RESUMEN
The investigation of molecular interactions between a silica surface and organic/inorganic polymers is crucial for deeper understanding of the dominant mechanisms of surface functionalization. In this work, attachment of various depolymerized polydimethylsiloxanes (PDMS) of different chain lengths, affected by dimethyl carbonate (DMC), to silica nanoparticles pretreated at different temperatures has been studied using 29Si, 1H, and 13C solid-state NMR spectroscopy. The results show that grafting of different modifier blends onto a preheated silica surface depends strongly on the specific surface area (SSA) linked to the silica nanoparticle size distributions affecting all textural characteristics. The pretreatment at 400 °C results in a greater degree of the modification of (i) A-150 (SSA = 150 m2/g) by PDMS-10/DMC and PDMS-1000/DMC blends; (ii) A-200 by PDMS-10/DMC and PDMS-100/DMC blends; and (iii) A-300 by PDMS-100/DMC and PDMS-1000/DMC blends. The spectral features observed using solid-state NMR spectroscopy suggest that the main surface products of the reactions of various depolymerized PDMS with pretreated nanosilica particles are the (CH3)3SiO-[(CH3)2SiO-]x fragments. The reactions occur with the siloxane bond breakage by DMC and replacing surface hydroxyls. Changes in the chemical shifts and line widths, as shown by solid-state NMR, provide novel information on the whole structure of functionalized nanosilica particles. This study highlights the major role of solid-state NMR spectroscopy for comprehensive characterization of functionalized solid surfaces.
RESUMEN
Three poly(organosiloxanes) (hydromethyl-, dimethyl-, and epoxymethylsiloxane) of different chain lengths and pendant groups and their mixtures of dimethyl (DMC) or diethyl carbonates (DEC) were applied in the modification of fumed silica nanoparticles (FSNs). The resulting modified silicas were studied in depth using 29Si, 1H, and 13C solid-state NMR spectroscopy, elemental analysis, and nitrogen adsorption-desorption (BET) analysis. The obtained results reveal that the type of grafting, grafting density, and structure of the grafted species at the silica surface depend strongly on the length of organosiloxane polymer and on the nature of the "green" additive, DMC or DEC. The spectral changes observed by solid-state NMR spectroscopy suggest that the major products of the reaction of various organosiloxanes and their DMC or DEC mixtures with the surface are D (RR'Si(O0.5)2) and T (RSi(O0.5)3) organosiloxane units. It was found that shorter methylhydro (PMHS) and dimethylsiloxane (PDMS) and their mixtures with DMC or DEC form a denser coverage at the silica surface since SBET diminution is larger and grafting density is higher than the longest epoxymethylsiloxane (CPDMS) used for FSNs modification. Additionally, for FSNs modified with short organosiloxane PMHS/DEC and also medium organosiloxane PDMS/DMC, the dense coverage formation is accompanied by a greater reduction of isolated silanols, as shown by solid-state 29Si NMR spectroscopy, in contrast to reactions with neat organosiloxanes. The surface coverage at FSNs with the longest siloxane (CPDMS) greatly improves with the addition of DMC or DEC. The data on grafting density suggest that molecules in the attached layers of FSNs modified with short PMHS and its mixture of DMC or DEC and medium PDMS and its mixture of DMC form a "vertical" orientation of the grafted methylhydrosiloxane and dimethylsiloxane chains, in contrast to the reaction with PDMS/DEC and epoxide methylsiloxane in the presence of DMC or DEC, which indicates a "horizontal" chain orientation of the grafted methyl and epoxysiloxane molecules. This study highlights the major role of solid-state NMR spectroscopy for comprehensive characterization of solid surfaces.
RESUMEN
Various nanostructured amorphous silicas [fumed silicas such as crude (A-300), hydro-compacted (cA-300, TS 100), and precipitated silica Syloid 244] were modified by different polydimethylsiloxanes such as PDMS5, PDMS100, PDMS200, PDMS1000, and PDMS12500 (the label numbers show the viscosity (η) values) using dimethyl carbonate (DMC) as a siloxane-bond-breaking reagent. In addition, hexamethyldisilazane was used to modify fumed silica cA-300. The nanocomposites were characterized using microscopy, infrared spectroscopy, thermodesorption, nitrogen adsorption-desorption, solid-state NMR spectroscopy, small-angle X-ray scattering, and zeta-potential methods. It was found that the morphological, textural, and structural characteristics of silicas grafted with PDMS depend strongly not only on the type and content of the polymers used but also on the organization of nonporous nanoparticles (NPNP) in secondary structures (aggregates of NPNP and agglomerated aggregates, ANPNP), as well on the reaction temperature (T r). Specifically, we determined that ANPNP with a macro/mesoporous character are favorable for the effective modification of the silicas studied with short polymers and no DMC addition but at higher temperatures or for a longer silicone polymer with the presence of DMC and at lower temperatures. In particular, the PDMS/DMC-modified silicas are of great interest from a practical point of view because they remain in a dispersed state with no strong compaction of the secondary structures after modification, and this corresponds to a better distribution of the modified nanoparticles in polymeric or other matrices.
RESUMEN
New nanocomposites have been prepared by combining tin selenide (SnSe) with graphene oxide (GO) in a simple aqueous solution process followed by ice templating (freeze casting). The resulting integration of SnSe within the GO matrix leads to modifications of electrical transport properties and the possibility of influencing the power factor (S 2σ). Moreover, these transport properties can then be further improved (S, σ increased) by functionalization of the GO surface to form modified nanocomposites (SnSe/GOmod) with enhanced power factors in comparison to unmodified nanocomposites (SnSe/GO) and "bare" SnSe itself. Functionalizing the GO by reaction with octadecyltrimethoxysilane (C21H46O3Si) and triethylamine ((CH3CH2)3N) switches SnSe from p-type to n-type conductivity with an appreciable Seebeck coefficient and high electrical conductivity (1257 S·m-1 at 539 K), yielding a 20-fold increase in the power factor compared to SnSe itself, prepared by the same route. These findings present new possibilities to design inexpensive and porous nanocomposites based on metal chalcogenides and functionalized carbon-derived matrices.