Bor triflorür dietil eterat'ın tiyofen (Th) ve türevlerinin elektropolimerizasyonuna etkisi: Elektrokimyasal empedans çalışması
The effect of boron tri-floride di-ethyl etherate on the electropolymerization of Th and their derivatives: The study of electrochemical impedance spectroscopy
Yazar: HACER DOLAŞ
Danışman: PROF. DR. ABDÜLKADİR SEZAİ SARAÇ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Bölümü / Kimya Ana Bilim Dalı / Fizikokimya Bilim Dalı
Derece: Doktora, 2013
Konu: Kimya = Chemistry ; Polimer Bilim ve Teknolojisi = Polymer Science and Technology

Özet:
Bilinen iletken polimerler arasında Tiyofen (Th), yükseltgenme potansiyeli diğer iletken polimerlere nazaran yüksek değere sahip bir monomerdir. Elektropolimerizasyonu sırasında yüksek potansiyel uygulandığından dolayı, elde edilen polimer oluşum sırasında aşırı potansiyele maruz kalmakta ve konjuge yapı bozulabilmektedir. Böylece, polimerin iletkenlik özelliği azalmaktadır. Bu nedenle Th nin elektropolimerizasyonunun daha düşük potansiyel değerlerinde gerçekleştirilmesi için alternatif yöntemler geliştirilmektedir. Literatürde bu tür yüksek potansiyel gerektiren monomerlerin ya uygun alkil zincirli sübstitüe türevlerini kullanarak ya yine uygun başka monomerlerle rastgele veya kenetlenme reaksiyonları ile komonomerlerini hazırlayarak ya da elektropolimerizasyon koşullarını kontrol ederek (çözelti ortamı, elektrolit, tarama hızı, tarama potansiyel aralığı, konsantrasyon, çalışma elektrodu gibi) yükseltgenme potansiyeli daha düşük değerlere çekilebilmekte ve polimerizasyon gerçekleştirilmektedir. Ayrıca son zamanlarda elde edilen polimerin kırılgan yapıda oluşu işlenebilirliğini sınırladığı için yüksek olan yükseltgenme potansiyelini daha düşük değerlere çekmenin  yanında serbest işlenmesi şekil verilmesi kolay olan filmlerin de elde edilmeye başlanması önem arzetmiştir. Bunun için Bor triflorür dietileterat (BFEE) kullanımı son zamanlarda hız kazanmıştır. Çünkü BFEE içeren elektrolit ortamında elde edilen iletken polimerler hem daha düşük potansiyelde polimerleşirken, daha esnek, serbest ve parlak filmlere de sahip olunabilmektedir.  
Bu tezde BFEE nin Th ve türevlerinin yükseltgenme potansiyelleri düşürmesi yanında yüzey morfolojisine ve değişen yüzey özelliklerinden dolayı bu iletken polimerlerle modifiye edilen elektrodun empedans parametrelerine, kapasitif özelliklerine dolayısıyla elektrokimyasal özelliklerine ne gibi bir değişiklik kattığını nasıl etkilediğini ve bu etkileşimin mekanizmasını yapıya girip girmemesi ile ilişkisinin araştırılması hedeflenmiştir. Bu amaçla Th ve türevleri olarak seçilen 3-metil Tiyofen (3-mTh), 3-heksil Tiyofen (3-hTh), 3-dodesil Tiyofen (3-dTh) ve Still kenetlenme Reakisyonu ile kimyasal olarak sentezlenen ve 3 th halkası taşıyan tri- Tiyofen İmidazol [Im(Th)3] komonomeri farklı BFEE içeren Asetonitril (ACN) çözeltisinde uygun elektrokimyasal koşullarda polimerizasyona uğratılmıştır. Bu işlem sırasında BFEE oranına bağlı olarak monomerlerin yükseltgenme potansiyelleri incelenmiştir. DV tekniği ile çalışma elketrotları farklı BFEE içeren çözeltide modifiye edilmiş ve elde edilen Elektrot/polimer/elektrolit sisteminin frekansa bağlı empedans değerlerinin değişimi ve kapasitanslık özelliği incelenmiştir. Bu değişimin % 0 BFEE ile % 100 BFEE ortamında elde edilen değerlerle kıyaslanması yapılmıştır. Hem türevler kendi arasında yükseltgenme potansiyeli ve Elektrokimyasal Empedans Spektroskopisi (EIS)  nden elde edilen parametreler açısından kıyaslanmış ve hem de monomerler kendi içlerinde farklı BFEE ortamına göre yine yükseltgenme potansiyeli ve empedans parametreleri bakımından irdelenmiştir. BFEE ortamında elde edilen farklı döngü sayısıyla kapasitansa ve empedans değerlerine etkinin yanında uygulanan potansiyelinde yine empedans verilerine etkisi incelenmiştir. Empedans verilerinin eşdeğer devre modellemesi yapılarak EIS parametreleri hesaplatılmış bulunan parametrelerle yüzey pürüzlülüğü ve Fourier Transform Infra Red ?Attenuated Transmittance Reflectance (FTIR-ATR) pikleri arasında BFEE oranına bağlı doğrusal bir ilişki olup olmadığı incelenmiştir.. Ayrıca tüm bu incelemeler her birimde 3 Th halkası taşıyan sentez maddesinde ayrıca incelenmiştir. Yüzeyin elektropolimerizasyon koşul ve monomerin yapısına nasıl değiştiği araştırılmıştır.

Abstract:
Polymers prepared from easily accesible or commercially available monomers, such as Thiophene and its derivatives, are of particular interest for use as electrochemical materials where complicated derivatives are not available. However, recent interest in these simple monomers has dwindled due to their high oxidation potential, which results in the irreversible over-oxidation of the polymer deposited on the electrode. 
Several methods have been employed  to overcome this problem. One such method is the polymerization of thiophene oligomers, such as 2,2?-bithiophene or 2,2-5,2??-terhiophene. Although thiophene oligomers can be electrochemically polimerized at lower potentials than the parent monomer, it was found that the resulting polymers exhibit low average conjugation lengths and poor mechanical properties.
A more synthetically demanding alternative, is the substitution of thiophene in the 3-position with substituents that decrease the monomer oxidation potential 
In some studies, to decrease the high oxidation potential of a monomer, the composite of the monomer are prepared electrochemically with another low oxidation potential monomers.   
For this reason having different electrical, optical, or other properties monomers are mixtured at convenient ratios and then electrochemically polymerized. Thus, the random copolymers of these monomers are prepared. In other way, firstly a comonomer is synthesized by convenient coupling reaction in suitable conditions. In copolymers prepared via this way, the arrangement of monomer units are more ordered in certain sequence.  
The oxidation potential of a monomer is affected from electropolymerization conditions such as electrolyte, scan rate, the potential range of scan, concentration, working electrode, the surface of electrode. 
In generally Conducting polymers are prepared in common organic solvents. The resulting polymers are  brittle. The processability of these polymers are limited. Thus to overcome this problem researchers were used BFEE or BFEE/ organic solvent solvent mixture. Obtained polymers in these medium are shiny metallic, flexible and free standing  films. So, BFEE medium affect the surface properties of obtained polymer such as grain radius or pore shape and radius. Macroscopically, the morphology are flat and compact. Microscopically, the polymer resembled a growth of aggregates shaped as blossoms. The growth of the nuclei was in the form of clusters. The film was also made of polymer grains with a grain size of several hundred nm. This morphology facilitated the movement of doping anions into and out of the polymer film during doping and dedoping and this agreed with the higher redox activity of conducting films in electrolyte solvent. 
In this thesis, the effects of BFEE on the oxidation potential, electropolymerization, electro active properties, Electrochemical Impedance parameters, morphology of Th and their derivatives were investigated.  
Firstly, the effect of BFEE on the oxidation potentials of monomers was investigated. For this reason, Th, 3-mTh, 3-hTh, 3-dTh were electropolymerized in 0.1M NaClO4-ACN (named %0 BFEE) solution or %60 BFEE solution at the range of 0.0-2.0V potential and 50mV/s scan rate. Obtained anodic polarization curves (LSV) were given at Figure 1.
 
Figure 1 : The obtained anodic polarization curves of the Th and its derivatives in 0.1 M NaClO4-ACN (%0 BFEE) or %60 BFEE (inset graph) by applying the 50 mV/s of scan rate. 
Th was oxidized at 1.71 V in %60 BFEE solution; it was oxidized at 1.81V in %0 BFEE solution. For 3-mTh, 3-hTh ve 3-dTh The oxidation potentials (Eox) 1.21 V, 1.52 V, 1.59 V in %0 BFEE; 1.01 V, 1.62 V, 1.45 V in %60 BFEE, respectively. As a result, it can be observed that Th and its derivatives were oxidized lower potential in BFEE solution than %0BFEE solution. 
To see how the Eox of Th monomer changes in different BFEE ratios included ACN solution, 0.1M Th was oxidized in different BFEE ratio solution. According to obtained results, Th was oxidized at 1.8V, 1.4V, 1.3V, 1.2V and 1.05V in %0 BFEE, %20 BFEE, %40 BFEE, %80 BFEE and %100 BFEE, respectively. 
The electrodes coated with the related monomers were prepared electrochemically at suitable range of voltage and scan rate from different BFEE-ACN solutions by using CV technique.   
From the obtained results ıt was seen that in %100 BFEE Th was oxidized at lower potential and was polymerized more regular structure on the surface of electrode than that in %0 BFEE.  To see the polymer formation on the electrode surface, the potential must be increased at 2.0V in %0 BFEE solution. During the polymerization observed the current value was higher  100 times in %100 BFEE than in %0 BFEE, approximately. According to this observation the polymer was accumulated more in %100 BFEE than in %0 BFEE.
The oxidation and reduction peak potential of the polymer were affected in electrolyte solution used during the polymerization. To investigate this effect, related experiments were carried out with 3-dTh monomer in %0 and %100 BFEE solutions, seperately.
According to obtained results it was seen that The oxidation potentials of both monomer and polymer were affected from the presence of BFEE in electrolyte solution. Thus, 3-dTh was oxidized at 1.4V in %0 BFEE, at 0.79 V in % 100 BFEE. The Poly(3-dTh) was oxidized 0.36V in % 100 BFEE. 
The oxidation potential of the polymer was lower than the monomer in the same BFEE ratio because of that the conjugation structure in the polymer more than in the monomer. Thus, the unit to affecting with the BFEE, sulfur atom number, more than monomer.  
To determine the effect of BFEE on the EIS parameters of the system formed with these electrode, the EIS  measurements of the electrodes were carried out in NaClO4-ACN solution.  
In Bode-phase graph Poly(3-mTh) has highest phase angle. Seen a new  peak in the graph for Poly(Th) signs that the electrochemical process was controlled by charge transfer at 100-10000 Hz. So, Poly(Th) has capacitance behaviour at low frequency, but it has resistance behaviour at a certain frequency range. This stuation was not observed for Poly(3-mTh) ve Poly(3-hTh).
When the calculated EIS parameters such as CLF, CDL, phase, Rs and Rp were investigated, the capacitance values for Poly(Th) and Poly(3-mTh) polymerized from 100% BFEE solution is lower than that of 0% BFEE. The reason of this situation can be that BFEE medium may cause deformation on structure and conjugation chain of polymer. The polarization resistance decreased depended on alkyl chain. This effect is the incorporating of BFEE in structure.
To investigate the effect of different BFEE ratio on EIS parameters (CLF, CDL and phase), EIS measurements were carried out with electrode coated Poly(Th) obtained from different ratio BFEE included ACN solution. According to obtained values, EIS parameters were showed the lowest values in 40% BFEE ratio for Poli(Th).  
To understand how the EIS results depend on the polymer surface morphology, the roughness value of the surface was calculated from AFM images of the obtained polymer from different conditions by using the related software program.
The surface morphology of Poly(Th) from 40% BFEE shows rather more compact structure. Because of that the surface feature decreases the reaction of the surface of polymer to the solution, the capacitance values decreases.
To examine the BFEE incorporate to the polymer structure, FTIR-ATR measurements were taken depended on BFEE ratios (%20, 60 ve 100). The C-O-C, due to ether, C=C, due to conjugation, peak ratios and the porosity of the polymer surfacevs. BFEE ratio is given in Figure 2.
 
Figure 2 : The graph of the changing the surface porosity, FTIR-ATR peak ratio and Rp value with BFEE ratio. [Poli(3-dTh)].
In experiments carried out with Poly(3-mTh), ıt was seen that the porosity of the polymer, the CDL and the SFE of the system formed from the polymer changed with BFEe ratio. As increase the ratio, as decrease the porosity, SFE and CDL. It can be understood that as incorporate BFEE into the polymer structure, compact and inporous surface formed. it was found that the surface morphology of the modified electrodes was changed, which observed different size with grains on surface. As a result, these different  surface properties caused the different EIS parameters for each of modified electrode conditions, due to different ion movement in  or out of the polymer surface
On the other hand, Im and Th comonomer was synthesized by using Pd catalyst in dry THF medium (Still Coupling Reaction). The product was dark yellow color (Yield = %10). The characterization of he product was carried out with 1H-NMR and FTIR-ATR measurements. Syntesized copolymer [poly(Im(Th)3] was affected by adding BFEE in electropolymerization solution due to that the copolymer bears 3 Th ring on each comonomer  unit. 
To investigate the effect of BFEE on electropolymerization of the comonomer, the copolymerization was carried out at the range of 0-1.8 V in BFEE:ACN (20:80 v/v) electrolyte solution.  
The oxidation potential of the comonomer was nearly the same value (0.8V)  in the two conditions (0% and 20% BFEE). The current value obtained from the 20% BFEE was the higher than that from the 0% BFEE (0.1 NaClO4 ?ACN). According to the results from the graph of the current value obtained from monomer-free voltammograms vs. the square root of of scan rate, ıt can be understood from the slope of graph that the redoxs behaviour of the copolymer is controlled by difussion.
From carried out EIS measurements for the copolymer coated electrode, ıt was found that the phase angle increased, CLF value decreased and CDL value was rested in the same value almost, in the presence of BFEE. When the values were compared, the Poly(Im(Th)3) films were more capacitive than Poly(Th) ones. Thay had the 87 mF of CLF and 58 mF of CDL. This result shows that the copolymer films is convenient for supercapacitor applications. 
The SEM and AFM images of the copolymer obtained from 20% BFEE solution showed that macroscopically, the film had flat and compact surface, microscopically, it had the surface formed by granules with some nanometers radius. This structure will provide the contribution on the efficiency of the film at the electroactive applications.