Aging behavior of optically transparent poly (methyl methacrylate) composites

The thermal aging between 25 and 115°C of hot pressed glass fiber reinforced poly (methy1 methacrylate)(PMMA) transparent composites was studied as a function of the temperature and time of hot pressing. Thermal aging at near the T_g of the PMMA matrix caused dimensional changes and a reduction in optical transmission and clarity. The reduction in transmission was attributed to gas bubbles that formed in the matrix, which may be due to the evaporation of residual MMA monomer or low T_g (∼ 75°C) polymer in the composites during aging. Thermal cycling between 25 and 100°C by eliminating low T_g of the PMMA matrix establishes the upper temperature limit to which the composite can be exposed without seriously damaging its optical properties.     

New developments in the melt rheology of nylons. I: Effect of moisture and molecular weight

Peculiar observations on the melt rheology of ultra-dry nylon resins, nylon 6 in particular, are reported. One aspect of this study deals with a sharp increase in zero shear melt viscosity (e.g. 2 to 5 times) as the nylon 6 resin moisture is taken from 0.10 down to 0.00%; the effect being reversible. Changes of such magnitude are unexpected considering that there are no detectable variations of the chemical/compositional/molecular weight type in the starting resin, when subjected to the imposed drying conditions. Another aspect of this study deals with a deviation of nylons (6, 6,6, and 12) from the Bueche (1952) relationship, well accepted for polymers to date. Under moderate drying conditions (e.g. 50°C/17 h/110 millitorr), the molecular weight exponent is found to be 3.8, which is within the range of 3.4 to 3.8 reported for nylon 6. However, under more severe drying conditions (e.g. 110°C/17 h/110 millitorr), the molecular weight exponents for nylon 6, nylon 66, and nylon 12 are 4.8, 5.4, and 4.6, respectively. We are proposing that a sharp increase in melt viscosity of ultra-dry nylon 6 is partly due to an increase in the molecular weight of the melt (extrudate) which then, has a more pronounced impact on melt viscosity in view of the 4.8 exponent. Such unique results, in contrast to polyethylene (free radical polymer) and poly(ethylene terephthalate) (condensation polymer) are tentatively attributed to H-bonding in nylon melts. Yet another aspect of this study deals with the rheology of supercooled molten polymers that can offer advantages for analytical characterization.     

无需微处理器控制的555定时器LED驱动器

从高端视频显示器到低端的照明,LED在很多领域得到了应用。设计者经常只需要一个专用LED驱动器功能的一部分,但不能承受用微控制器进行控制的成本。微处理器一般用于控制专用LED驱动器,实现例如对LED电流的模拟或PWM(脉冲     

Relating indices of knowledge structure coherence and accuracy to skill-based performance: Is there utility in using a combination of indices?

The authors examined the relative criterion-related validity of knowledge structure coherence and two accuracy-based indices (closeness and correlation) as well as the utility of using a combination of knowledge structure indices in the prediction of skill acquisition and transfer. Findings from an aggregation of 5 independent samples (N = 958) whose participants underwent training on a complex computer simulation indicated that coherence and the accuracy-based indices yielded comparable zero-order predictive validities. Support for the incremental validity of using a combination of indices was mixed; the most, albeit small, gain came in pairing coherence and closeness when predicting transfer. After controlling for baseline skill, general mental ability, and declarative knowledge, only coherence explained a statistically significant amount of unique variance in transfer. Overall, the results suggested that the different indices largely overlap in their representation of knowledge organization, but that coherence better reflects adaptable aspects of knowledge organization important to skill transfer. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Next-generation tools for evolutionary invasion analyses

Evolutionary invasion analysis is a powerful technique for modelling in evolutionary biology. The general approach is to derive an expression for the growth rate of a mutant allele encoding some novel phenotype, and then to use this expression to predict long-term evolutionary outcomes. Mathematically, such ‘invasion fitness’ expressions are most often derived using standard linear stability analyses from dynamical systems theory. Interestingly, there is a mathematically equivalent approach to such stability analyses that is often employed in mathematical epidemiology, and that is based on so-called ‘next-generation’ matrices. Although this next-generation matrix approach has sometimes also been used in evolutionary invasion analyses, it is not yet common in this area despite the fact that it can sometimes greatly simplify calculations. The aim of this article is to bring the approach to a wider evolutionary audience in two ways. First, we review the next-generation matrix approach and provide a novel, and easily intuited, interpretation of how this approach relates to more standard techniques. Second, we illustrate next-generation methods in evolutionary invasion analysis through a series of informative examples. Although focusing primarily on evolutionary invasion analysis, we provide several insights that apply to biological modelling in general.     

Analysis of a complete DNA–protein affinity landscape

Properties of biological fitness landscapes are of interest to a wide sector of the life sciences, from ecology to genetics to synthetic biology. For biomolecular fitness landscapes, the information we currently possess comes primarily from two sources: sparse samples obtained from directed evolution experiments; and more fine-grained but less authentic information from ‘in silico’ models (such as NK-landscapes). Here we present the entire protein-binding profile of all variants of a nucleic acid oligomer 10 bases in length, which we have obtained experimentally by a series of highly parallel on-chip assays. The resulting complete landscape of sequence-binding pairs, comprising more than one million binding measurements in duplicate, has been analysed statistically using a number of metrics commonly applied to synthetic landscapes. These metrics show that the landscape is rugged, with many local optima, and that this arises from a combination of experimental variation and the natural structural properties of the oligonucleotides.     

Analogical transfer from a simulated physical system.

Previous research has consistently found that spontaneous analogical transfer is strongly tied to concrete and contextual similarities between the cases. However, that work has largely failed to acknowledge that the relevant factor in transfer is the similarity between individuals' mental representations of the situations rather than the overt similarities between the cases themselves. Across several studies, we found that participants were able to transfer strategies learned from a perceptually concrete simulation of a physical system to a task with very dissimilar content and appearance. This transfer was reflected in better performance on the transfer task when its underlying dynamics were consistent rather than inconsistent with the preceding training task. Our data indicate that transfer in these tasks relies on the perceptual and spatial nature of the training task but does not depend on direct interaction with the system, with participants performing equally well after simply observing the concrete simulation. We argue that participants generated a spatial, dynamic, and force-based mental model while interacting with the training simulation and tended to spontaneously interpret the transfer task according to this primed model. Unexpectedly, our data consistently show that transfer was independent of reported recognition of the analogy between tasks: Although such recognition was associated with better overall performance, it was not associated with better transfer (in terms of applying an appropriate strategy). Together, these findings suggest that analogical transfer between overtly dissimilar cases may be much more common—and much more relevant to our cognitive processing—than is generally assumed. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Utilization of waste marble dust as an additive in cement production

In this experimental study, the usability of waste marble dust (WMD) as an additive material in blended cement has been investigated. For this purpose, waste marble dust added cements (WMDCs) have been obtained by intergrinding WMD with Portland cement clinker at different blend ratios: 2.5%, 5.0%, 7.5% and 10% by weight. 40 × 40 × 160 mm mortar prisms have been produced with the obtained cements. Strength tests have been carried out on mortar specimen at 7, 28, and 90 days. WMDCs have been compared to each other as well as to control cements of CEM I and CEM II with respect to their physical, chemical and mechanical properties. Obtained results showed that WMDCs conform to EN 197-1 standard and thus 10% WMD can be used as an additive material in cement manufacturing.     

用微距拍蝴蝶

所谓化茧成蝶,蝴蝶是经过4个复杂奇妙的生命阶段演化而来的情灵,它们也是绝佳的微距摄影主题.它们有复眼、6条腿、身体分三个部分,从身体结构到外形都美丽异常.