On Padé approximants to virial series

Padé approximants have long been used to predict virial series coefficients and to provide equations of state for low and high density materials. However, some justified criticism has appeared about this procedure. Although we agree to impose several restrictions on the use of Padé approximants, we indicate that the Padé approximant is still an excellent way to predict the first unknown virial series coefficients. As an example, we report a calculation of the B11=128.6 and B12=155 virial coefficients of the three dimensional hard sphere model that are in excellent agreement with the two most recent estimates. We also consider that the commonly used method to choose among Padé approximants is not completely reliable for this specific application and suggest an alternative new method.     

Transparent perfluoropolyethers for vacuum ultraviolet applications

After a broad scouting based on quantum chemical calculations, optical absorption measurements in the vacuum ultraviolet (VUV) wavelength region between 140 and 190 nm were performed on a narrower series of commercial and experimental liquids. By elimination of sources of external contamination, mainly due to atmospheric gases, the analysis of the contributions to the absorption related to the backbone structure and to the chain end composition allowed the synthesis of a novel family of linear perfluoropolyethers (PFPEs) with optical absorbance at 157 nm between 0.3 and 0.6 cm(-1) in a broad range of compositions and molecular weights. The dependence of the optical threshold on the PFPE composition demonstrates that -OCF2- is the most transparent segmental unit in the VUV region.     

Spontaneous Birefringence in Layer‐by‐Layer Films of Chitosan and Azo Dye Sunset Yellow

Layer‐by‐layer (LBL) films consisting of layers of the azo dye Sunset Yellow alternated with chitosan display spontaneous birefringence, which is attributed to the film anisotropy imparted by the LBL method. This is unusual for azobenzene‐containing materials as they normally form films with randomly oriented molecules, presenting birefringence only due to photoinduced isomerization cycles. Spontaneous birefringence does not appear in cast films, but occurs for LBL films obtained under various experimental conditions.

Chemical structures of (a) Sunset Yellow and (b) chitosan.