Search

We investigate the molecular origin of shear-thinning in melts of flexible, semiflexible and rigid oligomers with coarse-grained simulations of a sheared melt. Entanglements, alignment, stretching and tumbling modes or suppression of the latter all contribute to understanding how macroscopic flow properties emerge from the molecular level. In particular, we identify the rise and decline of entanglements with increasing chain stiffness as the major cause for the non-monotonic behaviour of the viscosity in equilibrium and at low shear rates, even for rather small oligomeric systems. At higher shear rates, chains align and disentangle, contributing to shear-thinning. By performing simulations of single chains in shear flow, we identify which of these phenomena are of collective nature and arise through interchain interactions and which are already present in dilute systems. Building upon these microscopic simulations, we identify by means of the Irving–Kirkwood formula the corresponding macroscopic stress tensor for a non-Newtonian polymer fluid. Shear-thinning effects in oligomer melts are also demonstrated by macroscopic simulations of channel flows. The latter have been obtained by the discontinuous Galerkin method approximating macroscopic polymer flows. Our study confirms the influence of microscopic details in the molecular structure of short polymers such as chain flexibility on macroscopic polymer flows.

Aqueous suspensions of sub-micron sized monodisperse particles exhibit structural ordering analogous that observed in atomic systems. Deionised suspensions of colloidal charged polystyrene particles exhibit long-range order for volume fraction ɸ> 0.001, whereas suspensions of hard-spheres undergo crystallization at ɸ> 0.48.In these dispersions the particle size is fixed and the temperature is not a controllable parameter to investigate the phase transitions like fluid to crystal (melting/freezing) and glass transition. Whereas aqueous suspensions of thermo-responsive poly(N- isopropylacrylamide) (PNIPAM) particles exhibit rich phase behaviour upon varying the temperature as well as osmotic pressure due to the variation in particle size as well as interparticle interaction U(r). Recently, Bergman et al [1]have proposed a Multi-Hertzian (MH) pair potential U(r), for PNIPAM microgels which are known to have an inhomogeneous dense polymer core and a thin shell of dangling polymer chains. MH pair-potential is expected to represent the interparticle interaction between PNIPAM microgel spheres up to their volume phase transition (VPT) temperature (VPT ɸ 34°C. The applicability of this potential in predicting the phase behaviour, is verified only in the fluid region (i.e. volume fractions upto ɸ=0.49) of PNIPAM microgels. Its applicability for understanding the ordering, dynamics and the phase behaviour of microgels under dense conditions needs to be investigated. With this as the motivation, we have performed Monte Carlo (MC) simulations with MH potential over a wide range of volume fractions and results are reported here. Quite surprisingly, we found that that the fluid phase freeze in to a glassy state instead of a crystalline state upon increasingɸ.

We report the results of the magnetisation reversal in Ising ferromagnet having thermal and field gradients by Monte Carlo simulation. We have studied the distribution of reversal times for different values of thermal and field gradients and compared the results with those obtained for uniform temperature. The movement of the domain wall of distinct domains and the growth of roughness of domain wall have also been studied statistically. The role of competing thermal and field gradients, in the reversal mechanism, was also studied and a line of marginal competition was obtained., Comment: 12 pages Latex and 13 captioned figures