1,721,118 research outputs found
Thermal properties and physical ageing of poly (l-lactic acid)
No full textPoly(l-lactic acid) (PLLA) was investigated by differential scanning calorimetry and dynamic mechanical and dielectric spectroscopies. In the dynamic mechanical and dielectric spectra no secondary relaxations were observed below the glass-to-rubber transition (70°C at 3 Hz). The thermal history strongly affects the physical properties of PLLA, inducing changes of crystalline: amorphous ratio as well as large physical ageing effects on the glassy amorphous phase. This latter phenomenon is clearly observed as a consequence of room-temperature storage and develops at a faster rate as the ageing temperature Ta approaches Tg. Ageing experiments carried out at the same undercooling (ΔT = Tg - Ta) on PLLA samples of different molecular weight ( M ̄v = 5300, 20 000, 691 000) show that a decrease in molecular weight increases the magnitude of the enthalpy relaxation at the glass transition. © 1992
Polymer crystallization: Fold surface free energy determination by different thermal analysis techniques
No full textThree different thermal analysis techniques (DSC, and hot-stage optical microscopy equipped with a photodiode to measure light depolarization or a video system to measure spherulitic growth) were used to study polymer crystallization. The fold surface free energy (σe) values from different analytical methods on five polymers are compared. The overall crystallization data always yield σe values 15-50% higher than those obtained from the more direct method of measuring growth rate. © 1995
1[2(RS)-3(RS)-4-Acetyl-2-hydroxy-3-(5-methoxy-4-methyl-3-phenyl-pyrazol-1-ylamino)-2,5-dimethyl-2,3-dihydrofuran-3-yl]ethanone
No full textPrimary crystal nucleation and growth regime transition in isotactic polypropylene
No full textWe studied nucleation and crystal growth in two polypropylene samples by analyzing the number of spherulites, their size, and shape for various treatments in a hot-stage microscope. The consistency of nucleation and growth data were tested by using overall crystallization experiments and the Johnson-Mehl-Avrami-Kolmogorov equation. In isothermal conditions, for temperatures, TC, varying between 123° and 138°C, the number of crystallites per unit area does not depend on crystallization time and temperature. Instead, at small undercoolings (for TC > 138°C), the total number of nuclei per unit area remains independent of crystallization time, but decreases with increasing temperature. We discuss the implications of this finding for the final microstructure of the crystallized samples. It is noteworthy that the passage between these two nucleation behaviors coincides with the transition from regime III to regime II, observed in the crystal growth kinetic analysis. This remarkable correlation between primary nucleation and crystal growth regimes is discussed
Sulfur containing polymers. Aromatic polydithiocarbonates and polythiocarbonates: Synthesis and thermal properties
No full textNew polydithiocarbonates and polythiocarbonates were obtained by interfacial polymerization of bis(4-mercaptophenyl)methane, bis(4-mercaptophenyl)ether and bis(4-mercaptophenyl)sulfide with phosgene, bisphenol A bischloroformate and bisphenol A polycarbonate oligomers (-OH/-O-CO-Cl terminated). Polymerization process was carried out under interfacial conditions using a phase-transfer catalyst, as earlier described for the synthesis of polydithiocarbonates and polythiocarbonates from 2,2-bis(4-mercaptophenyl)propane. The structures of the polymers were examined by IR and NMR spectroscopies; their thermal properties were investigated by thermogravimetric analysis and differential scanning calorimetry. In particular, the effect of the substitution of one or both the ethereal oxygen atoms of the carbonate group by sulfur has been analyzed by comparing the Tg values and the ability to crystallize of the sulfur containing polymers with those of the corresponding polycarbonates. © 2002 Elsevier Science Ltd. All rights reserved
Quantitative evaluation by fractal analysis of indentation crack paths in Si3N4-SiCW composites
No full textThe crack paths, generated by Vickers indentation in Si3N4-SiCW composites, were analyzed from a quantitative point of view, in order to distinguish between the possible toughening mechanisms involved. It was found that the roughness parameter RL and the fractal dimension D describe the geometrical features of the cracks and evidence the anisotropy of the microstructure, due to the whisker and Si3N4B-grain orientation. The relationships found between fracture toughness and these geometrical parameters emphasise the importance of the crack deflection mechanism in improving toughness and provide information on the role played by microstructure in determining the mechanical properties of a composite
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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