Optical and thermomechanical investigations on thermoplastic nanocomposites with surface modified silica nanoparticles

Dynamic mechanical thermal analysis (DMTA) and UV/VIS spectroscopy were applied to investigate the thermomechanical and optical properties of thermoplastic nanocomposites. The thermoplastic matrix material used was a copolymer derived from methylmethacrylate (MMA) and 2-hydroxyethylmethacrylate (HEMA). To improve the mechanical properties, especially in the high temperature region above the glass transition temperature (T-g) of the matrix, the copolymer was filled with spherical 10 nn silica particles (filler content 2, 5, and 10 vol.% respectively). The particles were introduced in the polymer matrix after appropriate surface coating to control the filler dispersion in the matrix and the filler/matrix adhesion. The coating was performed using acetoxypropyltrimethoxysilane (APTS) to achieve higher filler/matrix compatibility compared to unmodified silica particles dispersed in the polymer matrix. Methacryloxypropyltrimethoxysilane (MPTS) was used to improve filler/matrix adhesion by covalent bonding between the filler surface and polymer matrix. The appearance of the poly(MMA-co-HEMA) nanocomposites (denoted: PMH nanocomposites) changes from translucent for the systems containing uncoated silica to more transparent for the compositions containing silane coated silica. This is indicated by a decrease in scattering/absorbance losses from 1.48 dB/cm to 1.06 dB/cm at lambda = 650 nm. Investigations of the morphology of the same nanocomposites using transmission electron microscopy (TEM) showed that by coating the particles with silane an almost perfect dispersion of the fillers in the matrix can be realised. The more homogeneous dispersion of the silane coated particles in the polymer matrix compared to the uncoated silica is responsible for the increase in transparency of the systems. However, the composition dependence of the refractive index is in accordance with the expected behaviour and shows a decrease with increasing amounts of silica (0 % silica: n(e) = 1.5085, 10 % silica n(e) = 1.4965) whereas, the Abbe number remains almost constant at v(e) = 58 for all compositions. In addition, the fortyfold increase in the value for the storage modulus E' at T = 170 degrees C (derived from dynamic mechanical thermal analysis (DMTA)) for the system with 9.5 vol.% MPTS coated particles compared to the unfilled matrix indicates an increased thermomechanical stability of the nanocomposites.