收费全文 | 25801篇 |
免费 | 3977篇 |
国内免费 | 2433篇 |
化学 | 17979篇 |
晶体学 | 309篇 |
力学 | 1531篇 |
综合类 | 109篇 |
数学 | 2915篇 |
物理学 | 9368篇 |
2024年 | 70篇 |
2023年 | 524篇 |
2022年 | 946篇 |
2021年 | 965篇 |
2020年 | 1007篇 |
2019年 | 992篇 |
2018年 | 828篇 |
2017年 | 728篇 |
2016年 | 1203篇 |
2015年 | 1153篇 |
2014年 | 1349篇 |
2013年 | 1795篇 |
2012年 | 2409篇 |
2011年 | 2412篇 |
2010年 | 1586篇 |
2009年 | 1508篇 |
2008年 | 1652篇 |
2007年 | 1513篇 |
2006年 | 1367篇 |
2005年 | 1158篇 |
2004年 | 844篇 |
2003年 | 646篇 |
2002年 | 612篇 |
2001年 | 445篇 |
2000年 | 436篇 |
1999年 | 513篇 |
1998年 | 430篇 |
1997年 | 436篇 |
1996年 | 451篇 |
1995年 | 358篇 |
1994年 | 307篇 |
1993年 | 235篇 |
1992年 | 234篇 |
1991年 | 198篇 |
1990年 | 170篇 |
1989年 | 143篇 |
1988年 | 97篇 |
1987年 | 109篇 |
1986年 | 87篇 |
1985年 | 82篇 |
1984年 | 47篇 |
1983年 | 43篇 |
1982年 | 35篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1976年 | 9篇 |
1975年 | 11篇 |
1957年 | 4篇 |
Stereocomplex (SC) crystallization has been an effective way to improve the physical performances of stereoregular polymers. However, the competition between homo and SC crystallizations can lead to more complicated crystallization kinetics and polymorphic crystalline structure in stereocomplexable polymers, which influences the physical properties of obtained materials. Herein, we select the medium-molecular-weight (MMW) poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) asymmetric blends with different PDLA fractions (fD=0.01–0.5) as the model system and investigate the effects of fD and crystallization temperature (Tc) on the crystallization kinetics and polymorphic crystalline structure. We observe the fractionated (i.e., multistep) crystallization kinetics and the formation of peculiar β-form homocrystals (HCs) in the asymmetric blends under quiescent conditions, which are strongly influenced by both fD and Tc. Precisely, crystallization of β-form HCs is favorable in the MMW PLLA/PDLA blends with high fD (≥0.2) at a low Tc (80–100 °C). It is proposed that the formation of metastable β-form HCs is attributed to the conformational matching between β-form HCs and SCs, and the stronger constrain effects of precedingly-formed SCs in the early stage of crystallization. Such effects can also cause the multistep crystallization kinetics of MMW PLLA/PDLA asymmetric blends in the heating process.
相似文献Components of co-continuous phase can form an interpenetrating network structure, which has great potential to synergistically improve the mechanical properties of the blends, and to impart the functional blends superior electrical conductivity and permeability. In this work, the effects of shear rates (50–5000 s?1) at different temperatures on the phase morphology, phase size and lamellar crystallites of biodegradable co-continuous polybutylene terephthalate (PBAT)/polybutylene succinate (PBS) blend are quantitatively investigated. The results show that the above features of the PBAT/PBS have a strong dependence on the shear flow and thermal field. The co-continuous phase of the blend is well maintained at 130 °C. Interestingly, this phase structure transforms into a “sea-island” structure at 160 °C, which gradually recovers to a co-continuous phase when the shear rate increases from 1000 s?1 to 5000 s?1. The phase size decreases with the increase of shear rate both at 130 °C and 160 °C due to the refinement and deformation of phase structures caused by strong shear stress. Unexpectedly, a unique phenomenon is observed that the shear-induced lamellar crystallites are oriented perpendicular to shear direction in the range of 500–5000 s?1 at 130 °C, while the orientation of lamellar crystallites at 160 °C is along the shear direction within the whole range of shear rates. The degree of orientation for the PBAT/PBS blend crystals increases first and then decreases at both temperatures above. In addition, the range of shear rate has reached the level in the industrial processing. Therefore, this work has important guiding significance for the regulation of the co-continuous phase structure and the performance for the blend in the practical processing.
相似文献As a special subset of calix[4]arene, calix[4]resorcinarene is an excellent molecular platform which could be modified by introducing functional groups to multiple sites at the upper and lower rims. There are mainly three ways to build functionalized calix[4]resorcinarene derivatives: (1) modification on the C-2 sites of calix[4]resorcinarenes; (2) modification on the phenolic hydroxyl groups of calix[4]resorcinarenes; (3) modification on the bridging methylenes at lower rim of calix[4]resorcinarenes. Functionalized calix[4]resorcinarene derivatives play an important role in the development of self-assembly chemistry, among which hydrogen bonding and metal coordination are the two most common interactions to obtain multicomponent structures. Moreover, due to the excellent topological structures and various active substituents of functionalized calix[4]resorcinarene derivatives, their applications in various fields, such as nanoparticles, catalysts, fluorescent materials, and sensors, have been briefly presented in this paper.
相似文献