Tribocharged polymers display macroscopically patterned positive and negative domains, verifying the fractal geometry of electrostatic mosaics previously detected by electric probe microscopy. Excess charge on contacting polyethylene (PE) and polytetrafluoroethylene (PTFE) follows the triboelectric series but with one caveat: net charge is the arithmetic sum of patterned positive and negative charges, as opposed to the usual assumption of uniform but opposite signal charging on each surface. Extraction with n-hexane preferentially removes positive charges from PTFE, while 1,1-difluoroethane and ethanol largely remove both positive and negative charges. Using suitable analytical techniques (electron energy-loss spectral imaging, infrared microspectrophotometry and carbonization/colorimetry) and theoretical calculations, the positive species were identified as hydrocarbocations and the negative species were identified as fluorocarbanions. A comprehensive model is presented for PTFE tribocharging with PE: mechanochemical chain homolytic rupture is followed by electron transfer from hydrocarbon free radicals to the more electronegative fluorocarbon radicals. Polymer ions self-assemble according to Flory-Huggins theory, thus forming the experimentally observed macroscopic patterns. These results show that tribocharging can only be understood by considering the complex chemical events triggered by mechanical action, coupled to well-established physicochemical concepts. Patterned polymers can be cut and mounted to make macroscopic electrets and multipoles.
Isolated metals within Faraday cages spontaneously acquire charge at relative humidity above 50%: aluminum and chrome-plated brass become negative, stainless steel is rendered positive, and copper remains almost neutral. Isolated metal charging within shielded and grounded containers confirms that the atmosphere is an electric charge reservoir where OH(-) and H(+) ions transfer to gas-solid interfaces, producing net current. The electricity buildup dependence on humidity, or hygroelectricity, acts simultaneously but in opposition to the well-known charge dissipation due to the increase in surface conductance of solids under high humidity. Acknowledging this dual role of humidity improves the reproducibility of electrostatic experiments.
The electroneutrality principle 1 expresses the fact that all pure substances carry a net charge of zero. However, real substances in the environment are usually under significant static potential gradients and thus under nonzero electric potential. 2 The electrochemical potential (μ) (eq 1) of ions under a given electric potential (V)
Electrostatic potential at the surface of acidic or basic solids changes under higher relative humidity (RH), as determined by using Kelvin force microscopy (KFM). The potential on acid surfaces becomes more negative as the water vapor pressure increases, while it becomes more positive on basic solids. These results verify the following hypothesis: OH(-) or H(+) ions associated with atmospheric water ion clusters are selectively adsorbed on solid surfaces, depending on the respective Brønsted acid or base character. Therefore, Kelvin microscopy, under variable humidity, is a rigorous but convenient alternative to determine the acid-base character of solid surfaces, with a great advantage: it uses only one amphoteric and simple reagent to determine both the acid and base sites. Moreover, this technique provides information on the spatial distribution of acid-base sites, which is currently inaccessible to any other method.
Recebido em 30/6/10; aceito em 30/8/10; publicado na web em 8/11/10 ELECTROSTATIC CHARGING OF DIELECTRICS: NEW APPROACHES TO SOLVE PERSISTING PROBLEMS. Electrostatic phenomena were discovered long ago but their interpretation according to well-established atomic-molecular theory is still lacking. As a result, electrostatic phenomena are often irreproducible and uncontrolled, causing serious practical problems. Highly reproducible recent experimental results on electrostatic charging from this and other laboratories are reviewed in this work, together with a description of the relevant but not so usual Kelvin probe and Faraday cup techniques. These results support a new model for electrostatic charging of dielectrics and insulated metals, based on the role of moist atmosphere as a charge reservoir.Keywords: water ion partition; electrostatic charging; Kelvin probe. INTRODUÇÃO Eletrização de materiaisO primeiro relato de um fenômeno de eletrização é devido a Tales de Mileto e os resultados experimentais obtidos até o século 19 foram consolidados no tratado de Maxwell sobre Eletromagnetismo. 1 Hoje, a eletrostática está presente em muitas tecnologias de produtos e processos importantes: as máquinas de eletrocópias, impressoras a laser, 2 eletretos 3 usados como transdutores acústicos, reciclagem de políme-ros isolantes, 4 fabricação de nanofibras e nanotubos poliméricos por fiação eletrostática, 5 pintura eletrostática e filtração eletrostática. 6 A eletrização atmosférica também é conhecida de praticamente todas as populações humanas devido à sua associação às tempestades. Poderosas descargas eletrostáticas são ainda observadas em erupções vulcânicas e em tempestades de areia, em regiões áridas. 7 Apesar da onipresença dos fenômenos eletrostáticos e das suas consequências, o conhecimento sobre este tópico ainda é bastante empírico. 8,9 Persistem lacunas importantes na correlação de ideias fundamentais da teoria atômico-molecular com as observações experimentais da eletrostática de materiais isolantes 10 e muitas questões importantes sobre esse assunto ainda não foram satisfatoriamente respondidas. A principal delas é a identidade das espécies portadoras de cargas em um material isolante, bem como a sua detecção e identificação. Schein, um ex-dirigente de pesquisas na Xerox afirmou em uma importante e recente revisão: "Most researchers believe that insulator charging is a surface phenomenon. Creating a reproducible surface and obtaining experimental reproducibility among laboratories has been a challenge". 8 Portanto, não há consensos amplos sobre a teoria dos fenômenos eletrostáticos e também não há paradigmas experimentais reprodutíveis mesmo nos casos mais simples da eletrização por contato e triboeletrização.O desconhecimento dos mecanismos de acúmulo e dissipação de cargas eletrostáticas não é somente restrito à área de materiais. As pesquisas em geofísica não produziram ainda um modelo efetivo para explicar a eletrização da atmosfera, 11 o que está certamente associado às frequentes perdas de vidas e propried...
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