| Abstract |
Polymer nanocomposites are multi-constituent systems, where organic or inorganic nanomaterials are dispersed in a polymer matrix. These systems often exhibit improved physicochemical properties compared to the respective initial materials as well as to the conventionally filled systems. The understanding of the relationship between the properties of the neat materials and those of the nanocomposite as well as the interactions between the filler and the polymer matrix, are imperative in order to design new and improved polymer nanocomposites for more specific and targeted applications. In this current work, the rheological properties of five different polymer nanocomposite systems, comprised of poly(ethylene oxide), PEO and silica, SiO2, nanoparticles were investigated utilizing oscillatory shear rheology. More specifically, silica nanoparticles of two different sizes (R=7 nm and 67 nm) were dispersed in PEO of different molecular weights (3,350 (PEO3,35), 8,000 (PEO8), 35,000 (PEO35) g/mol), in several different compositions. Our goal was to study the effect of the concentration and size of the nanoparticles, the molecular weight of the polymer and the existence of entanglements on the rheological response of the nanohybrids. It should be noted that the presence of hydroxyl groups (-OH) on the surface of the nanoparticles, facilitates favorable interactions between PEO and the nano-additive. The state of the dispersion of the nanoparticles within the polymer matrix was studied utilizing Transmission Electron Microscopy (TEM) whereas the composition was confirmed via Thermogravimetric Analysis (TGA). The rheological response of the nanocomposites was studied within the limits of the linear viscoelastic regime. Initially, its boundaries were determined via dynamic strain sweep tests (dss), followed by the study of the dynamic equilibrium of the sample through consecutive dynamic frequency sweep tests (dfs). Finally, the dynamic response of each sample was evaluated though dynamic frequency sweep tests, spanning a range of temperatures. In the case, of the samples that had low viscosity where the above protocol could not be applied, its values were measured through Step Rate tests, for a range of shear rates. A similar rheological response was observed for all nanocomposite series with the addition of nanoparticles, leading to three distinct rheological behaviors. More specifically, at low nanoparticles loadings the nanocomposite response is polymer – like, while by increasing the concentration a weak gel-like response is observed. Finally, once a critical concentration is exceeded a strong gel-like behavior is observed due to the percolation of the nanoparticles. Even though, these rheological regimes are observed for all nanocomposite series, the critical concentrations that define their limits depend on both the size of the nanoparticles and the molecular weight of the polymer. Moreover, concerning the nanocomposites with PEO35, a significant decrease of the viscosity was observed, with the minimum value appearing at 0.5% wt NP7 and 2% wt NP67, respectively. This phenomenon appears to lessen in intensity as the molecular weight of the polymer decreases. More specifically, for nanocomposites with PEO8, the viscosity value remains almost constant with the addition of nanoparticles, while in the case of PEO3,35there is an increase in viscosity even with the addition of the minimum possible concentration of nanoparticles. Finally, the shift factors (aT) follow the Arrhenius model for all samples with the value of the flow activation energy increasing once the nanoparticle concentration threshold for the weak gel-like behavior is reached. The maximum activation energy value is observed for the nanocomposites with strong gel-like dynamics. These results were compared both with bibliographical research and results for PEOs with molecular weights Mw =100,000 and 300,000 g/mol with NP7 and NP67, which were systems previously studied in our lab. This comparison allows us to have a more well - rounded picture of the PEO/SiO2 system and it provides the opportunity to compare nanohybrids comprising of nanoparticles and polymer chains of similar size (PEO35 – NP7), polymer chains bigger in size as compared to the nanoparticles (PEO100, PEO300 – NP7) as well as far smaller (all polymers as compared to NP67). At the same time, the entire polymer range, as far as entanglements are concerned, from highly and marginally entangled to unentangled polymers was covered.
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