Search Results
The glass transition
Finite size effect
Abstract
The subjects of the paper are the mechanism of vitrification and the glass transition, and a definition of the temperature of the glass transition. A comprehensive description of the structural changes occurring in the amorphous phase (‘real’ and ‘semi-ordered’) in a vicinity of the glass transition is presented. One of the major motivation of our studies is to investigate the finite size effect of the glass transition that could be related to the cooperative motion in supercooled liquids. Also, new formula, describing the relaxation time temperature change, is applied in order to better reveal themechanismof the supermolecular formation under different internal and external factors. The results of the basic methods of thermal analysis, obtained for different polymeric systems, were used in this study. The proposed approach let us correlate the thermodynamic and the structural parameters, which are estimated from the experiments, and describe all well known shapes of the DSC traces, which can be recorded in the glass transition region. Based on positron annihilation lifetime spectroscopy and dilatometric results, the significance of the free and the specific volumes for the activation of the relaxing units is discussed.
Abstract
Enthalpic relaxation has been used to model the development of the glass transition in polymers, using kinetic parameters
determined separately. For this purpose the Kohlrausch-Williams-Watt stretched exponential function, relating the extent of
relaxation, Φ(t), to time t and an average relaxation time, τa, i.e.
Abstract
The glass transition temperature (Tg), measured by Modulated Differential Scanning Calorimetry (MDSC), is related to the flow characteristics of asphalt at low temperatures as is the rate of change of the creep stiffnessm. This study compared the glass transition temperature of different asphalts (neat, chemically modified, and crumb rubber modified asphalts) with the creep stiffness, the rate of change of creep stiffness, and the low specification temperature of the continuous PG grading of those asphalts. From the rheological data (BBR) and the thermal data (MDSC) for the virgin and the modified asphalts, the modified products had the same variations of values ofm, S, and Tg at lower temperatures as those of their corresponding virgin asphalts. A correlation between the Tg andm value was observed for both the modified and unmodified asphalts. Since DSC measurements for asphalt low temperature properties use less operator time, less sample, and have less measurement and operator error than rheological methods, Tg has promise to be considered as a fast and easy laboratory method to obtain the low temperature useful range of asphalts in pavements.
Glass transitions in crosslinked epoxy networks
Kinetic aspects
Epoxy resins of DGEBA type were thermally cured with diaminodiphenylmethane as crosslinking agent, and then analysed by Differential Scanning calorimetry (DSC) at various heating rates in order to determine the glass transition temperatureT
g of the final networks. First it was shown that during cyclingT
g is shifted towards higher values up to a maximum or
. Such a change is attributed to an increasing extent of cure which develops during the thermal analysis, and also to relaxation processes thermally activated inside the polymeric matrix. Then the dependence of
on the heating rateq imposed by the DSC apparatus was presented forq changing from 0.1 to10‡C min−1. At heating rates exceeding 3‡C min−1 only the classical temperatureT
g was detected, but at smallerq values, an additional endothermic transition was revealed, located at higher temperature and linked to a physical aging-like phenomenon, which takes place at low heating rates. The plot of
against logq is divided into two quasi-linear parts on each side ofq=3‡C min−1. In conclusions, an equation was given to describe the
vs. logq function.
separate overlapping transitions when one transition goes into the reversing signal and the other into the nonreversing signal. But what if both go into the reversing signal such as a melting transition overlapping a glass transition due to a phase
Investigations of the glass transitions of organic and inorganic substances
DSC and temperature-modulated DSC
The glass transitions of different materials (a silicate glass, a metallic glass, a polymer, a low molecular liquid crystal and a natural product) were investigated. By means of the temperature-modulated DSC (TM-DSC) mode, the frequency was varied. In the case of DSC, the cooling rate was changed. TM-DSC was shown to be a practicable tool for the acquisition of dynamic parameters of glass transitions for all kinds of materials.
Abstract
After a brief introduction of the terms supercooling, amorphous solid state, glass transition and devitrification, the known ways of production of amorphous solid water are discussed. DSC experiments with quench cooled aqueous solutions show the phenomenon of glass transition and devitrification.
Abstract
Crystallization and glass-transition phenomena were studied for amorphous chlorobenzene (CB)/toluene (TL) binary systems as the function of composition. Samples were prepared by vapor-deposition onto cold substrates, and their structural changes due to temperature elevation were monitored with Raman scattering and light transmission. It was found that the crystallization temperature (T c) of CB-rich amorphous samples increases as the TL concentration is increased. This is similar to the linear dependence of glass-transition temperatures (T g) of many organic compounds on the concentration of additive. Also found was that T c of TL-rich supercooled-liquids decrease as the CB concentration is increased. Issues related to the two kinds of T c are discussed briefly.
Abstract
Segmental dynamics around T
g in the 4- and 6-arm fullerene (C60) core star-like polystyrenes with different preset arm lengths was studied by DSC as compared to that in the linear PS and PS/C60 blend. The 
anomalies
in glass transition behavior were found for the stars including both suppression and facilitation of segmental motion, and pronounced dynamic heterogeneity within a transition range. The results are interpreted in terms of breakdown of intermolecular cooperativity of segmental motions and PS-C60 interactions.
glass transition kinetics of metallic alloys is of great importance to know its thermal stability, and finally to determine the useful range of operating temperatures for a specific technological application before the crystallization takes place [ 4
