Synthesis and Characterisation of Electrical Conducting Polymers/co-polymers Based on -Functionalised 3-Alkylthiophenes Ma YiFei National University of Singapore 2004 Synthesis and Characterisation of Electrical Conducting Polymers/co-polymers Based on -Functionalised 3-Alkylthiophenes Ma YiFei (B. Sc. (Hons.) NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY OF SCIENCE DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2004 I ACKNOWLEDGEMENTS I would like to express my sincere appreciation to Associate Professor Ng Siu Choon for his constant guidance and encouragement throughout the course of this project. I would like to extend my heartfelt gratitude to Professor Hardy Chan for his invaluable advice, patience and unwavering support. Without the help from Professor Chan, I would not have been able to complete my studies. I would also like to thank the members of Functional Polymer Group for their excellent support and stimulating advice, especially to Mr. Wang Yeang Chyn for his help and friendship. Many thanks to staff and technicians in Surface Science Laboratory, Honours Laboratory and Central Laboratory for their assistance. II Table of Contents Summary List of schemes List of figures List of tables Chapter Introduction 1. Overview of common conducting polymers 2. Brief introduction on the conducting mechanism 3. Heterocyclic conducting polymers 10 4. Polythiophene copolymers 15 5. Scope of thesis 17 6. Analytical method 20 References Chapter 28 Syntheses and characterisation of poly[3-( bromoalkyl)]thiophenes 1. Introduction 1.1 Conducting polythiophene 32 1.2 Functionalised poly(3-alkylthiophene) 33 1.3 Scope of the work in this chapter 35 2. Experiment 2.1 Monomer syntheses 37 2.2 Polymerisation 39 3. Results and Discussion II 3.1 Experiments on different protecting groups 40 3.2 Structural characterisation of synthesised monomers 41 3.3 Physical properties of polymers 43 3.4 FT-IR and 1H NMR characterisation 45 3.5 Electrical conductivity of the doped polymers 49 3.6 UV-Vis and fluorescence spectroscopy 50 3.7 Thermal stability of the neutral polymers 53 3.8 Polymer surface studies 56 3.9 Attempts to further functionalise the -bromo moiety on the polymers 58 4. Conclusion 59 5. Experimental 61 References 74 Chapter Graft copolymers of polythiophene and polystyrene based on 3-{ -[1-(p-vinylphenyl)]hexyl}thiophene 1. Introduction 79 2. Experiment 2.1 Monomer synthesis 81 2.2 Copolymer syntheses 83 3. Results and Discussion 3.1 Monomer synthesis and characterisation 89 3.2 Copolymer characterisation 93 III 4. Conclusion 122 5. Experimental 123 References 128 Chaper Graft copolymers of polythiophene and PMMA 1. Introduction 131 2. Experiment 2.1 Monomer synthesis 133 2.2 Copolymer syntheses 138 3. Result and Discussion 3.1 Monomer synthesis and characterisation 140 3.2 Copolymer characterisation 147 4. Conclusion 173 5. Experimental 174 References 179 Chapter Conclusion and suggestions for future work 1. Summary of the project 182 2. Comparison between the two series of copolymers 185 3. Suggestions for future work 189 Appendix 191 List of main compounds reported in this thesis IV Summary This thesis reports the syntheses and characterisation of three series of novel electrical conducting polymers/copolymers based on fuctionalised 3-alkylthiophene monomers. The aim is to study the viability of grafting commodity polymers with polythiophene through an alkyl linkage. These polymers are potentially useful as anti-static materials. 1. Poly[3-( -bromoalkyl)thiophene] (CH2)nBr S n = (pTHC4Br) = (pTHC6Br) = (pTHC8Br) = 10 (pTHC10Br) = 12 (pTHC12Br) m Initially, a series of monomers, 3-( -bromoalkyl)thiophenes, were synthesised and characterised based on reported methods. Subsequent oxidative polymerisation yielded a series of corresponding polymers, which were fully characterised. These polymers were soluble in polar solvent such as chloroform and had conductivities comparable to those of poly(3-alkylthiophenes). However, attempts to carry out Grignard reactions using these polymers in order to further functionalise them were unsuccessful. Hence 3-( -bromoalkyl)thiophenes were further functionalised in order to obtain graft copolymers of polythiophene and commodity polymers. 2. Graft copolymers of polythiophene and polystyrene (PS) based on 3-{ -[1-(pvinylphenyl)]hexyl}thiophene V In this approach, further functionalisation of 3-( -bromoalkyl)thiophenes resulted in a novel monomer, 3-{ -[1-(p-vinylphenyl)]hexyl}thiophene. C6H12Ph S This compound was polymerised in two steps: first, 2,2’-azobis(2- methylpropionitrile) (AIBN) was used as an initiator to polymerise the vinyl phenyl group. Subsequently an oxidative polymerisation step produced a polythiophene structure. However, the copolymer formed was not soluble and had poor conductivity. On the other hand, the monomer was copolymerised with styrene in different ratios. When the resultant copolymers were subjected to oxidative copolymerisation with thiophene or 3-alkylthiophene derivatives, a series of grafted copolymers of polystyrene and polythiophene with alkyl chain linkages were obtained. These copolymers have improved conductivity and processibility. A close look at the characterisation results revealed that the structure of the copolymers played an important role in determining their properties. 3. Polymethyl methacrylate (PMMA) and Poly(3-alkyl)thiophene co-polymer A novel monomer, 11-thiophen-3-yl-undec-1-en-3-one, was synthesised and characterised. VI O CH2 S Once again, a direct two-step co-polymerisation reaction of this monomer resulted in a polymer that was not soluble and with poor conductivity. Introducing methyl methacrylate MMA and thiophene or 3-alkylthiophene into the copolymer matrix using the process described above produced a series of grafted copolymers that have improved processibility and conductivity. The results from this work demonstrated the viable route of forming graft copolymers of polythiophene and PS or PMMA via functionalised 3- alkylthiophenes. Their properties largely depend on the backbone structure of the polymers. Soluble and conductive copolymers have been obtained by varying the polymer structure. These materials are potentially useful as anti-static plastic products. VII According to Ferraris and Guerrero [32], polythiophene copolymer can be summarised into the following categories: 1. Copolymers from thiophene and substituted thiophenes These include the copolymerisation of 3-alkylthiophene of different alkyl chain lengths. Polymerisation methods include chemical polymerisation either by Grignard coupling or oxidative polymerisation with FeCl3. Regioregular copolymer was also reported, e.g., by McCullough’s group [36]. As mentioned in the previous section, regioregular copolymers of 3-dodecyl/3methyl- and 3dodecyl/3-hexylthiophenes were formed to study the effect of chain lengths on the polymer backbone structure. This type of copolymer can also be synthesised using the electrochemical method. An interesting development was block copolymerisation of regioregular PAT copolymers with organosilanylenes in order to control luminescence wavelength. Copolymerisation of 3-alkylthiophene with functionalised 3-alkylthiophene, e.g., 3-( -acetic-acid-alkyl)thiophene or 3-( -ester-alkyl)thiophene have also been reported [33]. Such material will allow catalysts to be introduced into the system or can be further functionalised for desired properties. PAT copolymers with alkynes incorporated into the polymer backbone have been described. They are said to exhibit excellent optical properties. 2. Copolymerisation from substituted bi- or terthiophenes 16 Dimers or trimers of thiophene can be functionalised and used as monomers to produce copolymers with very well defined structures. For example, by polymerising a monomer with an alkyl group on one of the rings, an alternating structure can be obtained. This is another way of controlling the conjugation on the thiophene backbone. In order to fine-tune the band gap of copolymers for electroluminescent applications, various aryl groups have been introduced either as substituent groups or as substituents in the middle rings of -terthiophene. These aryl rings and electron donating or withdrawing groups attached to the aryl rings can alter the band gap of the resulting polymers to give the desired optical properties. Similarly, -terthiophene containing a central fused ring system has also been polymerised. A variety of fused aryl structures have been reported, including those with heteroatoms like O, N and even S. The heteroatoms not only function as electron donors or withdrawers in the resultant polymer backbone, but they also have steric effects comparative to =C-H bond. To further control the steric effect, alkyl groups can be introduced on the aryl rings as well. Polythiophene and polypyrrole copolymers have also been studied intensively. However these are not discussed here since they are not within the scope of this thesis. 5. Scope of this thesis The aim of this project is to study the viability of grafting commodity polymers with polythiophene through an alkyl linkage. These polymers are potentially 17 useful as anti-static materials. The copolymers are synthesised through a ‘bottom up’ approach, whereby the functionalised 3-alkylthiophenes are designed and synthesised first and are used as key monomers. The project commenced with the synthesis of 3-( -bromoalkyl)thiophene of different alkyl chain lengths following a reported procedure [34]. The polymers obtained from polymerisation of these monomers were characterised in order to determine if the bromo moiety has any effect on the overall electronic and optical properties. Attempts were also made to optimise the procedure for monomer synthesis. Much attention is paid to the bromo moiety. Subsequent substitution of the functional group with, e.g., styrene paves the way for creating grafted copolymers of commodity polymers and polythiophene. Such material may find its usefulness in various applications, e.g., as anti static material. Although the Grignard reaction of poly[3-( -bromoalkyl)thiophene] with 4-bromostyrene failed, coupling of its monomer with 4-bromostyrene through the same reaction, produced a monomer that gave a polystyrene co polythiophene graft copolymer after a two-step polymerisation process. The copolymer structure was modified to improve processability and electronic properties. A more detailed account is reported in Chapter 3. 18 In Chapter 4, monomers containing both thiophene and methacrylate structures were synthesised. A series of graft copolymers of PMMA and polythiophene was formed and characterised. These copolymers were found to have better processability and conductivity. Chapter concludes the thesis with the comparison of the two series of grafted copolymers and suggestions for future work. 19 6. Analytical method 1. NMR NMR spectroscopy is frequently adopted for the study of polymer structures. In this method, a sample is subjected to radio frequency radiation in a strong magnetic field environment. Absorption occurs due to transitions between quantised energy states of nuclei influenced by the external magnetic field. Upon Fourier Transformation, the interferogram or free induction decay (FID) NMR spectrum can be obtained. The chemical shifts, peak area, line width, coupling constants and relaxation rates all provide important structural information. In this thesis, NMR is the main method used to identify both monomers and (co)polymers. It also provides important information about the regioregularity of conducting polymers. 1H NMR and 13C spectra of monomers and polymers were recorded on a Bruker AM300 spectrometer. CDCl3 was used as solvent and tetramethylsilane (TMS) as internal reference [37]. 2. Infrared Spectroscopy In the analyses and characterisation of polymers, infrared (IR) spectroscopy is highly useful. The sample is irradiated with infrared radiation and the absorption of specific energy levels occur. The energy levels absorbed correlate to the frequencies of the various vibrational modes of the molecule. The IR spectra in this thesis were obtained using a Fourier-Transform Infrared (FT-IR) spectrometer, in which the interferogram produced by the interference of radiation between two beams was Fourier-Transformed to generate the spectra. The 20 polymer samples encountered in this thesis were all powder that were ground with KBr and pressed into pellets before being subjected to IR analysis. The midinfrared region (400-4000 cm-1) provided important information on the functional groups present in the monomers and (co)polymers. By utilising the absorbance spectrum, it is even possible to quantitatively determine the composition of copolymers and blends. FT-IR spectra of monomers and polymers were recorded on Bio-Rad TFS165 spectrometer. The spectra were obtained at room temperature (28 C) with the monomer samples dispersed in KBr disc pellets and polymer samples pressed into KBr pellets [38]. 3. Ultraviolet-Visible (UV-Vis) spectroscopy Polymers that are radiated with light that falls in the ultraviolet and/or visible region will be excited if the polymer structures contain chromophores. Chromophores, e.g., benzene rings and carbonyl groups when exposed to UV/visible light undergo n * or * transition resulting in electronic absorption. Hence, this method of analysis can be applied to determine polymer lengths if the polymer contains any series of conjugated bonds. Therefore this analytical method is particularly important for the characterisation of conducting polymers which have extended conjugation groups. UV-vis spectra provide information about the extent of conjugation length as well as information on band gap in the conducting polymer backbone. It is also useful in quantitative analysis by applying Beer-Lambert’s Law. The UV-vis spectra mentioned in this thesis were obtained from dilute solutions on a Perkin Elmer Lamda 900 21 spectrophotometer. Standard polymer solutions of 10-5 M in chloroform were used for studies in UV-Vis absorption and fluorescence spectroscopy [39]. 4. Thermal analysis The measurement of the physical property of a material as a function of temperature is classified as thermal analysis. On the other hand, the energy required for equalising the temperature of a reference and a sample as a function of temperature/time can be measured by differential scanning colorimetry (DSC). In the latter method, both the sample and the reference are placed in an isothermal chamber that is cooled or heated at a constant rate. Thermogravimetric analysis (TGA) is used primarily for determining the thermal stability of polymers. The samples are subjected to continuous measurement of weight on a sensitive balance (thermal balance) as the temperature of the surroundings is increased. This can be referred to as nonisothermal TGA. Data is recorded as a thermogram of weight versus temperature. Weight loss may arise from evaporation of residual moisture or solvent, but at higher temperatures it is a result of polymer decomposition. Besides providing information on thermal stability, TGA may be used to characterise polymers through the loss of a known entity. In this way, weight loss can be correlated to the percentage of vinyl chloride in a copolymer. TGA is also useful for determining volatilities of plasticisers and other additives. However, the major application of TGA are stability studies. Thermogravimetric analyses (TGA) of polymer powders were conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2960 thermogravimetric analyser. Analyses 22 were done both in air and in N2. A heating rate of 20 C min-1 with air (or N2) flow of 75 ml min-1 was used. The runs were conducted from room temperature to 1000 C [40]. 5. X-ray diffraction The spatial arrangements of atoms in a copolymer structure can be determined using x-ray diffraction. In this technique, there are modes of scattering involved, i.e. coherent and incoherent scattering. Coherent scattering or x-ray diffraction involves crystalline samples. Here, the wavelength and phase of the incident and scattered rays remain the same. On the other hand, incoherent scattering or Compton scattering usually occurs in semi crystalline samples. Incoherent scattering is accompanied by a change in wavelength and phase of incident and scattered rays. Wide-angle measurements determine coherent scattering whereas small angle measurements indicate incoherent scattering. The diffraction pattern observed gives considerable insight into polymer structure and morphology [41]. 6. X-ray Photoelectron Spectroscopy (XPS) The XPS sample is usually studied under high vacuum system (pressure 2; ease of sample handling, permitting almost any type of vacuum-compatible polymeric material to be investigated irrespective of shape, surface morphology, etc; minor problems with sample charging; negligible radiation damage in most cases; and ease of data quantification. The 24 limitations are its very poor spatial resolution (~200 m minimum diameter examined, in most instruments several square millimetres area) and relatively poor degree of molecular specificity. In this thesis, polymer powders are dried and mounted onto standard VG sample holders using double-sided adhesive tape. Core level spectra were obtained on a VG ESCA/SIMSLAB MKII spectrometer with a Mg K radiation source. All binding energies were referenced to the peak in the C1s envelope, defined at 285 eV, to compensate for surface charging effects. Spectral deconvolution was carried out using Gaussian component peaks with constant full-widths at half-maximum (FWHM) for all components in a particular spectrum. Surface elemental stoichiometries were obtained from peak area ratios corrected with the appropriate experimentally determined sensitivity factors, and are subject to 10% error [41]. 7. Fluorescence Luminescence is a process where some molecules lose their acquired energy upon excitation via emission of radiation. One of the luminescence pathways is fluorescence, which involves loss of energy from the lowest excited singlet state. Of the abovementioned molecules, polythiophene exhibit fluorescence property. In chapter 1, the effect of functional groups on this property was studied [43]. Fluorescence was measured on a Shimadzu RF5000 Spectrofluorophotometer using a xenon lamp as light source. 8. Elemental analysis 25 The elemental analysis of a new compound can be determined by microanalytical techniques. Bulk compositions of a substance can be monitored by the weight loss of selected elements in the sample through controlled combustion. Elemental analyses were performed on a Perkin Elmer 240C elemental analyser for C, H and S determinations. Halogen determinations were done either by ion chromatography or the oxygen flask method. 9. Gel permeation chromatography The gel permeation chromatography column was packed with finely divided solid particles, each particle being permeated by pores (tunnels). As the dissolved solute passes each particle, the smaller molecules will enter the pore openings and thus be retained by the column, while the larger ones cannot permeate pores and will be eluted with the solvent front. The degree of retention is determined by the size and molecular weight of the molecules. Thus, gel permeation method is essentially a process for the fractionation of polymers according to their size and, therefore, according to their molecular weight. The molecular weight as such, cannot be determined directly, but only after calibration of the system in terms of the elution time (or volume of solution eluted) expected for a particular polymer molecular-weight fraction with the use of this particular piece of equipment. Gel permeation chromatography (GPC) analyses were carried out using a Waters 600E system controller and Waters 410 differential reflectometer together with PhenogelTM MXL and MXM columns (300 mm 4.6 mm ID) calibrated using polystyrene standards and THF as eluant at 0.3ml/min [42]. 26 10. Electrical conductivity measurement The electrical conductivity of a sample is defined as the proportionality constant between the current density J and the electric field E applied to it: J =E Conductivity measurements were carried out using the four-point probe method [44]. The method uses a linear array of four equally spaced tips, which are depressed upon the surface of a pelleted polymer sample. A small current I from a constant-current source is passed through the outer two probe tips and the voltage drop, V is measured between the inner two probe tips. In the event where the probe tips’ spacing and the disc thickness ( ) are both small relative to the lateral dimension of the sample, the conductivity is given by: I ln2 =V In this thesis a four-point probe was connected to a Keithley voltmeter constantcurrent source system. The polymer samples were pressed into compact pellets, each of diameter 12.70 mm and approximately 0.05 cm thickness. 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Boca Raton, Fla. 2003, 207 31 [...]... structures of copolymers Graft 10 0 and 4 Fig 4 .1 1 H NMR of 3- (unde -10 -enyl)thiophene (3) Fig 4.2 1 H NMR of 11 -thiophen -3- yl-undec -1- en -3- ol (4) Fig 4 .3 13 Fig 4.4 1 Fig 4.5 13 Fig 4.6 1 H NMR spectrum of 11 -thiophen -3- yl-undec -3- en-2-one Fig 4.7 1 H NMR spectrum of copolymer 10 Fig 4.8 Overlaid IR spectra of copolymers 7, 9, 10 and 11 Fig 4.9 XPS spectrum of copolymer 9 C NMR spectrum of 11 -thiophen -3- yl-undec -1- en -3- ol... 11 -thiophen -3- yl-undec -1- en -3- ol (4) H NMR spectrum of 11 -thiophen -3- yl-undec -1- en -3- one (5) C NMR spectrum of 11 -thiophen -3- yl-undec -1- en -3- one (5) X Fig 4 .10 C1s core level of XPS spectrum of copolymer 11 Fig 4 .11 C1s core level of copolymer 7 Fig 4 .12 S 2p core level data of copolymer 10 Fig 4. 13 XRD spectrum of copolymer 7 Fig 4 .14 XRD spectrum of copolymer 9 Fig 4 .15 XRD spectrum of copolymer 10 Fig 4 .16 XRD... Fig 3. 7 1 H NMR spectrum of grafted copolymer 4 Fig 3. 8 1 H NMR spectrum of grafted copolymer of PS and polythiophene (5) Fig 3. 9 IR spectra of copolymers 4, 5 and the copolymer blend IX Fig 3. 10 IR spectrum of copolymer Graft 10 0 Fig 3. 11 XPS spectrum of copolymer 4 Fig 3. 12 C 1s core level of XPS spectrum of copolymer 5 Fig 3. 13 C 1s core level of XPS spectrum of copolymer Graft 10 0 Fig 3. 14 S 2p core... Deconvolution of C1s environment in XPS spectrum of pTHC4Br Fig 3. 1 1 Fig 3. 2 13 Fig 3. 3 NMR of precursor polymer 3a Fig 3. 4 NMR spectrum of precursor copolymer 3c Fig 3. 5 NMR spectrum of the resulted copolymer from 3- ( -bromohexyl)thiophene, H NMR spectrum of monomer 3 C NMR spectrum of monomer 3 monomer 3 and styrene in 1: 1 :10 mole ratio Fig 3. 6 1 H NMR spectrum of the polymer blend of PS and poly (3- octylthiophene)... level data of copolymer 11 Table 4.4 C1s core level data of copolymer 7 Table 4.5 C1s core level data of copolymer 9 XII Table 4.6 C1s core level data of copolymer 10 Table 4.7 Summary of the 2 peaks of the copolymers 7, 9, 10 , 11 Table 4.8 Thermogravimetric studies of the graft copolymers 7, 9, 10 , 11 Table 4.9 Thermal analyses results for copolymers 7, 9, 10 and 11 Table 4 .10 Atomic ratio of copolymers... development 2 A comparison of conductivity between doped polymers and selected metals is illustrated in table 1. 1 [4] Table 1. 1 Conductivities of metals and doped polymers Material Conductivity (S/cm) Copper 5.8x105 Gold 4.1x105 Polyacetylene 1 03- 10 5 Polyfuran 10 2 Poly(sulfur nitride) 1 03- 10 4 Poly(p-phenylene) 1 03 Poly(p-phenylenevinylene) 1 03 Polyaniline 10 2 -1 03 Polypyrrole 10 2 -1 03 Polythiophene 10 2 Polyacetylene... spectrum of copolymer 11 Fig 4 .17 TGA plot of copolymer 7 in N2 environment Fig 4 .18 TGA plot of copolymer 10 in N2 Fig 4 .19 TGA plot of copolymer 11 in N2 Fig 4.20 Stacked TGA plots for copolymers 7, 9, 10 and 11 (the % weight change is only indicative of the scale) Fig 4. 21 SEM image of (from top to bottom, left to right) copolymers 7, 9, 10 and 11 Fig 5 .1 Illustration of possible structures of copolymers... spectrum of copolymer 4 Fig 3. 15 XRD spectrum of copolymer Graft 10 0 Fig 3. 16 XRD spectrum of copolymer 4 Fig 3. 17 XRD spectrum of copolymer 5 Fig 3. 18 Thermal degradation pattern for copolymers (from top to bottom) Graft 21, Graft 51 and Graft 10 0 Fig 3. 19 TGA plot for copolymer 4 in air Fig 3. 20 Overlaid DSC plots for copolymers (from top to bottom) 4 (1) , Graft 10 0(2) and 5 (3) Fig 3. 21 Illustration of. .. ratio of 1: 1 and 5 :1 afforded 3b and 3c respectively, which can be further polymerised to give copolymers Graft 21 and Graft 51 Scheme 3. 4 Polymerisation of styrene and monomer 3 in the ratio of 10 :1 produced 3d, which can be further co- polymerised with 3- octylthiophene and thiophene Scheme 4 .1 Synthesis of monomer 5 Scheme 4.2 Possible mechanisms for the formation of 4 as the major product Scheme 4 .3 Synthesis. .. Synthesis of graft copolymer 7 from monomer 5 Scheme 4.4 Synthesis of graft copolymer 9 Scheme 4.5 Syntheses of graft copolymers 10 and 11 VIII List of Figures: Fig 1. 1 Proposed conducting unit of polyacetylene Soliton may be neutral (radical), positive (carbocation), or negative (carbanion) Fig 1. 2 The formation of polaron and bipolaron in polythiophene Fig 1 .3 Interchain hopping of bipolaron according . 2 .1 Monomer synthesis 13 3 2.2 Copolymer syntheses 13 8 3. Result and Discussion 3. 1 Monomer synthesis and characterisation 14 0 3. 2 Copolymer characterisation 14 7 4. Conclusion . Fig. 3. 10 IR spectrum of copolymer Graft 10 0 Fig. 3. 11 XPS spectrum of copolymer 4 Fig. 3. 12 C 1s core level of XPS spectrum of copolymer 5 Fig. 3. 13 C 1s core level of XPS. characterisation of synthesised monomers 41 3. 3 Physical properties of polymers 43 3. 4 FT - IR and 1 H NMR characterisation 45 3. 5 Electrical conductivity of the doped polymers 49 3. 6