| Abstract |
Polymer morphology and tuning of chain crystallinity are investigated in nanohybrids composed of hydrophilic poly (ethylene oxide) and silica nanoparticles of three different sizes, pursuing to understand the effect of the presence of the inorganic surfaces and of the severe confinement. Nanoparticles with size smaller, comparable and larger than that of the polymer coil are used in order to confine the polymer chains. Crystallinity was found to be affected by the degree of spatial confinement that the nanoparticles impose, their surface area as well as their curvature. The polymer chains that were able to crystallize under confinement showed a different crystalline behavior with a lower Tm and lower crystallinity. Small nanoparticles affect the crystallinity due to their high surface area which leads to large confinement while the large nanoparticles have the ability to adsorb the polymer chains on their surface, disrupting the crystallization process. For further understanding the crystallization process mixtures of two kinds of silica nanoparticles of different sizes were mixed with the polymer, in a three-component system, in an attempt to investigate the case of severely confined polymers. Under severe confinement, in the three component system, the polymer was not able to crystallize at all, resembling the case of intercalated PEO/Na+MMT. By varying the ratio of small vs large nanoparticles, in hybrids of constant polymer volume fraction, the tuning of crystallinity was achieved. The kinetics of crystallization, the chain conformations and the polymer dynamics in PEO/SiO2 nanohybrids were investigated as well, as a function of the inorganic surfaces and of the spatial confinement. The chain conformations in the hybrids showed an increase in the gauche population of the C-C bond along the polymer backbone, which increases with the increase of the nanoparticle concentration, indicating that the system becomes more disordered and liquid-like. The critical parameter was the confining length, expressed as the ratio of the interparticle distance over the chain radius of gyration, d/Rg and a d/Rg< 0.1-0.5 was found necessary to observe the effect. On the other hand, both local and segmental dynamics of PEO chains, close to the silica surfaces seems to remain mostly unaffected and does not exhibit significant differences compared to the respective of the pure polymer. The crystallization kinetics, reveals that silica nanoparticles act as nucleating agents promoting the nucleation process, with the large nanoparticles accelerating the primary crystallization process more than the small. At the same time, POM measurements show that the addition of nanoparticles affects the number and size of spherulites. A slightly retarded spherulitic growth rate, compared to that of the bulk polymer, was found with increasing the amount of inorganic surfaces.
|
Search
