251

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Table 2. Comparison of the Thermal Stabilities of PPBO and Various PBOs

a T5 and T10 are temperatures at which 5% and 10% weight loss were recorded by TGA in nitrogen atmosphere, respectively. bDetermined by TGA in flowing helium at the rate of 20 °C min−1, and there is no information about onset, 5% weight loss, and 10% weight loss.

Figure 5. (a) X-ray diffraction (XRD) profiles of TBS-prePBO after heating at 500 °C (red line). XRD patterns of TBS-prePBO (black line) and Zylon (blue line) are shown for comparison. For the XRD data upon heating, see Supporting Information, Figure S6. (b) TGA curves of PPBO film prepared from the TBS-prePBO film annealed at 500 °C for 24 h (10 °C min−1 in flowing N2). The TGA curves of PPBO and Zylon fiber are also shown as a comparison.

prePBO to PPBO upon heating at 500 °C for 1 h under vacuum. Transparent free-standing PPBO films were successfully fabricated from the TBS-prePBO film. Annealing of the PPBO film at 500 °C for 24 h provided a film with the thermal resistivity of 670 °C under nitrogen. To the best of our knowledge, this is the highest thermal stability among all organic polymers reported so far. The TBS-prePBO is able to be provided in various forms, such as PPBO nanofibers and nanocomposites. The presented study will open a way for the use of PPBO with the desired shape and form in many application fields including aerospace areas, high heat-resistance materials, such as firefighter cloths, microelectronics area, etc., in the next generation. It is expected that further improved monomer design would enable the synthesis of PPBO with higher molecular weight that has ultimate properties, and such study is currently under investigation in our laboratory.

(ethanol/hexane = 1/1 v/v), no such evident weight reduction was observed (see Supporting Information, Figure S7). Table 2 lists a comparison of the thermal decomposition temperatures of the PPBO and other processable PBOs reported in the literature. The PBOs containing an aliphatic main chain show a dramatically lower thermal stability; namely, the decomposition temperature is lower by about 200 °C than that of the PPBO.13 The PBOs containing a fluorinated group possess a higher thermal stability (∼620 °C)44−46 due to the strength of the C−F bonding over C−H bonding.47 To date, aromatic PBOs containing a cardo and triphenylamino structure show the highest thermal stability among the various processable PBOs reported so far (630−650 °C).16,21,48 To the best of our knowledge, our processable PPBO using the soluble PPBO precursor (TBS-prePBO) has the highest thermal stability (∼670 °C). It is evident that the PPBO film is a promising candidate for use as the thermal resistive membranes, thermal protective coating, and so on.

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ASSOCIATED CONTENT

S Supporting Information *

CONCLUSION In summary, we synthesized a new stable PPBO monomer, 4,6diaminoresolcinol functionalized with the tert-butyldimethylsilyl group (TBS-DAR). Polymerization of the TBS-DAR with terephthaloyl chloride gave a soluble PPBO precursor having a TBS group as the protecting group of the phenolic OH group (TBS-prePBO). A viscosity of 1.45 dL g−1 in NMP at 30 °C at a concentration of TBS-prePBO = 0.5 g dL−1 was obtained after the polymerization for 48 h at room temperature. The IR and TGA measurements revealed the conversion of the TBS-

FT-IR and 1H NMR spectrum of PPBO monomer (TBSDAR), TGA curves of PPBO from TBS-prePBO with various molecular weights, IR spectra of the TBS-prePBO after annealing for given time, DSC curve of TBS-prePBO, X-ray diffraction profiles of TBS-prePBO after annealing for given time, and SEM images of a cross section of PPBO film. This material is available free of charge via the Internet at http:// pubs.acs.org. 4252

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AUTHOR INFORMATION

Corresponding Author

*Tel +81-92-802-2840; Fax +81-92-802-2840; e-mail [email protected] (T.F.), nakashima-tcm@mail. cstm.kyushu-u.ac.jp (N.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Low-Carbon Research Network (LCnet), Nanotechnology Network Project (Kyushu-area Nanotechnology Network), Grant-in-aid for JSPS Research Fellow (No. 231617) (for TF), and Global COE Program (Science for Future Molecular Systems) funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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REFERENCES

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dx.doi.org/10.1021/ma3006526 | Macromolecules 2012, 45, 4247−4253