全文获取类型
收费全文 | 1055篇 |
免费 | 57篇 |
国内免费 | 269篇 |
专业分类
化学 | 1332篇 |
晶体学 | 3篇 |
综合类 | 1篇 |
物理学 | 45篇 |
出版年
2023年 | 11篇 |
2022年 | 12篇 |
2021年 | 22篇 |
2020年 | 17篇 |
2019年 | 33篇 |
2018年 | 21篇 |
2017年 | 24篇 |
2016年 | 33篇 |
2015年 | 35篇 |
2014年 | 37篇 |
2013年 | 123篇 |
2012年 | 66篇 |
2011年 | 66篇 |
2010年 | 57篇 |
2009年 | 66篇 |
2008年 | 73篇 |
2007年 | 61篇 |
2006年 | 88篇 |
2005年 | 65篇 |
2004年 | 70篇 |
2003年 | 50篇 |
2002年 | 66篇 |
2001年 | 44篇 |
2000年 | 48篇 |
1999年 | 41篇 |
1998年 | 28篇 |
1997年 | 23篇 |
1996年 | 14篇 |
1995年 | 12篇 |
1994年 | 18篇 |
1993年 | 11篇 |
1992年 | 14篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 1篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1984年 | 2篇 |
1981年 | 1篇 |
排序方式: 共有1381条查询结果,搜索用时 31 毫秒
1.
Starch belongs to the polyglucan group. This type of polysaccharide shows a broad β-relaxation process in dielectric spectra at low temperatures, which has its molecular origin in orientational motions of sugar rings via glucosidic linkages. This chain dynamic was investigated for α(1,4)-linked starch oligomers with well-defined chain lengths of 2, 3, 4, 6, and 7 anhydroglucose units (AGUs) and for α(1,4)-polyglucans with average degrees of polymerization of 5, 10, 56, 70, and so forth (up to 3000; calculated from the mean molecular weight). The activation energy (Ea) of the segmental chain motion was lowest for dimeric maltose (Ea = 49.4 ± 1.3 kJ/mol), and this was followed by passage through a maximum at a degree of polymerization of 6 (Ea = 60.8 ± 1.8 kJ/mol). Subsequently, Ea leveled off at a value of about 52 ± 1.5 kJ/mol for chains containing more than 100 repeating units. The results were compared with the values of cellulose-like oligomers and polymers bearing a β(1,4)-linkage. Interestingly, the shape of the Ea dependency on the chain length of the molecules was qualitatively the same for both systems, whereas quantitatively the starch-like substances generally showed higher Ea values. Additionally, and for comparison, three cyclodextrins were measured by dielectric relaxation spectroscopy. The ringlike molecules, with 6, 7, and 8 α(1,4)-linked AGUs, showed moderately different types of dielectric spectra. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 188–197, 2004 相似文献
2.
3.
The formation of the 28-membered ring cyclo[(6-amino-6-deoxy-d-galactonic acid)4] by cyclisation of a protected open chain fully hydroxylated nylon 6 linear tetramer in modest yield provides the first example of a new class of carbopeptoid-cyclodextrin. 相似文献
4.
The N-benzoyl-l-tyrosine ethyl ester (BTEE) anchored in cyclodextrin (β and β-methyl CD) serves as an excellent substrate for subtilisin
Carlsberg catalysis. The rate of hydrolysis was found to be approximately twofold higher than that of the methanolic substrate.
The Kmapp and Vmax values for the CD anchored substrates were significantly higher than the methanol-solubilized BTEE. 相似文献
5.
The block copolymer of polystyrene-b-poly(butyl acrylate) (PSt-b-PBA) with a well-defined structure was synthesized by atom transfer radical polymerization (ATRP); its structure was characterized, and the living polymerization was also validated by gel permeation chromatography, Fourier transform infrared, and 1H NMR measurements. Then, the amphiphilic block copolymer of polystyrene-b-poly(acrylic acid) (PSt-b-PAA) has been prepared by hydrolysis of PSt-b-PBA, and copolymers of PSt-b-PAA with longer PSt blocks and shorter PAA blocks were obtained by controlling the conditions of ATRP polymerization. The reversed micelle solution of PSt-b-PAA in toluene was prepared by using the single-solvent dissolving method, and the reverse micellization behavior of PSt-b-PAA in toluene was mainly investigated in this paper. The fluorescent probe technique was used by using polar fluorescence compound N-(1-Naphthyl)ethylenediamine dihydrochloride (NEAH) as a polar fluorescent probe to study the reverse micellization behavior of PSt-b-PAA. It was found that the reverse micellization behaviors of PSt-b-PAA in toluene can be clearly revealed by using NEAH as a polar fluorescence probe, and the critical micelle concentrations (cmcs) can be well displayed. The experimental results showed that the self-assembling behavior of PSt-b-PAA in toluene depends apparently on the microstructure of the macromolecules and is also influenced by the temperature. For the copolymers of PSt-b-PAA with the same length of hydrophobic PSt blocks, the copolymer with a longer hydrophilic block PAA has lower cmc, and at higher temperature, the copolymer has lower cmc. 相似文献
6.
7.
Immobilized polysiloxane-anchored permethyl-β-cyclodextrin (Chirasil-Dex) with a cyclodextrin content of approximately 30 % by weight, previously employed as a versatile chiral stationary phase for the separation of enantiomers by GC, has been used for the separation of enantiomers by capillary supercritical fluid chromatography (SFC). A considerable number of racemates could be resolved, e.g. aromatic alcohols, amino alcohols (TFA derivatives), and underivatized acids. Many pharmaceutical compounds were among those analyzed, including several NSAIDs (e.g. ibuprofen and ketoprofen), a steroidal drug (nor-gestrel), a barbiturate (hexobarbital), and others. Among the racemates resolved were many which cannot be analyzed by GC owing to low volatility or decomposition at elevated temperatures. For two racemates, analysis temperature and mobile phase density were systematically varied to give constant analysis times or capacity factors k. Low temperatures (ca 60 °C) yielded the best separation in term of separation factor, α, or resolution, Rs, even though higher densities had to be used. In comparison with GC, capillary SFC was able to furnish higher separation factors and similar resolution. The applicability of capillary SFC for the analysis of mixtures of cyclodextrin derivatives, e.g. those used in the synthesis of Chirasil-Dex, was, furthermore, demonstrated. 相似文献
8.
9.
Yingjin liu Naoto Horiuchi Yoshimi Sueishi Shunzo Yamamoto 《Journal of inclusion phenomena and macrocyclic chemistry》2006,54(3-4):233-239
The kinetics of the hydride transfer reaction between Methylene Blue (MB+) and 1-benzyl-1,4-dihydronicotinamide (BNAH) were studied in 10% ethanol-90% water mixed solvents containing β- and γ-cyclodextrins (β-CD and γ-CD). The pseudo-first order rate constant shows kinetic saturation at high initial concentration of BNAH. This indicates the formation of a complex between MB+ and BNAH. The reaction was suppressed by addition of β-CD, but enhanced by addition of γ-CD. MB+ and BNAH were separately accommodated within the β-CD cavity and the cavity walls may protect the activity site of the reactants. On the other hand, in the MB+-BNAH-γ-CD system, the inclusion of the complex between MB+ and BNAH with γ-CD occurred. This effect of γ-CD can distinguish between the productive and non-productive nature of the complex. 相似文献
10.
由环氧乙烷及烯丙基缩水甘油醚合成含双键侧基的聚氧乙烯用苯乙烯接枝共聚,接枝效率可达50%左右.接枝产物经纯化,用IR及~1H NMR表征.该产物与LiClO_4的络合物有较高的室温导电率,其PEO含量在70%及EO/Li=20/1时,25℃的导电率接近10~(-4)S/cm.此外,该接枝共聚物有较高的乳化能力.在Williamson固液反应中具有良好的相转移催化作用. 相似文献