Quan Wan

Light curable dental composites designed with colloidal crystal reinforcement

OBJECTIVES Methods to prepare dental composites with a periodic filler arrangement were developed following a strategy of colloidal crystallization. The aims of this study were to determine the influence of suspension medium, silane treatment and amine additive on colloidal particle redispersion and subsequent ordering, and to evaluate the effect of filler ordering on mechanical properties of composites. METHODS Dry monodisperse silica particles (spherical, approximately 500-nm diameter) were redispersed in selected solvents and monomers (e.g. triethyleneglycol dimethacrylate, TEGDMA) to form sediments or dispersions with ordered particle arrangements. Ordering was evaluated by microscopy and mechanical properties of the composites were measured using compression tests (n=6). RESULTS A face-centered cubic packed structure could form in both the sediment from silica dispersions in polar solvents and stable dispersions in TEGDMA. Dimethylaminoethyl methacrylate (DMAEMA) was found to disrupt an ordered structure when non-silanized silica particles were used. Silanization with 3-methacryloxypropyl trimethoxysilane (MPS) promoted filler ordering. Standard compression tests on composites containing 60wt% silica in TEGDMA with or without DMAEMA indicated that DMAEMA had a clearly significant effect (p<0.05) on failure strain, compressive strength, and toughness, and a marginally significant effect on modulus (p=0.12). SIGNIFICANCE Significant increases in compressive strength (16%), failure strain (71%), and toughness (135%) were observed for composites with ordered filler compared to non-ordered composites.

Understanding Stöber silica's pore characteristics measured by gas adsorption.

Controversial reports regarding Stöber silicas microporosity and specific surface area remain in the literature despite decades of widespread applications. In this work, Stöber silica samples prepared under controlled reaction time and postsynthesis washing/drying conditions were characterized by nitrogen adsorption at 77 K, transmission electron microscopy, elemental analysis, Fourier transform infrared spectroscopy, thermal analysis, and evolved gas analysis. Our experimental results demonstrated the important but often overlooked effects of reaction time and postsynthesis treatments on Stöber silicas pore characteristics, as evidenced by the strikingly large range of BET specific surface area (11.3-309.7 m(2)/g). A simple micropore filling and blocking mechanism compatible with an existing Stöber silica growth model incorporating both aggregation and monomer addition steps was proposed to explain all our experimental findings. The carbon and nitrogen contents appear to serve well as the indicative link between our experimental variables and the resulting pore blocking by TEOS and its derivatives. A suitable combination of experimental conditions is recommended in order to make microporous Stöber silica samples with large specific surface area, including a short reaction time, water washing, and drying at moderate temperature preferably under vacuum.

Unraveling the Mystery of Stöber Silica’s Microporosity

Puzzling aspects of the microporous structure of Stöber silica, including inconsistencies in the BET specific surface area and the long measurement time required for N2 adsorption, hinder further research on and potential applications of this material. In this work, Stöber silica samples prepared using systematic and detailed post-treatment methods were characterized by N2 adsorption, scanning electron microscopy, transmission electron microscopy, inductively coupled plasma optical emission spectrometry, elemental analysis, and Fourier transform infrared spectroscopy. We have found that the often overlooked sample preparation conditions may be the main causes that perplex the gas adsorption characterization results of Stöber silica samples. The pore-blocking processes associated with a variety of sample treatment methods are discussed in detail. Strong evidence for the particle growth model and pore-blocking mechanism involving ethoxyl groups, Si species, and condensation of silanols is provided. A remarkable result is that the measurement time is shortened from 1 month in our previous work to 2-3 days for samples with large specific surface areas. A suitable post-treatment condition is recommended to obtain microporous Stöber silica with a short measurement time, including water washing, low temperature drying without a vacuum, and a short storage time.