1,517 research outputs found
Negative Linear Compressibility
While all materials reduce their intrinsic volume under hydrostatic (uniform)
compression, a select few actually \emph{expand} along one or more directions
during this process of densification. As rare as it is counterintuitive, such
"negative compressibility" behaviour has application in the design of pressure
sensors, artificial muscles and actuators. The recent discovery of surprisingly
strong and persistent negative compressibility effects in a variety of new
families of materials has ignited the field. Here we review the phenomenology
of negative compressibility in this context of materials diversity, placing
particular emphasis on the common structural motifs that recur amongst known
examples. Our goal is to present a mechanistic understanding of negative
compressibility that will help inform a clear strategy for future materials
design.Comment: Submitted to PCC
Adaptive Optics Simulations for Siding Spring
Using an observational derived model optical turbulence profile (model-OTP)
we have investigated the performance of Adaptive Optics (AO) at Siding Spring
Observatory (SSO), Australia. The simulations cover the performance for AO
techniques of single conjugate adaptive optics (SCAO), multi-conjugate adaptive
optics (MCAO) and ground-layer adaptive optics (GLAO). The simulation results
presented in this paper predict the performance of these AO techniques as
applied to the Australian National University (ANU) 2.3 m and Anglo-Australian
Telescope (AAT) 3.9 m telescopes for astronomical wavelength bands J, H and K.
The results indicate that AO performance is best for the longer wavelengths
(K-band) and in the best seeing conditions (sub 1-arcsecond). The most
promising results are found for GLAO simulations (field of view of 180
arcsecs), with the field RMS for encircled energy 50% diameter (EE50d) being
uniform and minimally affected by the free-atmosphere turbulence. The GLAO
performance is reasonably good over the wavelength bands of J, H and K. The
GLAO field mean of EE50d is between 200 mas to 800 mas, which is a noticeable
improvement compared to the nominal astronomical seeing (870 to 1700 mas).Comment: 15 pages; accepted for publication in PAS
Nanostructure determination from the pair distribution function: A parametric study of the INVERT approach
We present a detailed study of the mechanism by which the INVERT method
[Phys. Rev. Lett. 104, 125501] guides structure refinement of disordered
materials. We present a number of different possible implementations of the
central algorithm and explore the question of algorithm weighting. Our analysis
includes quantification of the relative contributions of variance and
fit-to-data terms during structure refinement, which leads us to study the
roles of density fluctuations and configurational jamming in the RMC fitting
process. We present a parametric study of the pair distribution function
solution space for C60, a-Si and a-SiO2, which serves to highlight the
difficulties faced in developing a transferable weighting scheme.Comment: 15 pages, 7 figures, formatted for JPCM (RMC issue
Incommensurate chirality density wave transition in a hybrid molecular framework
Using single-crystal X-ray diffraction we characterise the 235\,K
incommensurate phase transition in the hybrid molecular framework
tetraethylammonium silver(I) dicyanoargentate, [NEt]Ag(CN). We
demonstrate the transition to involve spontaneous resolution of chiral
[NEt] conformations, giving rise to a state in which molecular
chirality is incommensurately modulated throughout the crystal lattice. We
refer to this state as an incommensurate chirality density wave (XDW) phase,
which represents a fundamentally new type of chiral symmetry breaking in the
solid state. Drawing on parallels to the incommensurate ferroelectric
transition of NaNO we suggest the XDW state arises through coupling between
acoustic (shear) and molecular rotoinversion modes. Such coupling is
symmetry-forbidden at the Brillouin zone centre but symmetry-allowed for small
but finite modulation vectors . The importance of
long-wavelength chirality modulations in the physics of this hybrid framework
may have implications for the generation of mesoscale chiral textures, as
required for advanced photonic materials.Comment: 5 pages, 3 figure
All-Acrylic Multigraft Copolymers: Synthesis and Structure-Property Relationship for Producing Next Generation Thermoplastic Elastomers
Polymer architecture and the advancement of molecular design using anionic and other controlled polymerization methods continues to be of significant research interest because of the tunable approach it provides, which can impact numerous applications ranging from thermoplastics to drug delivery systems. Among the numerous branched structures currently investigated, comb and graft copolymers continue to provide tailored materials which exhibit superior mechanical properties when compared to their di- and triblock linear counterparts. More specifically, the incorporation of two or more monomers into graft and multigraft constructions where the side chains are composed of a plastic (high Tg [glass transition temperature]) segment attached to a rubbery (low Tg) backbone has displayed much improved elastomeric properties for use in thermoplastic elastomer (TPEs) applications. These elastomeric materials continue to be dominated by compositions of styrene-isoprene or styrene-butadiene with little attention to all-acrylic systems in which both the soft and hard segments are made of acrylic monomers. By using anionic polymerization, methyl methacrylate macromonomers were synthesized and subsequently copolymerized with n-butyl acrylate using reversible addition-fragmentation chain transfer polymerization. In this manner we were able to construct the desired multigraft structures via a grafting-through methodology. The fundamental structure-property relationships were then studied to see how compositional changes such as branch point number, branch point functionality, side chain molecular weight, and volume percent of the glassy PMMA [poly(methyl methacrylate)] segments affects the overall mechanical performance of the branched material. This allowed us to show the ability to dramatically control the overall strength and elasticity of the all-acrylic multigraft copolymers, as well as to demonstrate a versatile synthetic technique that has the ability to be adapted for the synthesis of more complex architectures using a vast array of hard and soft segment
Photochemistry of lignocellulosic materials and related compounds
The primary photochemical processes of lignin-rich high-yield thermomechanical pulp (TMP)
have been investigated with a view to understanding and ultimately preventing the yellowing
(photoreversion) of such paper upon exposure to sunlight. This would enable their use in longer-life paper products and therefore represent large raw material and cost savings to the pulp and paper
manufacturer. [Continues.
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