首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   135篇
  免费   4篇
  国内免费   1篇
化学   85篇
晶体学   1篇
数学   14篇
物理学   40篇
  2023年   1篇
  2021年   2篇
  2019年   2篇
  2018年   2篇
  2017年   1篇
  2016年   2篇
  2015年   3篇
  2014年   1篇
  2013年   6篇
  2012年   7篇
  2011年   9篇
  2010年   6篇
  2009年   5篇
  2008年   9篇
  2007年   6篇
  2006年   10篇
  2005年   10篇
  2004年   6篇
  2003年   5篇
  2002年   6篇
  2001年   4篇
  2000年   3篇
  1999年   3篇
  1998年   1篇
  1996年   2篇
  1995年   3篇
  1992年   1篇
  1991年   1篇
  1990年   1篇
  1989年   1篇
  1987年   1篇
  1986年   1篇
  1985年   1篇
  1984年   2篇
  1982年   3篇
  1981年   1篇
  1980年   2篇
  1979年   1篇
  1978年   1篇
  1977年   1篇
  1976年   2篇
  1975年   1篇
  1974年   1篇
  1973年   1篇
  1970年   1篇
  1969年   1篇
排序方式: 共有140条查询结果,搜索用时 15 毫秒
101.
The aldol addition of dimethylsilyl enolates to aldehydes with alkali metal salt catalysts was examined. Several kinds of polyether derivatives were employed as solid solvents without a liquid one. In the presence of the polymer (Gr) obtained from poly(ethylene glycol) methyl ether methacrylate (PEGMA) or the crosslinked derivatives (GLs) from PEGMA, the aldol addition proceeded efficiently and yielded the product. The effective catalyst was lithium bromide for Gr and potassium bromide for GLs. The crosslinked polyether GLs were easily separated from the reaction mixture by simple filtration, which was preferable to simplifying the treatments of the reaction. The results demonstrated that the presence of polyether structures provided a favorable reaction environment and enabled the reaction under solvent‐free conditions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2075–2084, 2005  相似文献   
102.

Coumarin (C9H6O2) and coumarin derivative, coumarin 6H (C15H15NO2) samples were observed with a polarizing microscope at and around the melting points. The states of the two samples are crystalline at room temperature. When the temperature of the Coumarin sample was raised and a part was melted, the crystalline phases coexisted with the liquid phase. The crystal surface coexisting with the liquid phase went through a repetition of melting and crystallization on slightly modulating the sample temperature. The two-phase coexisting state was observed with a digital camera attached to a microscope. The changes in the number of pixels in the crystal part of the photographed image showed Debye relaxation with temperature change. It was found that the relaxation time was 4.9 s when the number of pixels decreased due to surface melting or increased due to surface crystallization. This relaxation time was in good agreement with that of the dynamic-specific heat measured by AC calorimetry. The coumarin 6H microcrystal also coexisted with the melt in a temperature range at and around the melting point. The change in the number of pixels of the coumarin 6H crystal part showed Debye relaxation with respect to the temperature, and the relaxation time was 15 s, which was the same as the relaxation time of the dynamic-specific heat measured in the melting temperature range of coumarin 6H.

  相似文献   
103.
The efficiencies of polyelectrolytes, i.e., polycations and polyanions, and several kinds of water-soluble polymers as fusogens on soybean phospholipid liposome (SL) and egg yolk phospholipid liposome (EL) were investigated by the fluorescence quenching method. There were optimal concentrations for the induction of fusion in every system. Polycations induced fusion of liposomes at very low concentration in comparison with other polymers. Poly(carboxylic acid)s induced fusion at relatively high concentration. A strong acidic polyanion with high molecular weight also induced fusion of liposomes. The induction efficiency of poly(ethylene glycol) on fusion was higher than other nonionic polymers. The efficiency of fusion of EL was lower than that of SL in all systems because of the higher stability of EL membrane. It was found that electrostatic interactions, hydrogen bonding and/or hydrophobic interaction between these water-soluble polymers and liposomal membranes played an important role on aggregation and fusion of liposomes.  相似文献   
104.
Polyethylene was prepared by γ-ray-induced polymerization in the temperature range 0–180°C. The morphology and the physical properties of the polymer as polymerized were studied by electron microscopy, differential scanning calorimetry, and gel permeation chromatography. Aggregates of small lamellar crystals with irregularly growing faces were produced below 55°C. Aggregates of large spherical particles were formed above 60°C together with hemispherical particles which adhered to the substrate. A few lamellar crystals of triangular or amoeba-like shapes were also found above 55°C. The polymers formed below 55°C showed a sharp single endothermic DSC peak and a bimodal molecular-weight distribution, while the sample above 60°C had a double endotherm and a unimodal molecular-weight distribution. These facts suggest that the mechanism of crystallization during polymerization below 55°C is different from that above 60°C. The melting point, however, decreased continuously with increasing polymerization temperature and was much lower than that of extended-chain crystals. The results show that the polyethylene, as polymerized, is composed of folded-chain crystals irrespective of the reaction temperature.  相似文献   
105.
The small crystal of n-hexatriacontane was observed by a polarizing microscope in the rotator phase transition temperature region. In the temperature region, the rotator phase coexists with the solid phase (low-temperature ordered phase). The boundaries of two phases move reversibly with the temperature change. The area fractional change of the rotator phase can be described by the Debye relaxation. The relaxation time decreases and the relaxation strength increases as the sample temperature is raised. The relaxation time agrees well with that of the dynamic specific heat, which was measured in the frequency range of 0.0003≤f/Hz≤0.05.  相似文献   
106.
107.
The solution behavior of hydrophobic-hydrophilic water-soluble graft copolymers consisting of poly[(ethylene-co-vinyl alcohol)-g-ethylene oxide] was investigated by high-resolution NMR spectroscopy and measurements of solution viscosity in tetrahydrofuran, water, and mixtures thereof at various temperatures. A graft copolymer with a backbone containing 2.7 mole% vinyl alcohol exists in an intramolecular phase-separated conformation in aqueous solution, independent of the temperature. Graft copolymers with backbones containing 20.3 mol% vinyl alcohol exhibit an intramolecular mixed-phase conformation in which the mobility of chains in the hydrophobic region depends on temperature. The temperature dependence of the intrinsic viscosity in water shows a maximum near 50°C.  相似文献   
108.
109.
A specific inhibitor of DNA cytosine C5-methylases would be useful for studying genomic imprinting, X-chromosome inactivation, carcinogenesis, and regulation of tissue-specific gene expression, for these physiological phenomena appears to be regulated through DNA methylation in promoter sequences. This paper reports a novel convenient in vitro assay method for screening DNA cytosine C5-methylase inhibitor. Our method uses a commercially available Hae III methylase (cytosine C5 methylase), its corresponding Hae III endonuclease, and lambda DNA as their substrate.  相似文献   
110.
The reaction of [(cod)M(mu-OMe)]2 (M = Rh, Ir; cod = cycloocta-1,5-diene) with calix[4]arenes (LH4) in the molar ratio of 0.5-0.6:1 gave the rhodium and iridium pi-arene complexes [(cod)M(eta 6-LH3)], while that in the molar ratio of 1.1-1.5:1 (M = Rh) led to the selective formation of the dinuclear complexes [((cod)Rh)2(eta 6:eta 2-LH2)] in which one of the Rh(cod)+ fragments is coordinated by an eta 6-aryl group and the other by two phenolic oxygen atoms; the stepwise synthesis of the Rh-Ir heterobimetallic analogue of the latter complex was further achieved.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号