Peter Mueller

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 (Tg) of the matrix, the copolymer was filled with spherical 10 nm 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) equals 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 realized. 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 behavior and shows a decrease with increasing amounts of silica (0% silica: ne equals 1.5085, 10% silica ne equals 1.4965) whereas, the Abbe number remains almost constant at ve equals 58 for all compositions. In addition, the fortyfold increase in the value for the storage modulus E at T equals 170 degrees Celsius [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.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Tantalum oxide nanomers for optical applications

The synthesis of transparent nanomers by the incorporation of nanoscaled tantalum oxide into an organic-inorganic composite matrix and their subsequent characterization are presented. The matrix materials used consist of a mixture of organically functionalized silanes and polymerizable monomers. The mixture does not exhibit phase separation, even down to the lowest nanometer scale, as revealed by SAXS measurements. The addition of nanoscaled Ta2O5 particles (mean particle diameter: 4 nm as determined by photon correlation spectroscopy) aims to increase the refractive index of the nanomers. The preparation of the oxide sol and the optimization of the synthesis with respect to compatibility with the matrix material, thereby avoiding agglomeration effects, is described. After incorporation of the particles in the monomer mixture, a photopolymerization step, followed by curing with a temperature program up to 90 degrees Celsius, led to colorless and transparent monoliths. The volume shrinkage, caused by polymerization, decreases from 8.2% for the unfilled matrix material to 5.8% for a nanomer containing 30 wt.% tantalum oxide. The shrinkage decreases linearly with increasing filler content of tantalum oxide. The increase in refractive index is about 7.4 X 10-4 per wt.% oxide (measured at a wavelength of 546.1 nm). The coloration of the monoliths is expressed as yellowness index G according to DIN 6167. Color values attained for nanometers with up to 15 wt.% tantalum oxide are comparable to values for commercial optical polymer materials. Nanomers containing 15 wt.% tantalum oxide show transparency losses at a wavelength of 850 nm below 0.1 dB/cm.

Fabrication of monolithic refractive optical lenses with organic-inorganic nanocomposites: relations between composition and mechanical and optical properties

Sol-gel derived organic-inorganic hybrid materials and their potential for the production of refractive optical elements are presented. The main components of the investigated compositions are precondensed silanes with polymerizable double bonds [e.g. methacryloxypropyltrimethoxysilane (MPTS)] and co-condensates thereof. Dimethacrylates like tetraethyleneglycoldimethacrylate (TEGDMA) were employed as organic monomers. Molar ratios of silanes to organic monomers between 10:90 and 90:10 were investigated. Nanoscaled titania was incorporated in the homogeneous mixture of silanes and organic monomers. The combination of different molecular hybrid matrices and inorganic nanoparticles allows the adjustment of material properties, for example: impact strength between 2 and 15 kJ/m2, Youngs moduli between 0.8 and 3.7 GPa and universal hardness in the range from 40 to 170 N/mm2. Phase separation could be kept in the nanometer range to minimize optical losses due to scattering effects. Depending on composition, ne could be varied between 1.50 and 1.54, whereby the corresponding Abbe numbers ranged from 57 to 45. Ophthalmic lenses were prepared in less than 10 hours by simple mould techniques and by applying a combination of photochemical and thermal curing processes.