100 research outputs found
Role of Chain Morphology and Stiffness in Thermal Conductivity of Amorphous Polymers
Designing thermally conductive polymer
is of scientific interest
and practical importance for applications like thermal interface materials,
electronics packing, and plastic heat exchangers. In this work, we
study the fundamental relationship between the molecular morphology
and thermal conductivity in bulk amorphous polymers. We use polyethylene
as a model system and performed systematic parametric study in molecular
dynamics simulations. We find that the thermal conductivity is a strong
function of the radius of gyration of the molecular chains, which
is further correlated to persistence length, an intrinsic property
of the molecule that characterizes molecular stiffness. Larger persistence
length can lead to more extended chain morphology and thus higher
thermal conductivity. Further thermal conductivity decomposition analysis
shows that thermal transport through covalent bonds dominates the
effective thermal conductivity over other contributions from nonbonded
interactions (van der Waals) and translation of molecules disregarding
the morphology. As a result, the more extended chains due to larger
persistence length provide longer spatial paths for heat to transfer
efficiently and thus lead to higher thermal conductivity. In addition,
rigid rod-like polymers with very large persistence length tend to
spontaneously crystallize and form orientated chains, leading to a
thermal conductivity increase by more than 1 order of magnitude. Our
results will provide important insights into the design of thermally
conductive amorphous polymers
High-Contrast, Reversible Thermal Conductivity Regulation Utilizing the Phase Transition of Polyethylene Nanofibers
Reversible thermal conductivity regulation at the nanoscale is of great interest to a wide range of applications such as thermal management, phononics, sensors, and energy devices. Through a series of large-scale molecular dynamics simulations, we demonstrate a thermal conductivity regulation utilizing the phase transition of polyethylene nanofibers, enabling a thermal conductivity tuning factor of as high as 12, exceeding all previously reported values. The thermal conductivity change roots from the segmental rotations along the polymer chains, which introduce along-chain morphology disorder that significantly interrupts phonon transport along the molecular chains. This phase transition, which can be regulated by temperature, strain, or their combinations, is found to be fully reversible in the polyethylene nanofibers and can happen at a narrow temperature window. The phase change temperature can be further tuned by engineering the diameters of the nanofibers, making such a thermal conductivity regulation scheme adaptable to different application needs. The findings can stimulate significant research interest in nanoscale heat transfer control
Postsynthetic Metalation of Bipyridyl-Containing MetalâOrganic Frameworks for Highly Efficient Catalytic Organic Transformations
We have designed
highly stable and recyclable single-site solid
catalysts via postsynthetic metalation of the 2,2â˛-bipyridyl-derived
metalâorganic framework (MOF) of the UiO structure (bpy-UiO).
The Ir-functionalized MOF (bpy-UiO-Ir) is a highly active catalyst
for both borylation of aromatic CâH bonds using B<sub>2</sub>(pin)<sub>2</sub> (pin = pinacolate) and <i>ortho</i>-silylation
of benzylicsilyl ethers; the <i>ortho</i>-silylation activity
of the bpy-UiO-Ir is at least 3 orders of magnitude higher than that
of the homogeneous control. The Pd-functionalized MOF (bpy-UiO-Pd)
catalyzes the dehydrogenation of substituted cyclohexenones to afford
phenol derivatives with oxygen as the oxidant. Most impressively,
the bpy-UiO-Ir was recycled and reused 20 times for the borylation
reaction without loss of catalytic activity or MOF crystallinity.
This work highlights the opportunity in designing highly stable and
active catalysts based on MOFs containing nitrogen donor ligands for
important organic transformations
Flexible Expectile Regression in Reproducing Kernel Hilbert Spaces
<p>Expectile, first introduced by Newey and Powell in <a href="#cit0017" target="_blank">1987</a> in the econometrics literature, has recently become increasingly popular in risk management and capital allocation for financial institutions due to its desirable properties such as coherence and elicitability. The current standard tool for expectile regression analysis is the multiple linear expectile regression proposed by Newey and Powell in <a href="#cit0017" target="_blank">1987</a>. The growing applications of expectile regression motivate us to develop a much more flexible nonparametric multiple expectile regression in a reproducing kernel Hilbert space. The resulting estimator is called KERE, which has multiple advantages over the classical multiple linear expectile regression by incorporating nonlinearity, nonadditivity, and complex interactions in the final estimator. The kernel learning theory of KERE is established. We develop an efficient algorithm inspired by majorization-minimization principle for solving the entire solution path of KERE. It is shown that the algorithm converges at least at a linear rate. Extensive simulations are conducted to show the very competitive finite sample performance of KERE. We further demonstrate the application of KERE by using personal computer price data. Supplementary materials for this article are available online.</p
Molecular Fin Effect from Heterogeneous Self-Assembled Monolayer Enhances Thermal Conductance across HardâSoft Interfaces
Thermal
transport across hardâsoft interfaces is critical to many modern
applications, such as composite materials, thermal management in microelectronics,
solarâthermal phase transition, and nanoparticle-assisted hyperthermia
therapeutics. In this study, we use equilibrium molecular dynamics
(EMD) simulations combined with the GreenâKubo method to study
how molecularly heterogeneous structures of the self-assembled monolayer
(SAM) affect the thermal transport across the interfaces between the
SAM-functionalized gold and organic liquids (hexylamine, propylamine
and hexane). We focus on a practically synthesizable heterogeneous
SAM featuring alternating short and long molecular chains. Such a
structure is found to improve the thermal conductance across the hardâsoft
interface by 46â68% compared to a homogeneous nonpolar SAM.
Through a series of further simulations and analyses, it is found
that the root reason for this enhancement is the penetration of the
liquid molecules into the spaces between the long SAM molecule chains,
which increase the effective contact area. Such an effect is similar
to the fins used in macroscopic heat exchanger. This âmolecular
finâ structure from the heterogeneous SAM studied in this work
provides a new general route for enhancing thermal transport across
hardâsoft material interfaces
MetalâOrganic Frameworks Stabilize Solution-Inaccessible Cobalt Catalysts for Highly Efficient Broad-Scope Organic Transformations
New
and active earth-abundant metal catalysts are critically needed
to replace precious metal-based catalysts for sustainable production
of commodity and fine chemicals. We report here the design of highly
robust, active, and reusable cobalt-bipyridine- and cobalt-phenanthroline-based
metalâorganic framework (MOF) catalysts for alkene hydrogenation
and hydroboration, aldehyde/ketone hydroboration, and arene CâH
borylation. In alkene hydrogenation, the MOF catalysts tolerated a
variety of functional groups and displayed unprecedentedly high turnover
numbers of âź2.5 Ă 10<sup>6</sup> and turnover frequencies
of âź1.1 Ă 10<sup>5</sup> h<sup>â1</sup>. Structural,
computational, and spectroscopic studies show that site isolation
of the highly reactive (bpy)ÂCoÂ(THF)<sub>2</sub> species in the MOFs
prevents intermolecular deactivation and stabilizes solution-inaccessible
catalysts for broad-scope organic transformations. Computational,
spectroscopic, and kinetic evidence further support a hitherto unknown
(bpy<sup>â˘â</sup>)ÂCo<sup>I</sup>(THF)<sub>2</sub> ground
state that coordinates to alkene and dihydrogen and then undergoing
Ď-complex-assisted metathesis to form (bpy)ÂCoÂ(alkyl)Â(H). Reductive
elimination of alkane followed by alkene binding completes the catalytic
cycle. MOFs thus provide a novel platform for discovering new base-metal
molecular catalysts and exhibit enormous potential in sustainable
chemical catalysis
Cycloastragenol restrains keratinocyte hyperproliferation by promoting autophagy via the miR-145/STC1/Notch1 axis in psoriasis
Psoriasis is characterized by inflammation and hyperproliferation of epidermal keratinocytes. Cycloastragenol (CAG) is an active molecule of Astragalus membranaceus that potentially plays a repressive role in psoriasis. Activated cell autophagy is an effective pathway for alleviating psoriasis progression. Thus, we investigated the role of CAG in the proliferation and autophagy of interleukin (IL)-22-stimulated keratinocytes. A psoriasis model was established by stimulating HaCaT cells with IL-22. Gene or protein expression levels were measured by qRT-PCR or western blot. Autophagy flux was observed with mRFP-GFP-LC3 adenovirus transfection assay under confocal microscopy. Stanniocalcin-1 (STC1) secretion levels were determined using ELISA kits. The apoptosis rate was assessed using flow cytometry. Interactions between miR-145 and STC1 or STC1 and Notch1 were validated by luciferase reporter gene assays, RIP, and Co-IP assays. CAG repressed cell proliferation and promoted apoptosis and autophagy in IL-22-stimulated HaCaT cells. Additionally, CAG promoted autophagy by enhancing miR-145. STC1 silencing ameliorated autophagy repression in IL-22-treated HaCaT cells. Moreover, miR-145 negatively regulated STC1, and STC1 was found to activate Notch1. Lastly, STC1 overexpression reversed CAG-promoted autophagy. CAG alleviated keratinocyte hyperproliferation through autophagy enhancement via regulating the miR-145/STC1/Notch1 axis in psoriasis.</p
Additional file 1: of trumpet: transcriptome-guided quality assessment of m6A-seq data
Source code of theĂÂ trumpet R package. (ZIP 2229ĂÂ kb
Additional file 2: of trumpet: transcriptome-guided quality assessment of m6A-seq data
Supplementary Material (including Table S1-S4) for trumept.ĂÂ (DOCX 21ĂÂ kb
DMRT1 Is Required for Mouse Spermatogonial Stem Cell Maintenance and Replenishment
<div><p>Male mammals produce sperm for most of postnatal life and therefore require a robust germ line stem cell system, with precise balance between self-renewal and differentiation. Prior work established <i>doublesex-</i> and <i>mab-3</i>-related transcription factor 1 (<i>Dmrt1</i>) as a conserved transcriptional regulator of male sexual differentiation. Here we investigate the role of <i>Dmrt1</i> in mouse spermatogonial stem cell (SSC) homeostasis. We find that <i>Dmrt1</i> maintains SSCs during steady state spermatogenesis, where it regulates expression of <i>Plzf</i>, another transcription factor required for SSC maintenance. We also find that <i>Dmrt1</i> is required for recovery of spermatogenesis after germ cell depletion. Committed progenitor cells expressing <i>Ngn3</i> normally do not contribute to SSCs marked by the <i>Id4-Gfp</i> transgene, but do so when spermatogonia are chemically depleted using busulfan. Removal of <i>Dmrt1</i> from <i>Ngn3</i>-positive germ cells blocks the replenishment of Id4-GFP-positive SSCs and recovery of spermatogenesis after busulfan treatment. Our data therefore reveal that <i>Dmrt1</i> supports SSC maintenance in two ways: allowing SSCs to remain in the stem cell pool under normal conditions; and enabling progenitor cells to help restore the stem cell pool after germ cell depletion.</p></div
- âŚ