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Crystalline Structure and Thermal Behavior of Water-Soluble Copolymers with Pendant Terthiophenes

Li Chen1, Tatsuo Kaneko1, Jian Ping Gong,1 Yoshihito Osada,* 1 Yutaka Ohsedo,2 Yasuhiko Shirota2

1 Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan2 Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka, 565-0871, Japan

Keywords: cross-linking reaction; crystalline structure; self-doping; terthiophene; water-soluble polymers;

IntroductionPoly(thiophene)s have been widely studied because oftheir high electric conductivitity by doping[1–3] and chro-mic properties.[4–9] Since the color changes of poly(thio-phene)s are based on the conformational change of p-con-jugated polymer backbones,[10–12] the molecular structuresand the intermolecular conjugation by cofacial stackingof poly(thiophene) derivatives were investigated.[13–17]

In recent years, new type of electrically conductingpolymers, non-conjugated vinyl-type polymers havingpendant p-electron systems have attracted wide attentiondue to their enhanced chemical stability, electrochemicalbehavior, and photoconductivities.[18–25] We have madesystematic studies on non-conjugated vinyl-type poly-mers containing pendant oligothiophene with the desig-nated molecular size and showed that the insulating poly-mers are transformed into electrically conducting poly-mers by electrochemical doping and demonstrated promi-nent possibility as novel electrochromic materials.[21, 26]

However, the structural approach of these interestingpolymers has been left unstudied.

In the present study, a new type of water-soluble poly-mers containing pendant oligothiophene was obtained by

copolymerizing vinyl-type oligothiophene monomer(2,29 :59,299-terthiophen-5-yl)methyl acrylate (AA3T) withacrylic acid (AA). One of the purpose of this study is toobtain the conductive polymers in which both electronsand ions participate as electric carriers. In this paper, andthe structures and thermal behaviors of the copolymerspoly(AA3T-co-AA) have been studied as a function ofmonomer composition.

Experimental Part

Materials

Tetrahydrofuran (THF; Junsei Chemical Co., Ltd.) andacrylic acid (AA; TCI) were distilled before use. 2,29-Azo-isobutyronitrile (AIBN; Tokyo Kasei Kogyo Co., Ltd.) usedas a radical initiator was recrystallized from ethanol. Acry-late monomer containing a terthiophene moiety, (2,29 :59,299-terthiophen-5-yl)methyl acrylate (AA3T), was prepared bythe methods described in the previous paper.[3]

Preparation of Poly(AA3T-co-AA)

Poly(AA3T-co-AA)s with various AA3T compositions wereprepared by radical copolymerization of AA3T and AA in

Full Paper: The water-soluble copolymers having pen-dant terthiophene, poly(AA3T-co-AA), were synthesizedby radical copolymerization of [(2,29 :59,299-terthiophen-5-yl)methyl acrylate] (AA3T) and acrylic acid (AA), andtheir molecular structures and thermal behaviors werestudied. X-ray diffraction study showed that the copoly-mers containing a certain amount of AA3T can form thecrystals with monolayer structure, while incorporation oftoo many AA units into the copolymers disrupted thisstructure. Possible mechanism of self-doping on heatingwas discussed in terms of interaction between AA3T unitwith AA unit in the copolymers.

Macromol. Chem. Phys. 2002, 203, No. 1 i WILEY-VCH Verlag GmbH, 69451 Weinheim 2002 1022-1352/2002/0101–0176$17.50+.50/0

Molecular structures of annealed poly(AA3T-co-AA).

Macromol. Chem. Phys. 2002, 203, 176–181

Crystalline Structure and Thermal Behavior of Water-Soluble Copolymers ... 177

THF. The total monomer concentration was 2.0 m and thepolymerization was carried out in the presence of AIBN(0.01 m) at 608C for 24 h under nitrogen. Poly(AA3T-co-AA)s were purified by repeated precipitation from THF intohexane. Number-average molecular weights, M

—n, were deter-

mined by gel permeation chromatography (eluent: THF)using polystyrene standards.

Measurements1H NMR spectra were measured in a 4 wt.-% of dimethylsulfoxide-d6 solution by NMR spectrometer (JEOL GSX-400) at 400 MHz. 1H NMR chemical shifts in parts per mil-lion (ppm) were recorded downfield from 0.00 ppm usingdimethyl sulfoxide (d = 2.51 ppm) as an internal reference.

Infrared (IR) spectra were recorded on microsamplinginfrared spectrometer (JASCO, MSX-2000) after 64 scan (4cm–1 resolution) over the range from 4600 to 600 cm–1. Sam-ples were mixed with potassium bromide and pressed to givetransparent pellets.

The thermal behavior was measured on a differential scan-ning calorimeter DSC (DSC22C, Seiko) at a scanning ratioof 5 K N min–1 from 20 to 2008C in nitrogen. The samples(about 10 mg) were dried in vacuo before being hermeticallysealed in aluminum pans. The enthalpy and entropy werecalculated with respect to AA3T unit. Temperature,enthalpy, and entropy calibration was made using meltingpeaks of In and Sn.

Wide-angle X-ray diffraction (WAXD) patterns weretaken using the imaging plate with a flat-plate cameramounted on a Shimazu X-ray generator (XD-610) emittingNi-filtered Cu Ka radiation (40 kV, 40 mA) in transmissiongeometry. The distance from the sample to the film wasdetermined by calibration with the silicon powder. Small-angle X-ray diffraction (SAXD) patterns were recorded on aRigaku X-ray diffractometer (RINT-2000) (40 kV, 200 mA)in transmission geometry. A 2h (h: diffraction angle) scan-ning speed of 1 degree N min–1 with a sampling interval of0.018 was used.

Electronic spectra were recorded using a Hitachi UV-visspectrophotometer (model U-3000). Thin film as a samplewas prepared by spin-coating a THF solution of the copoly-mer onto the glass substrate.

Results and DiscussionThe poly(AA3T-co-AA)s were obtained as pale or darkyellow powders. All these copolymers were soluble inTHF and DMSO. Some copolymers were also soluble inchloroform. The mole fraction of AA3T in the copoly-mers: F = ([AA3T]/([AA3T] + [AA])) was determinedfrom the peak ratio of terthiophene protons (7.2–7.7ppm) to that of the main chains (1.4–2.0 ppm) by 1HNMR spectra. The results are summarized in Table 1.The number-average molecular weights, M

—n, were deter-

mined by gel permeation chromatography and found asabout 2.06103 which is the same order as that of poly-(AA3T) reported previously.[24]

Molecular Structure

In order to study the molecular structure of poly(AA3T)and the poly(AA3T-co-AA)s with various F, the wide-angle X-ray diffraction (WAXD) study has been made.Figure 1a shows the WAXD pattern of the oriented sam-ple of poly(AA3T) obtained by rubbing it on the glassplate (rubbing direction is parallel to the meridian line).One can see five diffraction rings at 2h = 26.28, 22.88,21.78, 19.78 and 18.58 (h: diffraction angle) correspond-ing to spacings of 0.34, 0.39, 0.41, 0.45, and 0.48 nm.Besides, diffractions are observed at 2h = 15.58 (0.57nm), 10.88 (0.82 nm), and 5.58 (1.62 nm) on the equator-ial line. These patterns indicate that the poly(AA3T)forms the crystalline structure. Although the diffractionrings look almost homogeneous in this Figure, we couldconfirm on the screen of the imaging plate that three ringsat 2h = 26.28, 22.88, and 21.78 show strong diffractionson the equatorial line while two rings at 2h = 19.78 and

Table 1. Copolymer composition and molecular weight ofpoly(AA3T-co-AA)s.

f a) 1.0 0.70 0.40 0.20 0.11 0.05F b) 1.0 0.62 0.39 0.21 0.11 0.05Yield/% 45 50 51 22 55 49M—

n/103 c) 2.0 2.2 2.1 1.9 2.3 2.5

a) AA3T monomer composition in feed.b) AA3T composition in copolymers.c) Number-average molecular weight measured by GPC using

polystyrene standards.

Figure 1. Wide-angle X-ray diffraction image of poly(AA3T)and poly(AA3T-co-AA) with F of 0.62.

178 L. Chen, T. Kaneko, J. P. Gong, Y. Osada, Y. Ohsedo, Y. Shirota

18.58 show on the meridian line. Diffractions on theequatorial line [2h = 26.28 (0.34 nm), 22.88 (0.39 nm),21.78 (0.41 nm), 15.58 (0.57 nm), 10.88 (0.82 nm), and5.58 (1.62 nm)] have a spacing ratio of 1/5 :1/4 :1/3 :1/2:1 except for the diffraction with a spacing of 0.39 nm.This relation suggests the polymer forms a layer structurewith periodicity of d2 = 1.62 nm which is roughly equiva-lent to the length of AA3T side chain (1.5 nm) assumingit takes a fully extended conformation. The diffractionswith spacings of 0.45 and 0.48 nm on the meridian linewas attributed to the side-by-side stacking of terthio-phenes aligned perpendicularly to the main chain. Thus,WAXD and SAXD data show that poly(AA3T) forms theformation of a monolayer structure with a thickness of1.62 nm, as schematically shown in Figure 2a. The spac-ing (d = 0.39 nm) of the diffraction appearing on theequatorial line could be considered as the distancebetween main chains within the layer. WAXD pattern ofthe copolymer of F = 0.62 also shows clear crystallinepeaks with spacings of 0.39 and 0.45 nm as shown in Fig-ure 1b but the layer diffractions with the spacing ratio of1/5 :1/4 :1/3 :1/2 :1 cannot be observed any more. Thecopolymer of F = 0.39 shows the similar WAXD patternas the F = 0.62 sample. As schematically illustrated inFigure 2b, the copolymers with F = 0.62 and 0.39 formthe side by side stacking of the thiophene pendant groupswith d1 = 0.45 nm and the main chain stacking with aspacing of 0.39 nm without monolayer ordering. WAXDpattern of the copolymers below F = 0.21 show only thehalo ring, indicating they are amorphous (Figure 2c).

A small angle X-ray diffraction (SAXD) analysis wasfurther made. SAXD pattern of poly(AA3T) shows a sin-gle diffraction peak at 2h = 5.448 (1.62 nm) which couldbe seen in the WAXD pattern and no other peak with alonger spacing appeared. Poly(AA) and Poly(AA3T-co-AA) show no distinct diffraction regardless of F, indicat-ing the absence of the particular structure with the longrange periodicity.

Thus, only poly(AA3T) can form both molecular andsupra-molecular organized structure. The copolymershave only short-range ordering. The crystalline structureof the copolymers transforms to amorphous by furtherincorporation of AA.

Thermal Behavior

Thermal property of poly(AA3T) and the copolymerswith various F were investigated by DSC measurement.DSC thermograms in Figure 3 shows that poly(AA3T)exhibits a very sharp endothermic peak due to melting at1048C. Poly(AA3T-co-AA) of F = 0.62 also show themelting peak but the temperature slightly decreases to988C, instead, a new exothermic peak appeared at1758C. While these melting and exothermic peaksappeared for the copolymers of F = 0.39 and 0.21, theybecame less intensive and shifted to lower temperatureswith decrease in F. The temperatures (Figure 4a),enthalpy (DH), and entropy (DS) changes (Figure 4b) ofthe melting decrease with decreasing F, indicating thatthe thermal stability decreased by incorporation of AA,which might be associated with the disruption of theorganized structure.

One should note that the exothermic peaks appear onlyin the copolymers and not in poly(AA) nor in poly-(AA3T), suggesting that some special interaction betweenAA and AA3T units might be existing. Figure 5 shows aplot of the temperatures, DH, and DS of these exothermicpeaks as a function of F. The temperature of the exother-mic peaks decreases with decreasing F, while DH and DSonce increase and then decrease substantially. The copoly-mer samples of F = 0.21, 0.39 and 0.62 changed the colorfrom yellow to black after DSC measurements up to2008C, while poly(AA3T) showed no color change. Asimilar color change occurred when poly(3-thiopheneace-tic acid) is chemically doped.[14] These results suggest thatdoping has occurred between AA3T and AA by heating

Figure 2. Schematic illustration of structures of poly(AA3T-co-AA)s.

Crystalline Structure and Thermal Behavior of Water-Soluble Copolymers ... 179

presumably due to some interactions. In order to elucidatethis phenomenon, electronic spectra of the copolymerfilms were measured. The film was prepared using a spin-coater from a THF solution and annealed for 1–2 h belowand above the exothermic temperature, and compared tothose of unannealed samples. As shown in Figure 6, theunannealed copolymer films of F = 0.62, 0.39, and 0.21show only one absorption peak at kmax = 354 nm whichshould be attributed to the p-p* transition of the terthio-phene group. The copolymer film of F = 0.39 annealed for1 h at 1008C which is below the exothermic temperature(1158C) also showed the same peak. On the other hand,when the copolymers of F = 0.21 and 0.39 were annealedabove the exothermic temperature (1708C) for 1–2 h, newabsorption bands appeared at kmax = 460 and 580 nm andfor the sample of F = 0.62 at kmax = 460 nm in addition ofthe peak at kmax = 354 nm. Similar UV peaks appear at 577and 550 nm when poly(5-vinyl-2,29 :59,299-terthiophene)(PV3T) and poly(2,29 :59,299-terthiophen-5-yl)methylmethacrylate (PMA3T) are electrochemically doped.[21, 25]

Taking into account these results, it could be consideredthat the absorption peak at kmax = 580 nm observed for thecopolymers of F = 0.21 and 0.39 might be attributed to thependant terthiophene radical-cations due to doping

Figure 3. DSC thermograms of poly(AA3T-co-AA)s with var-ious F. Scanning rate: 5 K N min–1.

Figure 4. F dependence of melting behavior of poly(AA3T-co-AA)s. (a) temperatures; (b) enthalpy and entropy changes.

Figure 5. F dependence of exothermic behavior of poly-(AA3T-co-AA)s. (a) Temperatures; (b) enthalpy and entropychanges.

180 L. Chen, T. Kaneko, J. P. Gong, Y. Osada, Y. Ohsedo, Y. Shirota

through the interaction with AA units. Likewise, a newabsorption band at kmax = 460 nm for copolymers in F =0.21, 0.39 and 0.62 could be due to radical-cation from thesexithiophene chromophore since sexithiophene gives anabsorption band at kmax = 432 nm in benzene.[27] In the caseof poly(AA3T), this new absorption band of sexithiophenealso appeared by annealing at 1708C for 2 h.

In order to obtain further experimental informationabout the interaction of AA3T with AA, IR spectra of thecopolymers were investigated (Figure 7). IR spectra ofpoly(AA3T) and the poly(AA3T-co-AA) of F = 0.39show the peak at 1720 cm–1. This peak can be assigned tothe vibration of ester C2O of AA3T for poly(AA3T) andto the overlapped vibration of AA3T with carboxyl ofAA for F = 0.21.[28] When the samples were annealed at2008C for 0.5 h, no peak change occurred in the spectrumof poly(AA3T) while a new shoulder appeared at1650 cm–1 for the copolymer of F = 0.21 as shown in Fig-ure 7. According to the literature,[28] unionized carboxyl1COOH and ionized carboxylate 1COO– have IRabsorptions at 1720 and 1610 cm–1, respectively. Theabsorption band at 1650 cm–1 might be associated withthe carboxyl anion radial COO–9, due to the electrontransfer reaction from the terthiophenes to carboxyls inthe course of heating process. The carboxyl anion radicalis also formed by water radiolysis in the presence of for-

mate ions or by photoreduction of carbon dioxide.[29, 30] Ifthis is the case, the exothermic peak which has appearedat the temperature higher than 1008C might be associatedwith the doping reaction. The shift of the exothermicpeak to higher a temperature with increasing F (Figure5a) suggests that doping becomes more difficult to occurwhen the AA3T content in the copolymer increases. Themaximum DH and DS of the exotherms shown in Figure5b suggest that the copolymer of F = 0.21 would be anappropriate ratio. Although the copolymer films are solu-ble in THF, they become insoluble after annealing, sug-gesting a cross-linking reaction has occurred. We havepreviously reported that the cross-linkage is formed bythe coupling reaction of the pendant terthiophene moi-ety[25] (Figure 8). From the electronic absorption peaks ofannealed copolymers shown in Figure 6 we evaluated thedegree of cross-linkage and the results are summarized inTable 2. One can see that the degree of cross-linkageincreases with an increase in F and annealing time.

Figure 6. UV-vis absorption spectra of poly(AA3T-co-AA) films with various F. (—) unannealedsample, (- N -) annealed at 100 8C for 1 h, (- - -) annealed at 170 8C for 1 h, ( N N N ) annealed at170 8C for 2 h.

Figure 7. Change in IR spectra of the copolymers by annealingat 200 8C for 30 min. (a) poly(AA3T); (b) poly(AA3T-co-AA)of F = 0.39.

Figure 8. Molecular structures of annealed poly(AA3T-co-AA).

Table 2. Degree of cross-linkage of poly(AA3T-co-AA)sannealed at 170 8C.

F annealed for 1 h%

annealed for 2 h%

0.21 2 30.39 6 80.62 – 25

Crystalline Structure and Thermal Behavior of Water-Soluble Copolymers ... 181

The structure of annealed copolymers was furtherinvestigated by WAXD and SAXD. The annealed copoly-mer of F = 0.62 still kept the stacking structure of sidechains with d1 = 0.45 nm and main chains with d = 0.39nm, while that of copolymers below F = 0.39 had nomore distinctive diffractions. In poly(AA3T), the shortrange ordering was conserved but the layered ordering of1.62 nm was disrupted by annealing probably due to theformation of partial cross-linkage.

Acknowledgement: This research is supported by a Grant-in-Aid for the Special Promoted Research Project “Construction ofBiomimetic Moving System Using Polymer Gels” from the Min-istry of Education, Science, Sports, and Culture of Japan.

Received: May 10, 2001Revised: July 18, 2001

Accepted: July 30, 2001

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