Regular Dispersion Research Articles

Effect of Functionalized Polypropylene on Thermal, Mechanical, and Morphological Properties of Polypropylene/Nylon-6 Blends

Abstract The compatibilizing effect of polypropylene-graft-butyl acrylate (PP-g-BA) on the morphological, thermal, and mechanical properties of polypropylene and nylon-6 (Ny-6) blends was investigated. Addition of 2.4% (w/w) PP-g-BA compatibilizer in binary PP/Ny-6 blends results in more regular and finer dispersions of the two phases and improved mechanical properties. Various mathematical models have been used to explain the mechanical properties of these blends. Molau's test and MFI data support the formation of PP-g-Ny-6 due to chemical reaction between the terminal amino groups of Ny-6 and the ester groups of PP-g-BA copolymer.

The effect of a partly hydrolyzed styrene‐tert‐butyl acrylate diblock copolymer on the properties of poly(2,6‐dimethyl‐1,4‐phenylene ether) (PPE) and polyamide‐6 (PA) blends

AbstractThe compatibilizing effect of a polystyrene‐hydrolyzed poly(t‐butyl acrylate) diblock copolymer (SBAH) on the phase structure, rheological properties, and mechanical properties of immiscible poly(2,6‐dimethyl‐1,4‐phenylene ether) (PPE) and polyamide‐6 (PA) blends was investigated. The SBAH was prepared by sequential anionic polymerization of styrene and t‐butyl acrylate, followed by acid‐catalyzed hydrolysis of t‐butyl acrylate block. Scanning electron micrographs show that the blends exhibit a more regular and finer dispersion when the SBAH of 47% hydrolyzed t‐butyl acrylate block is added. By addition of small amount of the block copolymer, the blends show non‐Newtonian power‐law behavior, and the contribution of storage modulus (G′) to the total response increases. Solubility tests support the formation of graft copolymer by chemical reaction between amine groups of the PA and carboxyl groups of the SBAH. Both modulus and strength are improved about 20% with addition of the 3 wt% SBAH, while the elongation at break decreases notably; thus, the blends fail in a brittle manner.

Influence functions for rank-based procedures in the linear model

Throughout the development of rank-based methods, the influence function has often been ignored. In this paper, we give the influence functions for the scores rank estimate and the corresponding drop test (based on the difference in dispersion between the full and reduced fits). We show how the influence function measures resistance of the estimator to contamination and how proper score selection leads to more resistant procedures. Furthermore, the asymptotic distribution of the estimator and test can be inferred from the influence function. We also introduce the drop test statistic for the weighted rank-based procedure of Naranjo and Hettmansperger. This test statistic has an influence function that is bounded in both x and y space, and reduces to the regular rank dispersion test using Wilcoxon scores in the unweighted case.

Spatial pattern, regeneration and growth rates of Brachystegia spiciformis and Julbernardia globiflora

Past work on tree-to-tree interactions in semi-arid savannas, through the use of nearest neighbour analysis, has shown both a regular dispersion pattern and a positive correlation between the size of a tree and the distance to its nearest neighbour. From these results, the importance of competition has been inferred. In the present study, tree-to-tree interactions in a mesic savanna woodland at Marondera, Zimbabwe, dominated by relatively small-sized individuals of Brachystegia spiciformis and Julbernardia globiflora, were examined. Results show that the regular dispersion pattern is uncommon; that there are few significant positive correlations between size of individual and distance to nearest neighbour; and that growth rate is not consistently correlated with size and distance of nearest neighbour. Results of a study of regeneration relative to canopy cover suggest a possible mechanism for these findings: unlike semi-arid savannas, young plants often grow in under-canopy environments so that early spacing of young trees does not occur. Significant positive correlations between the size of a tree and the distance to its nearest neighbour were found in a stand of larger, presumably older, trees, suggesting that such a relationship only develops in mesic miombo woodland through a thinning process as the trees mature. Familial clumping, which is common in the study area, may mask spatial patterns.

Spatial Model of Movement and Foraging in Harvester Ants ( Pogonomyrmex) (II): The Roles of Environment and Seed Dispersion

Individual-based models provide a framework for understanding individual and collective behavior of a complex system, such as social insects. We applied an individual-based simulation model of foraging of the western harvester ant ( Pogonomyrmex occidentalis) to two environments, shrub-steppe and shortgrass-steppe, in which the movement characteristics of individual ants differed according to vegetation structure. Using regular, clumped, and fractal seed dispersions, we simulated the influence of environment and foraging speed on rates of seed harvest by a colony of ants. The relative importance of individual and co-operative foraging behavior, where ants use spatial memory or chemical recruitment, was also explored in conjunction with environment and seed dispersion. Our replicated modeling "experiment" permitted us to study the three-way interaction among environment, seed dispersion, and behavior in the rates of seed harvest by ants. When foraging individually or co-operatively at observed movement rates, ants had similar harvest rates of seeds in a regular dispersion. In contrast, ants rapidly removed seeds from a fractal dispersion of seeds when foraging co-operatively. This suggests that differences in ant behavior were less important to the rate of food intake when foraging in a homogeneous resource dispersion, whereas co-operative foraging was most responsive to food resources that were spatially correlated over a range of scales. A slight difference was found between observed and modeled distributions of ant foraging distances in the shrub-steppe environment, while observed and modeled distributions were not significantly different in the shortgrass-steppe. Thus, our simulation model captured many of the foraging properties of individual ants, although it did not always accurately describe the spatial distribution of foraging ants. Our application to P. occidentalis shows that processes operating in a social unit, such as the rate of food intake by an ant colony, are strongly influenced by the interplay of the environment, resource dispersion, and the behavioral responses of individuals to food resources. This interaction arises because of the spatial properties of both foraging ants and seed resources, which suggest that spatially explicit modeling approaches may yield different insights into foraging than those based on energetic criteria.

Microstructure and properties of polyester/urethane acrylate thermosetting blends, and their use as composite matrices

Thermosetting blends of a rigid polyester (PE) and a poly(urethane acrylate) elastomer (UA) have been investigated over a wide range of compositions in the cast condition, and in use as matrices of unidirectional glass-fibre composites. Transmission electron microscopy showed that blends in the composition range 20%–60% UA have a two-phase structure, probably with a phase inversion from PE-rich to UA-rich matrix between 40% and 50% UA. It was found that the observed variation in Young's modulus with blend composition could be represented by a simple geometrical model based on series/parallel combination of phases with a regular dispersion. This analysis provided supporting evidence for the proposed phase structures of the blends. Further evidence as to the phase structures was the rapid decline in indentation hardness and in temperature of distortion under load which occurred between 40% and 50% UA. Blends with up to 15% UA were found to have higher tensile strength, and slightly higher failure strain, than the unblended polyester. For composites with PE-rich matrices, the transverse Young's modulus exceeded that of the matrix in bulk, and it was found that the relationship could be expressed approximately by a version of the series/parallel model referred to above. Transverse tensile strengths of the composites were, in all cases, lower than the bulk matrix strength. To account for the observed relationship, a modified version of the Cooper and Kelly model for transverse strength is presented.

The effect of a styrene-methyl methacrylate block copolymer on the morphological, rheological and mechanical properties of poly(2,6-dimethyl-1,4-phenylene ether) (PPE) and poly(hydroxy ether of bisphenol A) (Phenoxy) blends

The compatibilizing effect of styrene-methyl methacrylate block copolymer (SM) on blends of poly(2,6-dimethyl-1,4-phenylene ether) (PPE) and poly(hydroxy ether of bisphenol A) (Phenoxy) was examined in terms of phase structure and rheological and mechanical properties. Scanning electron micrographs show that the blends exhibit a more regular and finer dispersion when a small amount of the SM block copolymer is added. The interfacial tension between two immiscible polymers in the blends is estimated by applying various theories of emulsion models to the results of rheological measurements. The interfacial tension is significantly reduced when 5 wt% of the copolymer is added. Thermal analysis suggests that the added block copolymer is located at the interface of the PPE and Phenoxy. The blends exhibit remarkably enhanced toughness with the addition of a small amount of SM, which seems to be attributable to both dispersed particle size reduction and improved interfacial adhesion.

Morphology, rheological and mechanical properties of poly(phenylene ether) and polyamide-6 blends with a compatibilizer

The compatibilizing effect of styrene-acrylic acid random copolymers (SAA) on the morphology, rheological and mechanical properties of poly(phenylene ether) (PPE) and polyamide-6 (PA6) blends was investigated. By addition of a small amount (ca. 3wt%) of SAA copolymer, the blends show non-Newtonian power-law behavior at low frequency and the contribution of the storage modulus (G′) to the total response increases. The blends containing SAA copolymer having acrylic acid content higher than 36mol% show more regular and finer dispersion and exhibit about 10% increased mechanical properties. It is believed from Molau test and MFI data that these results seem to attribute to the branch formation and MWD broadening by chemical reaction between terminal amine groups of PA6 and carboxyl groups of SAA.

Performance of a canopy light interception model for conifer shoots, trees and stands.

A hierarchical approach was used to evaluate the behavior of a canopy light interception model using submodels for interception at levels of the shoot, crown and stand. Mean silhouette areas of individual shoots of lodgepole pine (Pinus contorta var. latifolia Engelm.) calculated assuming spherical shoot orientation agreed well with silhouette areas measured with a video projection system. A theoretical mean silhouette to total leaf area ratio (STAR) calculated using measured shoot characteristics and a model of shoot geometry agreed reasonably well with STAR values from measured shoot silhouette and leaf areas, although the range of calculated STAR values was slightly lower than that of the measured values. The shadow areas of tree crowns were underestimated for lodgepole pine; however, a model using regular shoot dispersion in the crown gave better agreement than a model based on random shoot dispersion. The underestimates may have been caused by underestimation either of leaf or branch area for the crowns or of the extinction coefficient. Two equations used to calculate leaf area from sapwood area measurements both underestimated leaf area. Accordingly, there are too many uncertainties to determine whether crown interception is best represented by a random or regular distribution of shoots. Mean canopy transmission in the Engelmann spruce (Picea engelmannii Parry ex Engelm.) stand was estimated very well by a random (Poisson) model of the spatial distribution of trees, whereas, in the lodgepole pine stand, a regular (hexagonal) model gave the best results. The Engelmann spruce stand had less regular tree spacing than the lodgepole pine stand. Taken as a whole, the weakest submodel for calculating canopy interception was the model for individual crown transmission, which underestimated crown shadow areas. However, the problem may be in leaf area estimates rather than in model performance.

Compatibilizing effect of a styrene-methyl methacrylate block copolymer on the phase behavior of poly(2,6-dimethyl-1,4-phenylene oxide) and poly(styrene-co-acrylonitrile) blends

Scanning electron micrographs show that the blends exhibit a more regular and finer dispersion as the block copolymer is added. Thermal analysis with the aid of physical aging reveals that the block copolymer is located at the interface

The adaptive significance of acoustic spacing in male bushcrickets Tettigonia viridissima: a perturbation experiment

A perturbation experiment was carried out in which the spacing between singing male bushcrickets, Tettigonia viridissima, was artificially manipulated. The experiment entailed releasing virgin females into an area in which caged males were either spaced evenly or in which a proportion of the males were clumped. There was large variation among males in the proportion of time spent singing (range 39–91% of total singing time). Singing activity was correlated with male body weight, but was not influenced by male spacing. Out of a total of 108 females released during the experiment, the majority (N=90) moved from their release points onto the cage of one of the closest singing males. More distant signalers sometimes attracted females when one of the closest males did not sing, or sang very little, during the period in which females were moving. When clumped, males were less successful in attracting females than when regularly spaced within the experimental area. Therefore, within a homogeneous habitat in which females are randomly distributed, male mating success will be maximized when males space out as far as possible from their competitors. As predicted, a regular dispersion of signaling males is the pattern observed in the preferred microhabitats of this species in nature.

Competition, seed predation, and species coexistence

In 1970 Janzen proposed that species-specific seed predation, by eliminating establishment of seedlings near adult trees of the same species, should lead to a regular dispersion of trees and to increased species diversity in tropical forests. Seed predation would lead to increased species diversity by setting upper bounds to the densities of individual species: if each extant species reached its predation-determined limit and yet more canopy space was available, individuals of additional species would be able to invade. Here I derive a patch model of Janzen's mechanism that is simple in form yet captures its spatial flavor. The model predicts that this mechanism will tend to stabilize any number of competing species. As a corollary, the model predicts that Janzen's mechanism can lead to increased diversity without leading to regular spacing of individuals.

Hawk Occupancy of Disturbed Grasslands in Relation to Models of Habitat Selection

I counted nesting pairs of Ferruginous Hawks (Buteo regalis) and Swainson's Hawks (B. swainsoni) on 76 41-km2 study plots in 1982 and 1987 to study the effect of agricultural cultivation of grassland on hawk densities. The study plots were selected at random within a 74,686-km2 mixed-grass prairie region in southeastern Alberta. The results confirm a curvilinear relationship between hawk density and extent of cultivation. Nesting densities of both species were higher in areas of moderate cultivation than in grassland. On plots with extensive cultivation (>30%), Ferruginous Hawks declined and so did ground squirrels, their main prey. Swainson's Hawks did not decline with extensive cultivation but probably shifted to other prey where ground squirrels were scarce. The observed curvilinear relationship between hawk density and habitat is similar in shape to changes in the productivity of ecosystems subjected to varying degrees of disturbance. The pattern of hawk abundance in relation to habitat quality did not change between years despite a more than 50% increase in hawk densities. The hawks therefore did not conform to the general assumption in models of habitat selection that optimal habitats are saturated before suboptimal areas are occupied. Instead, new breeding pairs continued to settle in optimal habitat but not suboptimal habitat. To account for these results, I propose a graphical model which incorporates territoriality as a factor causing regular dispersion at medium population densities across the range of habitat types included in this study. Optimal habitat may be the last to reach saturation because only there can territory size be reduced furthest under increasing intruder pressure.

The regeneration niche of desert ants: effects of established colonies.

We marked the sites chosen by 338 foundress queens of two desert ant species (Veromessor pergandei and Myrmecocystus flaviceps) and monitored changes in the spacing of both species and the foraging activity at V. pergandei young nests. Although the long established colonies of both species tend intraspecifically toward regular dispersion, queens of both species were intraspecifically clumped. After 3 months, when the first workers emerged, the young colonies (reduced to a total of 42 colonies) were randomly spaced intraspecifically. We also followed the spatial patter of queens with respect to established colonies of both species. Queens founded nests away from the nests of all established colonies on the site. After three months, the young colonies were dispersed away from conspecifics only. During June through August 1986, we censused the number of foragers at the surviving V. pergandei nests. Young colonies that were more active also tended to be far from established conspecifics in July and August. There was no correlation of forating activity with distance to heterospecific established colonies in any month. These results indicate that established conspecifics may reduce the survivorship of young ant colonies.

Hood construction as a spacing mechanism inCleistostoma kuwaitense (Crustacea: Ocypodidae)

The ocypodid crabCleistostoma kuwaitense inhabits the upper shores of the mudflats of Kuwait, where it constructs a semi-permanent mud hood over the entrance to its burrow. Between September and November 1986, data were collected on the distribution and orientation of the hoods and their openings to investigate the crab's social spacing system. Irrespective of density, the distribution of burrow hoods is non-random, tending towards a regular dispersion. A contributory factor for this dispersion pattern is that the surface hoods tend to increase the distance between adjacent burrows. The hood openings are randomly distributed with respect to compass direction but avoid directly facing those of nearest neighbours. In the absence of burrow plugging and eviction as used by other ocypodid crabs, construction of elongated hoods appears to be a good alternative for ensuring social spacing.

Evenly dispersed social groups and intergroup competition for juveniles in a coral-reef fish

A model based on current knowledge of recruitment of settling juveniles of coral-reef fishes suggests that adult social groups far removed from their nearest neighbors can expect to attract larger numbers of juveniles than groups that are close to nearest neighbors. For protogynous fishes in which females change to males when intragroup, female-to-male sex ratios rise above threshold values, selection should favor individuals occupying groups that attract many juveniles. The model predicts that social groups should, in consequence, be regularly dispersed and that, in effect, groups compete with each other to attract juveniles. Dispersion of social groups was measured in two populations of the protogynous fish Anthias squamipinnis. The location of all groups were plotted on maps of the reef. A set of nearest-neighbor analyses applied to the data all showed significant departures of observed nearest-neighbor distances from randomness, in the direction of regularity. At both sites, neighboring groups were separated by much unoccupied space. In one population, the substrate consisted of discrete coral aggregates scattered over open sand bottom, while in the second population groups occupied a relatively continuous vertical reef face. Although regular dispersion could, in theory, result from regular distribution of food patches, current evidence does not favor this explanation. It is more likely that regular dispersion results either from the reproductive advantage of occupying groups that are highly attractive to juveniles, as seen in the model, or from limited perceptual and locomotor abilities of juveniles at the time of settlement.

The Role of Competition in the Spacing of Trees in a Burkea africana- Terminalia sericea Savanna

The Role of Competition in the Spacing of Trees in a Burkea africana- Terminalia sericea Savanna

Pattern in the distribution of Acacia tortilis (Forsk.) Hayne

The distribution pattern of the Acacia tortilis in several sites in Qatar was investigated using the Greig-Smith-Kershaw method of pattern analysis. Neither random nor regular dispersion were detected and the distribution of the species was of the contagious type in almost every site. Pattern analyses showed two consistent scales: a smaller scale pattern attributed to an intrinsic factor of restricted dispersal of seeds and a larger scale pattern related to past and present biotic interference.

Les structures, à grande échelle, de la végétation et du milieu en limite subhumide/semi-aride en Oranie (Algérie)

By utilization of a non parametric analysis method and a multi-variate analysis method on the floristic analysis of two linear transects, we set obviousness the floristic composition and ecology of some vegetal groups, and we observe the passage from a regular monospecific dispersion to an agrégative dispersion, according to the passage from relative mesophile conditions to more arid conditions.

Permeability of heterogeneous films cast from core-shell polymer latices

Techniques have previously been developed for the preparation of thin (10–100 μm) polymer latex films, and the measurement of their gas permeabilities. These methods have been used to study heterogeneous polymer films cast from core-shell polymer latices. Such films comprise a regular dispersion of core particles in a continuum of coalesced shell polymer. The behaviour of these films is compared with heterogeneous films cast from mixtures of two homopolymer latices, and homogeneous films cast from homopolymer and copolymer latices and from polymer solutions.