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排序方式: 共有948条查询结果,搜索用时 156 毫秒
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Raul Quijada Jairton Dupont Dione Corrêa Silveira Mrcia S. Lacerda Miranda Rosngela B. Scipioni 《Macromolecular rapid communications》1995,16(5):357-362
This work reports a comparative study of the catalytic behaviour for a series of metallocenes derived from Ti, Zr, Hf and Nb, which after activation with methylaluminoxane can polymerize ethylene. Results show that the Zr metallocene with a (CH3)2Si Bridge presents the highest activity, and the metallocenes based on Hf and Nb do not show any significant activity under the tested conditions. 相似文献
944.
Luis Quintanilla Jos Carlos Rodríguez-Cabello Juan Carlos Merino Jose María Pastor 《Macromolecular Symposia》1995,94(1):129-144
With the introduction of the Fourier Transform technique in Raman spectroscopy (FT-Raman) and the development of new methods in FTIR, such as FTIR with photoacoustic detection (PA-FTTR), vibrational spectroscopy has been launched into a new era of application in polymer chemistry and physics. This work provides a comparative study between PA-FTIR and FT-Raman techniques in which the potentials and limitations of these two methods are investigated by following and analyzing structural variations that take place in annealed injected-moulded poly(ethylene terephthalate) (PET). Both spectroscopic techniques appear to be suitable to detect structural changes that occur in PET submitted to different annealing temperatures. We suggest that these variations are directly related to conformational rather than crystallinity changes. The main drawbacks of PA-FTIR are the lengthy adquisition time of spectra and the photoacoustic saturation that can appear in regions with quite high optical absorption coefficient. Conversely, FT-Raman technique does not suffer from neither of these problems. However, the sensitivity to structural changes of certain infrared bands seems to be higher than that of analogous Raman bands and small structural details such as the regularity of chain folding, can only be detected in the infrared spectra. 相似文献
945.
J. Pablo Tomba Jos M. Carella Enrique Pardo Sonia Lopez Jos M. Pastor 《Macromolecular rapid communications》2000,21(14):983-989
Experiments designed to thoroughly test a recently proposedgeneralized method to calculate diffusion rates in polydisperse systems have been carried out. Polydisperse polystyrene (PS) samples were allowed to diffuse in a poly(phenylene oxide) (PPO) matrix. Designed blends were made from anionically polymerized PS with molecular weights which cover most of the ranges where Rouse dynamics control the diffusion processes. The diffusion temperatures range from (Tg – 1 K) to (Tg + 105 K), causing the monomeric friction factor values for PS to change by up to seven orders of magnitude along the diffusion coordinate. Calculations performed with the above mentioned method agree with Raman and DMA experimental data. 相似文献
946.
Since 1961, aliphatic polyisocyanates based on HDI-biurets are used as light stable hardeners for 2-K polyurethane coatings. The market introduction of the HDI-isocyanurates or simply 'trimers' in the early 1980's was driven by their lower viscosity and their better monomer stability. The most recent development in HDI-polyisocyanate research are the iminooxadiazinediones (asymmetric trimers). In this paper, a new product based on that technology with a very low viscosity (700 mPas@23°C) and a NCO-functionality of more than 3 is presented. Because of its low viscosity, this hardener can be easily incorporated into aqueous systems and is the solution for high and full solids coatings as well. 相似文献
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