Both bulk and mesoporous silica nanoparticles can be obtained in the form of granular aggregates using chitosan flakes as additive under very soft biomimetic reaction conditions.Biomineralization, that is the formation under extremely soft conditions of complex hierarchical inorganic materials by biological systems, is an exciting research field of increasing interest. 1 Indeed, while technological silica production usually requires vigorous conditions at high pH, 2 certain marine organisms and some higher plants are capable of forming silica skeletons under mild temperature and pressure conditions at circumneutral pH. 1,3,4 Such in vivo processes generating intricate silica nano and macro-scale patterns are speciesspecific, presumably encoded in the genome; 5-7 the possibility of controlling them in the laboratory remains a long-term major challenge. Notwithstanding, there have been different biomolecules identified like silicatein, silaffin and polyamines, which can play key roles in the hydrolysis, condensation and aggregation of the silica species. 3,4,8 Silica is widely used in industry, medicine and nanotechnology in a huge variety of forms. 2,7,9 This, coupled with the insufficiency of natural products of adequate purity and the scientific interest, justifies the search for bioinspired synthesis 8,10 leading to new silica-based materials with simple hierarchical structures or, at least, allowing silica production under soft-conditions (low-cost strategies). Then, inspired by nature, a variety of reagents have been added to check in vitro silicification processes in solution. 11,12 These additives, which intend to mimic the active natural molecules, can play different roles: catalysts, aggregation promoting reagents or structural directing agents. Otherwise, silicatein, silaffin and polyamines have been identified as constituents of the axial filaments in sponges and the cell walls in diatoms. 3,8,9 Although this fact strongly suggests that they process silica in the heterogeneous phase, as far as we know, there is no information about in vitro processes carried out under this conditioning (i.e. on the role of solid phase additives).Here we show how using solid flakes of chitosan (a commercial and low-cost biopolymer present in the shells of crustaceans and the cell walls of fungi and yeast, as well as in squid pens) 13 as additive, it is possible to promote silica polymerization and aggregation in the heterogeneous phase at room temperature, neutral pH and silica concentrations as low as those occurring in sea water or inside the diatom frustules. Our procedural approach leads to bulk biosilica nanoparticles or, alternatively, to similar mesoporous architectures when structural directing agents (SDA) are also added.In order to mimic frustule conditions, 6,14 we have performed the reactions in circulating silica solutions on chitosan flakes blocked in a part of the system in order to favour a certain confining effect (see ESIw). Summarized in Table 1 are the main parameters concerning syntheses of some sel...
Composites of silica nanospheres coated with crosslinked epoxy–amine were synthesised and examined by 29Si-magic-angle-spinning nuclear magnetic resonance spectroscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy and scanning electron microscopy. The most representative fact is that epoxy-modified nanospheres lost less weight at high temperatures. At temperatures greater than 300°C the loss of weight for epoxy-modified nanospheres was rather lower than for unmodified nanospheres. This helped them to retain their structures, as the loss of weight can have adverse effects on network defects, due to the loss of crosslinks by unit of volume.
The mechanical properties of epoxy-silica nanocomposites have been studied; the silica nanosphere fillers used were un-functionalised, functionalised with amine, with epoxy, or a mixture of both kinds. Dynamic mechanical analysis measurements revealed an increase in the shear storage modulus, for all samples with a filler content of 3–5%. Improvements were observed in the glassy and rubbery states, without affecting the glass transition temperature of the materials. Above these strengthening percentages, the mechanical properties began to deteriorate, but in all cases they remained superior to those of the pristine epoxy resin. For low strengthening percentages, samples reinforced with both nanospheres functionalised with amine and with epoxy showed better mechanical behaviour. As the strengthening percentage increased, materials reinforced with silica nanoballs functionalised with epoxy groups alone showed higher mechanical strength than the rest. To improve the mechanical properties of these systems, it was important to optimise both the percentage of added filler and the type of reinforcement. The parameter determining the flow stress was the cohesion of the solid state, which was represented by the storage modulus in shear.
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