Average Branch Length Research Articles

Power law tails in phylogenetic systems

Covariance analysis of protein sequence alignments uses coevolving pairs of sequence positions to predict features of protein structure and function. However, current methods ignore the phylogenetic relationships between sequences, potentially corrupting the identification of covarying positions. Here, we use random matrix theory to demonstrate the existence of a power law tail that distinguishes the spectrum of covariance caused by phylogeny from that caused by structural interactions. The power law is essentially independent of the phylogenetic tree topology, depending on just two parameters-the sequence length and the average branch length. We demonstrate that these power law tails are ubiquitous in the large protein sequence alignments used to predict contacts in 3D structure, as predicted by our theory. This suggests that to decouple phylogenetic effects from the interactions between sequence distal sites that control biological function, it is necessary to remove or down-weight the eigenvectors of the covariance matrix with largest eigenvalues. We confirm that truncating these eigenvectors improves contact prediction.

A Kolmogorov-Smirnov test for the molecular clock based on Bayesian ensembles of phylogenies.

Divergence date estimates are central to understand evolutionary processes and depend, in the case of molecular phylogenies, on tests of molecular clocks. Here we propose two non-parametric tests of strict and relaxed molecular clocks built upon a framework that uses the empirical cumulative distribution (ECD) of branch lengths obtained from an ensemble of Bayesian trees and well known non-parametric (one-sample and two-sample) Kolmogorov-Smirnov (KS) goodness-of-fit test. In the strict clock case, the method consists in using the one-sample Kolmogorov-Smirnov (KS) test to directly test if the phylogeny is clock-like, in other words, if it follows a Poisson law. The ECD is computed from the discretized branch lengths and the parameter λ of the expected Poisson distribution is calculated as the average branch length over the ensemble of trees. To compensate for the auto-correlation in the ensemble of trees and pseudo-replication we take advantage of thinning and effective sample size, two features provided by Bayesian inference MCMC samplers. Finally, it is observed that tree topologies with very long or very short branches lead to Poisson mixtures and in this case we propose the use of the two-sample KS test with samples from two continuous branch length distributions, one obtained from an ensemble of clock-constrained trees and the other from an ensemble of unconstrained trees. Moreover, in this second form the test can also be applied to test for relaxed clock models. The use of a statistically equivalent ensemble of phylogenies to obtain the branch lengths ECD, instead of one consensus tree, yields considerable reduction of the effects of small sample size and provides a gain of power.

Laser-induced construction of multi-branched CuS nanodendrites with excellent surface-enhanced Raman scattering spectroscopy in repeated applications.

We report on the successful fabrication of multi-branched CuS nanodendrites with average branch length of about 20 nm by laser ablation of bulk Cu target in thioacetamide (TAA) solution. During the nucleation of Cu and S species, the accurate anisotropic growth should be attributed to an ultra-rapid acid etching process by laser-induced TAA hydrolyzing reaction. Interestingly, the semiconductor CuS nanodendrites provide pronounced surface enhanced Raman scattering (SERS) properties with noble-metal comparable activity and a detection limit as low as ~10-10 M, approaching the requirement (~nM) for single molecule detection. More importantly, after SERS analysis, the crystal violet (CV) probe molecules can be effectively removed from the substrate by 1064nm laser irradiation-induced moderate thermal treatment. Therefore, the unique and distinctive advantage is that the as-prepared CuS nanodendrites exhibit excellent reusability for 60 cycles of repeated SERS analyses. The low-cost CuS semiconductor nanodendrites with enhanced SERS properties should be established as a prominent SERS-based ultrasensitive probe in the repeated applications.

Single-axon level morphological analysis of corticofugal projection neurons in mouse barrel field

Corticofugal projection neurons are key components in connecting the neocortex and the subcortical regions. In the barrel field, these neurons have various projection targets and play crucial roles in the rodent whisker sensorimotor system. However, the projection features of corticofugal projection neurons at the single-axon level are far from comprehensive elucidation. Based on a brain-wide positioning system with high-resolution imaging for Thy1-GFP M-line mice brains, we reconstructed and analyzed more than one hundred corticofugal projection neurons in both layer V and VI of barrel cortex. The dual-color imaging made it possible to locate the neurons’ somata, trace their corresponding dendrites and axons and then distinguish the neurons as L5 type I/II or L6 type. The corticofugal projection pattern showed significant diversity across individual neurons. Usually, the L5 type I neurons have greater multi-region projection potential. The thalamus and the midbrain are the most frequent projection targets among the investigated multidirectional projection neurons, and the hypothalamus is particularly unique in that it only appears in multidirectional projection situations. Statistically, the average branch length of apical dendrites in multi-region projection groups is larger than that of single-region projection groups. This study demonstrated a single-axon-level analysis for barrel corticofugal projection neurons, which could provide a micro-anatomical basis for interpreting whisker sensorimotor circuit function.

Overhead arbor tree shape affects tree development and increases fruit yields in sweet cherry

We evaluated different shapes of sweet cherry trees for their effects on cherry production, and tree development. The tree shapes were a palmette shape (4×7 m), a Y-palmette shape (7×4 m), an open-vase shape (5×4 m), and an overhead arbor shape (6×5 m). After inducing branches on trees of each shape in the first 3 years, tree growth, fruit production, flower bud formation, and labor hours to shape trees were assessed after seven years. The number of new branches was much higher in the overhead arbor shape (10,666 10 a-1) than in the palmette shape (2,867 10 a-1), the Y-palmette shape (6,178 10 a-1), and the open-vase shape (7,250 10 a-1). The average length of new branches was much smaller in the overhead arbor shape (39.6 cm) than in the palmette shape (55.5 cm), the Y-palmette shape (60.6 cm), and the open vase shape (44.0 cm). The results indicate that increasing numbers and decreasing length of new branches are related to tree shape of sweet cherry. Fruit yields were higher 1.86-3.15 times with the overhead arbor shape (916 kg 10 a-1) than with the palmette shape (291 kg 10 a-1), the Y-palmette shape (493 kg 10 a-1), and the open-vase shape (478 kg 10 a-1). A similar trend was observed among the four shapes in the levels of flower bud formation. The lowest amount of labor time was required for developing the open-vase shape (21.9 h 10 a-1) because new branches were not induced artificially in this shape, while the maximum labor time was required to develop overhead arbor shape (47.2 h 10 a-1). The study demonstrated that the overhead arbor shape has potential for small farmers in South Korea.

Evidence that remodeling of insular cortex neurovascular unit contributes to hypertension-related sympathoexcitation.

The intermediate region of the posterior insular cortex (intermediate IC) mediates sympathoexcitatory responses to the heart and kidneys. Previous studies support hypertension‐evoked changes to the structure and function of neurons, blood vessels, astrocytes and microglia, disrupting the organization of the neurovascular unit (NVU). In this study, we evaluated the functional and anatomical integrity of the NVU at the intermediate IC in the spontaneously hypertensive rat (SHR) and its control the Wistar‐Kyoto (WKY). Under urethane anesthesia, NMDA microinjection (0.2 mmol/L/100 nL) was performed at the intermediate IC with simultaneous recording of renal sympathetic nerve activity (RSNA), heart rate (HR) and mean arterial pressure (MAP). Alterations in NVU structure were investigated by immunofluorescence for NMDA receptors (NR1), blood vessels (70 kDa FITC‐dextran), astrocytes (GFAP), and microglia (Iba1). Injections of NMDA into intermediate IC of SHR evoked higher amplitude responses of RSNA, MAP, and HR. On the other hand, NMDA receptor blockade decreased baseline RSNA, MAP and HR in SHR, with no changes in WKY. Immunofluorescence data from SHR intermediate IC showed increased NMDA receptor density, contributing to the SHR enhanced sympathetic responses, and increased in vascular density (increased number of branches and endpoints, reduced average branch length), suggesting angiogenesis. Additionally, IC from SHR presented increased GFAP immunoreactivity and contact between astrocyte processes and blood vessels. In SHR, IC microglia skeleton analysis supports their activation (reduced number of branches, junctions, endpoints and process length), suggesting an inflammatory process in this region. These findings indicate that neurogenic hypertension in SHR is accompanied by marked alterations to the NVU within the IC and enhanced NMDA‐mediated sympathoexcitatory responses likely contributors of the maintenance of hypertension.

Association and Path Coefficient Analysis of Yield and Yield Attributes of Coffee (Coffea arabica L.) Under Sidama Specialty Coffee Growing Area, Awada, Southern Ethiopia

One hundred twenty germplasm collections of arabica coffee and four standard checks were evaluated for 19 quantitative traits using augmented design with four replications of the checks at Sidama specialty coffee ecology, Southern Ethiopia. The objective of the study was to address the nature of characters correlation and path coefficient among various quantitative traits. The result showed that in most of the traits, genotypic correlation coefficients were higher than the corresponding phenotypic correlation coefficients. Hundred green bean weight, stem diameter, canopy diameter, average length of primary branches, percent of bearing primary branches and leaf length were the most important traits which possessed positive association with coffee green bean yield. Likewise, path coefficient analysis was revealed that among the different yield contributing traits plant height, leaf length, leaf width, hundred green bean weight, percentage of bearing primary branches, stem diameter, average length of primary branches and coffee berry disease infestation influenced green bean weight directly and indirectly. Thus, selection for coffee green bean yield in coffee through these traits would be effective in order to boost the coffee production in Ethiopia.

Reducing long-branch effects in multi-protein data uncovers a close relationship between Alveolata and Rhizaria

Rhizaria is a major eukaryotic group of tremendous diversity, including amoebae with spectacular skeletons or tests (Radiolaria and Foraminifera), plasmodial parasites (Plasmodiophorida) and secondary endosymbionts (Chlorarachniophyta). Current phylogeny places Rhizaria in an unresolved trichotomy with Stramenopila and Alveolata (supergroup “SAR”). We assembled a 147-protein data set with extensive rhizarian coverage (M147), including the first transcriptomic data for a euglyphid amoeba. Phylogenetic pre-screening of individual proteins indicated potential problems with radically misplaced sequences due either to contamination of rhizarian sequences amplified from wild collected material and/or extremely long branches (xLBs). Therefore, two data subsets were extracted containing either all proteins consistently recovering rhizarian monophyly (M34) or excluding all proteins with ⩾3 xLBs (defined as ⩾2× the average terminal branch length for the tree). Phylogenetic analyses of M147 give conflicting results depending on the outgroup and method of analysis but strongly support an exclusive Rhizaria+Alveolata (R+A) clade with both data subsets (M34 and M37) regardless of phylogenetic method used. Support for an R+A clade is most consistent when a close outgroup is used and decreases with more distant outgroups, suggesting that support for alternative SAR topologies may reflect a long-branch attraction artifact. A survey of xLB distribution among taxa and protein functional category indicates that small “informational” proteins in particular have highly variable evolutionary rates with no consistent pattern among taxa.

Three-dimensional quantification of microglial ramification in the mouse brain after chronic restraint stress

Microglia play a pivotal role in the brain development and plasticity through synaptic pruning. Morphological change of microglia reflects a key feature in this process and has been used to define microglial activation. Most studies have applied two-dimensional (2D) imaging method to evaluate microglial morphology so far. We have established a 3D analytical approach in a mouse chronic restrain-stress (CRS, 2 h/day × 10 days) model with C57BL/6J male mice. Eighty microglia (20 microglia/brain area/treatment) in the infralimbic and M2 areas of the medial prefrontal cortex were imaged and reconstruct into 3D digital images, from which a semi-automatic method was used to track down microglial branches and several parameters were calculated by using the 3D analysis tools in ImageJ. In Ctrl animals, microglia in the infralimbic area had significantly lower average branch number and longer average branch length than in the M2 area (both p 0.05). After CRS, significant increase in the average branch number of microglia was only observed in the infralimbic area (p 0.05). However, the average branch length was decreased in the infralimbic microglia after CRS (p 0.05), whereas the M2 microglia showed increased maximal branch length (p 0.05). These results suggests that activated microglia increase their structural complexity in an area-specific manner, but it is hard to simply use over- or de-ramification to describe their activation status.

BRANCH DEVELOPMENT OF EUCALYPTS MANAGED FOR SAWLOGS

ABSTRACT Species of the genus Eucalyptus managed for sawlogs in fast-growing plantations show high potential for substitution for valuable native hardwoods. To obtain high quality wood, technical pruning is necessary. The objective of the study is to analyse the development of the branches for the first and second pruning lifts and to quantify the biomass of the pruned branches. For the study two stands of Eucalyptus grandis were selected (age 18 and 36 months) for evaluating a pruning lift from 0 to 3 m and 3 to 6 m. The average branch diameter and length were 18 mm and 2.1 m in the younger stand and 21 mm and 2.3 m in the older one. The relation between branch diameter and branch length could be expressed in a linear model (R² = 0.8). In both stands a higher proportion of branches were already dead. The oven-dry biomass of the pruned branches was 2.2 ton.ha-1 in the first pruning lift and 1.2 ton.ha-1 in the second. The results showed that branch development in wide spaced and early thinned eucalypt plantations is in line with the objective of high quality wood production. Pruning should take place before 18 month to avoid dead branches. The oven-dry branch biomass cut in the two pruning lifts shows a low volume making a commercial utilization difficult.

3D Analysis of PEM Fuel Cell Membrane Cracks Using X-Ray Computed Tomography

Crack and pinhole formation in polymeric membranes of fuel cells is understood to be caused by combined chemical, mechanical, and/or thermal mechanisms that are active during operation under typical automotive duty cycles [1] . This type of damage leads to gas leakage through the membrane thereby compromising its functionality and overall durability of the fuel cell system. Due to the innermost location of membrane within the fuel cell membrane electrode assembly, it has been impossible to explore the full features and morphology of membrane cracks with conventional 2D imaging techniques such as optical and electron microscopy [2]. Moreover, these techniques are destructive in nature, demand tedious sample preparation with risk of artifacts, operate under vacuum, and/or are suitable only for electrically conductive samples. The X-ray computed tomography (XCT) technique overcomes these traditional limitations and opens up a novel non-destructive 3D characterization method for imaging interior features of an object. Synchrotron facilities have been typically used for 3D imaging but their access is limited and costly. This work leverages the XCT technology using laboratory-based systems developed by ZEISS Xradia® to image membrane cracks in their natural state within the membrane electrode assembly (MEA) and gain a comprehensive insight into their features and characteristics. An end-of-life (EOL) MEA sample, subjected to cyclic open circuit voltage (COCV) accelerated stress test (AST) protocol [3], is analyzed upon failure using the XCT technique and compared with a similar beginning-of-life (BOL) sample. Based on their reach in the through-plane direction, cracks penetrating the entire membrane thickness are categorized into: (i) Exclusive cracks that remain confined within the membrane; and (ii) Non-exclusive cracks that extend into and penetrate through the anode and/or cathode catalyst layers. The very presence of Exclusive cracks (cf.Fig. 1), without any adjoining cracks in the catalyst layers, indicates the likelihood of crack initiation within the membrane. An examination of 9 membrane cracks spread over a 0.88 mm2area reveals that more than 50% of the cracks fall into the Exclusive category. This percentage is found to be significantly higher than the 13% overall interaction of catalyst layer cracks with the membrane cracks. In the in-plane direction, some membrane cracks are found to propagate as a single entity forming a curved I-shape while the others branch out once forming a Y-shape. Distribution of the two shapes is approx. equal among the analyzed cracks. The smaller Y-cracks (<20 μm average branch length), which are likely to be in their initial phase of development, tend to propagate at equal rate in all three directions. The total in-plane crack length in the membrane is found to have a linear relationship with the maximum crack width. The crack width remains almost uniform along its length and tapers sharply at the ends indicating that the crack propagation in the membrane is mechanical in nature caused by the cyclic in-plane stresses. The effect of these stresses seems to have been exacerbated by the observed non-uniform reduction in membrane thickness resulting in stress concentration locations within the membrane. During this analysis, a strong probability for crack development is observed at such locations with membrane thickness found to be approx. 30% lower at the crack sites than its average EOL bulk value. The work reported here is unprecedented from the perspective that, for the first time, a truly 3D view of a membrane crack is achieved; thus, making this comprehensive analysis possible. These results, establish the XCT as a breakthrough approach for reliable failure analysis of fuel cell membranes. Acknowledgements This research is financially supported by Ballard Power Systems and Automotive Partnership Canada (APC). The authors thank Chan Lim, Lida Ghassemzadeh and Erin Rogers for providing samples and technical support.

Clinical considerations of the glandular branch of the lacrimal artery.

The lacrimal artery is classically described as a branch of the ophthalmic artery supplied by the internal carotid. In this study, 25 orbits were dissected to identify variations in glandular branching and to compare them to previously published accounts. The glandular branching patterns of the lacrimal artery fall into two categories, those that branch (56%) and those that do not branch (44%). We found the medial and lateral glandular branches to be equal in diameter with a divergence of 2.67-40.58 mm proximal to the gland parenchyma. The long glandular branches run alongside the superolateral aspect of the orbit. The lateral branch runs lateral to the lateral rectus muscle. The medial branch runs superomedial to the lateral rectus muscle and lateral to the superior rectus muscle. In relation to the lacrimal gland, the medial branch enters the superior aspect of the gland parenchyma and the lateral branch enters its inferior aspect. The average branch lengths were 17.88 mm (medial) and 13.51 mm (lateral) as measured with a Mitutoyo Absolute 1/100 mm caliper. We could not confirm the existence of a third branch supplying the lacrimal gland, as posited by other authors. The key finding in this study is that the lacrimal gland is predominantly supplied by two significant arterial branches, both of which must be identified during procedures involving the lateral orbit.

Numerical study of a cylinder model of the diffusion MRI signal for neuronal dendrite trees

We study numerically how the neuronal dendrite tree structure can affect the diffusion magnetic resonance imaging (dMRI) signal in brain tissue. For a large set of randomly generated dendrite trees, synthetic dMRI signals are computed and fitted to a cylinder model to estimate the effective longitudinal diffusivity DL in the direction of neurites. When the dendrite branches are short compared to the diffusion length, DL depends significantly on the ratio between the average branch length and the diffusion length. In turn, DL has very weak dependence on the distribution of branch lengths and orientations of a dendrite tree, and the number of branches per node. We conclude that the cylinder model which ignores the connectivity of the dendrite tree, can still be adapted to describe the apparent diffusion coefficient in brain tissue.

Feasibility of Using External Jugular Vein and Its Branches as Y- and X-Shaped Vein Grafts for Bridging of Arterial Defects and Providing Additional Arterial Sources for Free Flap Applications in Rat Model

Venous grafting has been widely used in microsurgical training. Different types of vascular grafts have been described in experimental models. In this study we describe for the first time the Y- and X-shaped vein grafts (YVG, XVG) with accompanying drain-out branches as a new tool for the microsurgical training and free flap applications in rats. Twelve adult male Lewis rats were used in this study. The dissections were performed to determine the average diameter and harvestable length of vein grafts in eight rats. In four rats vein grafts were applied for bridging of the common carotid artery gap, whereas the drain-out branches were used as the arterial source for single and bilateral free groin flap applications. The venous anastomoses of groin flaps were performed in end-to-end fashion to the external jugular vein and its branches. The patency of anastomoses was checked 72 hours after repair. The average length of the harvestable vein branches ranged between 5.2 to 11.8 mm. The average surgery time for repair of the arterial gap with the vein grafts was 40 minutes. The ischemia time for single and bilateral groin flap transfer using YVG and XVG was 30 and 70 minutes, respectively. The patency of the interpositional vein graft was 100%. Flap survival rates were 50%. These vein grafts can be used as an alternative technique for reconstruction of tissue defects that require arterial gap repair with single or multiple free flap applications and also as a new microsurgical training model.

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The objective of this study was to characterize the damage caused by Oncideres saga in plantations of Acacia mearnsii . The percentage of trees attacked was evaluated and the diameter and length of the branches were measured. It was observed that 34.3 % of the trees of A. mearnsii presented branches cut by O. saga . The average diameter and length of the cut branches were 5.75 cm and 4.95 m, respectively. The lower third of the girdled branches showed 64.23% of the posture incisions.

Control of Perfusable Microvascular Network Morphology Using a Multiculture Microfluidic System

The mechanical and biochemical microenvironment influences the morphological characteristics of microvascular networks (MVNs) formed by endothelial cells (ECs) undergoing the process of vasculogenesis. The objective of this study was to quantify the role of individual factors in determining key network parameters in an effort to construct a set of design principles for engineering vascular networks with prescribed morphologies. To achieve this goal, we developed a multiculture microfluidic platform enabling precise control over paracrine signaling, cell-seeding densities, and hydrogel mechanical properties. Human umbilical vein endothelial cells (HUVECs) were seeded in fibrin gels and cultured alongside human lung fibroblasts (HLFs). The engineered vessels formed in our device contained patent, perfusable lumens. Communication between the two cell types was found to be critical in avoiding network regression and maintaining stable morphology beyond 4 days. The number of branches, average branch length, percent vascularized area, and average vessel diameter were found to depend uniquely on several input parameters. Importantly, multiple inputs were found to control any given output network parameter. For example, the vessel diameter can be decreased either by applying angiogenic growth factors--vascular endothelial growth factor (VEGF) and sphingosine-1-phsophate (S1P)--or by increasing the fibrinogen concentration in the hydrogel. These findings introduce control into the design of MVNs with specified morphological properties for tissue-specific engineering applications.

3D Tree Reconstruction from Simulated Small Footprint Waveform Lidar

Lidar-based 3D tree reconstruction enables the retrieval of detailed tree structure; however, many existing methods are based on high-density discrete return lidar datasets. In this paper, we propose the use of small footprint waveform lidar data to achieve branch-level tree reconstruction for both leaf-off and leaf-on conditions. The DIRSIG simulation environment was used for algorithm validation purposes. Leaf-off data served as reference, and leaf-on reconstruction for a particular tree resulted in an average branch length difference of 0.07 m and an average angular difference of approximately 6 degrees for both tilt and azimuth angles. Compared to in situ methods this approach may be used by an airborne system for accurate estimation of forest biomass, forest inventory, land degradation, etc. in large scale applications. Furthermore, since this approach can also be applied on leaf-on trees, the tree skeleton characterization eventually can be conducted year round and will be less dependent on seasonal changes.

Synthesis of well-defined long chain branched polyethylene via anionic polymerization combined with graft-onto method

Comb-like polyethylene(PE) was prepared via anionic polymerization combined with "graft-onto" process. The polybutadiene(PB) backbone underwent hydroxylation at 1,2-vinyl groups to obtain a controlled number of hydroxyl groups along the main chain. After the translation of hydroxyl groups to tosyl groups, a nucleophilic substitution by living anionic PB chains was achieved. The comb PE was finally obtained by the hydrogenation of the obtained unsaturated comb polymer. Since the living anionic polymerization was used to prepare the backbones and the branch chains, molecular weight to molecular weight distribution(M-w/M(n)1.5) can be well-controlled in the final comb polymer, including the average number and length of branches.

Tillage effects on topsoil structural quality assessed using X-ray CT, soil cores and visual soil evaluation

Soil structure plays a key role in the ability of soil to fulfil essential functions and services in relation to, e.g., root growth, gas and water transport and organic matter turnover. The objective of this paper was: (1) To quantify tillage effects on soil structural quality in the entire topsoil layer (0–20cm) using X-ray CT, visual evaluation and traditional core methods; and (2) To correlate pore network characteristics from X-ray CT imaging with the results from the visual evaluation and the core method. Samples were taken in May 2009 from a long-term rotation and tillage field experiment on a Danish sandy loam. The tillage treatments were direct drilling (D) and ploughing (P). For X-ray CT scanning, we sampled large soil cores (Ø=20cm, height=20cm) from the top layer. Small 100cm3 samples were taken from the 4–8 and 12–16cm layers for water content and bulk density measurements. Visual soil structure evaluation was carried out in the field at the same time as sampling. CT images (0.39×0.39×0.6mm3 voxels) were produced using a medical X-ray CT scanner. The visual assessment showed a good structural quality in the top 5–8cm for both treatments (Sq<2). A poorer soil structure was observed in lower part or the topsoil where a firm structure (Sq=2.9) was observed for D and relatively friable structure (Sq=2.2) for P. Lower bulk density was found for P than for D in the 4–8cm layer (1.34 and 1.52gcm−3, respectively), whereas relatively high bulk density values were observed for both treatments in the 12–16cm layers (1.50 and 1.56gcm−3, respectively). The X-ray CT image analysis showed that the P soil had more networks, branches and junctions but a lower degree of anisotropy and shorter average branch length than the D soil. The image data also confirmed a clear stratification of the 0–20cm topsoil layer for both tillage treatments. The stratification of the direct drilled soil was in accordance with our expectations whereas it was surprising for the ploughed soil. The dense lower topsoil layer for the ploughed soil was probably caused by compaction during secondary tillage and natural consolidation, and aggravated by a poor structural stability due to a low organic matter content. The visual soil evaluation scores were negatively correlated to soil porosity and number of pore networks estimated from X-ray CT imaging and positively correlated to the macropore characteristics of branch length and pore thickness.

SSA study of early polyethylenes degradation stages. Effects of attack rate, of average branch length, and of backbone polymethylene sequences length distributions

Low-density polyethylenes have been attacked by free radicals in the presence of oxygen to promote chains scission, aiming at showing effects of the molecular structure on the thermal behavior as determined by using a Successive Self-Nucleation and Annealing (SSA) technique. Three types of low-density polyethylenes were used: a hydrogenated polybutadiene (HPB), as a model for a statistical linear ethylene–butene copolymer, a linear ethylene–butene copolymer (LLDPE) with short branches not randomly distributed along the main chains, and a low-density polyethylene (LDPE) with randomly placed long and short branching. All the polymers were subjected to thermal oxidation in air. Additionally, the LDPE and HPB were exposed to gamma irradiation in air, while the HPB was also degraded with an organic peroxide. By virtue of the thermally activated oxidation, the three polymers show signs of molecular scission evidenced by the appearance of endotherms in the high temperature range of the final SSA thermogram. These endotherms are associated with the melting of thicker crystals formed by longer polymethylene sequences that were not available for crystallization in the original polymer. Similar indication of chain scission was detected in some of the irradiated HPB. In the case of the irradiated LDPE or the HPB modified with peroxide, the main effect observed was crosslinking that was identified by a reduction of the area of the higher melting endotherms in the final SSA thermogram.