Evaluation of Philips and Ziegler-Natta high-density polyethylene degradation during processing in an internal mixer using the chain scission and branching distribution function analysis

The oxidative and thermo-mechanical degradation of HDPE was studied during processing in an internal mixer under two conditions: totally and partially filled chambers, which provides lower and higher concentrations of oxygen, respectively. Two types of HDPEs, Phillips and Ziegler-Natta, having different levels of terminal vinyl unsaturations were analyzed. Materials were processed at 160, 200, and 240 degrees C. Standard rheograrns using a partially filled chamber showed that the torque is much more unstable in comparison to a totally filled chamber which provides an environment depleted of oxygen. Carbonyl and transvinylene group concentrations increased, whereas vinyl group concentration decreased with temperature and oxygen availability. Average number of chain scission and branching (n(s)) was calculated from MWD curves and its plotting versus functional groups' concentration showed that chain scission or branching takes place depending upon oxygen content and vinyl groups' consumption. Chain scission and branching distribution function (CSBDF) values showed that longer chains undergo chain scission easier than shorter ones due to their higher probability of entanglements. This yields macroradicals that react with the vinyl terminal unsaturations of other chains producing chain branching. Shorter chains are more mobile, not suffering scission but instead are used for grafting the macroradicals, increasing the molecular weight. Increase in the oxygen concentration, temperature, and vinyl end groups' content facilitates the thermo-mechanical degradation reducing the amount of both, longer chains via chain scission and shorter chains via chain branching, narrowing the polydispersity. Phillips HDPE produces a higher level of chain branching than the Ziegler-Natta's type at the same processing condition. (c) 2006 Elsevier Ltd. All rights reserved.

Characterization of the Microstructure of Bimodal HDPE Resin

In this work, two industrial bimodal high density polyethylene resins, resin A and resin B having similar molecular weight (M-w), molecular weight distribution (M-wD), and short-chain branching (SCB) content but different mechanical properties, were fractionated through cross-fractionation. The fractions were further, characterized by GPC, C-13 NMR, DSC AND FT IR techniques. These two resins were firstly fractionated into two franctions, i.e. high-temperature and low temperature fractions, via preparative solution crystallization fractionation. Resin A with much better mechanical properties contains more high-temperature fractions with longer crystalizable sequences. The SCB content in the low temperature fraction of resin A is lower than the of resin B. Both low-temperature fractions were then further fractionated using solvent gradient fractionation (SGF). The characterization of SGF fraction indicates that most of the branches fall into the high molecular weight chain in both low-temperature fractions.

Polydispersity of ethylene sequence length in metallocene ethylene/alpha-olefin copolymers. I. Characterized by thermal-fractionation technique

In this article, the polydispersity of the ethylene sequence length (ESL) in ethylene/alpha-olefin copolymers was studied by atomic force microscopy (AFM) and the thermal-fractionation technique. The crystal morphology observation by AFM showed that morphology changed gradually with decreasing average ESL from complete lamellae over shorter and more curved lamellae to a granular-like morphology, and the mixed morphology was observed after stepwise crystallization from phase-separated melt. This result indicated that the ethylene sequence with different lengths crystallized into a crystalline phase with a different size and stability at the copolymer systems. The thermal-fractionation technique was used to characterize the polydispersity of ESL. Three of the following statistical terms were introduced to describe the distribution of ESL and the lamellar thickness: the arithmetic mean (L) over bar (n), the weight mean (L) over bar (w), and the broadness index I = (L) over bar (w)/(L) over bar (n). It was concluded that the polydispersity of ESL could be quantitatively characterized by the thermal-fractionation technique. The effects of temperature range, temperature-dependent specific heat capacity C-p of copolymer, and the molecular weight on the results of thermal fractionation were discussed,

Improved thermal fractionation technique for chain structure analysis of ethylene/alpha-olefin copolymers

In this report, we describe an improved thermal fractionation technique used to characterize the polydispersity of crystalline ethylene sequence length (CESL) of ethylene/alpha -olefin copolymers. After stepwise isothermal crystallization, the crystalline ethylene sequences are sorted into groups by their lengths. The CESLs are estimated using melting points of known hydrocarbons. The content of each group is determined using the calibrated peak area. The statistical terms: the arithmetic mean (L) over bar (n), the weighted mean (L) over bar (w) and the broadness index I = (L) over bar (w)/(L) over bar (n) are used to describe the distribution of CESL. Results show that improved thermal fractionation technique can quantitatively characterize the polydispersity of CESL with a high degree of accuracy.

Using Inside-Outside Algorithm for Estimation of the Offspring Distribution in Multitype Branching Processes

Multitype branching processes (MTBP) model branching structures, where the nodes of the resulting tree are particles of different types. Usually such a process is not observable in the sense of the whole tree, but only as the “generation” at a given moment in time, which consists of the number of particles of every type. This requires an EM-type algorithm to obtain a maximum likelihood (ML) estimate of the parameters of the branching process. Using a version of the inside-outside algorithm for stochastic context-free grammars (SCFG), such an estimate could be obtained for the offspring distribution of the process.

Em Estimation of the Offspring Distribution in Mutitype Branching Processes - a Model in Cell Kinetics

2000 Mathematics Subject Classification: 60J80, 60J85, 62P10, 92D25.

Molecular dynamics studies of the effects of branching characteristics on the crystalline structure of polyethylene

Molecular dynamics simulations were carried out on single chain models of linear low-density polyethylene in vacuum to study the effects of branch length, branch content, and branch distribution on the polymer’s crystalline structure at 300 K. The trans/gauche (t/g) ratios of the backbones of the modeled molecules were calculated and utilized to characterize their degree of crystallinity. The results show that the t/g ratio decreases with increasing branch content regardless of branch length and branch distribution, indicating that branch content is the key molecular parameter that controls the degree of crystallinity. Although t/g ratios of the models with the same branch content vary, they are of secondary importance. However, our data suggests that branch distribution (regular or random) has a significant effect on the degree of crystallinity for models containing 10 hexyl branches/1,000 backbone carbons. The fractions of branches that resided in the equilibrium crystalline structures of the models were also calculated. On average, 9.8% and 2.5% of the branches were found in the crystallites of the molecules with ethyl and hexyl branches while C13 NMR experiments showed that the respective probabilities of branch inclusion for ethyl and hexyl branches are 10% and 6% [Hosoda et al., Polymer 1990, 31, 1999–2005]. However, the degree of branch inclusion seems to be insensitive to the branch content and branch distribution.

A description of the Mei2-like protein family; structure, phylogenetic distribution and biological context

The Schizosaccharomyces pombe Mei2 gene encodes an RNA recognition motif (RRM) protein that stimulates meiosis upon binding a specific non-coding RNA and subsequent accumulation in a “mei2-dot” in the nucleus. We present here the first systematic characterization of the family of proteins with characteristic Mei2-like amino acid sequences. Mei2-like proteins are an ancient eukaryotic protein family with three identifiable RRMs. The C-terminal RRM (RRM3) is unique to Mei2-like proteins and is the most highly conserved of the three RRMs. RRM3 also contains conserved sequence elements at its C-terminus not found in other RRM domains. Single copy Mei2-like genes are present in some fungi, in alveolates such as Paramecium and in the early branching eukaryote Entamoeba histolytica, while plants contain small families of Mei2-like genes. While the C-terminal RRM is highly conserved between plants and fungi, indicating conservation of molecular mechanisms, plant Mei2-like genes have changed biological context to regulate various aspects of developmental pattern formation.

Implications of geometric plasticity for maximizing photosynthesis in branching corals

Reef-building corals are an example of plastic photosynthetic organisms that occupy environments of high spatiotemporal variations in incident irradiance. Many phototrophs use a range of photoacclimatory mechanisms to optimize light levels reaching the photosynthetic units within the cells. In this study, we set out to determine whether phenotypic plasticity in branching corals across light habitats optimizes potential light utilization and photosynthesis. In order to do this, we mapped incident light levels across coral surfaces in branching corals and measured the photosynthetic capacity across various within-colony surfaces. Based on the field data and modelled frequency distribution of within-colony surface light levels, our results show that branching corals are substantially self-shaded at both 5 and 18 m, and the modal light level for the within-colony surface is 50 mu mol photons m(-2) s(-1). Light profiles across different locations showed that the lowest attenuation at both depths was found on the inner surface of the outermost branches, while the most self-shading surface was on the bottom side of these branches. In contrast, vertically extended branches in the central part of the colony showed no differences between the sides of branches. The photosynthetic activity at these coral surfaces confirmed that the outermost branches had the greatest change in sun- and shade-adapted surfaces; the inner surfaces had a 50 % greater relative maximum electron transport rate compared to the outer side of the outermost branches. This was further confirmed by sensitivity analysis, showing that branch position was the most influential parameter in estimating whole-colony relative electron transport rate (rETR). As a whole, shallow colonies have double the photosynthetic capacity compared to deep colonies. In terms of phenotypic plasticity potentially optimizing photosynthetic capacity, we found that at 18 m, the present coral colony morphology increased the whole-colony rETR, while at 5 m, the colony morphology decreased potential light utilization and photosynthetic output. This result of potential energy acquisition being underutilized in shallow, highly lit waters due to the shallow type morphology present may represent a trade-off between optimizing light capture and reducing light damage, as this type morphology can perhaps decrease long-term costs of and effect of photoinhibition. This may be an important strategy as opposed to adopting a type morphology, which results in an overall higher energetic acquisition. Conversely, it could also be that maximizing light utilization and potential photosynthetic output is more important in low-light habitats for Acropora humilis.

First Measurement of the Ratio of Branching Fractions B(Lambda_b to Lambda_c mu nu)/B(Lambda_b to Lambda_c pi)

The analysis uses data from an integrated luminosity of approximately 172 pb-1 of ppbar collisions at sqrt(s)=1.96 TeV, collected with the CDF II detector at the Fermilab Tevatron. The Lambda_b and B0 relative branching fractions are measured to be: B(Lambda_b to Lambda_c+ mu nu)/B(Lambda_b to Lambda_c+ pi) = 16.6 +- 3.0 (stat) +- 1.0 (syst) +2.6 -3.4 (PDG) +- 0.3 (EBR), B(B0 to D+ mu nu)/B(B0 to D+ pi) = 9.9 +- 1.0 (stat) +- 0.6 (syst) +- 0.4 (PDG) +- 0.5 (EBR), B(B0 to D*+ mu nu)/B(B0 to D*+ pi) = 16.5 +- 2.3 (stat) +- 0.6 (syst) +- 0.5 (PDG) +- 0.8 (EBR) This article also presents measurements of the branching fractions of four new Lambda_b semileptonic decays: Lambda_b to Lambda_c(2595)+ mu nu, Lambda_b to Lambda_c(2625)+ mu nu, Lambda_b to Sigma_c(2455)0 pi mu nu, Lambda_b to Sigma_c(2455)++ pi mu nu, relative to the branching fraction of the Lambda_b to Lambda_c mu nu decay. Finally, the transverse-momentum distribution of Lambda_b baryons produced in p-pbar collisions is measured and found to be significantly different from that of B0 mesons.

First measurement of the ratio of branching fractions B(Λb0→Λc+μ-ν̅ μ)/B(Λb0→Λc+π-)

This article presents the first measurement of the ratio of branching fractions B(Λb0→Λc+μ-ν̅ μ)/B(Λb0→Λc+π-). Measurements in two control samples using the same technique B(B̅ 0→D+μ-ν̅ μ)/B(B̅ 0→D+π-) and B(B̅ 0→D*(2010)+μ-ν̅ μ)/B(B̅ 0→D*(2010)+π-) are also reported. The analysis uses data from an integrated luminosity of approximately 172  pb-1 of pp̅ collisions at √s=1.96  TeV, collected with the CDF II detector at the Fermilab Tevatron. The relative branching fractions are measured to be B(Λb0→Λc+μ-ν̅ μ)/B(Λb0→Λc+π-)=16.6±3.0(stat)±1.0(syst)+2.6/-3.4(PDG)±0.3(EBR), B(B̅ 0→D+μ-ν̅ μ)/B(B̅ 0→D+π-)= 9.9±1.0(stat)±0.6(syst)±0.4(PDG)±0.5(EBR), and B(B̅ 0→D*(2010)+μ-ν̅ μ)/B(B̅ 0→D*(2010)+π-)=16.5±2.3(stat)± 0.6(syst)±0.5(PDG)±0.8(EBR). The uncertainties are from statistics (stat), internal systematics (syst), world averages of measurements published by the Particle Data Group or subsidiary measurements in this analysis (PDG), and unmeasured branching fractions estimated from theory (EBR), respectively. This article also presents measurements of the branching fractions of four new Λb0 semileptonic decays: Λb0→Λc(2595)+μ-ν̅ μ, Λb0→Λc(2625)+μ-ν̅ μ, Λb0→Σc(2455)0π+μ-ν̅ μ, and Λb0→Σc(2455)++π-μ-ν̅ μ, relative to the branching fraction of the Λb0→Λc+μ-ν̅ μ decay. Finally, the transverse-momentum distribution of Λb0 baryons produced in pp̅ collisions is measured and found to be significantly different from that of B̅ 0 mesons, which results in a modification in the production cross-section ratio σΛb0/σB̅ 0 with respect to the CDF I measurement.

Critical age-dependent branching Markov processes and their scaling limits

This paper studies:(i)the long-time behaviour of the empirical distribution of age and normalized position of an age-dependent critical branching Markov process conditioned on non-extinction;and (ii) the super-process limit of a sequence of age-dependent critical branching Brownian motions.

Supercritical age-dependent branching Markov processes and their scaling limits

This paper studies the long-time behavior of the empirical distribution of age and normalized position of an age-dependent supercritical branching Markov process. The motion of each individual during its life is a random function of its age. It is shown that the empirical distribution of the age and the normalized position of all individuals alive at time t converges as t -> infinity to a deterministic product measure.

Self-Assembly of Rod-Terminally Tethered Three-Armed Star-Shaped Coil Block Copolymer: Investigation of the Presence of the Branching in the Coil to the Self-Assembled Behavior

Self-assembled behavior of rod-terminally tethered three-armed star-shaped coil block copolymer melts was studied by applying self-consistent-field lattice techniques in three-dimensional (3D) space. Similar to rod-coil diblock copolymers, five morphologies were observed, i.e., lamellar, perforated lamellar, gyroidlike, cylindrical and sphericallike structures, while the distribution of the morphologies in the phase diagram was dramatically changed with respect to that Of rod-coil diblock copolymers.