Confinement Effects of Silica Nanoparticles with Radii Smaller and Larger than Rg of Adsorbed Poly(ethylene oxide)

The properties of polymers on nanoparticles are different than those of the bulk, so that the interphase region becomes more important as the surface/volume ratio of the nanoparticles increases. Knowledge of the modified polymer properties is relevant both in nanocomposite applications and for nanoparticles in suspension. Polymers have chain dimensions (characterized by the radius of gyration, Rg) similar to the nanoparticles (Rnanoparticle) themselves, so that chain conformation, mobility and crystallinity can be affected by Rg/Rnanoparticle. Here, both the glass transition temperature (Tg) and degree of crystallinity (Xc) of poly(ethylene oxide) (PEO) on individual SiO2 nanoparticles of nominal 15, 50, and 100 nm diameter (2RSiO2), in which Rg(PEO) was greater, equal to, or less than RSiO2 was investigated. Plateau adsorption of PEO on SiO2 nanoparticles (PEO−SiO2) increased in the order PEO−SiO2 (100 nm) > PEO−SiO2 (50 nm) > PEO−SiO2 (15 nm). At plateau adsorption, after melting and solidification, the samples were completely amorphous. The Tg of the adsorbed PEO increased in the order PEO−SiO2 (100 nm) > PEO−SiO2 (50 nm) > PEO−SiO2 (15 nm); since the Tgs were above 25 °C in all cases, the PEO behaved more like a brittle solid than an elastomer. For comparable amounts of PEO that were adsorbed from solution but not melted, the melt endotherm increased in the order PEO−SiO2 (15 nm) > PEO−SiO2 (50 nm) > PEO−SiO2 (100 nm). These trends were interpreted as due to an increase in loop/tail lengths and thus flexibility of the PEO chains, with a concomitant ability to crystallize, as Rg (PEO)/RSiO2 nanoparticles increased and which was the result of less hydrogen bond formation between the oxygens of PEO and the silanols (SiOH) of the SiO2 as the nanoparticle size decreased. This in turn was attributed to the energetically unfavorable conformations necessary for the PEO chains to adopt in order to hydrogen bond with silanols on the smaller nanoparticles.