| Abstract |
The investigation of polymer dynamics is an intriguing scientific problem, as it influences the structure to property relationship because of the complexity it exhibits over many length- and time- scales. Polymer dynamics includes vibrational motions, rotations of side groups, the segmental α-process as well as the overall chain dynamics, thus covering a very broad time regime of more than ten decades from the pico-second (ps) to the second (s) regime.
In recent years novel classes of polymers have emerged such as the biobased and biodegradable aliphatic polyesters, due to the combination of biodegradability and physicochemical properties comparable to those of conventional polymers, and the hyperbranched polymers, which show the unique features of dendrimers like low viscosity at high molecular weight, high solubility, miscibility, and reactivity influenced by the end groups, together with a cost-effective synthesis. Moreover, polymer nanocomposites, comprising a polymer matrix and nanoscale additives (e.g., nanoparticles, clays, nanotubes, graphene, etc.), possess improved and often innovative physicochemical properties compared to the bulk polymer or conventionally filled systems.
In the present thesis polymer dynamics is investigated in nanocomposites comprising of polymers with different architecture and various additives. The polymers examined were three linear polyesters with hydroxyl end groups and different repeating units, three hyperbranched polyester polyols of different generations and a hyperbranched poly (ester amide). Nanocomposites based on these polymers and three different additives, silica nanoparticles of spherical geometry, a multi-layer graphite oxide and a layered silicate, sodium montmorillonite, were prepared by solution mixing.
The structural and thermal characterization shows favorable polymer-additive interactions with dispersed nanoparticles within the polymeric matrix and with intercalated structures formed in the case of the layered additives. By increasing the concentration of the additive in all nanocomposites the thermal properties of the polymer are gradually suppressed as a result of a thin polymer layer formed close to the interface of the additive. Additionally strong confinement effect was observed depending on the system.
Polymer dynamics investigated by Broadband Dielectric Spectroscopy reveals multiple relaxation processes, observed in both the bulk polymer and the nanocomposites. Nevertheless, significant differences are observed due to the interactions with the additive and the effect of confinement. The local motions, attributed to the rotation of hydroxyls and the reorientation of carbonyls, possess lower activation energy in most of the nanocomposite cases in comparison to the bulk, because they are less constrained due to the reduced hydrogen bonding in nanohybrids. For the linear polyesters the segmental relaxation in nanocomposites retains its VFT temperature dependence whereas for the hyperbranched polymer nanocomposites, the segmental relaxation appears below the glass transition temperature of the respective bulk polymer with an Arrhenius temperature dependence. The confinement strongly affects the segmental dynamics of the hyperbranched polymers in both GO and clay nanocomposites. On the contrary, the segmental dynamics for the linear polyesters is similar to the bulk, not only in silica nanohybrids of lower confinement but in clay nanocomposites as well, due to their low molecular weight.
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Polymer dynamics in nanohybrid materials : effects of the polymer architecture and the additive geometry
