Polymeric nanocomposites: account for the effect of size distribution of nanoparticles

The essence of the work lies in the theoretical analysis of the influence of size distribution of nanoparticles in polymer nanocomposites on the glass transition temperature Tg and coefficient of thermal expansion CTE. Both of these characteristics are important for building materials containing polymers. If the values of these characteristics exceed the allowable value, the material will soften and you should not alter its size.

Methods: used the Poisson distribution applied to the radius of the nanoparticles. The analysis is performed at the expected mean values of 5 and 10 nm. As object of research used in the cured epoxy resin filled with nanoparticles of SiO2. The surface modified nanoparticles grafted polar groups possessing a dipole-dipole interaction and hydrogen bonds. The analysis is based on previously obtained relationships connecting the Tg and CTE with a set of atomic physical constants. This set depends on the chemical structure of the repeating unit of the polymer or molecular fragment of a polymer network. Among these constants, Van-der-Waals volume and the energy of the dispersion interaction of each atom, as well as energy dipole-dipole interaction or hydrogen bonds to polar groups.

The results of the study: they consist in the fact that the dependencies of Tg and CTE on the expected mean radius of nanoparticles are obtained. Considered part of the epoxy resin will be cured with usual methylhydrophthalic anhydride and another part of the epoxy resin will be cured with anhydride-modifier. The formulas for quantitative assessment of the values of Tg and CTE for such copolymer structures are obtained. The greatest increase in Tg of 430 to 465 K was observed when reducing the radius of the nanoparticles from 8 to 3 nm at the mean expected value of 5 nm. This is true when chemical interaction between the epoxy resin and anhydride-modifier takes place. The magnitude of the CTE increases from 1.95×10-4 up to 2.05×10-4 by increasing mean radius of nanoparticles from 3 to 7 nm.

Conclusions: The expression for the theoretical estimation of Tg value of nanocomposites was modified in order to account for the polydispersity of nanoparticles. The theoretical estimation shows that the influence of the size distribution of the silica nanoparticles on the both of Tg and CTE values of the epoxy/silica nanocomposite can be quite considerable. The most significant effect will be achieved at the conditions, when nanoparticles play the role of multifunctional curing agent. (paper)