PacSurf2022 Session NM-WeM2: Nanocomposites

It is well known that ABA triblock-type two fluoroalkyl end-capped oligomers [R_F-(M)_n-R_F; R_F = fluoroalkyl groups; M = radical polymerizable hydrocarbon monomers] can be synthesized by using fluoroalkanoyl peroxide as a key intermediate to form the nanometer size-controlled self-assembled molecular aggregates through the aggregation of terminal fluoroalkyl segments in aqueous and organic media [1, 2]. The fluorinated oligomeric aggregates can interact with a variety of inorganic guest molecules such as silica nanoparticles to afford the colloidal stable fluorinated oligomeric aggregates/guest molecule nanocomposites [2, 3]. Thus, it is of particular interest to develop the colloidal stable magnetite nanoparticles by using two fluoroalkyl end-capped oligomers. Here we report that fluoroalkyl end-capped vinyltrimethoxysilane oligomeric silica/magnetite composites [R_F-(VM-SiO_3/2)_n-R_F/Mag] can be prepared by the sol-gel reaction of the corresponding fluorinated oligomer [R_F-(CH₂CHSi(OMe)₃)_n-R_F; R_F =CF(CF₃)OC₃F₇ : R_F-(VM)_n−R_F] in the presence of magnetite (Fe₃O₄: Mag) nanoparticles. The R_F-(VM-SiO_3/2)_n-R_F/Mag composite powders thus obtained were applied to the surface modification of glass to exhibit a superoleophilic/superhydrophobic characteristic on the modified surface. These fluorinated composite powders are also well spread over water interface, not precipitated into water. Especially, it was found that these composite powders can adsorb effectively oil droplets spread on the water interface under a magnetic field (permanent magnet). In addition to the removal of oil over water interface, interestingly, these composite particle powders were applied to the facile removal of aromatic compounds such as bisphenol A and bisphenol AF from aqueous methanol solution under magnetic fields. More interestingly, it was demonstrated that we can observe the more efficient removal ability for the fluorinated aromatic compounds, compared to that of the corresponding non-fluorinated ones.

[1] Sawada H (1996) Chem. Rev. 96:1779-1808.

[2] Sawada H (2012) Polym. Chem. 3: 46-65.

[3] Oikawa Y, Saito T, Yamada S, Sugiya M, Sawada H (2015) ACS Appl. Mater. Interfaces 7: 13782-13793.

Polymer-based dielectric composites were synthesized using fillers of two dimensional (2D) nanomaterial, such as graphene and MXene, and barium titanium (BaTiO3, BTO)in a ferroelectric cyanoethylated pullulan (CEP) polymer matrix, in order to use the interface polarization of interfaces between fillers and a polymer matrix. For graphene, two materials of graphene oxide (GO) and reduced graphene oxide (rGO) and two mixing methods—simple mixing of graphene with BTO and elaborate mixing, i.e., encapsulation of BTO in graphene and subsequent mixing. rGO was synthesized by recuing GO chemically with hydrazine. Among four dielectric composite samples with graphene, the sample with rGO-encapsulated BTO (rGO@BTO) fillers showed better dielectric performance, such as high dielectric constants and low dielectric losses. Mixing conducting nanomaterials in a polymer matrix generally yields to a high dielectric constant due to metal-insulator transition (MIT), when the concentration of nanomaterials approaches to the critical concentration of percolation. However, around this critical concentration, it is difficult to suppress the soaring of the dielectric loss due to percolative connection among nanomaterials. Encapsulation can inhibit an increase in the dielectric loss by removing the main culprit of free graphene platelets during the chemical process of encapsulation. MXene was used as a filler in the simple mix method resulting in positive results comparable to the graphene cases—higher dielectric constant than rGO encapsulation (220 vs. 200) and slightly higher dielectric loss (0.08 vs. 0.04 in tan δ at 1 kHz). It is possibly due to its higher conductivity than graphene. AC driven inorganic electroluminescence devices, with a dielectric layer having the above five fillers (four graphene and one MXene), also yield to the similar trend to the dielectric composite. Details will be explained in the presentation.

Compared to alloy bulk phase diagrams, the experimental determination of phase diagrams for alloy nanoparticles (NPs), which are useful in various nanotechnological applications, involves significant technical difficulties, making theoretical modeling a feasible alternative. Yet, being quite challenging, modeling of separation nanophase diagrams is scarce in the literature. The task of predicting comprehensive nanophase-diagrams for Pd-Ir fcc-based three cuboctahedra is facilitated in this study by combining the computationally efficient statistical-mechanical Free-energy Concentration Expansion Method (FCEM) [1], which includes short-range order (SRO), with coordination-dependent bond-energy variations [2] as part of the input, and with rotationally symmetric site grouping for extra efficiency. This nanosystem has been chosen mainly because of the very small atomic mismatch that simplifies the modeling, e.g., in the assessment of vibrational entropy contributions based in this work on fitting to the Pd-Ir experimental bulk critical-temperature.

This entropic effect, together with SRO, leads to significant destabilization of Pd surface segregated Quasi-Janus (QJ) asymmetric configurations of the NP core, which transform to symmetric partially mixed nanophases. First-order and second-order intra-core transitions are predicted for dilute and intermediate-range compositions, respectively. The computed separation diagrams and intra-core solubility diagrams reflect enhanced elemental mixing in smaller QJ nanophases. In addition to these diagrams, the revealed surface and near-surface compositional variations are likely to be pertinent to the utilization of Pd-Ir NPs in heterogeneous catalysis, e.g., of CO oxidation in excess H₂.

[1] M. Polak and L. Rubinovich, Surface Science Reports, 38, 127, 2000.

[2] L. Rubinovich and M. Polak, Physical Review B 80, 045404 2009.