Intrinsically-polarized polymer LEDs

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Consideration of the structure of polyvinylidene fluoride (65) assuming a barrier of 3 kilo cal per mole for rotational minima of conformation of the chain by A. E. Tonelli (66) led to detailed conformation and its implications for dipole structure (Fig. 22). Indeed, the material can approximate a ferro electric. It is thus of interest in our expectations of the environments that polymers can provide for the creation of new phenomena. The total array of dipoles in polyvinylidene fluoride will switch in about 3 microseconds at 20°C with 200 megavolts per meter field. The system becomes much slower at lower temperatures and fields. But we do have a case of macroscopic polarization intrinsic to the polymer molecules, which thus supplements the extensive trapping and other charge of distribution phenomena that we have discussed in connection with electrets. 

Though most insulating polymers are not intrinsically ferroelectric, polar materials can be produced by charge injection into insulating polymers. Such electrets have been known, studied and utilized for many years. Notable exceptions are poly(vinylidene difluoride) (PVDF) and its copolymers which are ferroelectric. There have been numerous studies of orientation, poling, structure and phase transitions of these polymers. The large piezoelectric effect of PVDF when in the poled state has led to its application in acoustic and ultrasonic transducers. 

The advantages of PPO as a gas separation material has therefore been well appreciated. The search for new and improved membranes with highly efficient performance characteristics has however led many researchers to chemically modify PPO. Most chemical modifications of PPO reviewed in this chapter are for improvement of CO2/CH4 or O2/N2 permselectivities. Modifications have been made to improve the solubility selectivity of PPO without sacrificing its high intrinsic permeability. Many researchers have also tried to improve the solubility of PPO in polar solvents by structure tailoring the polymer. It may be noted that PPO can be easily modified via chemical reactions. The following section will describe the advances that have been made in the past few decades in the use of modified PPO as membrane materials for the separation of gases. 

---