Structure–property relationships of poly(vinyl alcohol). I. Influence of polymerization solvents and temperature on the structure and properties of poly(vinyl alcohol) derived from poly(vinyl acetate)

The solubility properties of poly(vinyl alcohol) (PVA) vary with the method of preparation of the poly(vinyl acetate) (PVAc) from which it is derived. PVAc was prepared with free-radical catalysts over a range of temperatures from ?78 to 90°C. with solvents of varying chain-transfer ability. The corresponding PVA samples varied in their resistance to dissolution in water. Their high-resolution proton nuclear magnetic resonance spectra showed on differences in tacticity. Data on 1,2-diol content showed only minor differences. Hence, the increase in resistance of PVA to dissolution in water arising from changes in chain-transfer activity of the solvent used in vinyl acetate polymerization is largely attributable to a decrease in molecular weight, and the increase in resistance of PVA to dissolution in water arising from a decrease in the temperature of the vinyl acetate polymerization is largely attributable to a decrease in both long and short branches. Evidently, with polar polymers having small side groups, tacticity is not the only factor influencing property variation; that is, variations in stereoregularity influence more the crystallinity of the sample as measured by density or x-ray methods than the ultimate crystallizability under conditions of mechanical and thermal treatment. In this regard polar polymers having small side groups differ from nonpolar polymers.     

纳米尺度CeO2在多相催化反应中的形貌效应

CeO2的催化性能不仅与粒子大小有关,而且与其形貌密切相关. 本文分析了CeO2的纳米结构和氧化还原特性,综述了CeO2在催化反应中的形貌效应,着重介绍了CeO2的结构和形貌对CO氧化、水气变换和乙醇水蒸气重整制氢以及二元醇脱水反应活性的影响.     

Sums of Three or More Primes

It has long been known that, under the assumption of the Riemann Hypothesis, one can give upper and lower bounds for the error in the Prime Number Theorem, such bounds being within a factor of of each other and this fact being equivalent to the Riemann Hypothesis. In this paper we show that, provided ``Riemann Hypothesis' is replaced by ``Generalized Riemann Hypothesis', results of similar (often greater) precision hold in the case of the corresponding formula for the representation of an integer as the sum of primes for , and, in a mean square sense, for . We also sharpen, in most cases to best possible form, the original estimates of Hardy and Littlewood which were based on the assumption of a ``Quasi-Riemann Hypothesis'. We incidentally give a slight sharpening to a well-known exponential sum estimate of Vinogradov-Vaughan.

     

The generalized Burnside ring and theK-theory of a ring with roots of unity

We prove that ifR is a suitable ring and a finite Abelian subgroup of the multiplicative group ofR, then the Hurewicz map from the stable homotopy ofB toK _* R factors geometrically through a mapK _* E K _* R, whereE is an appropriate finite field.     

《硅光电池特性》实验教学探索和实践

为了更好地培养全校本科生的创新思维和动手实践能力,探索了适合九零后学生的教学方法。针对实验中存在的一些问题,引导学生积极思考,找到一些快速完成实验并且大大降低仪器损耗率的实验技巧,实现学生、教师、学校三方共赢的目标。