Dynamics of associating polymer solutions above the reversible gelation point are studied. Each macromolecule consists of
a soluble backbone (B) and a small fraction of specific strongly interacting groups (A or C stickers) attached to B. A mixture
of B–A and B–C associating polymers with 1:1 stoichiometric ratio is considered. As a result of AC association, the polymers
reversibly gelate above the overlap concentration. It is shown that (1) the network strands are linear complexes (double chains)
of B–A and B–C; (2) “diffusion” of the network junction points is characterized by an apparent activation energy, which can
be significantly higher than the energy of one AC bond; (3) most importantly, the randomness of sticker distribution along
the chain can significantly slow down the network relaxation leading to a markedly non-Maxwellian viscoelastic behavior. The
theory elucidates the most essential features of rheological behavior of polysaccharide associating systems (with A = adamantyl
moiety, C = β-cyclodextrin, B = either chitosan or hyaluronan) including similar behavior of G′ and G″ in a wide frequency range, strong temperature dependence of the characteristic frequency ωx, and an extremely strong effect of added free stickers (fC) on the dynamics.
This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006. 相似文献
Intermediates with π-bonded Si or Ge are generated during gas phase pyrolysis of 1,2-disila- or 1,2-digerma-cyclohex-4-enes.Compounds such as R2MMR2 are successfully trapped by addition or [2 + 2] and [4 + 2] cycloadditions. These results were proved by mass spectrometry. 相似文献
The hydrolysis of maltotriose, maltose, cellobiose and gentiobiose, at a concentration of 1% (w/v), have been investigated in 0.20 N sulphuric acid and 0.2 N polystyrene sulphonic acid. The apparent rate constants determined by high pressure liquid chromatography are compared and their dependence on temperature is given. The thermodynamic parameters of reaction with both acids are deduced. The catalytic effect of the polyacid, expressed by the ratio of the rate constants at a given temperature, is discussed in terms of the condensation of the counterions. 相似文献
Pyridine-2-azo-p-dimethylaniline is suggested as an indicator in the titration of amines (aniline, butylamine, p-phenylenediamine and ethylenediamine) with perchloric acid in glacialacetic acid medium. The precision obtainable is much better than that with crystal violet indicator. The pyridine dye can also be used satisfactorily in the titration of sodium carbonate and sodium acetate. 相似文献
Leuconostoc mesenteroides NRRL B-1299 dextransucrase was fractionated into soluble (SGT) and insoluble (IGT) enzyme preparations differing by their dextran content. In spite of this, they displayed the same Km for sucrose (10 g/L) and the same activation energy (35 kJ/mol). But the presence of cells and insoluble dextran led to the IGT behaving like an immobilized enzyme: stabilization against thermal denaturation and diffusional limitations at low substrate concentrations were observed. On the other hand, the behavior of SGT was influenced by the presence, in the preparation, of soluble dextran that reduced enzyme inhibition by excess substrate. SGT and IGT present very different pH profiles. In the presence of 4 g/L of soluble dextran, IGT was activated and displayed the same susceptibility to pH as SGT. The activation of IGT was highly dependent on the nature of the acceptor added but also on the pH of the reaction medium. IGT and SGT synthesize both soluble and insoluble polymer containing α(l → 2), α(l → 3), and α(l → 6) linkages. A larger amount of insoluble dextran is elaborated by SGT. The polymer structures, examined by13C NMR spectrometry, revealed that they differ mainly by their α(l → 3) linkage content (from 0 to 11%). This linkage seems to be partly responsible for the dextran insolubility and can be completely eliminated by carrying out the synthesis of soluble polymer at pH 7.4 with SGT. 相似文献
The main techniques developed to characterize chitosan are recalled. The interaction of chitosan with oppositely charged surfactants was investigated giving very important surface activity effects. A few chemical modifications are described and the new properties obtained are mentioned: alkylation gives amphiphilic polymers having interesting thickening behavior; grafting cyclodextrin (chit‐CD) gives a polymer able to include hydrophobic molecules; grafting adamantane gives an amphiphilic polymer able to specifically interact with chit‐CD forming a temporary network with gel‐like behavior.
The viscosities of adamantane‐chitosan, cyclodextrin‐chitosan, and unmodified chitosan solutions studied here. 相似文献
An investigation on the sediment composition and grain size was carried out along the Bormida river (Piedmont, Northern Italy). The samples were taken both in the riverbed and on its banks. Multivariate statistical exploratory methods permitted to identify possible sources of primary pollution. In particular, principal component analysis (PCA) showed that there are probably at least three anthropogenic main causes of pollution, one connected with Cr, Ni and Sn, the second with cadmium and the last one with mercury. Some correlations were found between heavy metal ion concentrations and organic matter and/or the sediment grain size. Differences between the samples collected in the riverbed and on the banks were identified and the relationships between the principal components and the distance of the samples from the riverbed and from the hot spot represented by the ACNA industrial site were also analyzed. 相似文献