Track A Basic Science

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138319/1/jia218438.pd

Chain folding in cyclic oligo(oxyethylene)s Crystallinity of large unsubstituted crown ethers by X-ray scattering and differential scanning calorimetry

Uniform cyclic oligo(oxyethylene)s with chain lengths E15 to E18 (unsubstituted 45-crown-15 to 54-crown-18) have been prepared by reaction of corresponding linear glycols with tosyl chloride under alkaline conditions, purified by preparative gel permeation chromatography (GPC), and characterised by analytical GPC, NMR spectroscopy, and other methods. Wide-angle and small-angle X-ray scattering were used to investigate crystal structure and layer spacing. Comparison was made with scattering from linear oligomers crystallised in unfolded conformation. It was found that cyclo-E18 crystallised into the poly(oxyethylene) structure (monoclinic sub-cell), while the lower oligomers crystallised into different structures. In confirmation of this result, the layer spacing of cyclo-E18 was that expected for the twice-folded conformation. Melting temperatures and enthalpies of fusion of cydo-E18 and cydo-E27 (from previous work), measured by differential scanning calorimetry, were compared with those of linear oligomers to obtain values of the enthalpy and entropy of fold formation in the crystallisation of uniform oligo(oxyethylene)

Micellisation and gelation of diblock copolymers of ethylene oxide and propylene oxide in aqueous solution, the effect of P-block length

The aqueous solution properties of five diblock copolymers prepared by sequential anionic copolymerisation (i.e. E102P37, E104P52, E92P55, E104P60 and E98P73 where E denotes oxyethylene and P denotes oxypropylene) were studied across a wide range of concentration. The techniques used to study micellisation and micellar properties in dilute solution were static and dynamic light scattering, surface tension, and eluent gel-permeation chromatography. The gelation of concentrated solutions was also investigated. As expected, the critical micelle concentration (CMC) was lowered and the association number of the micelles was increased by an increase in P-block length. In contrast, the critical gel concentration was unchanged, consistent with the constant E-block length leading to micelles with essentially identical E-block fringes. Comparison of the CMCs of the diblock copolymers with those of triblock E(m)P(n)E(m) copolymers with the same P-block length shows the diblock copolymers to micellise more efficiently. A similar comparison of the CMCs of the diblock copolymers with those of E(m)B(n) copolymer (B denotes oxybutylene) shows the hydrophobicity of a P unit to be one-sixth that of a B unit. The possibility is explored of correlating the limiting association number of a spherical micelle with the hydrophobe block length of its constituent copolymer. Of the five copolymers, only dilute solutions of E98P73 were predominantly micellar at both room temperature and body temperature, and this copolymer must be a prime candidate in any consideration of the potential application of E(m)P(n) copolymers in the solubilisation and controlled release of drugs

Chain folding in semicrystalline oxyethylene/oxybutylene diblock copolymers

The chain folding behavior of oxyethylene/oxybutylene (E/B) diblock copolymers, with one crystallizable E block and one noncrystallizable B block, has been studied by simultaneous SAXS and WAXS, low-frequency Raman spectroscopy, and differential scanning calorimetry. Two models, normal density (ND) and crystal/liquid-crystal (CLC) models were used to explore the results, consideration being given to the difference in the cross-sectional area of the two chains and the need to fill space at approximately normal density. It was found that both models can describe the data for the longer copolymers, but the CLC model is better for the shorter copolymers

Kernels for Periodic Time Series Arising in Astronomy

We present a method for applying machine learning algorithms to the automatic classification of astronomy star surveys using time series of star brightness. Currently such classification requires a large amount of domain expert time. We show that a combination of phase invariant similarity and explicit features extracted from the time series provide domain expert level classification. To facilitate this application, we investigate the cross-correlation as a general phase invariant similarity function for time series. We establish several theoretical properties of cross-correlation showing that it is intuitively appealing and algorithmically tractable, but not positive semidefinite, and therefore not generally applicable with kernel methods. As a solution we introduce a positive semidefinite similarity function with the same intuitive appeal as cross-correlation. An experimental evaluation in the astronomy domain as well as several other data sets demonstrates the performance of the kernel and related similarity functions

MIS research directions: A survey of researchers' views

Several studies have addressed important issues for MIS Research, but until now no empirical studies have been conducted to asses how MIS researchers themselves view the relatively new field. This survey of 397 MIS researchers revealed preferences for research methods and current areas of concentration, current studies being conducted, publication history, and other factors of interest. It found the subjects' assessment of the quality of MIS research relatively low and that they feel there is an overemphasis on transient topics, rather than on topics of lasting significance. There is continuing evidence of fragmentation in the field. Few MIS researchers appear to rely on research frameworks. However, there is overall optimism that the quality of MIS research has been improving, and will continue to improve in the future. Implications for the future of MIS research are then discussed. © 1991, ACM. All rights reserved