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Synthesis and morphology control of

Synthesis and morphology control of self-condensable

naphthalene-containing polyimide by using reaction-induced

crystallization

Takashi Sawai a ,Kanji Wakabayashi a ,Shinichi Yamazaki a ,Tetsuya Uchida b ,Yoshimitsu Sakaguchi c ,Ryouhei Yamane c ,Kunio Kimura a ,?

Division of Sustainability of Resources,Graduate School of Environmental Science,Okayama University,3-1-1Tsushima-naka Kita-ku,Okayama 700-8530,Japan b Division of Chemistry and Biotechnology,Graduate School of Natural Science and Technology,Okayama University,3-1-1Tsushima-naka Kita-ku,Okayama 700-8530,Japan c

Toyobo Research Center,Toyobo Co.,Ltd.,1-1Katata 2-Chome,Ohtsu 520-0292,Japan

a r t i c l e i n f o Article history:

Received 2April 2013

Received in revised form 18May 2013Accepted 23May 2013

Available online 5June 2013Keywords:Polyimides Morphology Crystallization Self-condensable Naphthalene

a b s t r a c t

Poly(2,6-1H-benzo[f]isoindole-1,3(2H)-dione)(PBID),which was a self-condensable naph-thalene-containing polyimide,was synthesized as precipitated crystals from 6-amino-3-(ethoxycarbonyl)-2-naphthoic acid and 7-amino-3-(ethoxycarbonyl)-2-naphthoic acid by using reaction-induced crystallization during solution polymerization.The obtained PBID crystals possessed high crystallinity,excellent thermal stability and chemical resistance.They neither exhibited Tg nor Tm under the decomposition temperature.The morphology of the PBID crystals depended on not only the polymerization conditions but also the isomeric structure of the monomer,and the wide variety of the morphology appeared such as rods,plates and lathes.The plate-like crystals had ?ne protuberances on the surface.Copolymerization of PBID and poly(4-phthalimide)were also examined.

1.Introduction

Aromatic polyimides are in the highest class of high performance polymers owing to prominent thermal stabil-ity,mechanical properties,chemical stability,low conduc-tivity,and so on [1,2].Aromatic polyimides containing naphthalene moiety have been prepared to improve the heat resistance [3–6]and the linear coef?cients of thermal expansion [7,8]It is well known that six-membered imide ring has higher stability for hydrolysis compared with ?ve-membered imide ring.Hence,the naphthalene moiety is also introduced into the aromatic polyimides as a part of the six-membered imide ring [1,2,9]and these polyimides have been applied for polymer electrolyte membrane of fuel cell [10–15],as well as high-performance materials.

These aromatic polyimides are usually prepared from dian-hydrides and diamines by a two-step procedure including the formation of soluble poly(amic acid)precursors and the following imidization.Although many naphthalene-containing polyimides have been synthesized so far,the self-condensable naphthalene-containing polyimide has not been synthesized because of the instability of mono-mers due to the facile reaction between the amino group and the anhydride group attached in the same molecule.Morphology control of the polymer materials is of great importance to obtain the essential properties predicted from the polymer structure.However,the intractability of the rigid-rod polyimides makes them dif?cult to control the morphology by the conventional procedures.An iso-thermal polymerization can induce phase separation from homogeneous solution [16–23].The phase separation is determined by miscibility of the oligomer and the solvent being related to the decrease of solution mixing entropy.The interplay between the polymerization and the phase

?Corresponding author.Tel./fax:+81862518902.

E-mail address:polykim@cc.okayama-u.ac.jp (K.Kimura).

separation is responsible for the phase separation modes including crystallization and liquid–liquid phase separa-tion.Hence,the reaction-induced phase separation has been extensively studied to control the morphology by tuning the polymerization conditions in order to overcome the trade-off relation between high-performance and processability of aromatic polyimides.Nanoribbons of poly(4-phthalimide)(PPI)were prepared from self-con-densable4-amino-1-(ethoxycarbonyl)benzoic acid(1ECB) and4-amino-2-(ethoxycarbonyl)benzoic acid(2ECB)by means of the reaction-induced crystallization of oligomers during the isothermal polymerization[24].The obtained PPI nanoribbons were highly crystalline and molecular chains aligned along the long direction of the ribbon.

Poly(2,6-1H-benzo[f]isoindole-1,3(2H)-dione)(PBID) comprised of naphthalene moiety and imide linkage is analogous to PPI as shown in Scheme1,and it is expected to possess high-performance.In this paper,the synthesis of PBID was examined by using the reaction-induced crystal-lization of oligomers during solution polymerization from the view point of the morphology control.

2.Experimental

2.1.Materials

1ECB and2ECB were synthesized according to the pre-viously reported procedure[24].A mixture of isomers of dibenzyltoluene(DBT)was purchased from Matsumura Oil Co.Ltd.(Trade name:Barrel Therm400,MW:380, bp:382°C)and puri?ed by distillation under pressure (157–159°C/0.2mmHg).Diphenyl sulfone(DPS)was pur-chased from Tokyo Kasei Co.Ltd.,and puri?ed by recrystal-lization from a mixture of methanol and water.Triphenyl benzene was purchased from Alfa Aesar Co.Ltd.and puri?ed by recrystallization from toluene with activated carbon.

2.2.Measurement

Morphology of precipitates was observed on a HITACHI S-3500N scanning electron microscope(SEM).Samples for SEM were dried,sputtered with platinum/palladium and observed at20kV.Infrared(IR)spectra were recorded on a JASCO FT/IR-410spectrometer.NMR spectra were re-corded on a JEOL JNM-AL spectrometer operating at 300MHz.A solid-state13C NMR spectrum was measured on a Bruker AVANCE300WB spectrometer operating at 75.48MHz.Wide angle X-ray scattering(WAXS)was per-formed on a Rigaku Gaiger Flex with nickel-?ltered CuK a radiation(35kV,20mA).Thermogravimetric analysis (TGA)was performed on a Perkin–Elmer TGA-7with a scanning rate of10°C minà1in nitrogen atmosphere.Dif-ferential scanning calorimetry(DSC)was performed on a Perkin–Elmer DSC8000with a scanning rate of 10°C minà1in nitrogen.Inherent viscosities(g inh)were measured in97%sulfuric acid at a concentration of 0.1g dLà1at30°C.The content of2,6-1H-benzo[f]isoin-dole-1,3(2H)-dione(BID)moiety in the copolymer(v p) was estimated by IR analysis.

2.3.Monomer synthesis

2.3.1.6-Amino-3-(ethoxycarbonyl)-2-naphthoic acid(6EAN)

6-Nitronaphthalene-2,3-dicarboxylic acid(10.4g, 40mmol)synthesized according to the previously re-ported procedure[25]and acetic anhydride(200mL) were stirred at120°C for2h,and then excess of acetic anhydride and by-produced acetic acid were stripped off under reduced pressure.The obtained6-nitronaph-thalene-2,3-dicarboxylic anhydride was re?uxed in dried

T.Sawai et al./European Polymer Journal49(2013)2334–23432335

ethanol for24h.Recrystallization of the crude solid which was a mixture of3-ethoxycarbonyl-6-nitro-2-naphthoic acid(6ENN)and3-ethoxycarbonyl-7-nitro-2-naphthoic acid(7ENN)from ethanol6ENN with the yield of27%.Pd/C(500mg)was added into the suspension of 6ENN(0.5g, 1.73mmol)and dried methanol(150mL), and then the mixture was stirred at25°C for6h under H2atmosphere.After?ltration of Pd/C,methanol was evaporated from the?ltrate and the obtained solids were dried in vacuum to give yellow solids of6EAN with yield of97%.mp:181°C.Anal.Calc.for C14H13O4N(259.26):C, 64.86;H,5.05;N,5.40.Found:C,64.31;H,5.35;N,5.22. 1H NMR(300MHz,DMSO-d

):d8.24(s,1H),7.76(s, 1H),7.75(d,1H,J=9.3),7.04(dd,1H,J=8.4Hz), 6.88 (d,1H,J=1.8Hz),5.86(s,2H),4.23(q,2H,J=7.1),1.28 (t,3H,J=7.1).IR(KBr):3416,3334,2977,2596,1938, 1714,1627,1534,1501,1474,1390,1366,1289,1257, 1206,1133,1039,911,849,795,601.

2.3.2.7-Amino-3-(ethoxycarbonyl)-2-naphthoic acid(7EAN)

Recrystallization of the mixture of6ENN and7ENN ob-tained by the same procedure described above from THF gave7ENN with the yield of34%.Then,7-amino-3-(eth-oxycarbonyl)-2-naphthoic acid(7EAN)was converted from 7ENN by the procedure similar to6EAN with yield of99%. Anal.Calc.for C14H13O4N(259.26):C,64.86;H,5.05;N, 5.40.Found:C,64.22;H,5.02;N,5.30.mp:192°C.1H NMR(300MHz,DMSO-d6):d8.14(s,1H),7.76(s,1H), 7.65(s,1H),7.04(d,1H,J=8.4Hz),6.87(s,1Hz),5.87(s, 2H), 4.24(q,2H,J=7.1), 1.27(t,3H,J=7.1).IR(KBr): 3487,3381,2871,1721,1675,1624,1473,1395,1306, 1281,1257,1210,1151,1131,1041,911,889,822,786, 653,628,611,545,522.

2.4.Polymerization

DBT(10mL)was placed in a cylindrical?ack equipped with gas inlet and outlet tubes and a ther-mometer,and heat up to330°C under a slow stream of N2.6EAN(133mg,0.51mmol)was added to DBT at 330°C,and the mixture was stirred for5s to dissolve 6EAN entirely.Then the stirring was stopped and the polymerization was carried out at330°C for6h.Concen-tration of the polymerization,de?ned as(calculated polymer weight/solvent volume)?100,was1.0%in this polymerization.The yellow precipitates of PBID were col-lected by?ltration at330°C to avoid a disturbance of the morphology by the crystallization of oligomers dis-solved in the solution during cooling,and then washed with n-hexane and acetone.The obtained crystals were dried at50°C for12h in vacuum.The?ltrate was poured into n-hexane to precipitate the oligomers dis-solved in the solution at330°C.The precipitated oligo-mers were collected by centrifugation and washed with n-hexane.Polymerizations of7EAN and copolymeriza-tions were also carried out in a similar manner.When TPB and DPS were used as a solvent,precipitates were washed with toluene.3.Results and discussion

3.1.Morphology of PBID

6EAN and7EAN,which were structural isomers,were synthesized as the self-condensable monomers for PBID. These monomers were insoluble into the solvents at 25°C,but they were dissolved at the polymerization tem-perature.In order to induce the isothermal crystallization, the monomers were added into the solvent at the polymer-ization temperature and polymerized without stirring.The solution became turbid immediately after the addition of monomers due to the phase separation,and then the yel-low precipitates were obtained after6h.Results of the polymerization are presented in Table1.

The polymerizations of6EAN in DBT at330°C at con-centrations of1.0and2.0%afforded precipitates with the yields of45%and71%,respectively.PBID was insoluble in the common organic solvents but for sulfuric acid,and therefore solution viscosity was adopted as a criterion to discuss the molecular weight as commonly used.The g inh values of the precipitates measured in97%sulfuric acid were0.20and0.26dL gà1.The higher concentration made the yield and the g inh values higher.Certain amount of olig-omers was always left in the solution in spite of the con-centration,and therefore the higher concentration higher yield of the precipitates.The chemical structure was ana-lyzed by the IR spectroscopy and the solid-state13C NMR. In the IR spectrum of the precipitates shown in Fig.1a, the imide C@O was observed at1776and1716cmà1,and the imide C–N was done at1344cmà1.The deformation of the imide ring was also observed at736cmà1.A weak peak was observed at1846cmà1attributed to the C@O of anhydride end-groups,implying that PBID was not so high molecular weight.In the case of the PPI nanoribbon prepared at330°C,the anhydride end-group did not ap-pear at all[20].The PBID molecule is more rigid than the PPI molecule,and higher temperature is needed for the effective polymerization in the crystals.In a CP/MAS/TOSS 13C NMR spectrum of the PBID precipitates,corresponding imide carbons and other naphthalene carbons were ob-served,con?rming the formation of the PBID structure (see Supporting information).Concerning the morphology, the obtained precipitates were aggregates of rod-like crys-tals as shown in Fig.2a.The long rods existed in the precip-itates,of which the length and the width were ca.4l m and200–500nm.The many striations ran perpendicular to the long direction of the rod-like crystals suggesting the trace of the stacking lamellae of oligomers.These mor-phological features resemble the incipient crystals of poly(p-oxybenzoyl)whiskers[23],implying that these crystals might be formed by the spiral growth of lamellae caused by the screw dislocation.In the WAXS intensity pro?le of the rod-like crystals shown in Fig.3a,four shape and strong peaks were clearly observed at2h of13.7°(d=0.64nm),14.7°(d=0.60nm),23.5°(d=0.38nm)and 27.6°(d=0.32nm).Even though the diffuse halo attrib-uted from amorphous region was slightly observed,these crystals possess high crystallinity.It is considerable from the trace of the stacking lamellae that the polymerization

2336T.Sawai et al./European Polymer Journal49(2013)2334–2343

did not occur effectively between the lamellae in the crys-tals owing to the rigid molecular structure.The polymer-izations were next carried out in DBT at350°C to increase molecular weight.The yields of the precipitated PBID crystals increased to79–91%.Although the anhydride C@O peak was still observed in the IR spectrum as shown in Fig.1b,the intensity ratio of this peak to the imide peaks became relatively smaller,and the g inh values of0.55–0.57dL gà1became higher than those prepared at330°C. The molecular weight increased with the polymerization temperature.The re?ection peaks became sharper and the amorphous halo decreased in the WAXS intensity pro-?le of the PBID crystals prepared at350°C as shown in Fig.3b,indicating to possess the higher crystallinity than that prepared at330°C.The molecular chains packed more densely at350°C,owing to both the optimum mobility of oligomer molecules and the lower degree of super-cooling. TGA pro?les of the PBID crystals measured in nitrogen atmosphere are shown in Fig.4.5wt%loss temperature (T5)and10wt%loss temperature(T10)of the PBID crystals prepared in run no.2were474and610°C,respectively. The degradation occurred gradually from150°C due to the low molecular weight.In contrast to this,the T5and T10of the PBID crystals prepared in run no.4were617 and647°C respectively,and weight loss was not observed until400°C.Further,the char yield at700°C was over60%. The PBID having higher molecular weight and higher crys-tallinity possess excellent thermal stability.Glass transi-tion temperature(Tg)and melting temperature(Tm) were not detected under their decomposition by the DSC (see Supporting information).The morphology changed drastically,and the precipitates were spherical aggregate of plate-like crystals rather than rod-like crystals as shown in Fig.2b and c.It is noteworthy that the plate-like crystals prepared at a concentration of1.0%possessed many small protuberances on the surface.The higher molecular weight oligomers were precipitated at350°C and they might sup-press the occurrence of the screw dislocation,resulting in the formation of the plate-like crystals.The plate-like crys-tals having protuberances on the surface will be discussed later.

In order to clarify the in?uence of the solvent on the morphology,the polymerizations were carried out in DPS and TPB at350°C.In DPS,plate-like crystals having smooth surface were formed as typically shown in Fig.2d with the yields of81–93%.The g inh values were0.30–0.42dL gà1.The anhydride C@O peak was disappeared, but the C@O peak attributed to carboxylate group was newly appeared at1539cmà1as shown in Fig.1c.DPS is more polar solvent than DBT,and thereby the carboxylate group might be formed more stably.This suggests that the oligomers containing carboxylate groups in the end groups or the molecular chain were crystallized by the lateral growth to edge direction owing to the strong interaction, resulting in the formation of the plate-like crystals having smooth surface.In TPB at350°C,lath-like crystals and plate-like crystals having protuberances on surface were formed at a concentration of2.0%with the yield of66% as shown in Fig.2e.The g inh value was0.67dL gà1which was the highest.In TPB at370°C,spherical aggregates of

Table1

Results of polymerization of6EAN and7EAN.a

Run no.Polymerization condition Yield(%)g inh b(dL gà1)Morphology Monomer Solvent Conc.(%)Temp.(°C)

16EAN DBT 1.00330450.20Rod 26EAN DBT 2.00330710.26Rod 36EAN DBT 1.00350790.57PP c

46EAN DBT 2.00350910.55SP d

56EAN DPS 1.00350810.30Plate 66EAN DPS 2.00350930.42Plate 76EAN TPB 2.00350660.67Lath,PP 86EAN TPB0.50370480.33PP

96EAN TPB 1.00370600.53PP 107EAN DBT 2.00330570.25SP

117EAN DBT 2.00350810.46SP

a Polymerizations were carried out for6h.

b Measured in97%sulfuri

c aci

d at30°C at a concentration of0.1g dLà1.

c Plate-like crystal having protuberances on surface.

d Spherical aggregat

e o

f plate-like crystals.

T.Sawai et al./European Polymer Journal49(2013)2334–23432337

plate-like crystals having protuberances on surface were also formed at concentrations of 0.5%and 1.0%as shown in Fig.2f and g.The yields and the g inh values were 48–60%and 0.33–0.55dL g à1,respectively.

The plate-like crystals having protuberances on the sur-face were the most distinctive morphology,and therefore TEM and SEAD of these crystals were examined as shown in Fig.5.It is clearly observed that many ?ne plate-like

crystals were formed on the surface of the mother plate-like crystal as the protuberances.The length and the width

of the ?ne plate-like crystals were approximately 100and 20nm respectively,and they were very thin,of which the thickness was roughly 8nm.In the SAED pattern,several sharp spots were observed,suggesting high crystallinity.Among them,the spots of d -spacing 0.635nm and

0.385nm were intensively strong.The length of two repeating PBID units is estimated at 1.64nm by using MMFF94(Chem.draw 3D,12.0).The peak observed at 2h of 10.8°(d =0.83nm)in the WAXS pro?le shown in Fig.3is reasonably assigned as 002re?ection.With the assump-tion that the PBID crystal has some analogy of the PPI crys-tal structure [25],the spots of d -spacing of 0.635nm and

0.385nm might be assignable as 100and 010re?ection,

respectively.According to this speculation,the PBID mole-cules might align perpendicular to the plate plane.From

the TEM of the side view and the SAED taken from the edge

direction,the ?ne plate-like crystals of the protuberances possess the same crystal structure as the mother plate-like crystal.A dark ?eld image was taken from the edge direc-(e) 2 μm

5 μm

(a) 5 μm

(c)

(d) 5 μm 2 μm

(f)

(g) 2 μm (h)

5 μm

5 μm (b)precipitates prepared by 6EAN in (a)run no.2,(b)run no.3,(c)run no.4,(d)run no.6,(e)run no.7,(f)run no.8and (g)run 7EAN in (h)run no.10.

10 20 3040 50

2θ (degree)

intensity pro?les of PBID prepared in (a)run no.2,(b)run

run no.8.

2338T.Sawai et al./European Polymer Journal 49(2013)2334–2343

tion by using the re?ection of d -spacing of 0.635nm as shown in Fig.5c.The crystal is not uniformly bright.The mother plate-like crystal is bright,but the brightness of other ?ne plate-like crystals is ?uctuated,suggesting that some regions of internal distortions are present within the crystal and the ?ne plate-like crystals did not grow with speci?c crystallographic orientations on the mother plate-like crystal.The plate-like crystals having protuber-ances on the surface resembles the morphology of poly-benzimidazole crystals prepared by using reaction-induced crystallization during polymerization [26].According to the previous study,this unique morphology might be generated by the heterogeneous epitaxial mech-anism.However,the orientation of the molecular chains of PBID in the crystals has not been determined yet,and the details in the formation mechanism cannot be proposed.If the precipitated oligomers are not fully cyclized oli-goimides,the difference in the oligomer structure

derived

(a) (b)

(c)

SAED of plate-like PBID crystals having protuberances on surface prepared in run no.9taken from (a)though direction image (c)was taken by using a re?ection of d -spacing of 0.635nm.

T.Sawai et al./European Polymer Journal 49(2013)2334–23432339

from the isomeric monomers will in?uence the morphol-ogy of PBID.In order to clarify the in?uence of monomer structure on the morphology,7EAN was polymerized un-der the same conditions instead of6EAN as also summa-rized in Table1.The polymerizations in DBT at330and 350°C at a concentration of2.0%afforded the PBID crystals with the yields of57%and81%,respectively.Although rod-like crystals and plate-like crystals having protuberances on surface were formed from6EAN,spherical aggregates of plate-like crystals were formed from7EAN as shown in Fig.2h.The protuberances were not observed on the sur-face of the plate-like crystals from7EAN,and rod-like crys-tals were not formed at https://www.doczj.com/doc/7217457221.html,pounds dissolved in the solution at330°C were recovered after30min to estimate the structure of the precipitated oligomers.IR spectra of the recovered compounds polymerized from6EAN and 7EAN are shown in Fig.6.

The compounds recovered from the solution contained monomers and oligomers.The intensity ratio of the peak of imide C@O at ca.1720cmà1is quite different and the peak at ca.1720cmà1of the recovered compounds from 6EAN is relatively stronger than that from7EAN.Even though the precipitated oligomers could not be corrected, it can be reasonably thought that the precipitated oligo-mers from6EAN might be rich in imide linkage.On the other hand,the precipitated oligomers from7EAN might contain amic acid structure and amic ester structure.This difference in the oligomer structure makes the morpholog-ical difference understandable as follows;the rod-like crystals were formed by the spiral growth caused by the screw dislocation in the polymerization of6EAN,but the strong interaction between the oligomers though hydro-gen bonding prevented the screw dislocation and the plate-like crystals are mainly formed by the lateral growth of the oligomers from7EAN.

The obtained PBID was dissolved only in97%sulfuric acid as aforesaid.Owing to the rigid-rod structure,the PBID is expected to exhibit lyotropicity.However,maxi-mum concentration of PBID was ca2%,and therefore the lyotropicity was not observed in the97%sulfuric acid solution.

3.2.Morphology of P(BID-co-PI)

Copolymerizations of6EAN and1ECB,and those of 7EAN and2ECB were carried out in DBT at330°C at a con-centration of2.0%for6h with varying the content of BID moiety in feed(v f).Results of copolymerization are pre-sented in Table2.

With respect to the polymerization of6EAN and1ECB, P(BID-co-PI)precipitates were obtained in the whole range of the v f values with the yields of70–75%and the g inh val-ues of0.62–0.86dL gà1.Their morphologies were shown in Fig.7a–d.The copolymerization in?uenced the morphol-ogy signi?cantly.Although the rod-like crystals of PBID and the ribbon-like crystals of PPI were formed,plate-like crystals having protuberances on the surface were formed at v f of70mol%.The crystal habit extinguished at v f of 50mol%,and spheres fused each other were obtained at v f of30mol%.The content of BID moiety in the precipitates (v p)estimated from the IR analysis were ca.10mol%high-

Table2

Results of copolymerization of EAN and ECB.a

Run no.Polymerization condition Yield(%)v p c(mol%)g inh d(dL gà1)Morphology T10e(°C) Monomer v f b(mol%)

126EAN,1ECB7072780.44PP f647 136EAN,1ECB5070580.62Unclear634 146EAN,1ECB3075490.86Sphere608 151ECB0700–g Ribbon678 167EAN,2ECB7055760.35PP613 177EAN,2ECB5043590.45PP585 187EAN,2ECB3034450.55PP564 192ECB0500–Ribbon652

a Polymerizations were carried out in DBT at330°C at a concentration of2.0%for6h.

b Molar ratio of EAN in feed.

c Molar ratio of BID moiety in product.

d Measured in97%sulfuric acid at30°C at a concentration of0.1g dLà1.

e10wt%loss temperature measured by TGA at a heating rate of10°C minà1in nitrogen.

f Plate-like crystal havin

g protuberances on surface.

g Insoluble.

2340T.Sawai et al./European Polymer Journal49(2013)2334–2343

er than the v f values.The oligomers rich in BID moiety are precipitated more rapidly than those rich in PI moiety ow-ing to the lower solubility,causing the difference between the v f values and the v p values.The clear crystal habit is generally damaged by the copolymerization because of the lack of crystallizability of oligomers.Additionally the freezing point of the oligomers became lower by the copo-lymerization and the lower freezing point brings about to induce liquid–liquid phase separation rather than crystalli-zation resulting in the formation of spherical precipitates [23,27–30].Even though the morphology became spherical and unclear in the middle range of the v f value,they all possessed high crystallinity as shown in Fig.8.The charac-teristic peaks of the PBID crystals were clearly shown in the P(BID-co-PI)s,and those of PPI crystals were gradually visualized with the decrease in the v f value.

With respect to the polymerization of 7EAN and 2ECB,P(BID-co-PI)precipitates were obtained throughout the v f values with the yields of 34–55%and the g inh values of 0.35–0.55dL g à1.Even though the ribbon-like PPI crystals were formed from 2ECB,plate-like crystals having protu-berances on the surface were formed in the range of the v f value from 70to 30mol%as shown in Fig.7e–h.The v p values were also ca.10mol%higher than the v f values.It is of interest that the clear morphology appeared even in the middle range of the v f value in the copolymerization of 7EAN and 2ECB.As aforesaid,the strong interaction be-tween the oligomers from 7EAN attributed to the hydrogen bonding induced the crystallization regardless the v f vales,resulting in the formation of the plate-like crystals.

5 μm (a) 5 μm

(b) 5 μm

(c)

5 μm (d)10 μm (g)(e) 5 μm (f)

5 μm

(h)

co -PI)and PPI precipitates prepared by 6EAN and 1ECB in DBT at 330°C at v f of (a)70mol%,(b)50mol%,(c)30mol%,(d)7EAN and 2ECB in DBT at v f of (e)70mol%,(f)50mol%,(g)30mol%,and (h)0mol%.

I n t e n s i t y (a .u .)

10 20 3040 2θ (degree)

(a)

(b)

(c)

(d)

Fig.8.WAXS intensity pro?les of P(BID-co -PI)precipitates 6EAN and 1ECB in DBT at 330°C at v f of (a)70mol%,(b)30mol%and (d)0mol%(PPI).Re?ection peaks (d )and (s from PBID and PPI,respectively.

T.Sawai et al./European Polymer Journal 49(2013)2334–23432341

It is well known that the naphthalene moiety is useful to form the crankshaft structure into the polymer back-bone in order to lower the melting temperature with main-taining the rigidity of polymer structure[31–33].Aromatic copolyesters prepared from1,4-oxybenzoyl moiety and 2,6-oxynaphthoyl moiety are commercially available as thermotropic high-performance materials.The obtained P(BID-co-PI)precipitates exhibited neither Tg nor Tm un-der their decomposition(see Supporting information). The imide structure conjugated to naphthalene moiety cannot provide the crankshaft effect suf?ciently to reduce the melting temperature under the decomposition.

4.Conclusions

PBID was synthesized as precipitated crystals from 6EAN and7EAN by using reaction induced crystallization during polymerization.The PBID precipitates possessed high crystallinity,and they showed excellent thermal sta-bility and chemical resistance.They neither exhibited Tg nor Tm under the decomposition temperature and PBID was an infusible polyimide.The morphology of the PBID crystals depended on the polymerization conditions and the wide variety of the morphology appeared such as rods plates and lathes.The plate-like crystals had many protu-berances perpendicular to the surface.The morphology of PBID was also in?uenced by the structure of the monomer. The spherical aggregates of plate-like crystals were formed from7EAN,differing from the rod-like crystals from6EAN owing to the structure of the precipitated oligomers.The uncyclized oligomers were precipitated in the polymeriza-tion of7EAN and the hydrogen bonding between the olig-omers made the morphology different.Copolymerizations of EAN and ECB afforded the precipitates but they were not fusible.Crankshaft effect of naphthalene moiety was not effective to lower the Tm under the decomposition temperature in the case of polyimide.The morphology of copolymers was also in?uenced by not only the value of v f but also the structure of the monomer.Generally,copo-lymerization extinguished the clear morphology,espe-cially in the middle of the v f value owing to the lower crystallizability of oligomers and the tendency to induce the liquid–liquid phase separation instead of the crystalli-zation.Although the copolymerizations of6EAN and1ECB gave the precipitates showing unclear morphology and the spheres at the v f of50%and30%,those of7EAN and2ECB did the plate-like crystal having protuberances on surface. Appendix A.Supplementary material

Supplementary data associated with this article can be found,in the online version,at https://www.doczj.com/doc/7217457221.html,/10.1016/ j.eurpolymj.2013.05.021.

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