Key Role of the Dispersion of Carbon Nanotubes (CNTs) within Epoxy Networks on their Ability to Release

Carbon nanotube (CNT)-reinforced nanocomposites represent a unique opportunity in terms of designing advanced materials with mechanical reinforcement and improvements in the electrical and thermal conductivities. However, the toxic effects of these composites on human health have been studied, and very soon, some regulations on CNTs and on composites based on CNTs will be enacted. That is why the release of CNTs during the nanocomposite lifecycle must be controlled. As the releasing depends on the interfacial strength that is stronger between CNTs and polymers compared to CNTs in a CNT agglomerate, two dispersion states—one poorly dispersed versus another well dispersed—are generated and finely described. So, the main aim of this study is to check if the CNT dispersion state has an influence on the CNT releasing potential in the nanocomposite. To well tailor and characterize the CNT dispersion state in the polymer matrix, electronic microscopies (SEM and TEM) and also rheological analysis are carried out to identify whether CNTs are isolated, in bundles, or in agglomerates. When the dispersion state is known and controlled, its influence on the polymerization kinetic and on mechanical properties is discussed. It appears clearly that in the case of a good dispersion state, strong interfaces are generated, linking the isolated nanotubes with the polymer, whereas the CNT cohesion in an agglomerate seems much more weak, and it does not provide any improvement to the polymer matrix. Raman spectroscopy is relevant to analyze the interfacial properties and allows the relationship with the releasing ability of nanocomposites; i.e., CNTs poorly dispersed in the matrix are more readily released when compared to well-dispersed nanocomposites. The tribological tests confirm from released particles granulometry and observations that a CNT dispersion state sufficiently achieved in the nanocomposite avoids single CNT releasing under those solicitations.

Rheological Properties of Alkali-Activated Brick Powder Based Pastes: Effect of Alkali Activator and Silicate Modulus

The rheological behaviour of alkali-activated materials prepared by activation of a brick powder by alkaline solution (alkali + water glass) is described. The influence of the composition of activation solution (NaOH vs. KOH, varied silicate modulus) on the flow properties (yield stress, consistency coefficient, fluidity index) and the evolution of the elastic modulus (G ́) and the viscous modulus (G ́ ́) over time were studied. The rheological characterization was completed by frequency sweep tests with the aim of investigating the material behaviour more in detail. The results show that the pastes are thixotropic suspensions with very low yield stress. The potassium activator decreases the yield stress and viscosity of the pastes and retards the polymerization kinetic. The brick pastes become more rigid and more viscous with increasing silicate modulus. This also leads to an acceleration of gel formation in brick pastes.

Comparative Effect of Self- or Dual-Curing on Polymerization Kinetics and Mechanical Properties in a Novel, Dental-Resin-Based Composite with Alkaline Filler

Dental bulk-fill restorations with resin-composites (RBC) are increasing in popularity, but doubts concerning insufficient curing in depth still disconcert clinicians. An alternative might be offered by modern dual-cured RBCs, which additionally provide bioactive properties. This study assessed the impact of additional light-curing on polymerization kinetics, the degree of conversion (DC) and mechanical properties of a novel, dual-cured RBC with alkaline fillers. Since the bioactivity of a material often implies a release of compounds, the mechanical stability in simulated clinical environments was also evaluated. Polymerization kinetics and DC were assessed at 2- and 4-mm specimen depths in real-time up to one hour (n = 6). Incident and transmitted irradiance and radiant exposure were recorded at 2- and 4-mm depths. Micro-mechanical profiles (n = 6) were assessed in 100-µm steps along 6-mm deep specimens at 24 h post-polymerization. Flexural strength and modulus (n = 10) were determined up to three months of immersion in neutral (6.8) and acidic (4) pH conditions. DC variation in time was best described by a sigmoidal function (R2 > 0.98), revealing a retarded (3.4 ± 0.4 min) initiation in C=C double bond conversion in self-cured versus dual-cured specimens. The setting reaction kinetic was identical at 2- and 4-mm depths for the self-cure mode. For the dual-cure mode, polymerization initiated at 2-mm depth instantly with light-irradiation, while being retarded (0.8 min) at 4-mm depth. The material behaves similarly, irrespective of curing mode or depth, later than 11 min after mixing. Flexural strength and modulus was comparable to regular RBCs and maintained up to three months in both neutral and acidic conditions. Additional light-curing initially accelerates the polymerization kinetic and might help shorten the restauration procedure by hardening the material on demand, however with no effect on the final properties.

FUNCTIONAL ORGANOSILICON SUBSTANCES AS STABILIZERS OF POLYMERIC SUSPENSIONS

The review presents schemes for obtaining homologous series of the linear α,ω-carbofunctional oligodimethylsiloxanes with the silicone chain length from 6 to 60 siloxane units containing carboxydecyl, aminopropyl and glycidoxypropyl groups at the chain ends allowing to obtain organosilicon surfactants with reproducible structure and properties. Data on the surfactant colloidchemical properties and kinetic regularities of styrene polymerization in their presence are provided. Systematic research of heterophase styrene polymerization kinetic regularities in the presence of water-insoluble α,ω-carbofunctional oligodimethylsiloxane allowed to formulate the fundamental differences of polymerization kinetic regularities from those observed in the presence of water-soluble surfactants. The mechanism of interfacial adsorption layers formation with water-insoluble α,ω-carbofunctional oligodimethylsiloxanes on the surface of monomer drops and polymer-monomeric particles was considered. This mechanism consists in the forced surfactant replacement by the formed polymer (because of their incompatibility) to the interfacial adsorption layer and in the formation of the surfactant supermolecular structures. The latter in total with the polymer provide its high durability.

Polymer-graphene composites by photocuring of a system containing benzophenone macromer

Formulations incorporating benzophenone oligodimethacrylate (BP-DMA) and graphene structures (graphene oxide/GO, reduced graphene oxide/RGO) were exposed to UV/vis irradiation or femtosecond laser beam to achieve hybrid composites. All structures were characterized through various methods including 1H NMR and FTIR spectroscopies, optical microscopy, TEM, SEM/EDAX analysis, and DSC/XRD techniques. The photopolymerization of BP-DMA in monomer compositions with and without GO or RGO was investigated by photo-DSC and FTIR methods for determining the polymerization kinetic parameters. The photopolymerization experiments revealed a good photoreactivity of the monomers (degree of conversion: 65-77%) after 1 minute exposure to UV/vis irradiation and the addition of graphene (up to 0.5%), whereas the polymerization rate varied between 0.14 and 0.1 s-1. Moreover, two-photon photopolymerization of the formulations in presence/absence of GO or RGO nanosheets (0.1 wt.%) generated 2D microstructures by direct laser writing procedure. Also, the morphology and the properties of composites materials were analyzed.