Polymer dynamics and topology: Extension of stars and dendrimers in external fields

We study the influence of topology on the extension of branched polymers subjected to external forces. Such forces can be applied mechanically (by micromanipulation techniques such as laser tweezers) or electrically (in the case of charged polymers). We focus on the unfold dynamics of star and dendrimer type structures. Some of the dynamical quantities of interest are: (i) the structural average of the mean monomer displacement, (ii) the elastic and the loss moduli and (iii) the mean displacement of a specified monomer. In a Gaussian‐type approach, (i) and (ii) depend only on the eigenvalues of the adjacency matrix whereas (iii) also requires the knowledge of the eigenvectors. Thus one can sometimes dispense with a full diagonalisation and use efficient recursion procedures. We highlight how the dynamic properties depend on topology: the number of branches and their length for stars and the number of generations for dendrimers. The intermediate time (crossover) behavior turns out to be most revealing of the underlying structure. We compare our results to those for fractal structures in external fields.     

Synthesis,Characterisation and Phase Transition Studies of 50.9 and 90.5

The synthesis, characterisation of phases of the compounds N(p-n-pentyloxy benzylidene) p-n-nonylaniline, 50.9 and N(p-n-nonyloxy benzylidene) p-n-pentylaniline, 90.5 and phase transition studies using different experimental techniques are presented. The compound 50.9 exhibits a phase sequence NAB whereas 90.5 shows a phase variant ABG. A large density jump across IN transition is observed in 50.9 similar to that observed in 50.10 compound.     

Gelation and aggregation of mucin in relation to the stomach's protective barrier

Dynamic light scattering was applied to study the effects of pH and concentration on pig gastric mucin. At low concentration (C = 2.0 mg/ml), the distribution of diffusion constants is independent of pH, with an average diffusion constant D = 4 × 10⁻⁸ cm²/s. At pH2, very large aggregates with 100-time slower D (2 × 10⁻⁸cm²/s) are seen. Depolarized DLS can be observed at pH2 even at low concentration (C = 5.0 mg/ml). The rotational and translational diffusion constants were analyzed using the Kirkwood-Riseman model for rods and show that at pH2 mucin is a rod-like molecule of length 390 nm, diameter 55 nm, with an axial ratio (length/diameter) of 7. However, at pH7, no significant depolarized signals could be detected at any of the concentrations examined in the study. These results suggest that the mucin monomer undergoes a conformational change from a flexible to a rod-like conformation upon lowering pH from 7 (neutral) to 2 (acidic). Such conformational change and aggregation of the gastric mucin molecule at acidic pH may provide a mechanism by which the gastric mucin layer prevents the stomach from digesting itself, since the back diffusion of acid from the lumen to the gastric epithelium is retarded by the gel layer.