Morphology and Molecular Mobility

The morphology and molecular mobility of polyethylene nascent powder... 

The aggregate model would not appear to be generally applicable to high-density polyethylene and polypropylene. It appears that for polypropylene the aggregate model is applicable only at low draw ratios [78], As discussed above, there are simultaneous changes in morphology and molecular mobility at higher draw ratios. 

The focus of this report concerns nanocomposites from poly(propylene carbonate) (PPG) and multiwall carbon nanotubes (MWNTs). A solvent route using THF, ethoxylated non- ionic surfactants combined with sonication was found to be successful in deagglomerating and dispersing the nanotubes. The morphology and molecular mobility of the prepared nanocomposites (0.5, 3.0 and 5.0 wt% of nanotubes) were characterized by rheology, microscopy, low-field SS NMR, and electrical conductivity. The networking of nanotubes was highest with a steatyl alcohol ethoxylate surfactant, and was found to improve with the sonication... 

Kamaev, P. R, lordanskii, A. L., Aliev, 1.1., Was-serman, A. M., and Hanggi, U. (1999). Transport water and molecular mobility in novel barrier membranes with different morphology features. Desalination 26, 153-157. 

According to Kravelen,( l the fundamental characteristics of a polymer are the chemical structure and the molecular mass distribution pattern. The former includes the nature of the repeating units, end groups, composition of possible branches and cross-links, and defects in the structural sequence. The molecular mass distribution, which depends upon the synthesis method, provides information about the average molecular size and its irregularities. These characteristics are responsible, directly or indirectly, forthe polymer properties. They are directly responsible forthe cohesive force, packing density and potential crystallinity, and molecular mobility (with phase transitions). Indirectly, these properties control the morphology and relaxation phenomena (behavior of the polymer). 

Many polymers solidify into a semi-crystalline morphology. Their crystallization process, driven by thermodynamic forces, is hindered due to entanglements of the macromolecules, and the crystallization kinetics is restricted by the polymer s molecular diffusion. Therefore, crystalline lamellae and amorphous regions coexist in semi-crystalline polymers. The formation of crystals during the crystallization process results in a decrease of molecular mobility, since the crystalline regions act as crosslinks which connect the molecules into a sample spanning network. 

The treated phenomena and investigations summarized above are at the centre of the topic of this special two-volume edition of the Advances in Polymer Science on Intrinsic molecular mobility and toughness of polymers . However, little has been said about the mutual relations between molecular configuration, polymer morphology, nature, intensity and cooperativity of sub-Tg relaxation mechanisms, and mode of solicitation on the one hand and their effect on disentanglement, craze formation, yield behaviour and ultimate strength on the other. These subjects will be critically evaluated in the subsequent presentations of this Volume. 

It nevertheless remains difficult to discriminate morphological effects at the lamellar level from other factors, such as crystallinity and spherulite size [128], on the basis of the available evidence [21, 23, 24, 129, 130, 131, 132] (cf. Sect. 3.4.1). This also makes it difficult to discount alternative explanations for the improved ductility of the ft phase above Tg, based on its intrinsically higher molecular mobility, for example [133, 134, 135, 136]. One is therefore forced to conclude that any or all of the factors referred to here may play a significant role in the observed behaviour. 

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