41 research outputs found
SĂntese de resinas ligno-fenol-formaldeĂdo para aplicação em painĂ©is de mĂ©dia densidade.
bitstream/item/219766/1/TS2020-010-dis-MEPA.pdfDissertação (Mestrado em QuĂmica) - Universidade Federal do Ceará, Centro de CiĂŞncias, Fortaleza. Coorientador: Renato Carrhá LeitĂŁ
Vertical Phase Segregation Induced by Dipolar Interactions in Planar Polymer Brushes
We present a generalized theory for
studying structural properties
of a planar dipolar polymer brush immersed in a polar solvent. We
show that an explicit treatment of the dipolar interactions yields
a macroscopic concentration dependent effective “chi”
(the Flory–Huggins-like interaction) parameter. Furthermore,
it is shown that the concentration dependent chi parameter promotes
phase segregation in polymer solutions and brushes so that the polymer-poor
phase consists of a finite/nonzero polymer concentration. Such a destabilization
of the homogeneous phase by the dipolar interactions appears as vertical
phase segregation in a planar polymer brush. In a vertically phase
segregated polymer brush, the polymer-rich phase near the grafting
surface coexists with the polymer-poor phase at the other end. Predictions
of the theory are directly compared with prior reported experimental
results for dipolar polymers in polar solvents. Excellent agreements
with the experimental results are found, hinting that the dipolar
interactions play a significant role in vertical phase segregation
of planar polymer brushes. We also compare our field theoretical approach
with the two-state and other models invoking <i>ad hoc</i> concentration dependence of the chi parameter. Interplay between
the short-ranged excluded volume interactions and long-ranged dipolar
interactions is shown to play an important role in affecting the vertical
phase separation. Effects of mismatch between the dipole moments of
the polymer segments and the solvent molecules are investigated in
detail
Quantum Trajectory-Electronic Structure Approach for Exploring Nuclear Effects in the Dynamics of Nanomaterials
A massively parallel, direct quantum
molecular dynamics method is described. The method combines a quantum
trajectory (QT) representation of the nuclear wave function discretized
into an ensemble of trajectories with an electronic structure (ES)
description of electrons, namely using the density functional tight
binding (DFTB) theory. Quantum nuclear effects are included into the
dynamics of the nuclei via quantum corrections to the classical forces.
To reduce computational cost and increase numerical accuracy, the
quantum corrections to dynamics resulting from localization of the
nuclear wave function are computed approximately and included into
selected degrees of freedom representing light particles where the
quantum effects are expected to be the most pronounced. A massively
parallel implementation, based on the message passing interface allows
for efficient simulations of ensembles of thousands of trajectories
at once. The QTES-DFTB dynamics approach is employed to study the
role of quantum nuclear effects on the interaction of hydrogen with
a model graphene sheet, revealing that neglect of nuclear effects
can lead to an overestimation of adsorption
Quantum Trajectory-Electronic Structure Approach for Exploring Nuclear Effects in the Dynamics of Nanomaterials
A massively parallel, direct quantum
molecular dynamics method is described. The method combines a quantum
trajectory (QT) representation of the nuclear wave function discretized
into an ensemble of trajectories with an electronic structure (ES)
description of electrons, namely using the density functional tight
binding (DFTB) theory. Quantum nuclear effects are included into the
dynamics of the nuclei via quantum corrections to the classical forces.
To reduce computational cost and increase numerical accuracy, the
quantum corrections to dynamics resulting from localization of the
nuclear wave function are computed approximately and included into
selected degrees of freedom representing light particles where the
quantum effects are expected to be the most pronounced. A massively
parallel implementation, based on the message passing interface allows
for efficient simulations of ensembles of thousands of trajectories
at once. The QTES-DFTB dynamics approach is employed to study the
role of quantum nuclear effects on the interaction of hydrogen with
a model graphene sheet, revealing that neglect of nuclear effects
can lead to an overestimation of adsorption
A Novel and Functional Single-Layer Sheet of ZnSe
The
recently synthesized freestanding four-atom-thick double-layer
sheet of ZnSe holds great promise as an ultraflexible and transparent
photoelectrode material for solar water splitting. In this work, we
report theoretical studies on a novel three-atom-thick single-layer
sheet of ZnSe that demonstrates a strong quantum confinement effect
by exhibiting a large enhancement of the band gap (2.0 eV) relative
to the zinc blende (ZB) bulk phase. Theoretical optical absorbance
shows that the largest absorption of this ultrathin single-layer sheet
of ZnSe occurs at a wavelength similar to its four-atom-thick double-layer
counterpart, suggesting a comparable behavior on incident photon-to-current
conversion efficiency for solar water splitting, among a wealth of
potential applications. The results presented herein for ZnSe may
be generalized to other group II-VI analogues
Multicomponent Gas Storage in Organic Cage Molecules
Porous
liquids are a promising new class of materials featuring nanoscale
cavity units dispersed in liquids that are suitable for applications
such as gas storage and separation. In this work, we use molecular
dynamics simulations to examine the multicomponent gas storage in
a porous liquid consisting of crown-ether-substituted cage molecules
dissolved in a 15-crown-5 solvent. We compute the storage of three
prototypical small molecules including CO<sub>2</sub>, CH<sub>4</sub>, and N<sub>2</sub> and their binary mixtures in individual cage
molecules. For porous liquids in equilibrium with a binary 1:1 gas
mixture bath with partial gas pressure of 27.5 bar, a cage molecule
shows a selectivity of 4.3 and 13.1 for the CO<sub>2</sub>/CH<sub>4</sub> and CO<sub>2</sub>/N<sub>2</sub> pairs, respectively. We
provide a molecular perspective of how gas molecules are stored in
the cage molecule and how the storage of one type of gas molecule
is affected by other types of gas molecules. Our results clarify the
molecular mechanisms behind the selectivity of such cage molecules
toward different gases
Ab Initio Predictions of Hexagonal Zr(B,C,N) Polymorphs for Coherent Interface Design
Density
functional theory calculations are used herein to explore
the hexagonal (HX) NiAs-like polymorphs of ZrÂ(B,C,N) and compare them
with the corresponding ZrÂ(B,C,N) Hagg-like face-centered-cubic rocksalt
(B1) phases. Although all predicted compounds are mechanically stable
according to the Born–Huang criteria, only HX ZrÂ(C,N) are dynamically
stable according to ab initio molecular dynamics simulations and lattice
dynamics calculations. HX ZrN emerges as a candidate structure with
a ground-state energy, elastic constants, and extrinsic mechanical
parameters comparable with those of B1 ZrN. Ab initio band structure
and semiclassical Boltzmann transport calculations predict a metallic
character and a monotonic increase in electrical conductivity with
the number of valence electrons. Electronic structure calculations
indicate that the HX phases gain their stability and mechanical attributes
through Zr d–nonmetal p hybridization and broadening of the
Zr d bands. Furthermore, it is shown that the HX ZrN phase provides
a low-energy coherent interface model for connecting B1 ZrN domains,
with significant energetic advantage over an atomistic interface model
derived from high-resolution transmission electron microscopy (HRTEM)
images. The ab initio characterizations provided herein should aid
the experimental identification of non-Hagg-like hard phases. The
results can also enrich the variety of crystalline phases potentially
available for designing coherent interfaces in superhard nanostructured
materials and in materials with multilayer characteristics
Ab Initio Predictions of Strong Interfaces in Transition-Metal Carbides and Nitrides for Superhard Nanocomposite Coating Applications
Conceiving
strong interfaces represents an effective direction
in the development of superhard nanocomposite materials for practical
applications in protective coatings. In the pursuit of engineering
strong nanoscale interfaces between cubic rock-salt (B1) domains,
we investigate using density functional theory (DFT) coherent interface
models designed based on hexagonal (HX) NiAs and WC structures, as
well as experiment. The DFT screening of a collection of transition-metal
(M = Zr, Hf, Nb, Ta) carbides and nitrides indicates that the interface
models provided by the HX polymorphs store little coherency strain
and develop an energetic advantage as the valence-electron concentration
increases. Our result suggests that harnessing the polymorphism encountered
in transition-metal (M = Zr, Hf, Nb, Ta) carbides and nitrides for
interface design represents a promising strategy for advancing superhard
nanomaterials
Tuning from Half-Metallic to Semiconducting Behavior in SiC Nanoribbons
Half-metallic
nanoscale conductors, highly sought after for spintronic
applications, are usually realized through metal elements, chemical
doping, or external electric fields. By means of local and hybrid
density functional theory calculations, we identify pristine zigzag
silicon carbide nanoribbons (zSiC-NRs) with bare edges as a metal-free
monolayered material that exhibits intrinsic half-metallic behavior
without chemical doping or an external electric field. Ab initio molecular
dynamics simulations indicate that the half-metallicity is robust
at room temperature. We also demonstrate that edge termination with
O and S atoms transforms the zSiC-NRs into a full metal or a semiconducting
material, respectively, due to the presence of O dimerization only
on the Si edge and of S trimerization on both Si and C edges, the
latter being driven by an unusual Peierls-like distortion along the
functionalizing S atoms. The rich electronic properties displayed
by zSiC-NRs may open new perspectives for spintronic applications
using layered, metal-free, and light atom material
Thermodynamic Control of Two-Dimensional Molecular Ionic Nanostructures on Metal Surfaces
Bulk molecular ionic solids exhibit
fascinating electronic properties,
including electron correlations, phase transitions, and superconducting
ground states. In contrast, few of these phenomena have been observed
in low-dimensional molecular structures, including thin films, nanoparticles,
and molecular blends, not in the least because most of such structures
have been composed of nearly closed-shell molecules. It is therefore
desirable to develop low-dimensional ionic molecular structures that
can capture potential applications. Here, we present detailed analysis
of monolayer-thick structures of the canonical TTF–TCNQ (tetrathiafulvalene
7,7,8,8-tetracyanoquinodimethane) system grown on low-index gold and
silver surfaces. The most distinctive property of the epitaxial growth
is the wide abundance of stable TTF/TCNQ ratios, in sharp contrast
to the predominance of a 1:1 ratio in the bulk. We propose the existence
of the surface phase diagram that controls the structures of TTF–TCNQ
on the surfaces and demonstrate phase transitions that occur upon
progressively increasing the density of TCNQ while keeping the surface
coverage of TTF fixed. Based on direct observations, we propose the
binding motif behind the stable phases and infer the dominant interactions
that enable the existence of the rich spectrum of surface structures.
Finally, we also show that the surface phase diagram will control
the epitaxy beyond monolayer coverage. Multiplicity of stable surface
structures, the corollary rich phase diagram, and the corresponding
phase transitions present an interesting opportunity for low-dimensional
molecular systems, particularly if some of the electronic properties
of the bulk can be preserved or modified in the surface phases