Water uptake and transport studies in PVP-PMMA hydrogels

Hydrogels comprising 90 wt.% polyvinylpyrrolidone and 10 wt.% poly(methyl methacrylate) have been soaked in distilled water. By taking the mass change for different soaking times the number of water molecules entering the hydrogel can be calculated. A maximum in conductivity has been observed in the conductivity-soaking time plot. The increase and decrease in conductivity over time imply that some water molecules in the hydrogel are mobile (infrared band at 1598 cm-1) and immobile water band observed at 1687 cm-1. These can also be implied from the plot for number density of conductivity contributing water molecules versus soaking time. From the results shown water molecules diffuses at a faster rate when there are less conductivity contributing water molecules

Investigation of cell parameters, microstructures and electrochemical behaviour of LiMn2O4 normal and nano powders.

Nano materials are usually difficult to prepare. This work presents a simple way of preparing LiMn2O4 nano powders using the high-energy ball milling method. This method has the advantage of producing pure, single-phase and crystalline nano powders. The milling method is carefully controlled to avoid unwanted chemical reactions that may change the stoichiometry of the material. Nano powders of between 30 and 50 nm are obtained. Structural studies of the nano powders, as well as the more-conventional micron-sized LiMn2O4, are made using X-ray diffraction and neutron diffraction methods. Electrochemical evaluation of the materials is undertaken with a three-probe cyclic voltammetry technique and galvanostatic charge-discharge measurements. Structural studies reveal that not only are the crystallites of the nano powders much reduced in size from the normal powders, but their cell parameters are also smaller. The performance characteristics of the nano material show an improvement over that of the micron-sized material by about 17% in the 1st cycle and 70.6% in the 5th cycle, at which the capacity is 132 mAh g(-1). The normal material suffers from severe capacity fading but the nano material shows much improved capacity retention. © 2008, Elsevier Ltd