Properties and structure of elastomeric two-stage emulsion interpenetrating networks

Interpenetrating polymer networks (IPNs) consist of two crosslinked polymers which form a network within a network. Bulk-prepared IPNs are thermosetting and cannot be processed due to the formation of a macroscopic network. Emulsion IPNs, although thermosetting, can be processed as thermoplastic materials. This is due to a special particle-slippage flow mechanism which is practically insensitive to molecular weight. The morphology of the reported latex particles is unique in the sense that very small polystyrene domains are formed by phase-separation of polymerizing styrene added to a seeded flexible polyacrylate latex. Compression or injection moulded specimens of these crosslinked elastomeric materials show significant mechanical properties. Some properties and structural observations using a special electron microscopy technique are described in this article. This method is based on the differential radiation damage to various polymers embedded in ice and can be used as an analytical tool to determine the microstructure of certain multiphase systems.     

High-temperature thermodynamic properties of the vanadium carbides V2C and VC0.73 determined using a galvanic cell technique

The standard free energies of formation of V₂C and VC_0.73 have been obtained from electromotive force (emf) measurements on the following galvanic cells with BaF₂-BaC₂ solid solutions as the electrolyte: Ta, Ta₂CBaF₂-BaC₂V, V₂ (850 to 1200K) (D) VC_0.73, V₂C BaF₂-BaC₂ Cr, Cr₂₃C₆ (850 to HOOK) (E) VC_0.73, V₂C BaF₂-BaC₂ Mo, Mo₂C (890 to 1247 K) (F) Combining the results of this study with previous work1151 and those of Kukarniet al., ^[19.25] the following equations for ΔG_f° of V₂C and VC_0.73 have been determined: From cell (D), ΔG_v2c°(±1263) = -152,824(±9200) + 5.45(±0.27)7 Joule for the reaction 2V + C = V₂C. From cell (E), ΔG_{vc 0.73}°(±662) = -96,790.8(±6511.7) + 7.0(±0.3)r Joule/g * atom V From cell (F), ΔG_{vc 0.73}°(±665) = -97,000(±4606) + 6.79(±0.78)J Joule/g * atom V for the reaction V + 0.73C = VC_0.73.     

Human Resource Management: Guidelines for Effective Introduction of Microcomputer Technology

Microcomputer technology has matured to a point at which its successful assimilation depends more on the proper management of personnel and less on the features of the technology. This article presents a case study that demonstrates how lead users can be identified and recruited to provide a support infrastructure to peers throughout the initiation stage within the organization. The article recommends that managers of microcomputer technology develop an understanding of different user types and apply adaptive human resource management practices     

A possible defective estimation of antineutrophil cytoplasmic antibodies in systemic lupus erythematosus due to the coexistence of periodontitis: preliminary observations

The Family Environment Scale has been used extensively in family research since first being published. However, despite its appeal both conceptually and empirically, doubts have been raised over the scale's reliability. This article presents normative and reliability data for the Family Environment Scale from a large, combined sample of adolescents. Means and standard deviations were generally found to be in line with those reported in the scale's manual; however, estimates of internal consistency for most subscales could be considered inadequate for research purposes.     

Group B streptococcal meningitis in adults: report of twelve cases and review

Group B streptococcus (GBS) is the leading etiologic agent of bacterial meningitis and sepsis during the neonatal period, but it is an infrequent cause of meningitis in adults. We report 12 episodes of group B streptococcal meningitis in adults and review 52 cases reported in the literature. A total of 24 men and 40 women were included in the study; the mean age (+/- SD) was 49.2 +/- 20.5 years (range, 17-89 years). All the patients had cerebrospinal fluid cultures positive for GBS. Eighty-six percent of the patients had comorbid conditions, 50% had a distant focus of infection, and blood cultures yielded GBS for 78.7%. The overall case-fatality rate was 34.4% (22 patients). Factors associated with a poor outcome were advanced mean age (+/- SD) (61.5 +/- 17.4 years vs. 42.8 +/- 19.2 years; P = .0003) and the presence of complications on admission (P = .0001). Seven percent of survivors had neurological sequelae. Group B streptococcal meningitis in adults has become increasingly frequent in recent years; it tends to occur in patients with severe underlying conditions and is associated with a high case-fatality rate. Factors associated with a poor prognosis are advanced age and the occurrence of neurological and extraneurological complications.     

Interacting with electronic mail can be a dream or a nightmare: a user''s point of view

Diffusion of electronic mail (e-mail) is not yet universal. So far, e-mail has been implemented successfully within organisations, but its implementation for communications between organisations has been rather limited. This situation is surprising, given the great potential of e-mail for interorganisational communication. E-mail encounters from a user's point of view, reviewed in this paper, suggest that users of BITNET, one of the predominant e-mail networks in the academic world, face difficulties while interacting with e-mail. These include addressing difficulties, unreliability issues, medium limitations, and interface problems. BITNET is just one of many interorganisational networks and may not be representative. Still, e-mail technology is unlikely to survive if human engineering and reliability are not uniformly satisfactory across all e-mail systems. Poorly engineered e-mail systems frustrate not only their users, but also users of other networks because of gateways between the networks. Therefore, e-mail users might resort to other communication media like facsimile or the telephone, and abandon e-mail altogether.

For e-mail to be competitive in the communication arena, an interdisciplinary effort should be directed toward standardisation of features like better addressing conventions, international user directories, uniform user interfaces, and sophisticated management of e-mail messages.     

Elastomeric latex domain-interpenetrating polymer networks: Physical and rheological properties

The combination of rubbery and rigid polymers in a multiphase structure using staged emulsion polymerization has yielded materials with properties ranging from reinforced elastomers to high impact plastics. The many different particle morphologies that result from a two-stage latex (TSL) polymerization include core/shell, domain, interpenetrating polymer networks (IPN), and various combinations thereof. The sequence of polymerization, crosslinking, grafting, and composition are among the significant parameters that determine the particle morphology. Elastomeric TSL with soft polyacrylates (PA) as the seed particles and polystyrene (PS) as the second stage, with each stage lightly crosslinked, may yield IPN-microdomain particles. The particle morphology has been elucidated through a combination of microscopy and mechanical property analyses. The significant modulus of elastomeric latex interpenetrating polymer networks (LIPN) results from reinforcement by PS intra-particle microdomains and their significant tensile strength from a strength forming mechanism of PS inter-particle microdomains. The increase in the PA seed crosslinking increases the crosslinked PS (xPS) level of molecular mixing with, and grafting via residual unsaturation to, the crosslinked PA (xPA) network and decreases particle deformnability. At higher xPS concentrations the formation of an xPS-rich shell enhances xPS continuity in the molded material through the partial coalescence of the shells, diminishing the PA continuity, and yielding more PS-like properties. The submicron lightly crosslinked latex particles with these different morphologies flow as a pseudoplastie material through a particle slippage flow mechanism exhibiting neither a Newtonian plateau nor a yield stress at low shear rates. The deformable lightly crosslinked particles with interchangeable PS ties which disintegrate at elevated temperatures retain their identity and regain their shape at the cessation of shear. The LIPN can be processed using standard thermoplastic methods and machinery, with power law constants and shear insensitive flow activation energies that are similar to those of thermoplastics at high levels of shear. Uncrosslinked PS shells around crosslinked PA seed particles, on the other hand, completely coalesce upon molding to form a continuous thermoplastic PS matrix that may essentially flow through molecular deformation.     

Perspectives on allelopathy in mexican traditional agroecosystems: A case study in tlaxcala

Agroecosystems in Tlaxcala, Mexico, are surrounded by trees and water channels and have a great variety of cultivated and noncultivated plants. The main results of a study carried out on a traditional agroecosystem in Santa Inés, Tlaxcala are presented. Some ecological aspects of polycultures, plant covers (dry leaves ofAlnus firmifolia, Berula erecta, andJuncus sp.), and the allelopathic potential of crops and noncultivated plants (fresh and dry material) were analyzed. The main plants (trees, shrubs, and herbs) present in the agroecosystem were identified. The total number of weeds in plots where plant covers were added was reduced. The number of nodules ofRhizobium phaseoli and the production of bean and squash increased with plant covers. Corn, beans, and squash showed a clear allelopathic effect, as well asChenopodium murale, Tradescantia crassifolia, Melilotus indicus, andAmaranthus hybridus, among other weeds. The contribution of allelopathy in studies of traditional agroecosystems is of great importance for the management of species in space and time. Allelopathy can be the basis of biological control of pests and weeds and of the discovery of new useful substances.     

Sintering behavior of poly(methyl methacrylate) particles

The sintering behavior of poly(methyl methacrylate) (PMMA) particles was studied by photographic, means and mechanical testing. Interpretation of the data gives the sequence of events taking place during high temperature sintering (without pre-compaction). Optical observations, show essentially two main sintering regions while mechanical studies also give an intermediate transition zone separating these two regions. It is suggested that the flow mechanisms involved in the sintering process are viscoelastfc flow in the lower temperature range and shorter times and non-Newtonian viscous flow in the higher temperature range and longer times.     

Minima equalization: A useful strategy in automatic processing of microscopic images

Specifically stained features in microscopic images rarely have a unique range of grey levels which would permit selection of the features by simple thresholding. The spaces between features and immediately surrounding them are often as dark or darker than the more lightly stained features. An algorithm for minima equalization which facilitates the extraction and segmentation of such features was designed and is explained in intuitive terms. It is applied to the analysis of cross-sections of peripheral myelinated nerve fibres. It is shown that the binary image obtained can be combined with a gradient image to give a binary image which accurately reflects the thickness of the myelin in the original image. Using silver-impregnated nerve endings and bile canaliculi stained for thiamine pyrophosphatase, binary images like those prepared manually from thick specimens using a camera lucida can be obtained using the minima equalization procedure. The image processor is used to develop a composite image by combining images at various focal planes through the thick specimen. This image is then processed to obtain the binary image.