Dispersion and packing of carbon nanotubes

Most organic polymeric materials melt below 200 °C and most of them begin to degrade rapidly at temperatures only slightly above 200°G. Thermally stable polymers are generally considered to be those which will withstand much higher temperatures without loss of strength or change of structure. In general we expect these materials to withstand at least 300°C in air and up to 500°C or higher in inert atmospheres. Polymers which show these properties are usually highly aromatic in structure, often with heterocyclic units, high melting, sometimes infusible and usually with low solubility in all solvents. This makes their fabrication very difficult and as a consequence limits their usefulness.

Thermally stable polymers

Field-effect mobility of electrons as high as 0.1 cm2/(V s) is observed in n-channel thin film transistors fabricated from a solution spin-coated conjugated ladder polymer, poly(benzobisimidazobenzophenanthroline) (BBL), under ambient air conditions. This is the highest electron mobility observed to date in a conjugated polymer semiconductor. Comparative studies of n-channel thin film transistors made from a structurally similar nonladder conjugated polymer BBB gave an electron mobility of 10-6 cm2/(V s). These results demonstrate that electron transport can be as facile as hole transport in conjugated polymer semiconductors and that ladder architecture of a conjugated polymer can substantially enhance charge carrier mobility.

High Electron Mobility in Ladder Polymer Field-Effect Transistors

Materials with a ribbon- or ladder-type framework possess a two-dimensional geometry and are thus intermediate between linear and three-dimensional systems. The limited conformational freedom of ladder polymers is particularly relevant in the case of conjugated ladder-type materials since the steric inhibition of electron delocalization is drastically reduced.Up until today two general routes have been used to prepare ladder-type materials: (1) the polymerisation of multifunctional monomers, in which both strands of the ladder structure are generated in a single reaction; and (2) the cyclisation of suitably functionalized open-chain (single-stranded) precursor polymers in a polymer-analogous process. Both strategies pre-suppose certain essentials to arrive at structurally defined ladder polymers, especially the exclusion of side-reactions and an almost quantitative conversion of the starting materials.The main attention of this article is focused on an active physical function of ladder-type materials. Among others, ladder-type poly( p-phenylene)s (LPPPs) represent an outstanding class of ladder-type materials. They are characterized by an extraordinarily low concentration of active traps (topological defects, impurities) and display a set of attractive electronic properties (very intensive photo- and electroluminescence, high charge carrier mobilities). This unique performance has established the solution processable LPPPs as standard materials for organic polymer based light emitting diodes (LEDs) and optically pumped solid state lasers.

Ladder-type materials

Naphthalene diimides, which have for a long time been in the shadow of their higher homologues the perylene diimides, currently belong to the most investigated classes of organic compounds. This is primarily due to the initial synthetic studies on core functionalization that were carried out at the beginning of the last decade, which facilitated diverse structural modifications of the naphthalene scaffold. Compounds with greatly modified optical and electronic properties that can be easily and effectively modulated by appropriate functionalization were made accessible through relatively little synthetic effort. This resulted in diverse interesting applications. The electron-deficient character of these compounds makes them highly valuable, particularly in the field of organic electronics as air-stable n-type semiconductors, while absorption bands over the whole visible spectral range through the introduction of core substituents enabled interesting photosystems and photovoltaic applications. This Review provides an overview on different approaches towards core functionalization as well as on synthetic strategies for the core expansion of naphthalene diimides that have been developed mainly in the last five years.

Strategies for the Synthesis of Functional Naphthalene Diimides

Benzimidazo‐benzophenanthroline polymers

A new method of obtaining films of aromatic heterocyclic ladder polymers has been found which circumvents casting from high-boiling acidic solvents. The BBL ladder polymer has been formed into tough, durable films by collecting suspensions of the polymer obtained from acid reprecipitations upon a fritted glass funnel. After drying, the polymer can be removed as film, with thickness dependent upon the amount of material used. Such a film has a tensile strength of 9,600 lb/in.2 as compared to a tensile strength of 16,000 lb/in.2 obtained from the same polymer when cast from methanesulfonic acid solution.

Unusual film‐forming properties of aromatic heterocyclic ladder polymers

Polycondensations of 1,4,5,8-naphthalenetetracarboxylic acid (NTCA) with both 3,3′-diaminobenzidine (DAB) and 1,2,4,5-tetraaminobenzene tetrahydrochloride (TAB) in polyphosphoric acid (PPA) were found to produce soluble polymers which exhibit excellent thermal stabilities. Polymer structures were deduced from infrared, thermal, and elemental analyses of model compounds and polymers. Polymer derived from TAB had a ladder-type structure. Polymers with solution viscosities near 1 or above (determined in H2SO4) were obtained from polymerizations near 200°C., and analysis showed these to possess a very high degree of completely cyclized benzimidazobenzophenanthroline structure. Less vigorous reaction conditions gave polymers with lower solution viscosities which appeared to be less highly cyclized. Low-viscosity polymer was also prepared from DAB and NTCA by solid-phase polycondensation. Some advancements in the solution viscosities of polymers synthesized from DAB in PPA were caused by second staging in the solid phase.

Thermally stable polymers from 1,4,5,8‐naphthalenetetracarboxylic acid and aromatic tetraamines

In previous studies on poly{(7-oxo-7, 10H-benz[de]imidazo[4′,5′:5,6]-benzimidazo-[2,1-a]isoquinoline-3,4:10,11-tetrayl)-10-carbonyl} it was observed that the polymer exhibits very unusual film-forming properties. The ladder polymer forms supermolecular aggregates in solution which translates to a high degree of interchain packing in the bulk. New ladder imidazoisoquinoline polymers have been prepared in an effort to relate the variance of molecular structure to the aggregation behavior of this class of polymers. The polymers were prepared from the solution polycondensation of 1,4,5,8-naphthalene tetracarboxylic acid or dianhydride with 1,2,5,6-tetraaminonaphthalene, 2,3,7,8-tetraaminophenazine and 2,3,7,8-tetraaminodibenzofuran. It was found that the extent of interchain packing in the bulk is dependent to a large degree on the molecular geometry of the polymer backbone.

R. L. Van Deusen

Papers

Benzimidazo‐benzophenanthroline polymers

Unusual film‐forming properties of aromatic heterocyclic ladder polymers

Thermally stable polymers from 1,4,5,8‐naphthalenetetracarboxylic acid and aromatic tetraamines

New imidazoisoquinoline ladder polymers

Poly(perimidine‐tetrayls)