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.
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.
where T is the storage and T′ the fictive temperature, X is the structure factor and ΔH the activation enthalpy. Both have
been found to describe the process of enthalpic relaxation in polymer glasses and a direct comparison has been made with the
change in specific heat observed with different cooling rates in DSC experiments. The effect of variables, such as activation
enthalpies, pre-exponential factors, and the non-linear factors such as X and β on the observed Tgs and the temperature range over which the transition occurred have been determined.
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
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 temperatureTg of the final networks. First it was shown that during cyclingTg 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 temperatureTg 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 glasstransition due to a phase
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.
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.
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 (Tc) of CB-rich amorphous samples increases as the TL concentration is increased. This is similar to the linear dependence of
glass-transition temperatures (Tg) of many organic compounds on the concentration of additive. Also found was that Tc of TL-rich supercooled-liquids decrease as the CB concentration is increased. Issues related to the two kinds of Tc are discussed briefly.
Authors:V. Bershtein, V. Egorov, V. Zgonnik, E. Melenevskaya, and L. Vinogradova
Segmental dynamics around Tg 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.
Temperature-modulated calorimetry (TMC) allows the experimental evaluation of the kinetic parameters of the glass transition
from quasi-isothermal experiments. In this paper, model calculations based on experimental data are presented for the total
and reversing apparent heat capacities on heating and cooling through the glass transition region as a function of heating
rate and modulation frequency for the modulated differential scanning calorimeter (MDSC). Amorphous poly(ethylene terephthalate)
(PET) is used as the example polymer and a simple first-order kinetics is fitted to the data. The total heat flow carries
the hysteresis information (enthalpy relaxation, thermal history) and indications of changes in modulation frequency due to
the glass transition. The reversing heat flow permits the assessment of the first and higher harmonics of the apparent heat
capacities. The computations are carried out by numerical integrations with up to 5000 steps. Comparisons of the calculations
with experiments are possible. As one moves further from equilibrium, i.e. the liquid state, cooperative kinetics must be
used to match model and experiment.