Wuppertal, Universiẗat, Diss., 2003.

Th. 3e cycle--Chim. struct.--Besançon, 1982. N°: 402.

Made available in DSpace on 2014-06-11T19:27:12Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-04-20Bitstream added on 2014-06-13T19:35:01Z : No. of bitstreams: 1 oliveira_acm_me_guara.pdf: 813696 bytes, checksum: 291a32b0707aad7bbcf67e98ead0311c (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O presente trabalho aborda as condições de reticulação do polímero termoplástico polietileno e, para isso, foram avaliadas diferentes temperaturas de processo (60 e 90°C), bem como diferentes tempos de permanência em imersão em banho termostatizado (1, 3, 6 e 9 horas). Esta avaliação foi realizada a partir dos ensaios de teores de gel e fator de uptake, de massa específica, de calorimetria exploratória diferencial (DSC), de análise termogravimétrica (TGA), de espectrofotometria no infravermelho com transformada de Fourier (FT-IR) e de ensaios de tração. Desta forma, o grau de reticulação do polietileno de baixa densidade reticulado em água, foi correlacionado com sua morfologia, com seu desempenho térmico e mecânico. Os resultados mostraram que o teor de gel do polietileno reticulado aumentou com o aumento da temperatura e do tempo de exposição desta matriz polimérica em água, sendo que, este aumento ocorreu de forma significativa até 3 horas de exposição, não sendo observados ganhos significativos na formação de ligações cruzadas após este período / The present work deals with the crosslink conditions of polyethylene polymer using different process temperatures (60 and 90°C). For this different water immersion times in thermostated bath (1, 3, 6 and 9 hours) have been analyzed by using gel content and uptake factor, density tests, dynamic scanning calorimetric analyses (DSC), thermogravimetric analysis (TGA), infrared spectroscopy (FT-IR) and tensile tests. This way, it was evaluated the degree of crosslinking of low density polyethylene crosslinked into water, and these parameters were correlated with the morphology and its thermal and mechanical performance. According to found results, it was observed that the gel content of polyethylene crosslinked increased with increasing of exposition temperature and time of this polymeric matrix in water, therefore, this increase was significantly up to 3 hours of exposition, not being observed significant gains in crosslinking density after this time

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The temperature dependence of the reactions of halogen atoms Cl and Br with SiH4 and (CH3)3SiH have been investigated with the flash photolysis-resonance fluorescence technique. CCI4 and CH2Br2 were used as precursors to produce Cl and Br atoms, respectively. Experiments gave {k(Cl + SiH4) (295 - 472 K)} = (1.56 +0.11) x 10-1 exp[(2.0 + 0.2) kJ mol'/RT] cm3 s4, {k(Br + SiH4)(295 - 575 K)} = (9.0 + 1.5) x 10-" exp[-(17.0 + 0.6) Id mol'/RT] cm3 s', {k(Cl + (CH3)3SiH)(295 - 468 K)} = (1.24 0.35) x 104 exp[(1.3 + 0.8) Id mol4/RT] cm3 s', and {k(Br + (CH3)3SiH)(295 - 456 K)} = (7.6 + 3.3) x 1010 exp[-(28.4 + 1.3) Id mol'/RT] cm3 s'. The results were compared with values from earlier work.

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A nucleophilic cleavage reaction of 2,2,5,5,8,8-hexamethyl-2,5,8-trisilanona-3,6-diyne, I, with several bases, occurs rapidly at room temperature in both polar and non-polar solvents. An attack occurs exclusively at the central silicon regardless of the steric requirements of the bases (alkyllithium). The major products from the reaction of I with tert-butyllithium followed by hydrolysis were 2,2,5,5,6,6-hexamethyl-2,5-disilahepta-3-yne, II, and 2,2,5,5-tetramethyl-2,5-disilahexa-3-yne, VI.

Information technology has grown rapidly in the last few decades and is becoming one of the basic needs of humankind. As this technology grows, the demand for new materials to develop it also increases. So far, silicon is the main semiconductor material for electronic applications, while III-V semiconductors are the main materials for photonic applications. We readily find silicon integrated circuits (ICs) around us: in our cellular phones, personal computers, cars, home appliances, etc. For the further development of this technology, a material compatible with silicon microelectronics and suitable optoelectronic properties would play an important role. In particular, research should focus on a material which could be used for the next generation of photon emitters and could be integrated directly into an IC. Indeed, it is well-known that due to its indirect band-gap structure, bulk silicon is an extremely inefficient photon emitter. Therefore, scientists have turned their interests to other more complex and expensive semiconductor materials such as GaAs (gallium arsenide), InP (indium phosphide), GaP (gallium phosphide), etc. Even though these materials have allowed the realization of laser diodes, they cannot be easily associated with silicon integrated circuits. They are incompatible because the two materials have different crystal lattice constants, a socalled lattice mismatch. Another type of materials, which also give good luminescent efficiency are organic materials. In this case, one does not encounter the lattice mismatch problem due to their amorphous structure, but their processing into devices is not always compatible with microelectronics industry standards. Moreover, the stability of these materials is quite low compared to their inorganic counterparts.