Biomedical subjects

Johan L A Dubbeldam

Publications and source records attributed to Johan L A Dubbeldam.

3 recordsLinked to original sources

PubMed2006

Dynamic heterogeneity in hydrogen-bonded polymers.

We report on neutron spin echo experiments on hydrogen-bonded polymers and compare the experimentally found dynamical structure factor with theoretical predictions. Surprisingly, we find that in the melt phase the expected scaling of the Rouse dynamics is not satisfied. We propose an explanation based upon the large spatial volume occupied by the connecting groups. When the effects of these bulky groups on the local friction are taken into account, the usual scaling behavior is restored.

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PubMed2005

End-evaporation dynamics revisited.

We present analytical results on the so-called end-evaporation kinetics in equilibrium polymeric systems following a temperature jump (T jump). A T jump prepares the system with a nonequilibrium length distribution, after which it relaxes back to its equilibrium state. Starting from a master equation, we develop a mean-field analytical theory based on a generating function approach, which allows explicit approximate expressions for the monomer and dimer concentrations to be derived in a discrete setting; the concentrations of the other chains as well as the average chain length were shown to be entirely expressible in terms of the monomer and dimer concentrations. We find that the calculated monomer and dimer concentrations as well as the average chain length are in good agreement with numerical simulation results and do not suffer from some of the defects of earlier continuum theories. Furthermore, the relaxation was shown to take place in three different stages. The first stage comprises the very fast relaxation of the monomers to almost their equilibrium concentration; the other polymer chains have hardly relaxed. During the second stage, which is highly nonlinear, a redistribution of material at practically constant monomer density takes place. Only in the final stage of the relaxation process the chain concentrations approach their true equilibrium values. In this stage there are only very small shifts in the concentrations of chains, which are governed by extremely slow "indirect" monomer-mediated processes.

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PubMed2003

Self-consistent dynamics of wall slip.

A simple molecular model is studied to explain wall slip in a polymer melt. We consider a tube model for tethered chains in which the most important relaxation mechanisms: convective constraint release and chain stretching (retraction), are incorporated. Furthermore, we take the interactions between tethered chains and bulk flow self-consistently into account. Numerical simulations show that our model exhibits an entanglement-disentanglement transition, leading to a jump in the slip velocity which increases with the number of entanglements and the grafting density. The wall shear stress is found to be a nonmonotonic function of the slip and plate velocity, yielding the possibility of hysteresis and spurt instabilities. In a simplified version of the model we show via an analytical approach that the stick-slip transition is asymmetrical: the transition from stick to slip is much faster than the slip to stick transition. Our analysis reveals the existence of a dimensionless parameter that determines the time scale of the dynamics for the slowing down of the bulk flow. The relative rate at which relaxation of the tethered chains and slowing down of the bulk take place, seems to be quintessential for the slip behavior of the melt.

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