Molecular processes of fiber formation and network genesis in polyethylene (PE) are studied by molecular dynamics simulation of crystallization from highly stretched melt. We consider a system ...comprising 150 molecules of 500 united atoms, which is much larger than that of our previous work on fiber structures and their deformation Polymer 2013, 54, 3086, aiming to study crystal nucleation and growth separately. Rapid elongation of the isotropic melt gives rise to incipient crystal clusters or embryos much smaller than critical nuclei. Subsequent isothermal crystallization of the stretched melt leads to three distinct stages of structure formation. The initial stage is a period waiting for critical nucleation. In the second stage that follows, we observe independent growth of isolated lamellae and resulting polynomial increases in crystallinity. In the last stage the crystals come to collide with each other to give well-aligned stacked lamellae. By dividing the system into mesh cells (pixels) and using an algorithm for image processing, we analyze the growth of clusters in detail in terms of their sizes and shapes. Despite highly anisotropic chain conformation in the melt, we find rather isotropic growth of the clusters both along and perpendicular to the fiber axis in the early stage of crystallization, where crystallites show linear growth of similar rate for each direction. The exception is the crystallization under high tension, where lateral growth of lamellae seems to be hindered indicating characteristic diffusion-controlled growth. We also study genesis and development of polymer network in situ by properly defining fold, tie, and cilium segments connected to the growing crystallites. In the early stage of the network genesis, small crystal clusters are connected to very long cilia which are forming the dominant component of the system. In the following stage of network development, where the crystallites show marked growth in size, the long cilia are rapidly reeled into the crystalline region, and the folds and ties are continually tightened. Through statistical analyses of the folds and ties during crystallization, we find that both of them are initially slack and have broad length distributions, but they continually tighten and come to have characteristic asymptotic distributions. Of special interest is that the tie molecules have rather stretched conformation even after sufficiently long crystallization, which is an indication of memory of crystallization process from the highly stretched melt.
Molecular simulation is attracting great interest in these days as a powerful tool to reveal the molecular mechanism of polymer crystallization. Most of the studies reported hitherto have dealt with ...simple model polymers like polyethylene, and the majority of polymers having more complex chemical structures remain almost untouched. In this report, we make a new challenge to the crystallization in a typical helical polymer isotactic polypropylene (iPP). We consider a relatively small system of 40 iPP oligomers each made of 50 propylene monomers by use of a realistic flexible model. The crystallization of iPP has long been considered too sluggish to be investigated by molecular simulations. We here take advantage of the accelerated crystallization from the highly stretched amorphous state. By carrying out very long simulations of about 600 ns total, we succeed in observing the onset of crystallization and subsequent crystal growth. Very unexpectedly, we come to see the growth of the smectic mesophase, which is believed to grow at temperatures much lower than the melting point. For the growth of crystals of stable forms of definite chirality, such as the α-form or the β-form, the chirality selection process will need a much longer time. The hypothesis is that the very fast crystallization observed here does not allow the slower processes of helical selection and gives rise to the formation of the smectic mesophase with somewhat random disposition of helices.
Fiber formation from highly stretched melt and mechanical deformation of the fiber are investigated by molecular dynamics simulations. Polyethylene molecules of moderate chain length comprising 513 ...united atoms are simulated under constant temperature and stress (NTτ) conditions. Rapid crystallization into fiber structures of alternating crystalline amorphous layers is observed, where emergence of very thin lamellae and their subsequent thickening are found to be the basic mechanism of the fiber formation. The incipient lamellae are also suggested to have an intermediate degree of order. Mechanical elongation of the fiber along its axis reveals initial elastic deformation and large reorientation of the crystalline chains, which is followed by specific yielding due to chain slips in the crystals. Further elongation of the fiber leads to the formation of crazes and microfibrils. On the other hand, the transverse deformation of the oriented sample exhibits much lower Young's modulus and larger plastic deformation. During the transverse deformation, the fiber texture is severely distorted giving rise to the pronounced melting and recrystallization (or breaking and reformation) of the crystalline textures, through which the oriented fiber accomplishes nearly complete 90° reorientation.
Display omitted
Various linear hydrocarbons in nature such as n-alkanes are usually found as mixtures of molecules of various chain-lengths. Molecular mechanisms of self-organization and underlying molecular motions ...in the mixtures are of great scientific and technological interest. By molecular dynamics simulations we here study crystallization in mixtures and diffusion in crystals in multicomponent systems of n-alkanes CnH2n+2 (abbreviated as Cn): binary mixtures of C10C20, a tertiary mixture of C10C14C20, and a quaternary mixture of C10C14C20C25. We find clear phase-separated crystallization in the binary mixtures of C10 and C20, the phase diagram and the texture of which correspond well to the eutectic ones inferred from experiments. On the other hand, the tertiary mixture is found to show rather uniform co-crystallization but with nanoscale domains made of single species of molecules. We find that the domains of closer chain-lengths tend to be adjacent while those of distant chain-lengths prefer to be separated in space; the inclination to demix for the chains of distinct lengths is found to be mediated by the intervention of chains of in-between lengths acting as compatibilizers. Similar mechanism is found operative in the quaternary system also. Through detailed investigations of the molecular dynamics in the tertiary mixture, we find novel modes of long-range diffusion in the crystal. The diffusive motions parallel to the chain axis are essentially two steps of molecular escape from a lamella followed by reentry into the neighboring lamella. On the other hand, the chain motions perpendicular to the chain axis are found to be of quite unexpected three steps; escape from a lamella followed by diffusion in the inter-lamellar region and reentry into the lamella.
Display omitted
•Structures and dynamics in mixtures are studied by molecular dynamics simulations.•Molecular scale structures in eutectic binary mixtures are revealed.•Nanodomains in multicomponent mixtures of n-alkanes are first found.•Diffusion in crystals is found dominated by the novel lamellar-boundary-diffusion mechanism.
Bacillus thuringiensis (Bt) has been used as sprayable pesticides for many decades. Bt strains utilized in these products produce multiple insecticidal proteins to complement a narrow insect ...specificity of each protein. In the late 1990s, genes encoding Bt insecticidal proteins were expressed in crop plants such as cotton and corn to protect these crops from insect damage. The first Bt protein used in transgenic cotton was Cry1Ac to control Heliothis virescens (tobacco budworm). Cry1Ab was applied to corn to control Ostrinia nubilalis (European corn borer). Since these insects have developed resistance to Cry1Ac and Cry1Ab, new Bt proteins are required to overcome the resistance. In order to protect corn furthermore, it is desired to control Diabrotica virgifera (Western corn rootworm), Helicoverpa zea (corn earworm) and Spodoptera frugiperda (fall armyworm). Recently, many new Bt insecticidal proteins have been discovered, but most of them require protein engineering to meet the high activity standard for commercialization. The engineering process for higher activity necessary for Bt crops is called optimization. The seed industry has been optimizing Bt insecticidal proteins to improve their insecticidal activity. In this review, several optimization projects, which have led to substantial activity increases of Bt insecticidal proteins, are described.
Crystalline polymers are very interesting and useful materials with great versatility through their potential morphology control. Recent surge in computer modeling studies has its origin both in ...increasing need for efficient methods of materials' design and in tremendous developments in computer power that is expected to meet the need. In this paper, we briefly survey the present state of computer modeling of polymer crystallization with the aim to foresee future developments. We first review simulations of crystallization in simple polymers under quiescent conditions where most of the efforts have hitherto been devoted. We also examine recent studies on crystallization under flow or large deformation. Then we present our ambitious plans to extend the simulation methods to polymers having complex chemical structures, though it is still an uncultivated field of research. We also refer to the new modeling strategies which integrate macroscopic and microscopic methods, and to the possibilities of molecular modeling in polymer nanotechnologies. Though our goal seems very far, there are obviously very fertile lands for the computer simulation studies.
Display omitted
Superconducting nanowire single-photon detector(SNSPD) has been one of the important ingredients for photonic quantum information processing (QIP). In order to see the potential of SNSPDs, I briefly ...review recent progresses of the photonic QIP with SNSPDs implemented for various purposes and present a possible direction for the development of SNSPDs.
See article vol.27: 1264-1277 Severe hypertriglyceridemia usually accompanies hyperchylomicronemia and is classified into types I and V hyperlipidemia characterized by an increase in chylomicrons ...alone and an increase in both chylomicrons and very-low-density lipoproteins (VLDLs), respectively. The extremely rare, monogenic form of hyperchylomicronemia is often referred as familial chylomicronemia syndrome (FCS) that usually manifests as type I hyperlipidemia during childhood or adolescence. The genetic basis of FCS has been extensively investigated, revealing that the primary underlying causes common to FCS are critical defects in the lipolytic function of lipoprotein lipase (LPL), a crucial rate-limiting enzyme for hydrolysis of triglycerides (TG) in both chylomicrons and VLDLs. Accordingly, monogenic, recessive disease-causing mutations have been found in LPL itself or other genes essential for LPL function (such as APOC2, APOA5, GPIHBP1, and LMF1) in the majority of FCS patients. Although FCS is a very rare clinical condition that is found in approximately 1～10 patients in a million people, severe hypertriglyceridemia can be more frequently experienced in general clinics or hospitals, typically in adult patients with type V hyperlipidemia.
PDF
