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91.
P. D. Whaley S. Kulkarni P. Ehrlich R. S. Stein H. H. Winter W. C. Conner G. Beaucage 《Journal of Polymer Science.Polymer Physics》1998,36(4):617-627
Crystallization of isotactic polypropylene (iPP) from homogeneous solution in supercritical propane yields open-cell foams of high surface area (120–150 m2/g). Their morphology usually consists of microspheres with a dense core and a porous periphery of radiating fibrils. Pore radii covering the mesopore range (2–50 nm), making their largest contribution at 10–20 nm, were calculated from nitrogen adsorption isotherms. Surface areas of the correct order of magnitude are obtained by assuming that gas adsorption takes place on the surfaces of lamellar crystals. Crystallization of iPP from n-butane and n-heptane generates foams of lower mesoporosity and smaller surface area. These more “liquid-like” solvents do not allow the formation of an open network of mesopores or they promote its collapse upon their removal. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 617–627, 1998 相似文献
92.
M. A. RodríguezPrez O. Alonso A. Duijsens J. A. de Saja 《Journal of Polymer Science.Polymer Physics》1998,36(14):2587-2596
An experimental study on the thermal expansion of a collection of crosslinked low-density polyethylene (LDPE) foams with closed-cell structure is presented. The thermal characterization of these materials, the relationships between the linear thermal expansion coefficient and the structure of the foams, and the determination of the variables that can modify the thermal properties of these products are the goals of this work. The experimental results show that the linear thermal expansion coefficient decreases when the density of the foamed material increases. The gas expansion inside the cells is a mechanism that should be taken into account. Moreover, the thermal expansion also depends on the cellular structure. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2587–2596, 1998 相似文献
93.
94.
Ceramic open-cell foams were obtained from a preceramic polymer (a silicone resin) and blown polyurethanes, by pyrolysis at 1200°C in nitrogen. Silicon carbide submicron powders were also added to the silicone resin to give SiOC + SiC composite foams. The morphology of the foams was dependent on the architecture of the blown polyurethanes. The crushing strength as well as the elastic modulus increased with increasing relative density, reaching values as high as 14 and 450 MPa, respectively. Some of the foams displayed an excellent thermal stability (resistance to oxidation in air and decomposition in inert atmosphere) up to elevated temperatures. 相似文献
95.
Studies performed on strong polyelectrolytes and on a weak polyelectrolyte, sodium poly(acrylate), show that their stability
in presence of multivalent cations depends on the chemical nature of the charged side groups of the polymer. For sulfonate
groups (SO3
-) or sulfate groups (OSO3
-) phase separation generally occurs in presence of inorganic cations of valency 3 (as La3+) or larger and a resolubilization takes place at high salt concentration. The interactions of the polyelectrolyte with multivalent
cations are of electrostatic origin and the phase diagrams are weakly dependent on the chemical nature of the polymer backbone
and on the specificity of the counterions. For acrylate groups, (COO-), the phase separation was observed with inorganic cations of valency 2 (as Ca2+) or larger without resolubilization at high salt concentration. The phase separation is due to a chemical association between
cations and acrylate groups of two neighboring monomers of the same chain. This chemical association creates a hydrophobic
complex by dehydrating both monomer and cation. With organic trivalent cation, as spermidine +H3N(CH2)4NH2
+(CH2)3NH3
+, where no chemical association occurs with the charged side groups COO- or SO3
- of the polyelectrolyte, similar phase diagrams were observed whatever was the polyelectrolyte with a resolubilization at
high trivalent cation concentration.
Received 3 March 1999 and Received in final form 2 September 1999 相似文献
96.
A. K. Bledzki J. Zicans R. Merijs Meri D. Kardasz 《Mechanics of Composite Materials》2008,44(5):515-522
Blends of secondary rigid polyurethane foams (RPUFs) with soft polyurethane foams (SPUFs) were investigated. The effect of
SPUF content and its chemical nature on some physical and mechanical properties of the blends was evaluated. Owing to the
stronger intermolecular interaction and higher values of cohesion energy, the blends of RPUFs with polyester SPUFs showed
higher mechanical properties than those with polyether SPUFs. The density, hardness, ultimate strength, and the tensile, shear,
and flexural moduli increased, while the impact toughness, ultimate elongation, and damping characteristics decreased with
increasing RPUF content in the blends.
Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 5, pp. 737–746, September–October, 2008. 相似文献
97.
Tomoko Ikeda‐Fukazawa Daisuke Kita Kazushige Nagashima 《Journal of Polymer Science.Polymer Physics》2008,46(8):831-842
Raman spectra of poly methyl methacrylate (PMMA) in contact with compressed CO2 were measured, to investigate effects of CO2 sorption on structure of the PMMA chain. The solubility and diffusivity of CO2 in PMMA were estimated from temporal variations of intensities of CO2 peaks. The PMMA structure was analyzed from temporal variations of vibrational energies of PMMA peaks. The results show that the vibration energies of C H stretching modes of PMMA increase with CO2 sorption, whereas those for skeletal vibration modes decrease. These energy shifts are attributed to elongational deformation of PMMA. The PMMA structure is deformed with stretching of the chains as a bundle. From energy shifts of the CO2 peaks, the size of the CO2 accommodated space between the bundles is estimated to be 1.6–1.9 nm. Furthermore, it was observed that the vibrational energies of the PMMA modes in foamed glassy PMMA differ from the values in glassy PMMA without foams. This result suggests that the local structure of the PMMA chain changes with the process of the CO2 sorption and/or foaming. The local structure of the PMMA chain might be one of the dominant factors governing the properties of cellular materials. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 831–842, 2008 相似文献
98.
99.
100.
Fortes MA Rosa ME Vaz MF Teixeira PI 《The European physical journal. E, Soft matter》2004,15(4):395-406
We have carried out a systematic study of buckling-like mechanical instabilities in simple two- (2D) and three-dimensional (3D) symmetric foam clusters sandwiched between parallel planar walls. These instabilities occur when the wall separation w is reduced below a critical value, w*, for which the foam surface energy E reaches its minimum, E*. The clusters under investigation consist of either a single bubble, or of twin bubbles of fixed equal sizes (areas A in 2D or volumes V in 3D), which are either free to slide or pinned at the confining walls. We have numerically obtained w* for both free and pinned 2D and 3D clusters. Furthermore, we have calculated the buckled configurations of 2D twin bubbles, either free or pinned, and of 3D free twin bubbles, whose energy is independent of w and equal to the minimum energy E* of the unbuckled state. Finally, we have also predicted the critical wt* at which the terminal configurations under extension of 2D and 3D single and twin bubbles are realised. Experimental illustrations of these transitions under compression and extension are presented. Our results, together with others from the literature, suggest that a bubble cluster bounded by two parallel walls is stable only if the normal force it exerts on the walls is attractive, i.e., if dE/dw > 0; clusters that cause repulsion between the walls are unstable. We correlate this with the distribution of film orientations: films in a stable cluster cannot be too parallel to the confining walls; rather, their average tilt must be larger than for a random distribution of film orientations. 相似文献