W. James Feast

Synthesis and device characterisation of side-chain polymer electron transport materials for organic semiconductor applications

Improved syntheses and polymerisations are reported of monomers bearing electron transporting substituents based on 2,5-diphenyloxadiazole and 2,3-diphenylquinoxaline attached directly to a vinyl group. By copolymerisation and by use of mixtures of homopolymers, these materials have been incorporated into light emitting polymer devices in which hole conduction properties are provided by 4-vinyltriphenylamine groups. High luminescence efficiency is achieved by use of a fluorescent additive. The resulting devices show narrow emission bands and high brightnesses, except in the case of those based on a diphenyloxadiazole–triphenylamine polymer blend. Thermal analysis data are equivocal but we present evidence that in this system, but not the quinoxaline blend, phase separation occurs. The minority charge carrying capacity of the homopolymers is probed: it is shown that the quinoxaline derivative has hole blocking properties superior to those of the oxadiazole polymer and is a good candidate for use in optimised devices.

Soluble poly(para-phenylene)s. 2. Improved synthesis of poly(para-2,5-di-n-hexylphenylene) via Pd-catalysed coupling of 4-bromo-2,5-di-n-hexylbenzeneboronic acid

Abstract The synthesis of poly(para-2,5-di-n-hexylphenylene) via the palladium catalysed coupling of 4-bromo-2,5-di-n-hexylbenzeneboronic acid is described. The reaction yields structurally homogeneous products with the highest degrees of polymerization for poly(para-phenylene) derivatives yet recorded, namely DP ∼ 30 . The products are soluble and have been characterized by n.m.r., g.p.c. and osmometry.

Soluble poly(para-phenylene)s. 1. Extension of the Yamamoto synthesis to dibromobenzenes substituted with flexible side chains

Abstract This paper describes the synthesis of poly( para -2,5-di-n-alkylphenylene)s (2) by an extension of the Yamamoto route to poly( para -phenylene) (PPP); that is, the Ni-catalysed coupling of dibromoaromatics. These products, in contrast to PPP, are soluble and have been characterized in detail by 1 H and 13 C-n.m.r. and high pressure liquid chromatography. The structures are highly regular with repeat units connected exclusively in a 1,4-fashion. The maximum average degree of polymerization ( DP ) obtained by this method is of the order of 13.

New routes to conjugated polymers: 1. A two step route to polyacetylene

Abstract This paper describes routes to poly(acetylene) which involve the synthesis of soluble precursor polymers which may be purified and subsequently converted to poly(acetylene) via a symmetry allowed thermal elimination reaction. Poly(acetylene) produced in this way is free from catalyst residues and other impurities. The method allows the production of poly(acetylene) in the form of continuous uniform and featureless films as opposed to the disordered fibrillar mat morphology obtained as a result of the direct polymerization of acetylene.

Ring-opening metathesis polymerization of monocyclic alkenes using molybdenum and tungsten alkylidene (Schrock-type) initiators and 13C nuclear magnetic resonance studies of the resulting polyalkenamers

Abstract The ring-opening metathesis polymerization of a series of cycloalkenes initiated using well defined Schrock-type initiators is described. The products have been characterized by gel permeation chromatography, nuclear magnetic resonance and differential scanning calorimetry. The influence of initiator, reaction conditions and monomer structure is analysed in detail. The detailed microstructures of the polymers can be deduced from the 13C nuclear magnetic resonance spectra and apparent anomalies are accommodated in terms of the γ-gauche effect established by Tonelli.

Surface-controlled self-assembly of chiral sexithiophenes

We report on the self-assembly of two enantiomeric sexithiophenes in solution and on surfaces. Circular dichromism of aggregated sexithiophenes and drop-cast films reveals, as expected, mirror image spectra for both enantiomers. The aggregation in thin deposits from sexithiophenes molecularly dispersed in a solution on different types of substrates was investigated by atomic force microscopy (AFM). On graphite, one-dimensional objects (nanowires) are formed while on mica platelets are generated. Remarkably, we found that both enantiomers form left-handed helices on silicon. This observation depends on the hydrophilicity of the silicon. Furthermore, the achiral sexithiophene did not form helical aggregates suggesting that the stereocenter is required to obtain chirality in the fibers.

Synthesis, structure and properties of some hyperbranched polyesters

A new approach to the controlled synthesis of hyperbranched polyesters, via copolymerisation of AB2 and B3 monomers, has been demonstrated. This technique allowed relatively easy production of new families of hyperbranched materials, circumventing the laborious and time-consuming syntheses required for their dendrimer analogues. Copolymerisation of AB2 monomers with a B3‘terminator core’ restricted the hyperbranched growth and allowed an experimental assessment of points made in an earlier theoretical treatment of the subject.

Dielectric and mechanical characterization of aryl ester dendrimer/PET blends

Abstract The structure property relationships of miscible blends of two aryl ester dendrimers with poly(ethylene terephthalate) (PET) have been investigated. Although dielectric studies show relatively small effects on the glass transition process, the addition of dendrimers to PET produces a significant modification of the tensile drawing behaviour which may be interpreted in terms of the stretching of a molecular network. The smaller of the two dendrimers investigated acts as a plasticizer and reduces the chain entanglement density of the blend, whereas the larger dendrimer acts as an antiplasticizing agent and increases the entanglement density.

Ultrathin organic films for field effect transistors

Organic field effect transistors FET have been fabricated with active semiconducting organic thin films that are only a few monolayers thick. The motivation of this study has been to establish a direct correlation between transistor performance and the polymer microstructure in the ultrathin accumulation layer of the transistor. Monolayer thick films of a block copolymer and several model oligomers consisting of a rigid conjugated sexithiophene (6T) block and a flexible polyethyleneoxide (PEO) block have been deposited onto the surface of e.g. SiO2 gate dielectrics functionalized with a self-assembled monolayer. Block copolymer phase behavior and surface morphology has been studied as a function of chain length, solvent and film thickness. Operational transistors have been demonstrated with film thicknesses of only one or two monolayers. Typical device characteristics show a carrier mobility on the order of 10-2 - 10-3 cm2/Vs and ON-OFF current ratio of 103 - 105. Film microstructure, orientation of micro-crystallites and film thickness have been studied by atomic force microscopy (AFM), UV-Vis absorption spectroscopy and X-ray diffraction.

An approach to fluorinated surface coatings via photoinitiated cationic cross-linking of mixed epoxy and fluoroepoxy systems

Abstract The synthesis and characterisation of four polyfluorinated monoepoxides is described. These fluoroepoxy monomers were cationically polymerised either neat or as mixtures with diglycidylether of bisphenol-A using the initiator system (CH 3 CH 2 ) 3 Al/H 2 O (2/1) or the photoinitiator Ph 2 I + , PF 6 − . Contact angle measurements indicated that, in favourable cases, the fluorinated epoxide monomer surface segregated at low loadings to give significant lowering of the surface free energy of photocured films of diglycidylether of bisphenol-A.