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Atomistic details of disordering in superheated polymethylene crystals have
been studied using full molecular dynamics simulations of crystals
containing 9600 CH2-groups. The crystal size was about 227
nm3 Simulations were carried out for up to 100 ps, starting at
temperatures about 100 K above the melting temperature. Typically 1.5 h of
CPU time on a Cray X-MP were necessary per ps simulation. Superheating
causes a quick development of large-scale disorder throughout the crystal,
including reorientation, translation, and the destruction of crystal
symmetry. This is followed ultimately by surface Melting. Crystallization
centers with hexagonal packing are found in superheated, unconstrained
crystals. On cooling during the simulation, recrystallization processes
compete with the disordering, resulting in a reorientation of the molecular
chains and reorganization of the crystal. Neither the fully amorphous phase
nor the ordered crystal are reached during these short-time simulations
using an instantaneous temperature increase to above the melting
temperature, followed by a slow cooling into the crystallization temperature
Differential scanning calorimetry (DSC) was used to study blends of
polystyrene (PS) and poly (p-Methylstyrene) (PpmS). The presence of two
glass transitions on heating after quenching was interpreted as evidence of
phase separation at the temperature of the liquid before quenching. The
small difference between the glass transitions of the homopolymers in the
PS/PpmS system of “13 K limits the reliable detection of double glass
transitions for blends to concentrations between 30 and 70%. The results of
the DSC technique are supported by comparison with small angle neutron
scattering (SANS) data.
The nonequilibrium behavior of symmetrical tetra-n-alkylammonium halides is
described from a study of the transition behavior using thermal analysis and
hot-stage optical Microscopy. Results are discussed in terms of
reversibility, supercooling, sharpness of transitions on thermal cycling,
and glass transitions. The transitions of plastic to rigid
crystals have the largest supercoolings. Gradual disordering on heating and
incomplete ordering on cooling are found for some of the salts and represent
interesting examples of order/disorder and glass transitions with reduced
The deposition of large area multilayers with smooth surfaces is an important topic for the realisation of high quality multiturn coils for flux transformers. We have developed a co-sputtering process from two metallic targets for the deposition of large areas of extremely smooth YBaCuO thin films. Conditions for epitaxial growth of MgO layers on YBaCuO have been investigated. Epitaxial multilayers with good electrical properties (Tc,o > 82 K) have been fabricated.
The near field monitoring of an auditory evoked response from the cochlear (electrocochleography) is a tried and trusted clinical tool. Conventional techniques for performing electrocochleography are cumbersome to use and frequently uncomfortable for the patient. We present a simple, modified technique which provides more flexibility with regard to where and when electrocochleography may be performed and also improves patient comfort during the test.
Defects in polymers were observed by high resolution electron microscopy and inferred from their mechanical and dielectrical behavior. The details of their generation was not known, however, in the past. During the last few years we have been able to extend the molecular dynamics simulation of polyethylene to crystals containing up to 6100 atoms and to times as long as 100 ps. The major observation was that single bond rotations of more than 90° become possible already more than 100 K below the melting temperature. These defects have lifetimes of only a few ps. By coupling to kinks (2g1) they can extend their lifetime considerably. Addition of a thermal, mechanical or dielectric free energy gradient to the thermally created defects seems to be able to account for the microscopic motion needed to explain the macroscopically observed annealing, deformations and relaxation effects. Key to the mechanical and dielectric properties is thus the existence of conformational disorder (condis crystal).
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