High-Intensity Ultrasound-Assisted Low-Temperature Formulation of Lanthanum Zirconium Oxide Nanodispersion for Thin-Film Transistors.

The process complexity, limited stability, and distinct synthesis and dispersion steps restrict the usage of multicomponent metal oxide nanodispersions in solution-processed electronics. Herein, sonochemistry is employed for the in situ synthesis and formulation of a colloidal nanodispersion of high-permittivity (κ) multicomponent lanthanum zirconium oxide (LZO: La2Zr2O7). The continuous propagation of intense ultrasound waves in the aqueous medium allows the generation of oxidant species which, on reaction, form nanofragments of crystalline LZO at ∼80 °C. Simultaneously, the presence of acidic byproducts in the vicinity promotes the formulation of a stable as-prepared LZO dispersion. The LZO thin film exhibits a κ of 16, and thin-film transistors (TFTs) based on LZO/indium gallium zinc oxide operate at low input voltages (≤4 V), with the maximum mobility (µ) and on/off ratio (Ion/Ioff) of 5.45 ± 0.06 cm2 V-1 s-1 and ∼105, respectively. TFTs based on the compound dielectric LZO/Al2O3 present a marginal reduction in leakage current, along with enhancement in µ (6.16 ± 0.04 cm2 V-1 s-1) and Ion/Ioff (∼105). Additionally, a 3 × 3 array of the proposed TFTs exhibits appreciable performance, with a µ of 3-6 cm2 V-1 s-1, a threshold voltage of -0.5 to 0.8 V, a subthreshold swing of 0.3-0.6 V dec-1, and an Ion/Ioff of 1-2.5 (×106).

Interface Dominated Dielectric Response of PS-Fe3O4 Patchy Microspheres.

Polymeric-inorganic interface plays a vital role in enhancing dielectric properties of patchy microspheres, Janus particles, and nanocomposites. We performed the computational modeling and simulations along with experiments to understand the phenomena behind the improved dielectric permittivity of polystyrene-iron oxide (PS-Fe3O4) patchy microspheres. We addressed the fundamental insights into the role of the interfacial region on the dielectric properties. Based on the experimental outcomes and computational simulations on dielectric behavior including polarization and electric field formation, we propose a new mechanism of charge buildup at the interface. Computational results reveal that the creation of interface bound-charges at the inorganic-polymeric interface is responsible for the improved dielectric properties. We also fabricated PS-Fe3O4 patchy microspheres by Pickering emulsion polymerization using Fe3O4 particles as a solid stabilizer. The microstructure, composition, morphology, dielectric, and thermal properties of the synthesized patchy PS-Fe3O4 particles were investigated. The dielectric permittivity (k) of the neat PS increased from ∼2.9 to ∼14.8 after decorating with Fe3O4 particles. Impedance response of the patchy microspheres shows that the interface of PS-Fe3O4 stores more charges than bulk PS-Fe3O4. The dielectric behavior of patchy microspheres can be engineered by tuning the shape and position of the patches. The present studies on polymer-inorganic interface provide some insights into the mechanisms that control dielectric permittivity and nonlinear conduction in an applied electric field.

Nanoparticle Motion in Entangled Melts of Linear and Nonconcatenated Ring Polymers.

The motion of nanoparticles (NPs) in entangled melts of linear polymers and nonconcatenated ring polymers are compared by large-scale molecular dynamics simulations. The comparison provides a paradigm for the effects of polymer architecture on the dynamical coupling between NPs and polymers in nanocomposites. Strongly suppressed motion of NPs with diameter d larger than the entanglement spacing a is observed in a melt of linear polymers before the onset of Fickian NP diffusion. This strong suppression of NP motion occurs progressively as d exceeds a and is related to the hopping diffusion of NPs in the entanglement network. In contrast to the NP motion in linear polymers, the motion of NPs with d > a in ring polymers is not as strongly suppressed prior to Fickian diffusion. The diffusion coefficient D decreases with increasing d much slower in entangled rings than in entangled linear chains. NP motion in entangled nonconcatenated ring polymers is understood through a scaling analysis of the coupling between NP motion and the self-similar entangled dynamics of ring polymers.

Rouse mode analysis of chain relaxation in polymer nanocomposites.

Large-scale molecular dynamics simulations are used to study the internal relaxations of chains in nanoparticle (NP)/polymer composites. We examine the Rouse modes of the chains, a quantity that is closest in spirit to the self-intermediate scattering function, typically determined in an (incoherent) inelastic neutron scattering experiment. Our simulations show that for weakly interacting mixtures of NPs and polymers, the effective monomeric relaxation rates are faster than in a neat melt when the NPs are smaller than the entanglement mesh size. In this case, the NPs serve to reduce both the monomeric friction and the entanglements in the polymer melt, as in the case of a polymer-solvent system. However, for NPs larger than half the entanglement mesh size, the effective monomer relaxation is essentially unaffected for low NP concentrations. Even in this case, we observe a strong reduction in chain entanglements for larger NP loadings. Thus, the role of NPs is to always reduce the number of entanglements, with this effect only becoming pronounced for small NPs or for high concentrations of large NPs. Our studies of the relaxation of single chains resonate with recent neutron spin echo (NSE) experiments, which deduce a similar entanglement dilution effect.

Rouse Mode Analysis of Chain Relaxation in Homopolymer Melts.

We use molecular dynamics simulations of the Kremer-Grest (KG) bead-spring model of polymer chains of length between 10 and 500, and a closely related analogue that allows for chain crossing, to clearly delineate the effects of entanglements on the length-scale-dependent chain relaxation in polymer melts. We analyze the resulting trajectories using the Rouse modes of the chains and find that entanglements strongly affect these modes. The relaxation rates of the chains show two limiting effective monomeric frictions, with the local modes experiencing much lower effective friction than the longer modes. The monomeric relaxation rates of longer modes vary approximately inversely with chain length due to kinetic confinement effects. The time-dependent relaxation of Rouse modes has a stretched exponential character with a minimum of stretching exponent in the vicinity of the entanglement chain length. None of these trends are found in models that allow for chain crossing. These facts, in combination, argue for the confined motion of chains for time scales between the entanglement time and their ultimate free diffusion.

Nanoparticle diffusion in polymer nanocomposites.

Large-scale molecular dynamics simulations show that nanoparticle (NP) diffusivity in weakly interacting mixtures of NPs and polymer melts has two very different classes of behavior depending on their size. NP relaxation times and their diffusivities are completely described by the local, Rouse dynamics of the polymer chains for NPs smaller than the polymer entanglement mesh size. The motion of larger NPs, which are comparable to the entanglement mesh size, is significantly slowed by chain entanglements, and is not describable by the Stokes-Einstein relationship. Our results are in essentially quantitative agreement with a force-level generalized Langevin equation theory for all the NP sizes and chain lengths explored, and imply that for these lightly entangled systems, activated NP hopping is not important.

Universal viscosity behavior of polymer nanocomposites.

Nonequilibrium molecular dynamics simulations are used to show that the shear viscosity of a polymer melt can be significantly reduced when filled with small energetically neutral nanoparticles, apparently independent of the polymer's chain length. Analogous to solvent molecules, small nanoparticles act akin to plasticizers and reduce the viscosity of a polymer melt. This effect, which persists for particles whose sizes are as large as the chain size or the entanglement mesh size, whichever is smaller, can be overcome by making the chain-nanoparticle interactions significantly attractive. Our simulations allow us to systematically organize the viscosity data of filled polymer melts, and thus provide a strong basis from which to predict the flow behavior of these commercially important class of materials.