Relationship between Intrinsic Viscosity and Molecular Weight for Fractionated Cellulose

A relationship between intrinsic viscosity and molecular weight for fractionated cellulose is established. The exponent a of the Mark-Houwink equation is 0.80. The unperturbed dimensions of cellulose are discussed on the basis of the Stockmayer-Fixman and Kurata-Stockmayer viscometric theories, and it is determined that cellulose is not a very rigid macromolecule in nature. This conclusion disagrees with the results obtained by statistical mechanics procedures in which the restriction of β-glucose residues to the C1 conformation was kept. The steric hindrance factor, σ is about 2, and it seems to be independent of the substituents in cellulose if it is evaluated in both cellulose and cellulose derivatives from viscometric data. From statistical mechanics data reported in the literature, σ is estimated as 4.4 for cellulose and cellulose derivatives, such as cellulose nitrate, and this means that the steric hindrances of the substituents have little influence on the rigidity of cellulose derivatives. This view disagrees with the results obtained from the non-Newtonian behavior of cellulose and cellulose nitrate in dilute solutions. By applying the Noda-Hearst theory, the conclusion is reached that cellulose nitrate is a more extended macromolecule than cellulose.     

An Unusual Fullerene–Carbene Adduct: Thermal Motion,Disorder, or Both?**

A single-crystal X-ray study of a fullerene-imidazole adduct at nine temperatures (80 K≤T≤480 K), accompanied by energy calculations, strongly suggested thermal motion of the C₆₀ moiety with respect to the imidazolium heterocycle. Analysis of the anisotropic displacement parameters, calculations of frequencies, and the refinement of disorder models for the crystal at four temperatures (230 K≤T≤380 K) lead to the conclusion that the rotator is moving at all temperatures. The rotation barrier is low, with one preferred crystallographic site and several other energy minima.     

Blends of isotactic polypropylenes and a plastomer: crystallization and viscoelastic behavior

Blends covering the entire range of compositions of a metallocenic ethylene-1-octene, CEO, copolymer and two conventional isotactic polypropylenes, iPP, of different molecular weights have been prepared, analyzing the effect of composition and molecular weight on the crystallization (studied by DSC and X-ray diffraction) and viscoelastic behavior (DMTA). It was found that those blends rich in the iPP component show a behavior practically coincident with the weighted addition of the two components. On the contrary, significant deviations were found for the blends where the CEO copolymer is the major component. These deviations are considerably more important in the case of the blends with the iPP of higher molecular weight. Moreover, both components are not miscible, exhibiting the glass transitions of the two neat components. The area under the loss tangent curves provides a preliminary information about how the toughness is enhanced using this type of impact modifier, though it provokes a significant reduction of stiffness.