Preparation and rheology of liquid blends and elastomers based on polydimethylsiloxane
Creators: name: Lavallata, Vito
Publisher: name: Universita degli studi di Salerno
Year: 2021
Subjects: subject: Reologia dei polimeri
subject: Polidimetilsilossano (PDMS)
subject: Viscoelasticità di blend polimeriche
subject: CHIM/06 CHIMICA ORGANICA

description: 2019 - 2020
description: To improve the performance of elastomers, the blending of pure polymers is often used during the cross-linking or processing phase. The resulting blend has chemical-physical properties that can be modulated over a wide range of values; moreover, they are advantageous from an industrial point of view since they do not involve the synthesis of new elastomers. In this thesis work, we have focused our attention on homologous blends based on polydimethylsiloxane (PDMS) with different viscosity and average molecular weights, which we will call low, medium, and high. PDMS is an organosilicon polymer widely used in various fields and with unique physicochemical properties. In particular, we first present methodologies for the preparation of elastomeric blends: liquid uncross-linked or slightly crosslinked and cross-linked solid. Next, we investigate the viscoelastic rheological behaviour by using the parallel plate rheometer for the fluid state blends and pure reference elastomers, and the dynamic-mechanical thermal analyser for the cross-linked ones. We then process the experimental data and interpret them in the light of the rheology models of polymeric fluids and rubber elasticity, in order to consider the implications from the point of view of modelling, processing, and applications. For the liquid elastomeric blends, our rheological experiments have revealed unexpected effects of great interest using the low molecular weight component or low cross-linker concentrations. In the first case, the blend has shown a marked increase in elasticity component; in the second, however, we have found a noticeable increase in viscosity properties. As we will see, the medium/high blend exhibits the ordinary rheological behaviour, according to the literature. In contrast, the low/high blend shows the inversion behaviour of viscosity, compliance, modulus, relaxation time, and dynamic moduli. The Entanglement Swelling Tube (EST), the nonlinear dynamical mechanism at entanglements and the Deswelling-Solvation at higher temperature models will be proposed. The importance of our approach lies in having obtained liquid or crosslinked blends with unprecedented rheological behaviours, subject to publication. These blends may find application in industrial and biomedical fields; moreover, our study may provide an important contribution to the rheological modelling of homologous polymer blends. As regards the cross-linked elastomer blends, the dynamic-mechanical measurements have indicated, also in this case, a dependence of the elastic properties on the low molecular weight component. At low deformation and concentration of blending agent (the minor component), an increase in the elastic component has been obtained, then with increasing concentration a decrease, finally at high concentrations, the viscoelastic properties of the blends become lower than those of the pure elastomer. The dynamic-mechanical investigation conducted on the cross-linked blends has also revealed unexpected and interesting behaviours from the point of view of possible applications in the biomedical field, allowing a fine modulation of the final viscoelastic properties of elastomer blends. Finally, we have applied the state equations of several molecular and phenomenological models for the rubber elasticity. Particularly, we have compared the theoretical predictions with our outcomes, in order to understand the structure-property relationship that determines elastomeric performance. [edited by Author]