Synthesis and characterization of biodegradable lactic acid‐based polymers by chain extension

BACKGROUND: Poly(lactic acid) (PLA), coming from renewable resources, can be used to solve environmental problems. However, PLA has to have a relatively high molecular weight in order to have acceptable mechanical properties as required in many applications. Chain‐extension reaction is an effective method to raise the molecular weight of PLA. RESULTS: A high molecular weight biodegradable lactic acid polymer was successfully synthesized in two steps. First, the lactic acid monomer was oligomerized to low molecular weight hydroxyl‐terminated prepolymer; the molecular weight was then increased by chain extension using 1,6‐hexamethylene diisocyanate as the chain extender. The polymer was characterized using ¹H NMR analysis, gel permeation chromatography, differential scanning calorimetry and Fourier transform infrared spectroscopy. The results showed that the obtained polymer had a M_n of 27 500 g mol^?1 and a M_w of 116 900 g mol^?1 after 40 min of chain extension at 180 °C. The glass transition temperature (T_g) of the low molecular weight prepolymer was 47.8 °C. After chain extension, T_g increased to 53.2 °C. The mechanical and rheological properties of the obtained polymer were also investigated. CONCLUSION: The results suggest that high molecular weight PLA can be achieved by chain extension to meet conventional uses. Copyright © 2008 Society of Chemical Industry     

Numerical study of the auroral particle transport in the polar upper atmosphere

Starting from the Boltzmann equation and with some reasonable assumptions, a one-dimensional transport equation of charged energetic particles is derived by taking account of major interactions with neutral species in the upper atmosphere, including the processes of elastic scattering, the excitation, the ionization and the secondary electron production. The transport equation is numerically solved, for a simplified atmosphere consisting only of nitrogen molecules (N₂), to obtain the variations of incident electron fluxes as a function of altitude, energy and pitch angle. The model results can describe fairly the transport characteristics of precipitating auroral electron spectra in the polar upper atmosphere; meanwhile the N2 ionization rates calculated from the modeled differential flux spectra also exhibit good agreements with existing empirical models in terms of several key parameters. Taking the energy flux spectra of precipitating electrons observed by FAST satellite flying over EISCAT site on May 15, 1997 as model inputs, the model-calculated ionization rate profile of neutral atmosphere consists reasonably with that reconstructed from electron density measurements by the radar. Supported by the National Natural Science Foundation of China (Grant Nos. 40404015, 40390150), the Ministry of Science and Technology of PRC (Grant No. 2006BAB18B06) and Key Lab of Geospace Environment and Geodesy, Ministry of Education of PRC     

北京：玩具生意有亮点 整体看来是平淡

六一儿童节，悠悠球的销售也达到了高潮，“奥迪双钻”在京负责培训的“四大高手”中两名来天乐玩具市场现场表演，精湛的技艺引入驻足，6米长的专柜前一直挤满人。     

悠悠球火爆京城探究

悠悠球在市场上成功地转起来了。笔者作为北京红桥天乐玩具市场的经营管理者。他们从旁观者的角度观察和分析其运作成功的原因，值得一读。     

Bilayer carbon-based structure with the promotion of homogenous nucleation for lithium metal anodes

Lithium metal anodes (LMAs) are considered as the promising alternatives for next-generation high energy density batteries, but are still hampered by the severe growth of uncontrollable lithium dendrites. The growth of lithium dendrites induces poor cycling lifespan and serious safety concerns, dragging lithium metal batteries out of practical applications. We designed a bilayer carbon-based structure covered with Co/C nanosheets and vertical graphene sheets (VGS). The enormous specific surface area and uniformly distributed Co nanoparticles of the CC@Co/C-VGS host are derived from its unique design, which can reduce local current density and nucleation overpotential, resulting in a dendrite-free morphology and exceptional cycling stability. Symmetric cells exhibit over 400 cycles (800 h) at a high current density/capacity of 10 mA cm^?2/10 mA h cm^?2. Full cells using LiFePO₄ as the cathode have an enhanced rate capability and a prolonged lifespan, reaching 90 mA h g^?1 after 1000 cycles at 2 C with 73.5% capacity retention. This unique design sheds light on developing high-performance LMAs.

     

基于要素模型的GIS平台的研究与实现

分析了GIS空间数据模型的基本问题,对空间数据模型的类型和含义进行了界定。研究了要素模型的特点和空间关系;分析了一个GIS通用平台应该具有的功能,设计并实现了一个GIS系统,该系统实现了一个GIS平台的基本功能,并具有可扩展性。     

校园前区绿地系统规划问题

本文就郑州粮食学院校园前区绿地系统规划设计,阐述了设计的指导原则,绿地规划布局,建筑小品设计构思,以及植物种植处理等问题。     

高效液相色谱法测定循环水中葡萄糖含量

本方法采用甲醇作为内标物，二次蒸馏水为流动相，色谱柱温度为85℃，利用差示折光检测器进行检测。通过本方法测得的葡萄糖的回收率为95％～105％，变异系数为0．96％。     

Prediction of SYM-H index by NARX neural network from IMF and solar wind data

SYM-H is one of the important indices for space weather. It indicates the intensity of magnetic storm, similarly to Dst index but with much higher time-resolution. In this paper an artificial neural network (ANN) of Nonlinear Auto Regressive with eXogenous inputs (NARX) has been developed to predict SYM-H index from solar wind and IMF data. In comparison with usual BP and Elman network, the new NRAX model shows much better prediction capability. For 15 testing great storms including 5 super-storms of Min. S...     

基于ArcGIS VBA在面状空间要素变更中的应用

空间要素的变更早已成为了TGIS领域一个重要的研究方向,众多的学者将注意力放在研究单个空间实体的变更过程上,固然,从微观上研究单个实体变更类型及其对应的操作固然是好。其实,在关注整体而非注重个体变更的情况下,并考量到操作效率方面,其不一定是个行之有效之方法。本文在分析传统的单个实体多种变更类型后,从区域空间要素整体性出发,将空间要素在时间轴上划分为两种状态,并结合面状要素实例,运用ArcGIS VBA二次开发将两种不同状态下空间要素集区分开来,实验取得了较好效果。