全文获取类型
收费全文 | 1803篇 |
免费 | 191篇 |
国内免费 | 126篇 |
专业分类
化学 | 2066篇 |
晶体学 | 3篇 |
力学 | 5篇 |
综合类 | 6篇 |
物理学 | 40篇 |
出版年
2024年 | 2篇 |
2023年 | 11篇 |
2022年 | 16篇 |
2021年 | 40篇 |
2020年 | 54篇 |
2019年 | 51篇 |
2018年 | 41篇 |
2017年 | 53篇 |
2016年 | 71篇 |
2015年 | 75篇 |
2014年 | 79篇 |
2013年 | 151篇 |
2012年 | 111篇 |
2011年 | 109篇 |
2010年 | 97篇 |
2009年 | 91篇 |
2008年 | 127篇 |
2007年 | 113篇 |
2006年 | 109篇 |
2005年 | 93篇 |
2004年 | 101篇 |
2003年 | 87篇 |
2002年 | 53篇 |
2001年 | 52篇 |
2000年 | 47篇 |
1999年 | 44篇 |
1998年 | 26篇 |
1997年 | 40篇 |
1996年 | 28篇 |
1995年 | 31篇 |
1994年 | 37篇 |
1993年 | 33篇 |
1992年 | 25篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有2120条查询结果,搜索用时 281 毫秒
71.
Noritaka Ohtani Makoto Nakaya Kenji Shirahata Tsuyoshi Yamashita 《Journal of polymer science. Part A, Polymer chemistry》1994,32(14):2677-2686
Polystyrene-based crosslinked cationic ionomers containing ammonium or phosphonium chlorides (AxRCI and PxBuCI) were reacted with decyl methanesulfonate. The kinetic data were correlated with the swelling behavior of the ionomers and the solution viscosity of the corresponding linear ionomers. The reactivity of the ionomers was independent of the particle size of the ionomer beads, indicating no diffusion control of the reaction. The solvent and the ion content of the ionomers greatly affect the reactivity. In nonpolar solvents with a low acceptor number, AN, such as toluene, the aggregation of ionic groups with an increasing ion content reduces the reactivity. A solvent with a high value of AN, such as chloroform, led a very low reactivity independent of the ion content. Aprotic polar solvent, such as acetonitrila, promoted the dissociation of the ionic groups and furnished a relatively high reactivity independent of the ion content. Several catalytic substitution reactions were carried out under liquid-solid-solid triphase conditions. The kinetic results were accounted for in terms of slow nucleophile transport and fast chemical reaction within the ionomer particles. © 1994 John Wiley & Sons, Inc. 相似文献
72.
Summary The surface dyeability of the poly(vinyl alcohol) fiber with different degree of formalization was studied by measuring -potential of the fiber in alkaline aqueous solutions (pH 10) of a cationic dye Methylene Blue. With the increase in the dye concentration, the sign of the -potential of the fiber changed from negative to positive and thereafter the positive value approximated to saturated value, and the amount of dye adsorbed (expressed in mol/cm2-fiber) increased also. These results may possibly be attributed to the formation of the electrostatic bond between the fiber and the dye. The linear relations were found between and logC
d
and its slope did not change with rise of temperature. The free energy of dyeing G (negative) calculated from the slope of the -logC
id
curve, etc. increased with increasing the degree of formalization of the fiber. This fact corresponds to the increase in the surface dye adsorption with increasing the degree of formalization. With increasing the degree of formalization, the heat of dyeing H (negative) increased and the entropy of dyeing S (positive) decreased. The positive value of S for each fiber, suggests the formation of hydrophobic bond as a driving force to the dyeing of Methylene blue on the fiber surface in addition to the electrostatic bond.This paper is Part XXVII in a series on Studies on -potentials and Surface Dyeability of Natural and Synthetic Fibers in Dye Solution. Part XXVI: T. Suzawa andK. Kawakami, Nippon Kagaku Kaishi1975, No. 7,1134. 相似文献
73.
C. Damas M. Adibnejad A. Benjelloun A. Brembilla M. C. Carré M. L. Viriot P. Lochon 《Colloid and polymer science》1997,275(4):364-371
Cationic amphiphilic polymers (e.g. polyvinylpyridinium bromides and polyvinylimidazo-lium bromides) adopt a compact coiled
form in aqueous solutions. In the case of former polymers, the resulting hydrophobic microdomains are evidenced only by fluorescence
spectroscopy if a molecular rotor (DMAC) is used as a fluorescent probe, while the behaviour of the latter can be studied
in aqueous solution by using both types of fluorescent probes, i.e., pyrene and the molecular rotor. The purpose of the present
investigation deals with a comparative study between the magnitude of the local viscosity and the polarity of the hydrophobic
microdomains generated by poly(3-hexadecyl-1-vinylimidazolium bromide) in an aqueous medium using pyrene and DMAC as fluorescent
probes. Moreover, the results are compared with the data obtained with micelles of conventional surfactants such as the homopolymer
repetitive unit model and CTAB.
Received: 3 June 1996 Accepted: 29 September 1996 相似文献
74.
Yu Chen Melania Bednarek Przemysaw Kubisa Stanisaw Penczek 《Journal of polymer science. Part A, Polymer chemistry》2002,40(12):1991-2002
The cationic ring‐opening polymerization of 3,3‐bis(hydroxymethyl)oxetane (BHMO) and the copolymerization of BHMO with 3‐ethyl‐3‐(hydroxymethyl)oxetane (EOX) were studied. Medium molecular weight polymers (number‐average molecular weight ≈ 2 × 103) were obtained in bulk polymerization. Poly[3,3‐bis(hydroxymethyl)oxetane], as highly insoluble, was only characterized by gel permeation chromatography and NMR methods in the esterified form. Copolymers of BHMO and EOX that were slightly soluble in organic solvents were characterized in more detail. In a copolymerization from a 1:1 mixture, the comonomers were consumed at equal rates. Matrix‐assisted laser desorption/ionization time‐of‐flight analysis confirmed that a random 1:1 copolymer was formed. 13C NMR analysis indicated that in contrast to previously described homopolymers of EOX in which the degree of branching was limited, the homopolymers of BHMO were highly branched. This pattern was preserved in the copolymers; EOX units were predominantly linear, whereas BHMO units were predominantly branched. The copolymerization of BHMO with EOX provides, therefore, a route to multihydroxyl branched‐polyethers with various degrees of branching containing ? OH groups exclusively as ≡C? CH2? OH units. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1991–2002, 2002 相似文献
75.
Shokyoku Kanaoka Nobuyuki Ikeda Akira Tanaka Hitoshi Yamaoka Toshinobu Higashimura 《Journal of polymer science. Part A, Polymer chemistry》2002,40(14):2449-2457
Random copolymers with high molecular weights of indene and p‐methylstyrene (pMeSt) were synthesized by cationic polymerization with trichloroacetic acid/tin tetrachloride in CH2Cl2 at low temperatures. When indene and pMeSt (1:1 v/v), for example, were polymerized at ?40 °C, both monomers were consumed at very similar rates to give a copolymer with high molecular weight [number‐average molecular weight (Mn): 8–9 × 104]. This is indeed quite unexpected behavior for the combination of these two monomers because pMeSt polymerized over 1000 times faster than indene in the homopolymerization under the reaction conditions previously described. The product copolymer of indene and pMeSt had a random monomer sequence in it that was confirmed by NMR analyses and thermal‐property measurements. In sharp contrast with pMeSt, styrene and p‐chlorostyrene, which have no electron‐donating groups on the phenyl ring, led to low molecular weight polymers (Mn < 10,000) in the copolymerization with indene (1:1 v/v). © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2449–2457, 2002 相似文献
76.
硅氧烷乳液聚合过程中大颗粒形成机理研究:Ⅰ.阳离子型乳液的耐电解质… 总被引:5,自引:1,他引:5
探讨了二甲基聚硅氧烷阳离子型乳液耐电解质稳定性的影响因素。结果表明,加入少量的非离子型表面活性剂与阳离子型乳化剂并用进行乳液聚合,可以保护乳液粒子,防止由于电解质引的乳液粒子的相互凝聚而形成大颗粒。 相似文献
77.
Eri Yoshida Kazuaki Nakamura Toshikazu Takata Takeshi Endo 《Journal of polymer science. Part A, Polymer chemistry》1993,31(6):1505-1512
Oxoaminium salt ( 1 ), derived from 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 2 ) by one-electron oxidation, could be an initiator for cationic polymerization of vinyl monomers such as isobutyl vinyl ether (IBVE), 2,3-dihydrofuran, p-methoxystyrene, N-vinyl pyrrolidone, etc., to give the corresponding polymers, when 1 had a low nucleophilic counter anion. Formation of the adducts of 1 and IBVE as well as 1H-NMR and IR data suggested the formation of polymers containing N? O? C structure as the polymer head group. In the polymerization of IBVE, the effects of solvent and concentration of 1 were little observed, however the polymerization rate was dependent on temperature. Furthermore, the thermal reaction of the polymers obtained, which were regarded as prepolymers for block copolymerization and polymeric initiators for radical polymerization, was studied. For example, poly(2-benzylidene-1,3-dioxane) obtained by the polymerization of 2-benzylidene-1,3-dioxane with oxoaminium hexafluoroantimonate ( 1, X = SbF6) was employed as an initiator for radical polymerization of MMA to give its block copolymer with PMMA. © 1993 John Wiley & Sons, Inc. 相似文献
78.
Francesca Giuntini Daniele NistriGiacomo Chiti Lia FantettiGiulio Jori Gabrio Roncucci 《Tetrahedron letters》2003,44(3):515-517
The base-catalyzed cyclotetramerization of 3-, 4- and 4,5-dimethylaminophenylthio phthalonitriles with zinc(II)acetate afforded 1(4),8(11),15(18),22(25)- and 2(3),9(10),16(17),23(24)-tetrasubstituted and 2,3,9,10,16,17,23,24-octasubstituted Zn(II)phthalocyanines, respectively. The statistical mixed condensation of the same phthalonitriles with 1,2-dicyanobenzene gave the corresponding mono- and disubstituted derivatives. Methylation of such products afforded a series of cationic Zn(II)phthalocyanines with different pattern of substitution, with potential use as photodynamic agents in microbial infections. 相似文献
79.
Organometallic rhodium complexes are described which are highly efficient initiators for the ring-opening polymerization of expoxides and other heterocyclic compounds. A cocatalyst, consisting of a compound or polymer containing silicon–hydrogen bonds must also be present. These same catalyst–cocatalyst mixtures are also highly active for hydrosilylation reactions. Other complexes bearing phosphine ligands have been discovered, which while active for hydrosilylation, are not catalysts for epoxide polymerizations. Polymer supported rhodium catalysts are also described which permit the synthesis of epoxy-functional silanes in high yields without competing ring-opening polymerization. 相似文献
80.
Noritaka Ohtani Yukihiko Inoue Hiroshi Mizuoka Kenya Itoh 《Journal of polymer science. Part A, Polymer chemistry》1994,32(13):2589-2595
Dilute-solution viscosities of polystyrene-based cationic ionomers containing ammonio or phosphonio groups were measured in several solvents. In polar solvents with dielectric constant (εr) beyond 10, the ionomers showed a typical polyelectrolyte behavior, indicating that a large part of ionic groups were dissociated into ions. In nonpolar solvents with low εr, the reduced viscosity of the ionomers linearly decreased with a decreasing ionomer concentration. At low polymer concentrations, every ionomer gave a reduced viscosity lower than that of the corresponding chloromethylated polystyrene. With an increasing ion content, the intrinsic viscosity progressively decreased if the nonpolar solvents had a low acceptor number (AN), such as toluene or tetrahydrofran (THF). In the halogenated solvents with high AN value, such as chloroform, however, the intrinsic viscosity was hardly dependent on the ion content. This indicates that the intramolecular aggregation among the ionic groups is inhibited in the halogenated solvents due to a strong anion solvation. An addition of a protic solvent to a nonpolar solvent eliminates the aggregation between ionic groups and leads to polyelectrolyte behavior. © 1994 John Wiley & Sons, Inc. 相似文献