Composite of Poly ( Dithienopyrrole ) and Poly ( Vinylchloride ) Obtained by the Electrode Coating Method

Filmes compósitos obtidos pela polimerização eletroquímica de ditienopirrol sobre eletrodos de platina ou ITO recobertos com um filme de poli(cloreto de vinila) foram estudados neste trabalho. Os resultados obtidos da análise térmica e de microscopia indicam a natureza compósita do ma te rial. As propriedades eletrocrômicas foram estudadas por espectroeletroquímica dinâmica e são semelhantes às propriedades do poli(ditienopirrol) puro.

in ter est ing electrochromic prop er ties 6 .Films of pDP depos ited on PVC coated elec trodes also showed electrochromic prop er ties in com par i son to the pure polymer.On the other hand, they did not char ac ter ize the new ma te rial in de tail and con se quently it was not clear whether it was a com pos ite, a blend or a co pol y mer.Fur ther more, they did not do mor pho log i cal and ther mal anal y sis for the de ter mi na tion of the sta bil ity and the com po si tion of this poly meric ma te rial.
In this pa per we re port on re sults ob tained by sev eral tech niques, which better ex plain the char ac ter is tics and the com po si tion of the prod uct ob tained by the electropolymerization of dithienopyrrole on PVC films.We con firm the com pos ite na ture of the ma te rial and its electrochromic prop er ties.

Ex per i men tal
4H-dithieno[3,2-b;2',3'-d]-pyrrole (dithienopyrrole) was syn the sized as de scribed in the lit er a ture 8 .Films of poly(dithienopyrrole) on PVC were pre pared by cy cling Ar ti cle *mdepaoli@iqm.unicamp.brITO (glass slides coated with in dium-tin ox ide) elec trodes coated with PVC films (thick ness 2-7 µm) be tween -0.5 and <%-2>1.5 V in acetonitrile so lu tions of dithienopyrrole (1 x 10 -2 M) and 0.1 M tetrabutylammonium per chlor ate (TBAP) or 0.1 M tetrabutylammonium tetrafluorborate (TBAF).The electrode po ten tials in this work are re ferred to the sat u rated cal o mel elec trode, more over, a salt bridge was used to connect the ref er ence elec trode and the or ganic elec tro lyte solu tion.
Acetonitrile (Merck p.a.) was de hy drated with CaCl 2 , twice dis tilled over CaH 2 , and stored un der ni tro gen at mosphere.TBAP and TBAF were pu ri fied by crys tal li za tion from meth a nol.
Dy namic spectro-electrochemical ex per i ments were car ried out us ing an elec tro chem i cal cell with op ti cally trans par ent win dows in the sam ple com part ment of an Hewlett-Packard 8452A di ode-array spectrophotometer with a re sponse time of 0.1 s.The elec trode po ten tial was con trolled with a FAC 200A potentiostat driven by a computer and a home made pro gram.Cy clic-voltammetric exper i ments were done with an AMEL Electrochemolab in stru ment.
Thermogravimetry (TGA) de ter mi na tions and the exper i ments of Dif fer en tial Scanning Cal o rim e try (DSC) were per formed with a Dupont 9900 Ther mal Anal y sis system.The Fou rier-transform in fra red spec tra were ob tained with a FT-IR DX Nicolet in stru ment.

Re sults and Dis cus sion
The cy clic voltammetries of dithienopyrrole dis solved in acetonitrile and of films of poly(dithienopyrrole)/PVC, pDP/PVC, are shown in Fig. 1.These were pre pared on ITO elec trodes, as de scribed in the ex per i men tal part, both with ClO 4 -and BF 4 -as counterions.These were used to study the ef fect of an ions with dif fer ent ionic ra dius on the re dox pro cesses of the com pos ite ma te rial.Curves a and b dem on strate the small in ter fer ence of the an ion of the electro lyte on the elec tro chem i cal prop er ties of the mono mer.Curve 1c clearly dem on strate that, the PVC ma trix produces a strong broad en ing of the re dox waves, prob a bly caused by the hin drance for ion dif fu sion.The in su lat ing host, how ever, does not hin der the re vers ibil ity of the re dox pro cess of pDP.
The vari a tion of the op ti cal spec trum as a func tion of ap plied po ten tial for a film of pDP (BF 4 -as counterion)/PVC on ITO elec trode is re ported in Fig. 2. The po ten tial was scanned at 50 mV s -1 from 0 to +1.5 V and then back to 0 V (in re la tion to SCE).A good agreement is ob served when com par ing the spec tra of Fig. 2 with Fig ure 2. Vari a tion of the absorbance spec tra of a film of pDP on PVC (7 µm thick) as a func tion of elec trode po ten tial.Scan rate = 50 those pre vi ously ob tained at sta tion ary con di tions with a film of pDP(ClO 4 -as counterion)/PVC 9 .In these pre vi ous ex per i ments, 2 or 3 s af ter the po ten tial change, the equi librium at the new con di tion was nearly reached, with out ev idence of in ter me di ate spe cies be tween the ini tial and fi nal states.The re dox pro cess of the con duc tive poly mer depend on the dif fu sion of ions be tween the poly mer film and the elec tro lyte so lu tion.The re sults ob tained with pDP/PVC is an in di ca tion that the hin drance to the ion diffu sion pro cess caused by the PVC phase is not suf fi ciently large to im pede the op ti cal re sponse of the ma te rial.Moreover, the change of the counterion (ClO 4 -in the sta tion ary, BF 4 -in the dy namic ex per i ments) did not af fect the electrochromic re sults.The dy namic ex per i ments (measure ment of the ab sorp tion spec tra dur ing a cy clic voltammetry scan) re veal in ter me di ate ab sorp tions at 650 nm un der an ap plied po ten tial of 1.3 to 1.4 V (in re la tion to SCE), in cath odic or an odic scan, Fig. 2.This ab sorp tion was also ob served in the sta tion ary ex per i ments 9 .When the film is po lar ized at 1.5 V the ab sorp tion is lower pro duc ing a val ley in this re gion of the curve, in di cat ing a high chromatic con trast be tween the re duced and the ox i dized species.
Thermogravimetric ex per i ments on pure pDP sam ples, with ClO 4 -and BF 4 -as counterions, are re ported in Figs. 3  and 4, re spec tively.In both cases two dif fer ent thermogravimetric steps are vis i ble.The first one is assigned to dedoping of the poly mer with a mass loss cor respond ing to counterion elim i na tion.This step is cen tered at 232 °C in the case of ClO 4 -, and badly de fined but cen tered in the range 232-300 °C in the case of BF 4 -.The mass loss is 22.5% for ClO 4 -and 18% for BF 4 -, re spec tively.These values can be cor re lated to the mass loss due to dedoping.Assuming a charge of 0.5-0.6 mC per mono mer unit (as de ter mined pre vi ously for ox i dized pDP 6 ), to tal dedoping would lead to a the o ret i cal mass loss of 22-25% for pDP(ClO 4 -) and 19-22% for pDP(BF 4 -).
The sec ond thermogravimetric step is due to de com posi tion of the poly mer, and be gins at ca. 600 °C, for both com pos ites.For these sam ples a mass de crease can be detected in the range from 60 to 80 °C.This mass loss decrease is not ob served if the sam ple is pre vi ously heated to 100 °C, and is as signed to sol vent or H 2 O evap o ra tion, as ob served pre vi ously for other con duct ing poly mers 10,11 .
Dif fer en tial scan ning cal o rim e try mea sure ments, DSC, for pDP/ClO 4 -re veal an en do ther mic pro cess, not de tectable in the sec ond heat ing cy cle, in the range from 60 to 80 °C, due to the H 2O or/and sol vent evap o ra tion and to a cor re lated re ar range ment of the poly mer chains, Fig. 5.An exo ther mic pro cess in the range from 200 to 230 °C is ev ident in the case of ClO 4 -doped poly mer.It is as signed to the counterion loss, as dem on strated by the thermogravimetric ex per i ments on the same sam ple.The loss of the ClO 4 -dopant at this tem per a ture is also re ported in a pre vi ous work 10,11 .The same pro cess is not de tect able in the BF 4 - doped poly mer at 200 °C.The tem per a ture was not in -Fig ure 4. Thermogravimetric anal y sis of a pow der sam ple of pure pDP (BF 4 -as counterion) un der the same con di tions as Fig. 3; bro ken line is the first de riv a tive of the TGA curve.creased over this value be cause of sam ple de com po si tion.In this case, it is pos si ble that the exo ther mic pro cess related to the counterion loss oc curs at higher tem per a tures.No other ther mal pro cesses were de tected in the range of -120 °C to 200 °C; this is ev i dence of the amor phous na ture of the ma te rial.
Ob ser va tions by op ti cal and elec tron scan ning mi croscopy of the prod ucts of the electropolymerisation of dithienopyrrole on PVC coated elec trodes also give ev idence for com pos ite for ma tion.These ex per i ments sug gest that the poly meri sa tion of dithienopyrrole oc curs at the inter face be tween the so lu tion and the PVC film, and along a num ber of pores cre ated by the sol vent swell ing into the host poly mer.When the elec tro-oxidation pro ceeds, pDP grad u ally fills these pores, and goes out from the poly mer film at the side op po site to the elec trode in the form of amor phous ag gre gates, as shown in the scan ning elec tron micrography in Fig. 6.The dis tri bu tion of pDP into the host poly mer is uni form, as ob served by op ti cal mi cros copy (not shown in this pa per).A fur ther in di ca tion that the electropolymerization of pDP in PVC gives rise to a polymeric com pos ite can be drawn by the ob ser va tion of the electrochromic be hav iour of the prod uct.This is very sim ilar to that of pure pDP, re ported pre vi ously 7 .Also, the FTIR spec trum of the com pos ite re sem bles a su per po si tion of the spec tra of its sin gle com po nents.

Con clu sions
The prod uct of electropolymerization of dithienopyrrole on an elec trode coated with a PVC film is a poly meric com pos ite.More over, while the elec tro chem ical poly mer iza tion of dithienopyrrole do not give good electrodic films, the films of the com pos ite are ad her ent and self sup ported.The con duct ing poly mer grows from the elec trode in the di rec tion of the elec tro lyte and across the in su lat ing poly mer film.Ad justing the poly mer iza tion time, the pDP film reaches the side op po site to the electrode for a PVC thick ness of 2-3 µm.Mi cro scopic ob ser vations sug gest the for ma tion of ag gre gates of con duct ing poly mer into the PVC "sponge", with an ho mo ge neous distri bu tion of pDP into the host poly mer.The elec tri cal conduc tiv ity of these films (0.6 S cm -1 ) is not much lower than that for pure pDP 6,7 .Pure pDP shows no mass loss dur ing heat ing up to 200 °C.Above this tem per a ture the thermogravimetric mea sure ments re veal poly mer dedoping.Chain de com po si tion oc curs at 600 °C.DSC mea sure ments do not show phase tran si tions for pure pDP.This may in di cate a very low de gree of crystallinity for pDP.Finally, the above re sults in di cate that the pro duc tion of a com pos ite ma te rial of a ther mo plas tic poly mer and electrochromic pDP was achieved.The pro cess yields a self-supported and ad her ent electrochromic poly meric mate rial.

Ac knowl edg ments
This work was de vel oped with the sup port of the exchange programm be tween Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq-Brazil) and Consiglio Nazionale delle Ricerche (CNR-Italy).
Fig ure 6. Scanning elec tro micrography (mag ni fi ca tion of 750 times) of the sur face op po site to the ITO elec trode for the PVC-pDP compos ite pro duced af ter 5 potentiodynamic cy cles at 0.100 V s -1 in a 0.1 M TBAP/acetonitrile so lu tion contanining 10 mM dithienopyrrole.

Fig ure 1 .
Fig ure 1. Cy clic voltammetries of 10 mM so lu tions of dithienopyrrole in acetonitrile: a) TBAP 0.1 M, b) T BAF as sup port ing elec tro lytes, and c) 0.7-0.8µm thick film of pDP on PVC coated Pt elec trode.Scan rate = 50 mV s -1 .

Fig ure 3 .Fig ure 5 .
Fig ure 3. Thermogravimetric anal y sis of a pow der sam ple of pure pDP (ClO 4 -as counterion) un der Ar flow; bro ken line is the first deriv a tive of the TGA curve.Heating rate = 10 °C min -1 .