Search

"Hairy" nanoparticles (HNPs), i.e. inorganic NPs functionalized with polymer chains, are promising building blocks for the synthesis of advanced nanocomposite (NC) materials having several technological applications. Recent evidence shows that HNPs self-organize in a variety of anisotropic structures, resulting in an improvement of the functional properties of the materials, in which are embedded. In this paper, we propose a three-step colloidal synthesis of spherical SiO2-HNPs, with controlled particle morphology and surface chemistry. In detail, the SiO2 core, synthesized by a modified Stöber method, was first functionalized with a short-chain amino-silane, which acts as an anchor, and then covered by maleated polybutadiene (PB), a rubbery polymer having low glass transition temperature, rarely considered until now. An extensive investigation by a multi-technique analysis demonstrates that the synthesis of SiO2-HNPs is simple, scalable, and potentially applicable to different kind of NPs and polymers. Morphological analysis shows the overall distribution of SiO2-HNPs with a certain degree of spatial organization, suggesting that the polymer coating induces a modification of NP-NP interactions. The role of the surface PB brushes in influencing the special arrangement of SiO2-HNPs was observed also in cis-1,4-polybutadiene (cis-PB), since the resulting NC exhibited the particle packing in "string-like" superstructures. This confirms the tendency of SiO2-HNPs to self-assemble and create alternative structures in polymer NCs, which may impart them peculiar functional properties.

Composite materials based on hybrid filler systems can be a promising approach to produce advanced rubber nanocomposites (NCs). Recently, the positive effect of the combined use of silica and sepiolite on NCs mechanical properties has been reported, compared to those of compounds reinforced with the same amount of the only silica filler. In this context, the present work aims at studying the possible synergistic self-assembly of nanosilica and sepiolite in the generation of a cooperative hybrid filler network in rubber-based NCs, in connection with material performance for tires application. In detail, the influence of the introduction of a secondary anisotropic filler in conjunction with isotropic nanosilica on their dispersion and interaction with the rubber matrix has been comprehensively investigated, in order to define the best formulation ensuring low rolling resistance and significant fuel saving. NCs, containing simultaneously silica and sepiolite, either pristine (Sep) or chemically modified (mSep) by an acid treatment, were prepared by combining latex compounding technique (LCT) and melt blending. Rheological and dynamic-mechanical analyses highlighted that the use of a double white filler, constituted by particles with different aspect ratio, affords a good balance between efficient reinforcement and low Payne effect. Tansmission electron microscope (TEM) analysis evidenced the formation, within the rubber matrix, of cooperative superstructures due to the self-assembly of sepiolite and silica nanoparticles containing occluded rubber, when the secondary filler is mSep. The peculiar characteristics of mSep, characterized by fibers shorter than Sep and higher surface silanol bonding sites, bring about significant interactions between mSep and silica, which promote self-assembly of the two fillers in a collaborative hybrid network, improving dynamic-mechanical performances. These results, even if related to sepiolite/silica NCs, demonstrate the effective role of the collaborative filler network, based on the hybrid double fillers, able to lend enhanced properties to rubber materials.

Sepiolite (Sep)&ndash, styrene butadiene rubber (SBR) nanocomposites were prepared by using nano-sized sepiolite (NS-SepS9) fibers, obtained by applying a controlled surface acid treatment, also in the presence of a silane coupling agent (NS-SilSepS9). Sep/SBR nanocomposites were used as a model to study the influence of the modified sepiolite filler on the formation of immobilized rubber at the clay-rubber interface and the role of a self-assembled nanostructure in tuning the mechanical properties. A detailed investigation at the macro and nanoscale of such self-assembled structures was performed in terms of the organization and networking of Sep fibers in the rubber matrix, the nature of both the filler&ndash, filler and filler&ndash, rubber interactions, and the impact of these features on the reduced dissipative phenomena. An integrated multi-technique approach, based on dynamic measurements, nuclear magnetic resonance analysis, and morphological investigation, assessed that the macroscopic mechanical properties of clay nanocomposites can be remarkably enhanced by self-assembled filler structures, whose formation can be favored by manipulating the chemistry at the hybrid interfaces between the clay particles and the polymers.

Global industry is showing a great interest in the field of sustainability owing to the increased attention for ecological safety and utilization of renewable materials. For the scientific community, the challenge lies in the identification of greener synthetic approaches for reducing the environmental impact. In this context, we propose the preparation of novel biocomposites consisting of natural rubber latex (NRL) and sepiolite (Sep) fibers through the latex compounding technique (LCT), an ecofriendly approach where the filler is directly mixed with a stable elastomer colloid. This strategy favors a homogeneous dispersion of hydrophilic Sep fibers in the rubber matrix, allowing the production of high-loaded sepiolite/natural rubber (Sep/NR) without the use of surfactants. The main physicochemical parameters which control Sep aggregation processes in the aqueous medium were comprehensively investigated and a flocculation mechanism was proposed. The uniform Sep distribution in the rubber matrix, characteristic of the proposed LCT, and the percolative filler network improved the mechanical performances of Sep/NR biocomposites in comparison to those of analogous materials prepared by conventional melt-mixing. These outcomes indicate the suitability of the adopted sustainable procedure for the production of high-loaded clay&ndash, rubber nanocomposites with remarkable mechanical features.

The development of advanced polymer nanocomposites requires a strong filler-polymer interfacial interaction and an optimal filler nanodispersion. The incorporation of the clays into a polymer matrix frequently does not improve the composite mechanical properties, owing to both poor dispersion and macroscopic particle dimensions. In this work, pristine and organically-modified sepiolites (Sep) were structurally modified by an acid treatment, which provides nano-sized sepiolite (NS-Sep) fibers with reduced particle size and increased silanol groups on the surface layer. NS-Sep fibers were used to prepare styrene-butadiene rubber nanocomposites with enhanced mechanical properties. Dynamic-mechanical analysis of clay polymer nanocomposites demonstrated that the NS-Sep fibers provided an excellent balance between reinforcing and hysteretic behavior, compared to the large-sized pristine Sep and isotropic silica. This was related to the enhanced interfacial chemical interaction between NS-Sep and rubber, as well as to the size and self-assembly of anisotropic nanofibers to form filler network structures, as supported by transmission electron microscopy analysis. The preparation of nanocomposites, based on Sep nanofibers obtained by a simple and versatile acid treatment, can thus be considered an alternative approach for the designing of advanced clay polymer nanocomposites.

A novel hybrid nanofiller, SiO2@POSS, where the silica nanoparticles (NPs) and the POSS belong to the same functional structure, has been synthesized by grafting different loadings of OctaMethacrylPOSS onto silanized commercial SiO2, using a surface reaction mediated by dicumylperoxide (DCP). The peroxide, besides anchoring the nanocages onto the silica surface, ensures the presence of methacryl functionalities in the final structure, which are still available for cross-linking reactions with a polymer host. The hybrid SiO2@POSS NPs were used to prepare, by ex situ blending, SBR nanocomposites. The dynamic-mechanical analysis performed on the cured SBR/SiO2@ POSS composites indicated that the presence of POSS induces a remarkable increase of modulus either at low or at high strain, and a considerable decrease of hysteresis. This has been associated with the peculiar hybrid structure of the SiO2@POSS filler, in which silica NP aggregates are partially interconnected and surrounded by a thin shell of POSS nanounits which, thanks to their high number of reactive functionalities, promote the formation of "sticky regions'' among the silica aggregates and, consequently, a tight filler network wherein rubber is immobilized. This grants a relevant reinforcement and increased hysteretic properties, suggesting SiO2@POSS as a promising filler system for decreasing the energy loss under strain and for leading to a potential reduction of filler utilization in rubber composite formulations.

Rubber compounds enhance their mechanical properties when a rigid solid particulate (filler) is finely dispersed in the rubber structure, particularly in case of nanostructured silica. In fact, most of nanosilica properties that impact on rubber compounds have been investigated, in particular the size of the nanoparticle, its state of aggregation and surface chemistry. An extensive effort was paid to understand how nanostructured silica exert a reinforcing action on the resulting silica/rubber nanocomposite material. The intrinsic difficulty in obtaining silica nanoparticles with well defined shape has limited the discussion to nanoparticles different from silica, for example carbon nanotubes, nanoclays, thermoplastic polymeric fibres, cellulose nanowhiskers, ecc. It is therefore necessary to create a model system in which silica nanoparticle shape can be tuned adequately and its influence on the reinforcing mechanism can be worked out. In this context, the aim of the thesis is to investigate the impact of the anisotropic silica particles on the dynamic-mechanical behavior of model SBR nanocomposites, considering first the networking of differently shaped nanoparticles, and second the formation of the nanoscale rigid rubber at the filler interface. Silica nanoparticles featuring different shapes were obtained by sol-gel method, using a structuring agent based on an acqueous micellar solution of surfactant CTAB, that allowed to control the growth of the particles giving them different anisotropies, including spherical and rod-like particles with aspect ratio ranging between 2 and 7. Nanocomposites of silica nanoparticles and styrene butadiene rubber (SBR) were prepared by compounding methods, in order to evaluate the effect of shape on the mechanical properties of the material. The nanocomposites morphology and its effect on the dynamic-mechanical behavior were studied by a multi-technique approach. The combined use of TEM and AFM tapping mode analyses revealed that in all the nanocomposites spherical and anisotropic NPs are surrounded by a layer of about 15 nm of immobilized rubber. Spherical or nearly spherical particles show small contact zones sharing thin rubber layers. Anisotropic particles show instead oriented domains of rods preferentially aligned along the main axis separated by several layers of immobilized rubber. The formation of these self assembled domains causes an increase of rubber fraction trapped between the aligned particles. Low field 1H NMR measurements evidences that polymer chain dynamics of the rubber layers tightly bound to the silica particles are restricted by the interaction with the filler, increasing the stiffness with respect to that of polymer far from the particles. Dynamic- Mechanical measurements confirm that anisotropic particles with higher aspect ratio give a high reinforcement, as a result of spontaneous alignment of anisotropic particles that implies higher amounts of immobilized rubber, when compared to the system containing spherical particles. A similar approach was applied to anisotropic nanoparticles based on Sepiolite, a naturally occurring nanostructured silicate. Nanorods were prepared by a surface modification methodology and successfully compared with the model system proposed before showing improved reinforcement, particularly related to energy dissipation, that makes them potentially interesting for an industrial application. To conclude, the reinforcement mechanism passes through the immobilization of rubber on the surface of the nanoparticles, and it was found that the self-assembly of particles in anisotropic domains is crucial in order to further immobilize rubber and ensure a high reinforcement. The study confirmed that the control of filler morphology is mandatory to adequately modulate the nanocomposite mechanical properties.

Silica-styrene butadiene rubber (SBR) nanocomposites were prepared by using shape-controlled spherical and rod-like silica nanoparticles (NPs) with different aspect ratios (AR = 1-5), obtained by a sol-gel route assisted by a structure directing agent. The nanocomposites were used as models to study the influence of the particle shape on the formation of nanoscale immobilized rubber at the silica-rubber interface and its effect on the dynamic-mechanical behavior. TEM and AFM tapping mode analyses of nanocomposites demonstrated that the silica particles are surrounded by a rubber layer immobilized at the particle surface. The spherical filler showed small contact zones between neighboring particles in contact with thin rubber layers, while anisotropic particles (AR > 2) formed domains of rods preferentially aligned along the main axis. A detailed analysis of the polymer chain mobility by different time domain nuclear magnetic resonance (TD-NMR) techniques evidenced a population of rigid rubber chains surrounding particles, whose amount increases with the particle anisotropy, even in the absence of significant differences in terms of chemical crosslinking. Dynamic measurements demonstrate that rod-like particles induce stronger reinforcement of rubber, increasing with the AR. This was related to the self-alignment of the anisotropic silica particles in domains able to immobilize rubber. This journal is

Silica–natural rubber nanocomposites were obtained through a novel non-aqueous in situ sol–gel synthesis, producing the amount of water necessary to induce the hydrolysis and condensation of a tetraethoxysilane precursor by esterification of formic acid with ethanol. The method allows the synthesis of low hydrophilic silica nanoparticles with ethoxy groups linked to the silica surface which enable the filler to be more dispersible in the hydrophobic rubber. Thus, high loaded silica composites (75 phr, parts per hundred rubber) were obtained without using any coupling agent. Transmission Electron Microscopy (TEM) showed that the silica nanoparticles are surrounded by rubber layers, which lower the direct interparticle contact in the filler–filler interaction. At the lowest silica loading (up to 30 phr) silica particles are isolated in rubber and only at a large amount of filler (>60 phr) the interparticle distances decrease and a continuous percolative network, connected by thin polymer films, forms throughout the matrix. The dynamic-mechanical properties confirm that the strong reinforcement of the rubber composites is related to the network formation at high loading. Both the improvement of the particle dispersion and the enhancement of the silica loading are peculiar to the non-aqueous synthesis approach, making the method potentially interesting for the production of high-loaded silica–polymer nanocomposites.

Silica/styrene butadiene rubber (SBR) nanocomposites were prepared by blending method using shape-controlled spherical and rod-like nanoparticles with different aspect ratios as filler for the rubber reinforcement. The differently shaped silica particles were synthesized by sol-gel method using tetraethoxysilane (TEOS) and (3-mercaptopropyl) trimethoxysilane (MPTSM) as silica precursors, and cetyltrimethylammonium bromide (CTAB) as structure directing agent. This strategy allowed to study the influence of the particle morphology on the reinforcing effect independently of the silica surface chemistry and considering the aspect ratio as the only geometrical variance. Spherical and anisotropic rod-like particles, dispersed in the nanocomposites, formed a network of particles bridged by thin rubber layers throughout the SBR matrix. Moreover, differently oriented domains of aligned rods are observed when the aspect ratio of particles increases and is >=2. Dynamic-mechanical properties demonstrated that the rod-like particles with the higher aspect ratio provided stronger reinforcement of the rubber. This was related to the self-alignment of the anisotropic particles and to the consequent larger filler/polymer interface, compared to that of spherical ones. © 2014 Elsevier Ltd. All rights reserved.

Silica-rubber nanocomposites were obtained by in situ sol-gel synthesis, using trialkoxysilanes with different functional groups as precursors. The functionalities were selected in order to favor the formation of differently shaped silica particles and/or to modulate the filler-filler and the filler-rubber interactions. The functional groups included (a) alkyl and alkenyl groups: triethoxy(vinyl) (VTEOS), triethoxy(propyl) (PTEOS), triethoxy (octyl) (OCTEOS); (b) N-containing alkyl groups: triethoxy(3-aminopropyl) (APTEOS), triethoxy(3- cyanopropyl) (CPTEOS), triethoxy(3-propylisocyanate) (ICPTEOS); (c) S-containing alkyl groups: trimethoxy(3-mercaptopropyl) (TMSPM), bis(3-triethoxysilylpropyl) disulfide (TESPD), bis(3-triethoxysilylpropyl) tetrasulfide (TESPT); triethoxy(3-octanoylthio-1-propyl) (NXT). Transmission electron microscopy (TEM) investigation suggested a relationship between the morphology of the filler network and the used trialkoxysilanes, as a function of the particle shape and of the interaction of the particle surface groups between them and with the matrix. The dynamic-mechanical properties of nanocomposites, both uncured and vulcanized, were discussed in relation to the network morphology, suggesting a connection between the used silica precursors and the functional properties. The filler-rubber interaction due to substituents which chemically interact with the polymer, promotes the homogeneous distribution of the silica particles in the matrix, while the filler-filler interaction, favored by the shape induced physical interactions or by the chemical interaction among surface groups, mainly contribute to the filler networking and to the dynamic-mechanical properties of the composites. © 2013 The Royal Society of Chemistry.