5 research outputs found
Wetting Regimes for Residual-Layer-Free Transfer Molding at Micro- and Nanoscales
Transfer
molding offers a low-cost approach
to large-area fabrication of isolated structures in a variety of materials
when recessed features of the open-faced mold are filled without leaving
a residual layer on the plateaus of the mold. Considering both macroscale
dewetting and microscale capillary flow, a proposed map of wetting
regimes for blade meniscus coating provides a guide for achieving
discontinuous dewetting at maximum throughput. Dependence of meniscus
morphology on the azimuthal orientation of the stamp provides insight
into the dominant mechanisms for discontinuous dewetting of one-dimensional
(1-D) patterns. Critical meniscus velocity is measured and residual-layer-free
filling is demonstrated for 1-D patterned soft molds (stamps) with
periods ranging from 140 nm to 6 μm. Transfer of isolated lines,
and multilayer woodpile structures were achieved through plasma bonding.
These results are relevant to other roll-to-roll compatible processes
for scalable production of high-resolution structures across large
areas
Core@Double-Shell Structured Nanocomposites: A Route to High Dielectric Constant and Low Loss Material
This work reports the advances of
utilizing a core@double-shell nanostructure to enhance the electrical
energy storage capability and suppress the dielectric loss of polymer
nanocomposites. Two types of core@double-shell barium titanate (BaTiO<sub>3</sub>) matrix-free nanocomposites were prepared using a surface
initiated atom transfer radical polymerization (ATRP) method to graft
a polyÂ(2-hydroxylethyle methacrylate)-<i>block</i>-polyÂ(methyl
methacrylate) and sodium polyacrylate-<i>block</i>-polyÂ(2-hydroxylethyle
methacrylate) block copolymer from BaTiO<sub>3</sub> nanoparticles.
The inner shell polymer is chosen to have either high dielectric constant
or high electrical conductivity to provide large polarization, while
the encapsulating outer shell polymer is chosen to be more insulating
as to maintain a large resistivity and low loss. Finite element modeling
was conducted to investigate the dielectric properties of the fabricated
nanocomposites and the relaxation behavior of the grafted polymer.
It demonstrates that confinement of the more conductive (lossy) phase
in this multishell nanostructure is the key to achieving a high dielectric
constant and maintaining a low loss. This promising multishell strategy
could be generalized to a variety of polymers to develop novel nanocomposites
Thermomechanical Properties of Bimodal Brush Modified Nanoparticle Composites
The
enthalpic incompatibility of organic polymer matrices and high
surface energy inorganic nanoparticles often leads to phase separation
in polymer nanocomposites (PNC) precluding the realization of anticipated
property enhancements. The grafting of polymer chains to nanoparticles
holds promise as a means for controlling dispersion. A single population
of polymer chains with tunable graft density (σ) and molecular
weight (<i>N</i>) is observed to have antithetical enthalpic
and entropic effects on interface compatibility. We report the use
of bimodal polymer brushes with fundamentally decoupled enthalpic
and entropic parameters. Bimodal polystyrene brushes were grafted
from 15 nm colloidal silica nanoparticles by a previously reported
consecutive RAFT (reversible addition–fragmentation chain transfer)
polymerization technique. The combination of a high graft density
short brush and a low graft density long brush was found to cause
improved nanoparticle dispersion in a polystyrene matrix when compared
to single populations of long and short brushes of corresponding graft
densities and molecular weights. A new quantitative model was developed
to understand these results and was found capable of predicting dispersions
in grafted nanoparticle composites in the allophobic dewetting regime.
The bimodal-brush-graft particles were also found to be remarkably
well dispersed in an entropically unfavorable higher molecular weight
matrix. This facilitated a study of the role of matrix–brush
entanglement on the thermomechanical properties of PNCs, isolated
from the effects of particle dispersion. The best enhancements in
glassy properties resulted from improved matrix–brush entanglement,
attained by lowering the long chain graft density and increasing the
long chain to matrix molecular weight ratio
Bimodal Surface Ligand Engineering: The Key to Tunable Nanocomposites
Tuning the dispersion of inorganic nanoparticles within
organic
matrices is critical to optimizing polymer nanocomposite properties
and is intrinsically difficult due to their strong enthalpic incompatibility.
Conventional attempts to use polymer brushes to control nanoparticle
dispersion are challenged by the need for high graft density to reduce
particle core–core attractions and the need for low graft density
to reduce the entropic penalty for matrix penetration into the brush.
We validated a parametric phase diagram previously reported by Pryamtisyn
et al. (Pryamtisyn, V.; Ganesan, V.; Panagiotopoulos, A. Z.; Liu,
H.; Kumar, S. K. Modeling the Anisotropic Self-Assembly of Spherical
Polymer-Grafted Nanoparticles. <i>J. Chem. Phys.</i> <b>2009</b>, <i>131</i>, 221102) for predicting dispersion
of monomodal-polymer-brush-modified nanoparticles in polymer matrices.
The theoretical calculation successfully predicted the experimental
observation that the monomodal-polyÂ(dimethyl siloxane) (PDMS)-brush-grafted
TiO<sub>2</sub> nanoparticles can only be well dispersed within a
small molecular weight silicone matrix. We further extended the parametric
phase diagram to analyze the dispersion behavior of bimodal-PDMS-brush-grafted
particles, which is also in good agreement with experimental results.
Utilizing a bimodal grafted polymer brush design, with densely grafted
short brushes to shield particle surfaces and sparsely grafted long
brushes that favor the entanglement with matrix chains, we dispersed
TiO<sub>2</sub> nanoparticles in high molecular weight commercial
silicone matrices and successfully prepared thick (about 5 mm) transparent
high-refractive-index TiO<sub>2</sub>/silicone nanocomposites
Tunable Multiscale Nanoparticle Ordering by Polymer Crystallization
While ∼75% of commercially
utilized polymers are semicrystalline,
the generally low mechanical modulus of these materials, especially
for those possessing a glass transition temperature below room temperature,
restricts their use for structural applications. Our focus in this
paper is to address this deficiency through the controlled, multiscale
assembly of nanoparticles (NPs), in particular by leveraging the kinetics
of polymer crystallization. This process yields a multiscale NP structure
that is templated by the lamellar semicrystalline polymer morphology
and spans NPs engulfed by the growing crystals, NPs ordered into layers
in the interlamellar zone [spacing of O (10–100
nm)], and NPs assembled
into fractal objects at the interfibrillar scale, O (1–10
μm). The relative fraction
of NPs in this hierarchy is readily manipulated by the crystallization
speed. Adding NPs usually increases the Young’s modulus of
the polymer, but the effects of multiscale ordering are nearly an
order of magnitude larger than those for a state where the NPs are
not ordered, i.e., randomly dispersed in the matrix. Since the material’s
fracture toughness remains practically unaffected in this process,
this assembly strategy allows us to create high modulus materials
that retain the attractive high toughness and low density of polymers