David Djurado

Controlled polymerization of aniline at sub-zero temperatures

Aniline is polymerized by oxidation with ammonium persulfate in aqueous hydrochloric acid solutions at sub-zero temperatures down to −43°C in the presence of LiCl and ethanol. The polymerization is controlled by concomitant potential, temperature and pH profiling. The concentrations of LiCl and ethanol are so determined to enable complete dissolution of the reactants in solution and to prevent freezing. Six polymerization steps were identified at low temperatures. The reduction of primarily synthesized pernigraniline to emeraldine with FeCl2 is investigated. The as-synthesized and deprotonated emeraldine is purified by extraction with chloroform. The polyaniline is characterized by viscosity, gel permeation chromatography (GPC), UV-Vis-NIR spectroscopy, conductivity, X-ray diffraction and chlorine analysis. High molecular weight polyanilines are synthesized at temperatures lower than −25°C having inherent viscosities within 2.0–2.15 dl/g, Mn = 30 000 g/mol, Mw = 140 000–170 000 g/mol, with Mw/Mn = 2−3. The polymers in the emeraldine base form dissolved in NMP-0.5% LiCl show a more planar conformation as indicated by the wavelength of the exciton peak reaching 680 nm. An enhanced crystallinity in terms of structural perfection is observed by means of X-ray diffraction spectra. The UV-Vis-NIR spectra of polyanilines reduced with FeCl2, protonated with (±)-10-camphorsulfonic acid (CSA), in m-cresol show a smeared polaron peak shifted into the UV. They show a broad convex NIR band peaking far beyond 2700 nm indicating an enhanced polaron delocalization. The conductivity of the films of emeraldine protonated by CSA cast from m-cresol are higher than 300 S/cm for polymers with inherent viscosities exceeding 1.8 dl/g and show an enhanced resistance to ageing. The purification of the emeraldine base by extraction with chloroform increases the molecular weight of the polymers and leads to an enhanced conductivity of the films. Polymers synthesized at low temperatures with FeCl2 reduction have better physical properties probably on account of less branching.

Ageing of PANI: chemical, structural and transport consequences**

Chemical, structural and transport consequences of PANI have been studied for two types of samples: hydrochloric acid protonated PANI powders and camphorsulfonic acid protonated PANI films cast from m-cresol solutions. Different characterization techniques have been applied: elemental analysis, FTIR, XPS, XRD, and conductivity measurements. The combination of these techniques enables us to identify three main degradation processes: dedoping, oxidation/hydrolysis/scission of the chains, and crosslinking. The impact of ageing on transport properties is briefly discussed and a simple model taken into account the kinetics of the conductivity decay is postulated.

Polyaniline protonated with camphorsulfonic acid: modelling of its crystalline structure

Polyaniline (PANI) (in the form of emeraldine) protonated with (±) camphorsulfonic acid (CSA) was subjected to computer modelling of its crystalline structure. Diffraction patterns were calculated for some modelled structures with the aim of obtaining the best agreement with observed X-ray diffraction patterns (recorded on films cast from m-cresol solutions) and with measured density. The model structures consist of layers of parallel PANI chains intercalated by CSA molecules, that do not contain m-cresol. Two particular models are presented and their features are discussed.

Is granularity the determining feature for electron transport in conducting polymers

From several studies of transport properties of conducting polymers (CP) as a function of a parameter related to their structure or microstructure we show that in strongly disordered CP, electron transport is dominated by hopping between conducting grains separated by insulating barriers. In CSA-protonated polyaniline, a weakly disordered CP which exhibits a metallic-like behaviour, several results indicate that heterogeneities play a major role in the transport process. Thus, we conclude that heterogeneous disorder appears as a key parameter which controls the conductivity of a large majority of CP.

The role of chain and dopant engineering in the preparation of processible conducting polymers with desired properties

We discuss spectroscopic, structural and processing peculiarities of polyaniline (PANI) doped with two types of protonating agents, namely diesters of 4-phthalosulfonic acid (abbreviated as DEPSA) and diesters of sulfosuccinic acid (abbreviated as DESSA). In out approach PANI doped with DEPSA or DESSA is considered as a comb-shaped macromolecular system in which dopant anions are regarded as side groups attached to the main chain via ionic bonds. This type of supramolecular architecture facilitates the self-organization of doped macromolecules into layered or in some cases lamellar structures. Plasticizing ester groups, present in the dopant anions, lower the glass transition temperature, T g , which in all cases is below room temperature. This low T g induces several features which previously were not reported for doped PANI. First, the polymer can be stretched at room temperature up to three times its original length. Second, it exhibits low temperature termochromism which is manifested by an abrupt transformation of the UV-vis-NiR spectrum characteristic of delocalized charge carriers into a spectrum typical of localized charge carriers. An interesting correlation can he found between the temperature of the onset of this low temperature thermochromism, the temperature of the metal to insulator transition determined from the σ = f(T) relationship and the temperature of the freezing of the dopant molecular motion derived from the analysis of the temperature dependence of elastic neutron scattering intensity. The coincidence of these three temperatures underlines a key role of the dopant motion in the transport properties of low T g , plasticized PANI.

X-ray scattering study of CSA protonated polyaniline films and powders

An X-ray scattering study was carried out on camphorsulfonic acid-protonated emeraldine base samples in water, m-cresol and in their mixtures. The X-ray profiles strongly depend on the protonation conditions and on the molecular weight of the emeraldine base (EB). The protonation of the EB in solution in m-cresol enables a better crystallization than inside the EB powder suspended in aqueous acidic solution. X-ray profiles evolved when protonated EB m-cresol-free samples were exposed to m-cresol vapors, indicating that m-cresol enhances the crystallinity and the structural order. All different X-ray profiles of the films could be indexed with a sole orthorhombic cell with a = 21.5 A, b = (2x) 3.5 A and c = 18.6 A.

Mapping the 3D distribution of CdSe nanocrystals in highly oriented and nanostructured hybrid P3HT–CdSe films grown by directional epitaxial crystallization

Highly oriented and nanostructured hybrid thin films made of regioregular poly(3-hexylthiophene) and colloidal CdSe nanocrystals are prepared by a zone melting method using epitaxial growth on 1,3,5-trichlorobenzene oriented crystals. The structure of the films has been analyzed by X-ray diffraction using synchrotron radiation, electron diffraction and 3D electron tomography to afford a multi-scale structural and morphological description of the highly structured hybrid films. A quantitative analysis of the reconstructed volumes based on electron tomography is used to establish a 3D map of the distribution of the CdSe nanocrystals in the bulk of the films. In particular, the influence of the P3HT-CdSe ratio on the 3D structure of the hybrid layers has been analyzed. In all cases, a bi-layer structure was observed. It is made of a first layer of pure oriented semi-crystalline P3HT grown epitaxially on the TCB substrate and a second P3HT layer containing CdSe nanocrystals uniformly distributed in the amorphous interlamellar zones of the polymer. The thickness of the P3HT layer containing CdSe nanoparticles increases gradually with increasing content of NCs in the films. A growth model is proposed to explain this original transversal organization of CdSe NCs in the oriented matrix of P3HT.

Organic semiconductors for field-effect transistors (FETs): tuning of spectroscopic, electrochemical, electronic and structural properties of naphthalene bisimides via substituents containing alkylthienyl moieties

A series of new naphthalene bisimides containing alkylthienyl groups as a part of their N-substituents have been prepared. Two synthetic routes were used. In the first one appropriate alkylthienyl anilines were synthesized from bromoalkylthiophene derivatives and then condensed with 1,4,5,8-naphthalenetetracarboxylic acid bisanhydride. The second route involved condensation of thienylmethylamine with the same bisanhydride, followed by bromination of the product and its Suzuki-type coupling with alkylthiophene. UV-vis and FTIR spectroscopic investigations showed that the extent of the conjugation in the substituents is very sensitive to their regiochemical arrangement. The presence of alkylthienyl groups lowers the bisimide LUMO level to −3.94 eV with respect to the vacuum level. This property combined with their solution processibility makes the new bisimides excellent candidates for their application as active layers in all-organic field-effect transistors, fabricated by solution processing (spin coating or printing techniques). X-Ray diffraction measurements have shown that the bisimide molecules, deposited on either polyethylene naphthalate (PEN) or untreated Si 〈100〉 substrates, adopt regular arrangements giving rise to crystalline domains, which, in their overwhelming part, are highly oriented respectively to the substrate with the long period of their structure being in a plane perpendicular to the substrate. As a result a tendency to align π-stacked moieties in a direction parallel to the substrate can clearly be observed. Bottom contact staggered organic field-effect transistors have been fabricated and tested in air environment. All layers, except the source and drain electrodes, have been deposited from solution. The obtained transistors exhibit charge carriers mobility approaching 10−2 cm2 V−1 s−1 and the on/off ratio exceeding 105, indicating that this new class of bisimide semiconductors can be considered as promising solution processible materials for n-channel transistors.

X-ray study of plasticized polyaniline

Polyaniline (PANI) doped with diisooctyl phosphate (DiOHP) has been investigated by X-ray diffraction measurements. It was found that a lamellar-like structure is stabilized when x, the molar ratio of DiOHP to PANI is equal to 0.2. For x ¼ 0:3 the samples are singularly disordered. For 0:3 < x < 0:5, some molecular ordering is recovered and results in an ill defined layered structure. Finally, for molar ratios greater than 0.5 this ordering is progressively lost. The aged samples show exactly the same sequence as a function of the doping rate but globally with a higher degree of crystallinity for the ordered phases. Results are qualitatively discussed in terms of interplaying of protonation and plasticization effects induced by the doping process. 2002 Elsevier Science Ltd. All rights reserved.

Charge transport in poly(3-hexylthiophene):CdSe nanocrystals hybrid thin films investigated with time-of-flight measurements

Charge carrier mobilities and transport dispersivity are studied in hybrid films composed of poly(3-hexylthiophene) and CdSe nanocrystals by the time-of-flight method, using a field range of 105–106 V/cm. It is found that charge transport parameters pass through an optimum for 75 wt. % (36 vol. %) of nanocrystals in the polymer matrix, yielding balanced hole and electron mobilities around 10−3 cm2/V s. Changing the nanocrystal shape from spherical to branched increases the intersite coupling disorder in the hybrid whereas changing the surface ligands from stearate/oleylamine to pyridine decreases it. Both these modifications have an impact on the electric field dependence of the measured mobilities.