Crystallization kinetics of some new olefinic block copolymers

Blocky ethylene-octene copolymers synthesized by chain-shuttling polymerization differ from statistical copolymers in their rapid rate of crystallization and in the formation of space-filling spherulites even when the crystallinity is as low as 7%. The bulk crystallization rate, measured with DSC, was rapid even in copolymers with a relatively large fraction of non-crystallizable soft block and only slowed somewhat as the amount of crystallizable hard block decreased from 100 to 18 wt%. As measured with the polarized optical microscope, the linear spherulite growth rate exhibited the same dependence on soft block content as the bulk crystallization rate. The fold surface energy was extracted from an analysis of the growth rate according to the Lauritzen-Hoffman theory. A gradual increase in the fold surface energy with soft block content reflected some increasing disorder of the fold surface. In contrast, even a small amount of statistically distributed comonomer was very effective in disrupting the fold surface regularity as demonstrated by the high fold surface energy.     

Recrystallization studies on isotropic cold-crystallized PET: Influence of heating rate

The nanostructural changes associated to the multiple melting behaviour of isotropic cold-crystallized poly(ethylene terephthalate) (PET) have been investigated by means of simultaneous wide- and small-angle X-ray scattering, using a synchrotron radiation source. Variations in the degree of crystallinity, coherent lateral crystal size and long period values, as a function of temperature, for two different heating rates are reported for cold-crystallized samples in the 100-190 °C range. The Interface Distribution Function analysis is also employed to provide the crystalline and amorphous layer thickness values at various temperatures of interest. Results suggest that samples crystallized at both low (T_a = 100-120 °C) and high (T_a = 160-190 °C) temperatures are subjected to a nearly continuous nanostructural reorganization process upon heating, starting immediately above T_g (≈80 °C) and giving rise to complete melting at ≈260 °C. For all the T_a investigated, a melting-recrystallization mechanism seems to take place once T_a is exceeded, concurrently to the low-temperature endotherm observed in the DSC scans. For low-T_a and slow heating rates (2 °C/min), a conspicuous recrystallization process is predominant within T_a + 30 °C ≤ T ≤ 200 °C. In contrast, for high-T_a, an increasingly strong melting process is observed. For both, high- and low-T_a, an extensive structural reorganization takes place above 200 °C, involving the appearance of new lamellar stacks simultaneously to the final melting process. The two mechanisms should contribute to the high-temperature endotherm in the DSC scan. Finally, the use of a high heating rate is found to hinder the material's overall recrystallization process during the heating run and suggests that the high-temperature endotherm is ascribed to the melting of lamellae generated or thickened during the heating run.     

A novel approach of measuring the dielectric properties of PET preforms for stretch blow moulding

Polyethylene terephthalate (PET) is the most common plastic material used in injection stretch blow molding (ISBM) process for the manufacturing of bottles. The injection‐molded PET preform has to be preheated uniformly to a pliable state before the blowing stage of the process. Microwave technology offers a potentially more economical means for preheating the preform in place of the conventional infrared oven heating. For such applications, determination of dielectric properties of PET preform of a given geometry is essential. This article describes a novel approach of measuring the dielectric properties of PET preforms at microwave frequencies instead of the common experimental procedures used for such measurements. The dielectric properties are determined by using both CST Microwave Studio electromagnetic simulation software and the automatic network analyzer together. No complex mathematical solutions are required. Validation of this new approach has been made by comparing the measured dielectric properties of some polymers with those available in the published literature. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Synthesis of α‐Carbolines Starting from 2,3‐Dichloropyridines and Substituted Anilines

9H‐α‐Carbolines have been prepared via consecutive intermolecular Buchwald–Hartwig reaction and Pd‐catalyzed intramolecular direct arylation from commercially available 2,3‐dichloropyridines and substituted anilines. The combination of a high reaction temperature (180 °C) and the use of DBU were found to be crucial for the intramolecular direct arylation reactions of the 3‐chloro‐N‐phenylpyridin‐2‐amines as no reaction was observed at 120 °C and 180 °C using different inorganic and other organic bases. On the other hand, nitrogen‐methylated pyridine analogues of these substrates {N‐[3‐chloro‐1‐methylpyridin‐2(1H)‐ylidene]anilines} do undergo ring closure at 120 °C, with K₃PO₄ as base, affording the respective 1‐methyl‐1H‐α‐carbolines in good yields.     

Thermal stability of nanocrystalline Ni- and Co-based pulsed current electrodeposits: correlation of calorimetric measurements and microstructure development upon annealing

The exceptional properties associated with nanocrystalline materials are, to a large extent, a result of their high inter-crystalline volume fraction. However, the intrinsic instability of the nanostructured state may compromise the gain in properties by the occurrence of grain growth during exposure at elevated temperatures. Thermal stability is, therefore, a fundamental materials issue for nanocrystalline materials. This article describes what can be deduced from calorimetric measurements in the context of what is known about the microstructural evolution upon annealing of nanocrystalline Ni- and Co-based pulsed current electrodeposits. Special emphasis is put on interpreting the shape of the curves obtained by a differential scanning calorimetry (DSC). The temperature ranges for relaxation, segregation, precipitation, as well as abnormal and normal grain growth can be predicted. Also, by evaluating the shift in peak temperature with heating rate (Kissinger plot), the activation energies for grain growth can be obtained for the different materials.     

Effect of Al content on high temperature erosion properties of arc-sprayed FeMnCrAl/Cr_3C_2 coatings

Three types of FeMnCrAl/Cr3C2 coatings with different Al content were deposited on 20# steel substrates by the high velocity arc spraying (HVAS) process. Surface microstructures of the coatings were analyzed by optical microscopy (OM) and X-ray diffractometry (XRD). High temperature erosion (HTE) tests were performed in an erosion tester at different impact angles. The surface morphologies of the eroded coatings were observed on a field emission scanning electron microscope(FE-SEM). The laminated structure is found on all the prepared coatings with the porosity and oxide fraction in the coatings decreasing with the Al content from 0 to 15% (mass fraction). Sample FA3 with 15% Al, possessing the lowest porosity and oxide fraction, has the best HTE resistance, which demonstrates that Al addition can improve the HTE resistance of the coatings. The erosion rate of sample FA1 exhibits a maximum value at 90° impact angle. The maximum erosion rates of both FA2 and FA3 samples appear in the range of 60°-90° impact angles. Erosion loss of the coatings occurs through brittle breaking, cutting and fatigue spalling.     

粉末粒度对碳/碳基体上羟基磷灰石涂层的影响

采用等离子喷涂技术在碳纤维增强碳复合材料(简称碳／碳复合材料)上制备了羟基磷灰石涂层,研究了原始粉末粒度对涂层的表面形貌、剪切强度、相组成等方面的影响,并分析了涂层与基体的界面结合状况。结果表明：采用粗粉末喷涂后得到的涂层结合强度较高．结晶程度也较高,但涂层与基体的结合机制仍为机械嵌合。