Device degradation studies

With this collection of short review papers we would like to present a broad overview of research on polyfluorenes and related heteroanalogues over the last two decades. The collection begins with papers on the synthesis of polyfluorenes and related polyheteroarenes, then reports photophysical properties of this class of conjugated polymers both at the ensemble and the single chain level, continues with a discussion of the rich solid state structures of polyfluorenes, and finally switches to device applications (e.g. in OLEDs). In addition, two chapters are devoted to defined oligofluorenes as low molecular weight model systems for polyfluorenes and also to degradation studies. 

Despite the complexity (discussed earlier) in the quantitative interpretation of photoluminescence measurements, they proved to be very effective in mechanistic studies of OLED degradation. A seminal work by Aziz, Popovic, and coworkers demonstrated that Alq cation-radicals are unstable in a solid-state, OLED-like environment [62]. They designed mostly monopolar devices to study the separate effects of charge carriers on the stability of Alq. 

Holland, S.J. Jolly, A. M. Yasin, M. and Tighe, B. J. Rolymers for biodegradable medical devices. II. Hydroxybutyrate-hydroxyvalerate copolymers hydrolytic degradation studies. Biomater. 1987,8(4), 289-295. 

Li, D. Guo, G. Fan, R. Liang, J. Deng, X. Luo, F. Qian, Z. PLA/F68/Dexamethasone implants prepared by hot-melt extrusion for controlled release of anti-inflammatory drug to implantable medical devices I. Preparation, characterization and hydrolytic degradation study. Int. J. Pharm. 2013,447(1-2), 365-372. 

As documented in numerous studies on device degradation behavior both the organic layers [129,130] and the low work function metal electrodes [131—134] were found to be prone to degradation upon exposure to oxygen or water, leading to the introduction of chemical defects into the conjugated backbone or metal electrode decomposition and ultimately delamination. 

Giebeler, C., S.A. Whitelegg, D.G. Lidzey, RA. Lane, and D.D.C. Bradley. 1999. Device degradation of PLEDs studied by electroabsorption measurements. Appl Phys Lett 75 2144—2146. 

Although the above picture raises some questions with respect to modeling of LEDs, it could also provide an answer to inconsistencies often found [42] in quantitative studies. It is also possible that the existence of fast and slow paths contributes to device degradation and, understanding these dispersive phenomena could help in screening for long-lived materials. 

GA devices degrade heterogeneously, with the degradation being faster in the internal part than at the surface where a layer of less degraded material is formed. Studies earried out by Gri/./.i et al. eonfirmed the formation of an outer skin layer as demonstrated in Fig. 5.4. 

From a review of the literature, which includes both animal and human clinical study data, it is possible to construct a composite model of how absorbable devices degrade in living tissues. One version of such a model is presented below which we believe accounts for the majority of the published observations with PL devices. With the exception of degradation rate, this same model can also be generally applicable to devices made from PG and PLG copolymers. For additional information specific to PG and PLG polymers, the reader is referred to Chu (1985) and Katz Turner (1970). Published observations of inflammation associated with devices made from PG and PLG copolymers may or may not be pertinent to the model described below for PL. Observations of inflammation will be discussed in the section on HUMAN CUNICAL STUDIES. 

The primary results of this study are 1) pinholes can be observed by TOF-SIMS analysis in films that appear to be of high optical quality 2) indium penetration into the EL polymer (which has been observed by others, but not as a consequence of stressf79,20 is driven by electrical operation and 3) the amount of calcium strongly bound to the polymer surface increases with electrical stress. Each of these observations has relevance to one or more modes of device degradation. 

An early version of the in vitro collagenase degradation assay was based on measurement of stress relaxation of a strip of test polymer immersed in a standardized collagenase bath. Previously, Tobolsky had used such a device to study the chemical stress relaxation of natural rubber in the presence of ozone. In spite of its immediate utility as a screening probe in our early research this experimental arrangement was eventually replaced by a more reproducible assay. The latter is an adaptation of a colorimetric procedure originally developed by Mandl and coworkers. ... 

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