Effects of Entanglement on Polymer Crystal Growth and Intercrystalline Phase Formation

We apply atomistic molecular dynamics simulations to investigate the structural evolution in the intercrystalline phase of polyethylene (PE) during crystallization. Two crystalline seeds with various relative orientations and distances are placed in molten PE samples to trigger instantaneous crystallization under quiescent conditions. Using the Z1+ algorithm, we monitor the distributions and relaxation of entangled chains near the seeds during crystallization. We show that crystal growth requires alignment and disentanglement of polymer strands. As crystallization proceeds, the polymer dynamics become hindered near the crystals. A layer of trapped entanglement consisting of loops and ties accumulates near the crystal surface and, in turn, impedes the crystal growth. Our work also reveals the formation of the stress transmitters, including tie chains and entangled loops, in the amorphous regions. The tie-chain fraction increases with increasing molecular weight and decreasing intercrystal distance, which is well-described by a modified Huang–Brown model

Tension-Induced Nematic Phase Separation in Bidisperse Homopolymer Melts

We use an analytical mean-field theory and all-atom molecular dynamics (MD) simulations to predict that external tension, together with the nematic coupling interactions, can drive phase separation of long chains from short ones in bidisperse homopolymer melts. The nematic coupling parameter α for polyethylene (PE) oligomers under applied tension is extracted from the MD simulations and used in the mean-field free energy to predict the phase boundary for bidisperse melts in which the longer chains are stretched by uniaxial tension. The predicted phase diagram is validated by direct MD simulations. We also show that extensional flow, and possibly even shear flow, may lead to nematic phase separation in molten PE oligomers, because the flow can impose a stronger tension on the longer chains than the short ones

Direct All-Atom Molecular Dynamics Simulations of the Effects of Short Chain Branching on Polyethylene Oligomer Crystal Nucleation

Using all-atom molecular dynamics (MD) simulations, we demonstrate that short alkyl branches can hinder the nucleation of polyethylene (PE) oligomers. Although one methyl and ethyl branch in a 50-carbon oligomer may only slow the nucleation kinetics mildly, bulkier side chains, such as propyl, butyl, and hexyl branches, disturb the arrangement of neighboring backbone atoms, preventing these atoms from joining a growing crystal, and therefore significantly suppress the nucleation of PE crystals, with no clear evidence of nucleation being observed over a 20 ns simulation run when hexyl side chains are present. The degrees of branching and the distributions of short alkyl groups on PE backbones can also affect the crystallization kinetics, with well-spaced branches having a greater impact on crystallization than branches that are grouped within a shorter distance along the backbone that is similar to or shorter than the length of the branch. We show that the linear portions of PE crystallize first and the branched monomers may be regarded as “defects” that impede crystallization by slowing chain conformational and diffusive relaxations and limiting the lengthening of crystalline stems

Interfacial Oriented Precursor to Secondary Nucleation of Alkane Oligomer Crystals Revealed by Molecular Dynamic Simulations

In atomistic molecular dynamics simulations, alkane oligomers (C50 or C100) rapidly form an oriented interface when placed in contact with a crystal slab of stretched periodic polyethylene chains. The oriented atoms in this interface have a similar order parameter to those of nematic atoms. After a quench below the melting point, we show that this oriented “nematic” interface thickens from around two to three layers thick and crystalline order nucleates from this layer onto the crystal-slab surface and spreads as a two-dimensional patch. Once a crystal patch is large enough, the oriented interface above it advances by forming a stable nematic patch three layers above the crystal nucleus which grows and eventually nucleates a crystal patch within it. Simulation snapshots and mean-first-passage time (MFPT) results prior to reaching steady-state growth suggest that the nematic-to-crystal transition is rate-determining, as it is much slower than the thickening of the induced oriented interface. After steady state is established, the rate of crystallization of C100 at 360 K is determined roughly equally by the rates of nucleation and of spreading of a new crystal patch to the size large enough to propagate the nematic growth front. These findings, along with those of Bourque and Rutledge (Bourque, A. J.; Rutledge, G. C. Macromolecules 2016, 49, 3956−3964) contrast sharply with the stem-by-stem growth assumed in the Hoffman–Lauritzen theory of secondary nucleation, with the work reported here indicating the importance of the oriented “nematic” layer in the propagation of the crystalline front

Predicting Chain Dimensions of Semiflexible Polymers from Dihedral Potentials

We develop a numerical and an analytical approach to estimate the persistence length lp and mean-square end-to-end distance ⟨R2⟩ of complex semiflexible polymers. Numerically, lp and ⟨R2⟩ are determined by averaging a large set of single chain conformations with the correct dihedral angle distributions p(ϕi). Analytically, lp and ⟨R2⟩ are extracted from the tangent–tangent correlation function. We apply both approaches to two semiflexible conjugated polymers, poly­(3-hexylthiophene) (P3HT) and poly­((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis­(thiophen-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (PFTBT). Results obtained via the two methods agree for polymers with any degree of polymerization N. Our methods can be applied to any semiflexible polymers with any number of distinct moieties

Synthesis of Palladium–Tungsten Metallene-Constructed Sandwich-Like Nanosheets as Bifunctional Catalysts for Direct Formic Acid Fuel Cells

Although great efforts have been made over past decades, it is very challenging to develop advanced bifunctional electrocatalysts for direct formic acid fuel cells. Herein, we report the preparation of PdW metallene-constructed sandwich-like nanosheets (S-PdW NSs) via a facile wet-chemical method. This nanostructure shows excellent electrocatalytic activity and durability in both formic acid oxidation and oxygen reduction reactions. The remarkable enhancement of catalytic activity could be mainly attributed to its unique metallene-constructed sandwich-like nanosheets, whose layered structure could provide appropriate distances at the atomic scale close to the diameter of oxygen and formic acid molecules to realize bridge adsorption. The significantly improved durability could be due to the modification of the electronic structure of Pd via the introduction of W into the Pd lattice to generate strong bonding interactions. This work offers a low-Pd-loading, highly active, and stable bifunctional catalyst for direct formic acid fuel cells

Additional file 1 of Identification and functional analysis of the LEAFY gene in longan flower induction

Supplementary Material

Coupling Oxygen Vacancies and Heterophase Homostructure Achieving High-Rate-Endurable Aqueous Zinc-Ion Storage

In recent years, manganese dioxide cathodes have demonstrated unparalleled benefits in aqueous zinc-ion batteries (AZIBs) and aqueous zinc-ion hybrid capacitors (AZICs) owing to their high discharge voltage (∼1.4 V), abundant resources, nontoxicity, high theoretical specific capacity (308 mAh g–1), and various crystal types (α-/β-/δ-/γ-MnO2). Unfortunately, their intrinsic shortcomings, including low conductivity and poor structural stability, lead to unsatisfactory electrochemical performance (poor rate performance and rapid capacity decay). Herein, a novel manganese dioxide cathode material with oxygen vacancies and a heterophase homostructure was designed and produced by a one-step hydrothermal process. This unique design could enhance conductivity and accelerate electron transfer. As expected, AZIBs showed excellent cycle performance with a capacity decay rate of 0.014% per cycle during 2800 cycles as well as outstanding rate performance (76.6 mAh g–1 at 10 A g–1). Furthermore, AZICs offer an energy density of 48.8 Wh kg–1 at a power density of 100 W kg–1 and a capacity retention rate of up to 73.4% even after 10,000 cycles. These discoveries pave the way for the rational design of high-performance electrode materials and provide an innovative option for next-generation energy storage systems