Synthesis and characterization of amphiphilic poly(p phenylene) based nanostructured materials 1

Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers Chapter Poly(p-phenylene)s and Derivatives: Promising Blue Light Emitting Conjugated Polymers Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers C onjugated polymers are a new class of processible, film-forming semi-conducting or metallic organic macromolecules which consist of a backbone with alternating doubleand single-bonds. The pz-orbitals of the carbon atoms which form the π-orbitals of the alternating double and single bonds mesomerize more or less, i.e. the single and double bonds becomes similar, double bonds overlap over the single bonds. The essential structural characteristic of all conjugated polymers is their quasi-infinite π-system extending over a large number of recurring monomer units. The π-electrons can be easily moved from one bond to the other, which makes conjugated polymers to be onedimensional semiconductors. Similar to inorganic semiconductors, they can be doped to drastically increase their conductivity. For the discovery and development of such conducting polymers, Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa received the Nobel Prize in chemistry in the year 2000. 1.1 Conjugated polymers-an overview The history of conducting polymers began after the discovery of poly(sulfur nitride) [(SN)x] in 1975 which becomes superconducting at low temperatures.1 Research into the electronic, optical, and magnetic properties of conjugated polymers intensified after a number of seminal experimental achievements. One of the first discovery in this direction is the polyacetylene (PA) thin films (initially discovered by Shirakawa et al., using a Ziegler Natta type polymerization catalyst) by MacDiarmid and Heeger.2 In their seminal work, they demonstrated that the electrical conductivity of PA (10−9 S cm−1) could be enhanced by several orders i.e. 105 S cm−1 by simple doping with oxidizing agents e.g. I2, AsF5, NOPF6 (p-doping) or reducing agents (n-doping) e.g. sodium napthalide. Later on, in 1990, Friend et al. reported the synthesis of phenyl-based Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers polymers and discovery of electroluminescence under low voltages in these systems established the field of polymer optoelectronics.3 The later discovery of photoinduced electron-transfer from a conducting polymer to buckminsterfullerene opened a new direction toward photo-detector and photovoltaic cells.4 In addition to this, work done by several other polymer and materials scientists around the world has generated renewed interest of the scientific community towards the study and discovery of new conducting polymeric systems. The electronic and optical properties of conjugated polymers, coupled with their processability and interesting mechanical properties make these attractive materials for the electronics industry. The attractiveness of using organic materials in semiconductor devices emerge from the desired properties such as low-cost processing, mechanical flexibility, light weight and color-tunability.5 The discovery and development of conducting polymers was recognized by the award of the Nobel prize for chemistry in 2000 to Heeger, MacDiarmid, and Shirakawa.2 π-Conjugated polymers are often intrinsic semiconductors due to their delocalized π-electrons. Most of the CPs studied today have alternating single and double bonds on the main chain. Such one dimensional π-systems are often conceived, as having a twoband structure using the one electron model approximation.6 The highest occupied molecular orbitals (HOMO) form the occupied π-band (valence band) of the polymer and the lowest unoccupied molecular orbitals (LUMO) form the π*-band (conduction band) of the polymer. As a consequence of the band alternation, band gap of the neutral polymer lies in the range of c.a 1.5 (near IR) to eV (UV), resulting in semiconducting properties. The high values of the electrical conductivity obtained with this organic macromolecules have led to the name ‘synthetic metals’. The alternating bonds provide the pathway for Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers charge transport along such a chain. The chemical structures of these polymers have the intrinsic characteristics as materials with the electronic structure of semiconductors. Semiconductors Metals Conductivities of conjugated polymers are summarized in Figure 1.1. Silver Copper Iron Mercury Graphite TTF-TCNQ Doped Germanium Silicon 105 Doped PA 10-5 Doped PANI Doped Polypyrrole and polythiophene Doped PPP Non-doped PA Non-doped polythiophene 10-10 Insulators SiO2 Nylon Non-doped PPV Non-doped PPP 10-15 Polyethylene Polystyrene PTFE 10-20 σ(Ω-1cm-1) Figure 1.1. Conductivities of conjugated polymers compared with other common materials From the beginning of 1980’s chemists started to synthesize new conducting polymers with improved/desired properties. In conjugated polymers, the energy difference between the highest occupied state in the π band and the lowest unoccupied state in the π* Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers band is the π - π* energy gap, Eg, depends upon the molecular structure of the repeating unit. This provided synthetic chemists opportunities to control the energy gap through molecular level design. Since the discovery of conjugated polymers, many new areas of potential applications for these materials such as polymer light emitting diodes (LEDs), photoconductors, nonlinear optical materials, laser dyes, scintillators, piezoelectric and pyroelectric materials, optical data storage, optical switching and signal processing, solar energy conversion, transparent antistatic coating, molecular wires, and chemical and biosensors have been identified.7-8 This creates many commercial interests as well as academic research that led to the development of conjugated polymer derivatives having a base structure of alternating single and double/triple bonds.9-10 Some of the parent structures of conjugated or conducting polymers such as polyacetylene, poly(p-phenylene) (PPP), poly(p-phenylenevinylene) (PPV), polyaniline (PANI), polypyrrole (PPy) and polythiophene (PT) are shown in Figure 1.2. Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers n n PA PDA S n n PT PPV n n PPP PF N N H H N N n PANI Figure 1.2. Chemical structures of some of the important conjugated polymers. PA = poly acetylene, PDA = polydiacetylene, PPV = polyphenylenevinylene, PT = polythiophene, PPP = poly(p-phenylene), PF = polyfluorene, PANI = polyaniline 1.2 Poly(p-phenylene)s the simplest aromatic conjugated polymer 1.2.1 A brief history of PPP Poly(p-phenylene)s, PPPs, constitute the prototype of rigid-rod polymers.11 Over the past few decades, numerous chemists have been exploring the synthesis of this simplest aromatic conjugated polymer, poly(p-phenylene) (PPP), with new and novel molecular architectures.12 The key advantages of PPPs arise from their conceptually simple and appealing molecular structure, high chemical stability, and interesting physical properties. In addition, PPP and its derivatives have the large HOMO-LUMO energy gaps required for obtaining blue emission and have been used as blue-light emitting materials in LED devices.13 PPPs also showed high quantum yield and good charge transport Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers properties.14 However poor solubility of the unfunctionalized PPPs limited the processability for device fabrications. n Figure 1.3. Chemical structure of unsubstituted PPP There are many attempts to improve the solubility of PPPs by the introduction of alkyl substituents on PPP backbone. However substituents caused deplanarization of the polymer backbone and reduced the extent of π- conjugation resulting in hypsochromic shift of the emission wavelength.15 The tilt angle and the reduction in effective conjugation length strongly influence the energy gap, which increases with the degree of substitution. To circumvent such limitations, the planarization of the PPP backbone was investigated through various methods such as incorporation of additional covalent bonds, and weak interactions such as hydrogen bonds along the polymer backbone.99 Through the planarisation of PPPs, it was possible to minimize the tilt angle between neighboring phenyl rings leading to reduction of the energy gap and photoluminescence energy.93(b) The details of the soluble substituted PPPs and further efforts to minimize the tilt angle by modification on the PPP backbone are summarized in the following section, which describes various synthetic efforts reported. 1.2.2 Synthetic strategies of PPP and its derivatives The attempts to synthesize PPP may be classified as either direct or indirect methods.11b In the direct method the monomers that contain the phenylene moiety will become the repeating unit of the final polymer. In the indirect method, a precursor Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers polymer is first synthesized from which PPP is then released, e.g., by thermal treatment. Unfortunately both methods have serious drawbacks. For most of the direct syntheses, the reaction conditions are too harsh for a regiospecific coupling reaction to take place. Thus, linkages between wrong sites, cross linking, and other side reactions occur. The molecular weights of the polymers synthesized are very low, which is specifically due to solubility problems. Typically degrees of polymerization (DP) range from to 15 owing to the low solubility of higher oligomers. Precursor method is superior to all direct methods, in which high molecular weights are achieved. A serious limitation of this method is that the structural irregularities contained in the precursor are inevitably transplanted into the final polyarylene. The conversion of the precursor polymer does not proceed as cleanly as desired and it is either incomplete or leads to chain fracture. There are only a small number of suitable precursor polymers available. From this consideration it becomes evident that chemists had to develop solutions for the individual problems of both approaches. The following section summarize different methodologies developed for the synthesis of PPPs and its derivatives 1.2.2.1 Oxidative condensation of benzene derivatives The first attempts to generate poly(p-phenylene) were undertaken in 1960s by Kovacic et al.16 They reported that the oxidative treatment of benzene with copper (II) chloride in presence of strong Lewis acids (aluminium trichloride) led to condensation of the aromatic rings. Benzene subunits are preferentially connected in the 1, 4-position, however, cross-linking and oxidative coupling to form polycyclic aromatic hydrocarbons occur as side reactions. Based on this initial procedures, other 1,4-sustituted benzene derivatives were coupled to poly(p-phenylene)s. In another oxidative condensation Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers method, Katsuya et al,27 reported the oxidative coupling using (copper(II) chloride/ aluminum chloride) of electron-rich benzene derivatives such as 2,5-dimethoxy-benzene to poly(2,5-dimethoxy-1,4-phenylene). The polymer obtained by this method also had limited solubility and only soluble in concentrated sulfuric acid, and is fusible at 320 οC. OMe OMe CuCl2/AlCl3 or FeCl3 MeO n MeO Scheme 1.1. Oxidative coupling of 2,5-dimethoxy-benzene using copper(II) chloride/ aluminum chloride 1.2.2.2 Transition metal-mediated couplings One of the early and easy methods reported by Yamamoto et al.18 described the nickel(0)-catalyzed or -mediated coupling between dihaloaromatic compounds and Mg metal. In a typical reaction, 1,4-dibromobenzene, equivalent each of Mg and dichloroNi(bipyridyl) complex were refluxed in tetrahydofuran. Even though, the coupling was mild and yielded exclusively para-linked polyphenylenes, the low molecular weight of the obtained polymer was a drawback of this method. Later, the adoption of Pd (0)-catalyzed coupling of various bromobenzene derivatives with benzene boronic acid developed by Suzuki19 et al have been used for the synthesis of high molecular weight PPPs with improved materials properties.20-21,23a Generally, Suzuki polycondensation (SPC) is a stepgrowth polymerization of bifunctional aromatic monomers to poly(arylene)s and related polymers. The general outline for SPC reaction is shown in Scheme 1.2. The mechanism of the SPC involves oxidative addition, transmetallation, and reductive elimination. Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers Ar-X Ar-Ar’ Pd(0) Reductive elimination Oxidative Addition Ar-Pd(II)-Ar’ Ar-Pd(II)-X Ligand Substitution Ar’-B(OH)2 Ar-Pd(II)-OH NaOH NaX Scheme 1.2. The general outline of Suzuki polycondensation (SPC) reaction. Since the development of Suzuki polycondensation (SPC) for the synthesis of poly(p-phenylene), the methodology have been modified to incorporate flexible groups on the polymer backbone. One of the great advantages of this method is that during polymerization, the solubilizing groups keep the growing polymer chain in solution and therefore accessible for further growth. Such synthetic methodologies led design of structurally diverse, processable PPP derivatives by the introduction of solubilizing side groups and of electro active groups.21-22 According to the structural features, the modified PPPs which correlate with certain properties can be classified as (a) polymers with alkyl or alkoxy chains, (b) amphiphilic PPPs, (c) polyelectrolytes, (d) PPP precursors for ladder polymers, (e) polymers with main-chain chirality, (f) dendronized PPPs, and (g) poly(arylene vinylene)s and poly(arylene ethinylene)s. During the same period of development of SPC, Kaeriyama et al.22 reported the synthesis of PPP using Ni(0)catalyzed coupling. The strategy of Kaeriyama represents a so-called precursor route, and was developed in order to overcome the shortcomings (insolubility, lack of processability) 10 Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers O n PPP EHO-PPP O DO-PPP O O n O n O Poly[2,5-bis(3’methylbutyloxy 1,4-phenylene)] O DB-PPP n PBP O n O Poly[2,5-diheptyloxy, 1,4-phenylene] Scheme 1.23. Mono and dialkoxy substituted PPPs derivatives which explored for the photophysical studies. Dialkoxy substituted PPPs such as poly(2,5-dibutyloxy-1,4-phenylene) (DB-PPP), poly[2,5-bis(3’-methylbutyloxy- 1,4-phenylene)], and poly(2,5-diheptyloxy-1,4- phenylene) with alkoxy pendant groups having carbon numbers ranging from to 12, exhibit absorption maxima at ca 336 nm irrespective of the length of the alkyl groups.21(d) The PL maximum of the polymers in chloroform solution appears at 410 nm but the 47 Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers intensity changes with the length of the alkoxy groups. There are a few other important classes of PPPs such as polyfluorenes, ladder type PPPs and PPP copolymers. Brief descriptions about their optical properties are summarized below. The first PLED for blue emission was fabricated in 1991.94 The increase in conjugation and the reduction of geometrical defects was the main motivation to incorporate a poly(p-phenylene)(PPP) backbone into a ladder polymer structure.95 The PL maximum of the ladder PPP in toluene is at 450 nm.96 However the polymeric film shows excimeric PL emission with the maximum at 600 nm. This suggests that the ladder structures stack easily with each other. The observed solution quantum yield was 61%. An alternating copolymer with a structure of dodecyl terphenylene and unsubstituted phenylene units exhibits a PL maximum at about 400 nm and a band gap of 3.5 eV that is substantially larger than that of PPP (2.7 eV).97 The two adjacent bulky side groups repel each other forming a larger tilt angle between the adjacent phenylene units than those without the side groups. PPP which is perfluoropropylated phenylene units (PF-PPP) on the backbone was and soluble in organic solvents and showed the PL maximum at 450 nm on photoexcitation at 300 nm.98 A blue shift of the PL maximum by 10 nm from that of PPP at 460 nm has been attributed to a decrease in the effective conjugation length in PF-PPP rather than to the electron withdrawing nature of the perfluoropropyl group. 1.5 Scope and outline of the thesis Controlled self-assembly of organic molecules especially conjugated polymers has attracted increasing attention in view of their potential utility for the fabrication of nanostructured materials for application in electronic and optoelectronic devices, 48 Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers photovoltaic cells, biosensors etc. Due to the photoluminescence efficiency and attractive physicochemical properties, PPPs have attracted much interest in recent years. Over the years, a series of soluble alkoxy substituted PPPs has been introduced by many research groups. Recently our group synthesized a new class of amphiphilic PPPs (CnPPPOH), by the incorporation of a hydrophilic phenolic head group and hydrophobic alkoxy tail to the PPP backbone. Significantly, the new approach explored the planarisation of the PPP backbone and anticipated improved processibility and optoelectronic properties. The scope of the thesis is to study various aspects of a new class of planar PPPs such as the thin film deposition, morphology of the deposited films and its dependence on the optoelectronic properties, development of mixed π-conjugated polymer network films and preparation of PPP-silica nano-composites by tailoring appropriate side chains to the amphiphilic PPPs. The thesis consists of six chapters including the above discussed detailed summary of PPPs and its various aspects. Chapter describes the investigation of an optimum chemical structure of a homologous series of conjugated poly(pphenylene), CnPPPOH, for the formation of ultra thin films at the air water interface using Langmuir Blodgett technique. Some of the key features of this study involve, • The formation of uniform monolayer at the air water interfaces and subsequent transfer on to various solid substrates. The influence of alkoxy groups on the PPP backbone toward the organization of polymer chains in thin films is investigated. • Alkoxy chains with different length (C6H13O-, C12H25O- and C18H37O-) and phenolic –OH groups were incorporated onto a PPP backbone to provide an optimum amphiphilicity for the layer by layer deposition of LB films. 49 Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers • UV-Vis, PL, AFM and SPR techniques were used to characterize the formation and properties of monolayer and multilayers of the polymers. In Chapter we investigated the correlation between photophysical properties and thin film morphology of a homologous series of planar amphiphilic poly(p-phenylene)s, CnPPPOH. Some of the important features of this chapter are • Comparison of optical properties of the polymers in solution and thin film. • Investigation of quantum yield for the polymers • Photophysical properties such as fluorescence, charge carrier mobility time resolved PL measurements etc. The main highlights of chapter are • Preparation of the electroactive groups bearing asymmetric poly(p-phenylene) (C6PPPC5Cb) by the incorporation of an alkoxy carbazole group (O(CH2)5Cb) onto poly(p-phenylene) (C6PPPOH) back bone. • Investigation of the Langmuir-Blodgett-Kuhn (LBK) film of the polymer, C6PPPC5Cb, compared with the parent polymer C6PPPOH. • Monolayers of C6PPPC5Cb were transferred to different substrates such as quartz, gold coated LaSFN9 and ITO. The observed increase in absorbance from UV-Vis studies and change in peak shifts (Δθ) from SPR data showed that the film transfer was uniform. 50 Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers • Multilayers of C6PPPC5Cb were transferred to ITO and gold substrates to prepare a multilayer conjugated polymer network (CPN) using electropolymerization of the carbazole group on the precursor polymer. • The electrochemical studies revealed that the scan rate was found to have a significant effect on the electropolymerization. In addition, the number of layers of the precursor polymer deposited on the substrate has also influenced the formation of the stable cross linked network film. The combined SPR-cyclic voltametry studies supported these observations. In Chapter 5, the formation mechanisms in biogenic systems (e.g., silicatein induced silica polymerization) are utilized for the synthesis of conducting polymer-silica nanocomposites, where the conducting polymer acts as both catalyst and template for the deposition of silica. A series of functionalized poly(p-phenylene) (PPP) were employed for environmentally benign synthesis of neutral polymer silica composites. Some of the key features of these investigations are, • Formation of spherical silica particles was observed from the precursor TEOS in presence of C12PPPC11OH without an added catalyst (e.g., H+ or –OH), which implies that the polymer plays a key role as template and catalyst in this silicification process. The absence of activity in structurally similar polymer, C12PPPOH, highlights the importance of structure in the aggregation and reactivity of polymers. • The blue light-emitting conjugated polymer, C12PPPC11OH, was incorporated into spherical silica particles through an ambient solution synthesis route. The formation of the silica particles as well as the incorporation of C12PPPC11OH were established using UV, FTIR, fluorescence, DLS, TEM and AFM studies. 51 Poly(p-phenylene)s and derivatives: promising blue light emitting conjugated polymers • The spherical silica particles were dispersable in organic solvents as well as in water and appear to be luminescent. Such luminescent silica nanoparticles with controlled size and tunable optical properties are of great importance in optoelectronic device fabrications as well as in biology, biomedical sciences and biotechnology as fluorescent biological labels. 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PPP derivatives The details of the synthetic scheme involved substituted endiynes, e.g poly(2-phenyl -1, 4phenylene) starting from 1- phenyl-hex-3-en -1, 5-diyne or the structurally related poly(2phenyl -1, 4-naphthalene) starting from 1- phenylethynyl-2-ethynyl-benzene (Scheme 1. 16) H Ph Ph Ph Ph n n Scheme 1. 16 Synthesis of poly(2-phenyl -1, 4 -phenylene) 1. 2.2.4 Hyperbranched poly (phenylene) derivatives The... weights of up to 50000 (Mn) allow for the fabrication of thick films and strips (up to 10 μm) The devices showed enhanced stability and lasing over a period of more than 10 7 pulses.48d 1. 2.2.3 Other synthetic strategies for poly(p- phenylene)s One of the very early reported methods for the synthesis of unsubstituted PPP by Marvel et al.49 involves the polymerization of 5,6-dibromocyclohexa -1, 3-diene... precursor polymer into PPP and a free-standing PPP film was obtained However these films contained large amounts of the acidic reagent, polyphosphoric acid Absorption maximum (λmax) of 336 nm was observed for new PPP materials Another class of phenylated PPPs were developed by Stille and co-workers using a set of suitable monomers such as 1, 4-diethynylbenzene and 1, 420 Poly(p- phenylene)s and derivatives: promising... poly(5,6-dibromo -1, 4-cyclohex-2-ene) followed by a thermally induced, solid state elimination of HBr to form PPP (Scheme 1. 13) However the obtained products were not 19 Poly(p- phenylene)s and derivatives: promising blue light emitting conjugated polymers defect free and showed several types of structural defects (incomplete cyclization, cross linking etc.) n n Br Br Br Br Scheme 1. 13 Synthesis of unsubstituted... by Kim and Webster.54 Trifunctional benzene -based monomers (1, 3,5-trisubstituted benzene cores) 21 Poly(p- phenylene)s and derivatives: promising blue light emitting conjugated polymers could be used to synthesize hyperbranched poly(phenylene)s structures The selfcondensation of 1, 3-dibromophenyl-5-boronic acid leads to the formation of soluble, hyperbranched PPP-type macromolecule (Scheme 1. 17) The... layer construction ITO/LPPP 12 /Ca; showed quantum efficiency of ca 1. 0% at an applied voltage: 4-6 V.46 18 Poly(p- phenylene)s and derivatives: promising blue light emitting conjugated polymers R' CH3 R CH3 R R' n R' R CH3 R' R: -aryl, R’:-alkyl Scheme 1. 12 Chemical structure of Me-LPPP The preparation of blue LEDs from LPPP materials is still limited due to the emission of the yellow light Thus further... Poly(p- phenylene)s and derivatives: promising blue light emitting conjugated polymers benzyloxy-4-dodecyloxyphenyl-2,5-bis(boronic acid) and the corresponding dibromoro pyridyl monomers.32 C12H25O OH C12H25O n N OH RO N R= CH2(CH3 )11 CH2(CH3 )15 CH2(CH3 )17 n OH C12H25O n N (A) OH (B) Scheme 1. 7 Amphiphilic PPPs synthesized using Suzuki polycondensation (A) Polymer chains with phenolic groups and alkyl... culminated by the logical continuation of the ‘step-ladder’ strategy, minimizes the mutual distortion of adjacent main chain phenylene units Maximum conjugation was achieved through flattening of the conjugated π-system by bridging all subunits The synthesis of a soluble conjugated ladder polymer of the PPP-type (LPPP) was reported by Scherf and Müllen in 19 91 (Scheme 1. 11) . 41 The polymer, LPPP, obtained (number... formation of [4+2] cyclization adducts followed by a simple aromatization of the cyclohexene moieties 22 Poly(p- phenylene)s and derivatives: promising blue light emitting conjugated polymers led to the synthesis of branched oligo (phenylene). 56 The phenylated, two-dimensional arylene structures based on a tetrabenzoanthracene core with interesting topologies were achieved (Scheme 1. 18).55 11 0°C DDQ (A)... COOMe CuO n n Scheme 1. 3 Precursor route synthesis of PPP using Ni(0)-catalyzed couplings Another approach on the synthesis of structurally homogeneous, processable PPP derivatives started with the pioneering work of Schlüter and Wegner.23 The preparation of soluble PPPs were achieved via introduction of solubilizing side groups to prepare poly (2,5-dialkyl -1, 4-phenylene)s The coupling of aromatic compounds . Ni(0)- OH R O C H 2 (C H 3 ) 11 CH 2 (CH 3 ) 15 C H 2 (C H 3 ) 1 7 R = OH R O C H 2 (C H 3 ) 11 CH 2 (CH 3 ) 15 C H 2 (C H 3 ) 1 7 R = (A) (B) N C 12 H 25 O O H n N C 12 H 25 O O H n N C 12 H 25 O O H n N C 12 H 25 O O H n N C 12 H 25 O O H n N C 12 H 25 O O H n P P o o l l y y ( ( p p - - p p h h e e n n y y l l e e n n e e ) ) s s . Polypyrrole and polythiophene PTFE 10 -20 10 -15 10 -10 10 -5 1 10 5 Polystyrene Polyethylene Nylon SiO 2 Silicon Doped Germanium TTF-TCNQ Graphite Mercury Iron Copper Silver σ(Ω -1 cm -1 ) Insulators Semiconductors Metals Non-doped. Polypyrrole and polythiophene PTFE 10 -20 10 -15 10 -10 10 -5 1 10 5 Polystyrene Polyethylene Nylon SiO 2 Silicon Doped Germanium TTF-TCNQ Graphite Mercury Iron Copper Silver σ(Ω -1 cm -1 ) Insulators Semiconductors Metals Non-doped