Time scales and mechanisms of relaxation in the energy landscape of polymer glass under deformation: direct atomistic modeling

Mol.-dynamics simulation is used to explore the influence of thermal and mech. history of typical glassy polymers on their deformation. Polymer stress-strain and energy-strain developments have been followed for different deformation velocities, also in closed extension-recompression loops. The latter simulate for the first time the exptl. obsd. mech. rejuvenation and overaging of polymers, and energy partitioning reveals essential differences between mech. and thermal rejuvenation. All results can be qual. interpreted by considering the ratios of the relevant time scales: for cooling down, for deformation, and for segmental relaxation. [on SciFinder (R)

Understanding Mott's law from scaling of variable-range-hopping currents and intrinsic current fluctuations

We have used the master equation to simulate variable-range hopping (VRH) of charges in a strongly disordered d-dimensional energy landscape (d=1,2,3). The current distribution over hopping distances and hopping energies gives a clear insight into the difference between hops that occur most frequently, dominate quant. in the integral over the mobility distribution, or are crit. ones that still need to be considered in that integral to recover the full low-temp. mobility. The recently reported scaling with temp. of the VRH-current distribution over hopping distances and hopping energies is quant. analyzed in 1D and 2D, and accurately confirmed. Based on this, we present an anal. scaling theory of VRH, which distinguishes between a scaling part of the distribution and an exponential tail, sepd. by crit. currents that set the scale and that follow self-consistently at each temp. This naturally renders Mott's law for the low-temp. mobility, in a way and with a phys. picture different from that of the established crit.-percolation-network approach to VRH. We argue that current fluctuations as obsd. in simulations are intrinsic to VRH and play an essential role in this distinction. [on SciFinder (R)

Direct atomistic modelling of deformed polymer glasses

We use molecular-dynamics computer simulations to explore the influence of thermal and mechanical history of typical glassy polymers, atactic polystyrene (PS) and (bis)phenol A polycarbonate (PC), on their deformation. Polymer stress-strain and energy-strain developments have been followed for different deformation velocities, also in closed extension-recompression loops. The latter simulate for the first time the experimentally observed mechanical rejuvenation and overaging of polymers. Energy partitioning reveals essential differences between mechanical and thermal rejuvenation. All results are qualitatively interpreted by considering the ratio's of relevant timescales: for cooling down, for deformation, and for intrinsic segmental relaxation

Wanorde en structuren

In het afscheidscollege wordt ingegaan op de rol en de drijfveren van de theoretisch natuurkundige in door toepassing geïnspireerd onderzoek. Dit leidt zowel tot een terugblik als een vooruitblik: op eigen keuzes, gezamenlijk verworven wetenschappelijke inzichten, de organisatie van publiek-private samenwerking, nieuwe thema’s van onderzoek en op het belang van onderwijs in de theoretische natuurkunde aan een technische universiteit

Universality of AC conductivity: random site-energy model with Fermi statistics

The universality of the frequency-dependent (AC) conduction of many disordered solids in the extreme-disorder limit has been demonstrated exptl. Theor., this universality has been established with different techniques and for various models. A popular model that has been extensively investigated and for which AC universality was established is the sym. random-barrier model without Fermi statistics. However, for the more realistic model of random site-energies and Fermi statistics AC universality has never been rigorously established. In the present work we perform a numerical study of the latter model for a regular lattice in two dimensions. In addn., we allow for variable-range hopping. Our main conclusion is that AC universality appears to hold for this realistic model. The obtained master curve for the cond. and the one obtained for the random-barrier model in two dimensions appear to be the same. [on SciFinder (R)

Density functional theory for the elastic moduli of a model polymeric solid

We apply a recently developed density functional theory for freely hinged, hard polymeric chains to calculate the elastic moduli of an idealized polymeric solid lacking long-range bond order. We find that for such a model packing effects dominate the elastic behavior of the polymeric solid in a similar way as is the case in the hard-sphere crystal, which we reexamine. Our calculations show that the elastic stiffness of the model polymeric solid is essentially determined by how far one is removed from its melting point. The main role of the chain connectivity is to destabilize the solid relative to the equivalent solid of hard monomers. Comparison of our results with experimental data on semicrystalline polymers shows order-of-magnitude agreement

Dopant-induced crossover from 1D to 3D charge transport in conjugated polymers

The interplay between inter- and intrachain charge transport in bulk polythiophene in the hopping regime has been clarified by studying the conductivity s as a function of frequency ¿/2p (up to 3 THz), temperature T, and doping level c. We present a model which quantitatively explains the observed crossover from quasi-one-dimensional transport to three-dimensional hopping conduction with increasing doping level. At high frequencies the conductivity is dominated by charge transport on one-dimensional conducting chains