Dinamica e reatividade de superficie de poli (dimetilsiloxano)s

AUTOR(ES)
DATA DE PUBLICAÇÃO

1994

RESUMO

Crosslinked polydimethylsiloxane (PDMS) heated at temperatures between 200 and 280°C for short periods (one-half to two hours), reveals a partially thermoplastic behavior. Earlier studies on silicone describe the phenomena occurring under heating as due to chemical reactions leading to the formation of oligomers and oxidation products, which cause degradation of the rubber. However, well-determined conditions of heating temperature and heating time lead also to self-adhesion, in silicone rubbers. Heating at 250°C for 2 hours gives rise to reactive groups, which can diffuse into the polyrner network and recombine. At 200°C and the same heating time, self-adhesion is not observed. At 280°C, PDMS loses its elasticity and is embrittled. PDMS behavior and reactivity under heating were studied by the following techniques: thermogravimetry, scanning electron microscopy, transmisson electron microscopy, atomic force microscopy, mechanical stress tests, infrared spectroscopy, nuclear magnetic ressonance, x-ray photoelectron spectroscopy, x-ray diffraction, sorption and permeability rneasurements. The results obtained by these differents techniques allow us to explain the silicone self- adhesion phenomena according to the dynamics of polymer chains. The opening of main-chain bonds within the polymer network allows the migration of chain ends across the network, with subsequent recombination at the interface. In this process, one distinguishes the contribution of surface and bulk polymer chains: siloxane chains can change conformation easily and the Si-O bonds are highly labile, so that surface chains can recombine at the interface. On the other hand, due to the formation of linear and cyclic oligomers in the bulk as well as to ring opening, diffusion and recombination, it is possible to weld crosslinked PDMS. Silicone thermoplasticity is thus concurrent with the rapid approach to polymerization equilibria in these polymers, above 200°C.

ASSUNTO(S)

polimeros

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