全文获取类型
收费全文 | 113篇 |
免费 | 0篇 |
专业分类
化学 | 101篇 |
晶体学 | 1篇 |
力学 | 3篇 |
物理学 | 8篇 |
出版年
2022年 | 2篇 |
2021年 | 3篇 |
2020年 | 1篇 |
2016年 | 2篇 |
2015年 | 1篇 |
2014年 | 2篇 |
2013年 | 1篇 |
2012年 | 1篇 |
2011年 | 5篇 |
2010年 | 8篇 |
2009年 | 3篇 |
2008年 | 5篇 |
2007年 | 6篇 |
2006年 | 2篇 |
2005年 | 4篇 |
2004年 | 1篇 |
2003年 | 5篇 |
2002年 | 3篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 3篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 1篇 |
排序方式: 共有113条查询结果,搜索用时 31 毫秒
1.
J. Guzmn E. Riande M. Rico J. Santoro 《Journal of polymer science. Part A, Polymer chemistry》1985,23(8):2283-2289
The microstructure of tetrahydrofuran (A)-3,3 dimethyloxetane (B) copolymers was studied by 13C-{1H}-NMR spectroscopy. Only the methyl carbons corresponding to the 3,3 dimethyloxetane unit appear as a singlet, whereas the other carbons present a more complicated spectral pattern than it would be expected if ? effects were negligible. The assignment of the resonance signals allowed the determination of the values of the probabilities of the different triads, which were in good agreement with those obtained from the reactivity ratios. 相似文献
2.
M. T. Iglesias J. Guzmn E. Riande 《Journal of polymer science. Part A, Polymer chemistry》1994,32(13):2565-2576
The synthesis of new unsaturated monomers containing functional groups has been theoretically analyzed by considering a simple kinetic scheme. In the reactions between acetyl and acryloyl chloride with neopentylglycol, it was shown that the comparison between theoretical and experimental results, does not allow us to conclude that the two rate constants controlling the reactions are equivalent. Kinetic experiments of the polymerization of 3-hydroxyneopentyl acrylate were carried out in benzene and 1,4-dioxane solution at different temperatures. Dilatometric techniques and nonlinear least-squares methods were used to obtain kinetic data and to determine the kinetic constants, respectively. The values of $k_p /k_t^{1/2}$ for this monomer were found higher in dioxane than in benzene due, probably, to the fact that in the last solvent the polymerization is heterogeneous. The activation energy, determined by using different values of $k_p /k_t^{1/2}$ was found 7.6 kcal/mol. The stereostructure of the polymers derived from 3-hydroxyneopentyl acrylate and 3-acetoxyneopentyl acrylate was determined by 13C-NMR spectroscopy from the analysis of the resonance signals belonging to the carbonyl groups, obtaining values for the fraction of isotactic dyads in the range 0.36 ± 0.03 for both polymers. Finally, the glass transition temperatures of both polymers, determined calorimetrically, were found 281 and 255 K, respectively. © 1994 John Wiley & Sons, Inc. 相似文献
3.
The relaxation behavior of poly(5-acryloxymethyl-5-methyl-1,3-dioxacyclohexane), a polymer containing highly flexible side groups, is studied by broadband dielectric spectroscopy in the frequency and temperature ranges 10(-1)-10(9) Hz and 123-473 K, respectively. Above the glass transition temperature T(g) the dielectric loss in the frequency domain exhibits a prominent alpha absorption, followed in increasing order of frequencies by two secondary absorptions called beta and gamma. At temperatures slightly higher than T(g), the a relaxation is well separated from the beta, but as temperature increases overlapping between both relaxations augments forming an alphabeta absorption in the vicinity of 420 K. This latter absorption displays a shoulder on its high-frequency side corresponding to the y relaxation. The strength of the a relaxation decreases with increasing temperature, eventually vanishing at the temperature at which the alphabeta absorption is formed. The time retardation spectra of the isotherms are calculated and further used to facilitate the deconvolution of the overlapping relaxations. The fact that the temperature dependence of the beta relaxation also describes that of the alphabeta absorption suggests that both relaxations have the same nature. It seems that as temperature increases, the a relaxation feeds on the beta absorption until its complete disappearance. The gamma relaxation, in turn, seems to increase at the expense of the alphabeta process at high temperature. 相似文献
4.
Molecular dimensions,configurational partition functions and entropies of linear aromatic polyesters
E. Riande 《European Polymer Journal》1978,14(11):885-888
The values of the configurational partition function Z and of the entropy S. as well as the unperturbed molecular dimensions 〈r2〉o/M, were calculated for aromatic polyesters having the repeat unit -OC(C6H4)COO(CH2)mO-. From a qualitative point of view, the values of Z are useful in interpreting the effect of the number of methylene groups on the unperturbed dimensions. For the lower members of the series (2 ? m ? 10), the melting temperatures show some correlation with the flexibility of the chains as measured by the configurational entropy, calculated at low temperature. 相似文献
5.
6.
7.
Lianet Contreras Rodríguez Evaristo Jos Madarro-Cap
Carlos Miguel Legn-Prez
Omar Rojas Guillermo Sosa-Gmez 《Entropy (Basel, Switzerland)》2021,23(5)
Entropy makes it possible to measure the uncertainty about an information source from the distribution of its output symbols. It is known that the maximum Shannon’s entropy of a discrete source of information is reached when its symbols follow a Uniform distribution. In cryptography, these sources have great applications since they allow for the highest security standards to be reached. In this work, the most effective estimator is selected to estimate entropy in short samples of bytes and bits with maximum entropy. For this, 18 estimators were compared. Results concerning the comparisons published in the literature between these estimators are discussed. The most suitable estimator is determined experimentally, based on its bias, the mean square error short samples of bytes and bits. 相似文献
8.
Beatriz Jurado-Sánchez Evaristo Ballesteros Mercedes Gallego 《Analytical and bioanalytical chemistry》2010,396(5):1929-1937
Humans are exposed to endogenous or exogenous formation of aromatic amines (AAs) and N-nitrosamines (NAms), which are considered to be potent carcinogens. The objective of this study was to monitor AAs and NAms
in human urine to obtain a way to assess exposure. However, while NAms can be directly detected in urine samples, AAs require
hydrolysis to convert their conjugates into free amines. A semiautomatic flow-base method is proposed for the simultaneous
determination of aliphatic and aromatic NAms, anilines and chloroanilines in human urine in one analytical run. Conjugated
AAs are released from urine by on-line microwave-assisted acid hydrolysis without degradation of NAms; all amines were then
preconcentrated using solid-phase extraction. Separation/determinations are carried out by gas chromatography and mass spectrometry
operating in selected ion monitoring mode. The method is fast (∼15 min for 25 mL of sample) and provides low limits of detection
(from 2 to 26 ng/L) with good precision (relative standard deviation within and between days less than 7%). Finally, the proposed
method was successfully applied to check AAs and NAms in the human urine of exposed and unexposed researchers. The kinetics
of amine excretion in the urine of the researchers exposed is calculated after termination of the exposure and shows half-life
times between 1.3 and 2.1 h, and that the dosage absorbed was eliminated within 6 h after exposure. 相似文献
9.
Ricardo Díaz-Calleja Evaristo Riande Julio Guzmn 《Journal of Polymer Science.Polymer Physics》1991,29(1):93-98
The mechanical and dielectric relaxation behavior of strained and unstrained networks, prepared from hydroxyl-terminated poly(diethylene glycol-trans-1,4-cyclohexane dicarboxylate) (PDGC), is studied over a wide interval of frequencies and temperatures. The mechanical relaxation spectrum exhibits a glass-rubber absorption, designated β, located in the vicinity of 0°C at 0.1 Hz, followed by a β relaxation which appears to be the result of two overlapping peaks centered at ?80°C (β1) and ?110°C(β2). These two peaks coalesce into a single peak in the case of strained networks. The dielectric relaxation spectrum also exhibits an α absorption followed by a subglass β relaxation whose width decreases as the elongation ratio λ increases. The activation energy associated with the mechanical β1 appears to increase as λ increases. However, the activation energy of the dielectric β process does not show a clear dependence on the elongation ratio. The analysis of the conformational characteristics of PDGC chains indicates that rotational transitions through the Ccy? C* bonds of the acid residue would give rise to high dielectric activity. Conformational changes about the CH2? CH2 bonds of the glycol residue would produce significant mechanical activity but, comparatively, low dielectric activity. The glass-rubber absorption is slightly displaced toward the high-temperature side as the elongation ration increases, suggesting that the entropic effects overcome the volume effects. The glass-rubber transition is interpreted in terms of the free volume theory. 相似文献
10.
Enrique Saiz María P. Tarazona Evaristo Riande Julio Guzmn 《Journal of Polymer Science.Polymer Physics》1984,22(12):2165-2174
The stress-optical behavior of an unswollen elastomeric network of poly(3-methyltetrahydrofuran) was measured for elongation ratios a in the range 1.182–1.549, at several temperatures between 20 and 60°C. No evidence of strain-induced crystallization was found; moreover, the dependence of birefringence Δn on true stress f/A was linear in the interval of α investigated. Values of Δa ranged from 2.4 to 2.8 in units of 10?24 cm3, in the temperature range studied, with a temperature coefficient 3.1 × 10?3 K?1. Theoretical calculations carried out with the rotational-isomeric-state model gave values of Δa noticeably smaller than the experimental results; however, a small increase in the backbone valence angles θ improved the theoretical result of Δa without worsening that of the dipole ratio. Analysis of the Δa results seems to corroborate the conclusion obtained through the study of dipole moments concerning the preference for nucleophilic attack on the less hindered α carbon in the monomer. Theoretical and experimental values of the temperature coefficient of Δa were in clear disagreement; a qualitative explanation for this discrepancy is discussed. 相似文献