Polymer doping with functional dopants

Hole transport in polymers occurs by charge transfer between adjacent donor functionalities. The functionalities can be associated with a dopant molecule, pendant groups of a polymer, or the polymer main chain. Most literature references are of doped polymers. The more common donor molecules include various arylalkane, arylamine, enamine, hydrazone, oxadiazole, oxazole, and pyrazoline derivatives. Commonly used polymers are polycarbonates, polyesters, and poly(styrene)s. Transport processes in these materials are unipolar. The mobilities are very low, strongly field and temperature dependent, as well as dependent on the dopant molecule, dopant concentration, and the polymer host This chapter reviews hole transport in polymers and doped polymers of potential relevance to xerography. The organization is by chemical classification. The discussion mainly includes molecularly doped, pendant, and... 

Bipolar transport requires both donor and acceptor functionalities. This may be accomplished by doping a polymer with separate donor and acceptor molecules, or by the use of a dopant molecule that contains both donor and acceptor functionalities. The functionalities can be either associated with a dopant molecule or incorporated within the polymer chain. Most literature references are to doped polymers where the donor and acceptor functionalities are associated with separate donor and acceptor molecules. There are few references of bipolar transport in the literature. Because of its importance to single-layer photoreceptors, however, the subject is of considerable technological relevance. 

Depth-distribution of fluorescent dopants in cast polymer film (4) Fluorescence spectra of poly(N-vinylcarbazole) (PVCz) film doped with perylene are shown in Fig. 6. They consist of two broad structureless excimer bands of the polymer with a shoulder at 375 nm and a peak at 420 nra, and perylene band with a vibrational structure above 450 nm. It is worth noting that the perylene fluorescence intensity under the TIR condition is relatively weaker than that under the normal one. Since the boundary surface is selectively excited under the former condition, the structure near the surface should be different from the bulk. It is well known that the excitation energy migrates over carbazolyl chromophores and is trapped in the doped perylene efficiently. Therefore, the present result means that energy migration efficiency in the host polymer and/or the dopant concentration are a function of the depth from the interface. 

More recently, polymers soluble in the doped (i.e. conducting) state have been prepared by the use of appropriately functionalized dopants. For example, it was demonstrated that acid-base doping (protonation) of polyemeraldine base with sulphonic acids [9] or phosphoric acid diesters [10] results in the fabrication of soluble conducting polyaniline. 

The formation of hydrogen bonds between the carboxylic acid groups of the functionalized liquid crystal copolymers and the pyridine portion of the dopants leads to stable, none separating mixtures. In mixtures containing up to 30 % of the dopants no separation was observed. Induction of a nematic mesophase is observed in the case of a smectic polymer matrix doped with low molecular weight photochromic dyes. 

A number of composite systems have been reported in which a polymer possessing one of the requisite functionalities, e.g., covalently attached NLO chromophores, is doped with the others, e.g., CG and CT dopants, or a photoconductive polymer, e.g., poly(N-vinylcarbazole), is doped with the sensitizer and NLO dopants (1, 4-7). The high dopant loading levels necessary (up to 50 wt%) result in severe limitations of doped systems, including diffusion, volatilization, and/or phase separation (crystallization) of the dopants. In addition, plasticizers and compatibilizers are often used to lower the glass transition temperature (Tg) of the polymer and increase solubility of the dopants in the host polymer, respectively. This, in turn, dilutes the effective concentration of CT, CG, and NLO moieties, diminishing the efficiency and sensitivity of the photorefractive polymer composite. 

Common protonic acids such as hydrochloric and sulfuric acids leave the polymer highly polar. Dopants with long polymer chains such as dodecylbenzenesulfonic acid can lessen the polar character and change the interactions of the polymer [7,15]. The effect is clearly evident from the solubility/dispersibility of polyaniline doped with dodecylbenzenesulfonic acid in nonpolar solvents such as toluene and xylene [7,16]. Certain functionalized protonic acid solutes such as camphorsulfonic acid have been shown to be highly effective in rendering the doped polymer soluble in polar organic solvents such as m-cresol [7]. 

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