It is evidenced that due to the kinetic character of the glass transition as a ‘freeze in’ process, PVT measurements extended over the glass transition range depend not only on the thermal history but also on the pressure acting during the formation of the polymeric glasses. As a consequence metastable glasses are formed which show during heating of the glassy polymer through the glass transition range ‘volume relaxation zones’, characterized by a retarded increase or even decrease of the volume. The width of the ‘relaxation zone’ increases with increasing pressure and depends additional on the mode of operation used during the PVT measurements. In the same time a pressure induced shift of the glass temperature to higher temperatures is observed, the shift being the greater the stiffer the polymer, i.e. the higher the glass temperature of the polymer at atmospheric pressure. Due to the metastable character of polymeric glasses the evaluation of universal equations of states is thus not ingenious for polymeric glasses, because the deduced EOS will be valid only for that given glass characterized by a well defined thermal and pressure history. Additionally the EOS is influenced by the unknown time dependent aging and relaxation processes within polymeric glasses.
PVT-measurements of anionic polystyrenes on heating have shown that, depending on the mode of operation, specific ‘relaxation zones’ within the glass transition range are observed. Isothermal PVT curves exhibit always ‘relaxation zones’ independent whether the pressure is increased or decreased during the scan. The shift of the relaxation zones to higher temperatures is, however, higher for isothermal scans with increasing pressure. These ‘relaxation zones’ are explained by pressure-dependent changes of the state of the polymeric sample isothermally scanned within the glass transition range. At lower pressures the polymer is actually in the molten state, whereas at higher pressures it may be in the metastable glassy state and the actual state depends on the rate of pressure change. In isobaric PVT curves ‘relaxation zones” in heating scans are exhibited only if the pressure applied during glass formation differs from the pressure applied during the heating scan. The observed pressure-dependent shift of the glass temperatures to higher temperatures was higher for the studied polystyrenes of different molecular weight that had a higher glass temperature at normal pressure. But the specific molecular weight influence on the width of the ‘relaxation zone’ could not be ascertained. An attempt to calculate characteristic volume relaxation times failed because of insufficient precision of the measurements.
Authors:H. L. Anderson, H. A. Schneider, A. Kemmler, and R. Strey
The method of computer-experiments can be applied successfully on the radical depolymerization under inclusion of initiation, depropagation and termination. The comfortable PC-program ‘TA-kin’ for the non-linear estimation of parameters for TG- or DSC-experiments was applied to the determination of activation parameters of depropagation and termination. Therefore the overall-evaluation of three or more data sets is a prerequisite. The determination of kinetic parameters runs satisfactory if the measured curves are strongly different, e.g. by varying the heating rate, including acceptable experimental errors. Several recommendations for laboratory experts are given. A great support for a very sufficient estimation is the inclusion of simultaneous analysis of the radical concentration.
Authors:E. Urbanovici, H. A. Schneider, D. Brizzolara, and H. J. Cantow
A DSC study was carried out of the isothermal melt crystallization kinetics of poly(L-lactic acid), PLLA, at 110, 115, 120, 125 and 130‡C. The experimental data were evaluated within the framework of the well-known Avrami kinetic model and an extended model involving an additional third kinetic parameter . In order to perform the necessary numerical calculations, a number of functions built into the Mathematica® software system were used. The results showed that the isothermal melt crystallization kinetics of PLLA can be described adequately by both these kinetic models. It should also be stressed that the kinetic model of Urbanovici and Segal offers a better description of the experimental melt crystallization data of PLLA than the classical Avrami model.
Authors:T. Vilics, H. A. Schneider, Virginia Manoviciu, and I. Manoviciu
A new criterion for evaluating different plasticizers the ability of suppression of the Β transition in plasticized PVC blends. Accordingly, the Β suppression ability is proportional to the PVC-plasticizer compatibility, expressed either by the critical solution temperature, CST, or by the interaction parameter related to the difference between the solubility parameters of the blend components. The criterion is, however, valid for low plasticizer contents (<5%w/w) only, as long as the Β transitions are not overlapped by the α transitions, shifted towards lower temperatures due to the effect of the plasticizer. For higher plasticizer contents the α transition starts to overlap the Β transition and the Β suppression ability of the plasticizer depends increasingly on the efficiency of the plasticizer i.e. on the depression of the glass transition temperature of PVC (related to theTg of the plasticizer). Accordingly, plasticizers with both good efficiency (lowTg) and compatibility are more effective in the Β suppression than plasticizers which have only a higher compatibility but also a highTg (i.e. reduced efficiency).
Authors:Luke A. Schneider, Daniel L. King, and Paul H. Delfabbro
Background and aims
Familial influences are known to affect the likelihood of an adolescent becoming a problem gamer. This systematic review examined some of the key findings in empirical research on family factors related to adolescent problem gaming.
A total of 14 studies in the past decade were evaluated. Family-related variables included: (a) parent status (e.g., socioeconomic status and mental health), (b) parent–child relationship (e.g., warmth, conflict, and abuse), (c) parental influence on gaming (e.g., supervision of gaming, modeling, and attitudes toward gaming), and (d) family environment (e.g., household composition).
The majority of studies have focused on parent–child relationships, reporting that poorer quality relationships are associated with increased severity of problem gaming. The paternal relationship may be protective against problem gaming; therefore, prevention programs should leverage the support of cooperative fathers.
The intergenerational effects of problem gaming require further attention, in light of adult gamers raising their children in a gaming-centric environment. Research has been limited by a reliance on adolescent self-report to understand family dynamics, without gathering corroborating information from parents and other family members. The very high rates of problem gaming (>10%) reported in general population samples raise concerns about the validity of current screening tools.
Interventions for adolescents may be more effective in some cases if they can address familial influences on problem gaming with the active co-participation of parents, rather than enrolling vulnerable adolescents in individual-based training or temporarily isolating adolescents from the family system.
Authors:J. Major, H. Schneider, A. Seeger, A. Siegle, H. Stoll, I. Billard, M. Koch, and U. Lauff
Correlated measurement of the lifetime and of the lineshape of the 511 keV annihilation radiation of positrons (age-momentum correlation, AMOC) has become a powerful tool for investigating reactions of positron or positronium in condensed matter as a function of time. The beam-based+
AMOC method installed at the Stuttgart relativistic positron beam facility (Ekine+
4MeV) offers substantial advantages over the conventional
E coincidence technique resulting in a fast data taking due to the unity detection efficiency of the+ scintillator which provides the start signal for the lifetime measurement. In this paper the application of AMOC to positronium chemistry is illustrated by the study of spin conversion of positronium in the system methanol/HTEMPO as a function of HTEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, a paramagnetic solute) concentration. The same technique can be applied to other systems and other reactions in positronium chemistry, e. g., oxidation or complex formation.
Authors:H. Vogg, H. Braun, R. Löffel, A. Lubecki, A. Merz, J. Schmitz, J. Schneider, and J. Vehlow
The works done by the “Laboratorium für Isotopentechnik” of the Nuclear Research Centre, Karlsruhe, is discussed in the field
of the application of radioisotope techniques in chemistry and chemical engineering.
Authors:P. Castellaz, J. Major, C. Mujica, H. Schneider, A. Seeger, A. Siegle, H. Stoll, and I. Billard
Correlated measurements of the individual positron lifetimes (so-called positron ages) and of the Doppler shift of the 2-annihilation photon energy (Age-Momentum Correlation, AMOC) allow the evolution of the positron states to be observed in the time domain. The AMOC technique is thus especially useful when chemical reactions of positronium induce transitions between positron states. The full information contained in the AMOC data may be extracted from the two-dimensional AMOC histogramme, the so-called AMOC relief, by fitting a suitable two-dimensional model function. In this way quantitative information on the spin conversion of positronium by a paramagnetic radical in the systems HTEMPO/methanol and HTEMPO/benzene and on the formation of positron bound states in aqueous solutions of various sodium halides has been deduced.