| Abstract |
Polymer nanocomposites, comprised of a polymer matrix and inorganic or organic additives (e.g., nanoparticles, nanotubes, clays, graphene, etc.) as the nanofiller, possess improved and often innovative physicochemical properties compared to conventionally filled systems. Understanding the relationship between the physicochemical attributes of the nanofillers and the host matrix and the final properties of the hybrid is of great importance for the design of new materials with specific functionalities. In this work we report on the rheological behavior of a series of poly(ethylene oxide)/silica, PEO/SiO2 nanocomposites through oscillatory shear rheology measurements. The nanohybrids were synthesized by dispersing spherical SiO2 nanoparticles of two different radii in high molecular weight (Mw) poly(ethylene oxide) in different compositions to investigate the effect of the additive on the material’s rheological properties. The two different poly(ethylene oxide) polymers that were used have a molecular weight of 100.000 and 300.000 g mol-1 , respectively. Silica nanoparticles of two different sizes, i.e. R~7nm and R~67nm were utilized and they are noted as NP7 (Ludox LS, Sigma-Aldrich) and NP67 (Snowtex ZL, Nissan Chemicals), respectively. In both cases, the surface of the nanoparticles had -OH groups so that they can interact favorably with hydrophilic polymers such as PEO. Transmission Electron Microscopy (TEM) verifies the good dispersion of the additive within the polymeric matrix. Dynamic time sweep and dynamic strain sweep tests were carried out in every temperature to verify the thermal stability and the linear viscoelastic state of the material and finally a dynamic frequency sweep was performed to probe its dynamic response. The effect of the nanoparticle size and the hybrids’ composition on the behavior is examined seeking to correlate the morphological changes to the rheological response of the materials in an attempt towards the better understanding of the structure-properties relation. Specifically, for all series of materials, the addition of nanoparticles at a low weight percentage of the overall composition, regardless of their radius and the molecular weight of the respective polymer, yielded a notable decrease of the storage modulus, loss modulus, and complex viscosity values compared to the respective values of the matrix, while, by increasing the weight percentage, the properties of the materials approach a polymer-like behavior. In even higher percentages, different for each series of materials, solidlike behavior was observed in all four series, possibly due to the formation of an internal framework leading to the percolation of the nanoparticles thus amplifying the mechanical properties. The viscosity of the nanoparticle compositions with lower weight percentages (PEO100/NP67 4%, PEO300/NP67 2% and PEO300/NP67 2%) was calculated to be more than ten times lower than the viscosity of the matrix, which, to our knowledge, is the largest decrease yet reported. Furthermore, the shift factors (aT) of all materials that 9 did not present solid-like behavior, showed a similar temperature dependency following the Arrhenius model. Finally, regarding the attempt to correlate the morphological changes of the nanocomposite materials with their rheological response, it is worth noting that no clear relation could be confirmed between the observation of solid-like behavior and the decrease of the degree of crystallinity in these materials. The two factors that clearly showed to affect the rheological response was the interparticle distance of the nanoparticles, with the formation, or absence, of an internal framework leading to the increase or decrease of the relaxation time respectively, and also, the volume of the polymer adsorbed on the inorganic surface of the nanoparticles, which is expressed in an inversely proportional manner by the E/VPEO ratio, according to which, as the volume increases, so thus the tendency of the material to show liquid-like behavior, similar to the behavior of the matrix.
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