Since the physical properties of lithium borate glasses xLi2O-(1-x)B2O3 (0<x< 0.28) vary over a wide range with the composition, this binary system is particularly suitable for studying the relationship
between vibrational anharmonicity and fragility. The density, the linear expansion coefficient, the longitudinal and transverse
ultrasonic velocities and their respective temperature coefficients of the velocities are measured, from which the vibrational
anharmonicity in lithium borate glasses is evaluated with the help of the Grneisen parameter at the Debye cut-off frequency
and the Anderson-Grneisen parameter: these two parameters plotted vs. composition have the same characteristics with minima at x≈0.08. The fragility is evaluated from the temperature width of the glass transition; the fragility also shows a minimum at
x≈0.08. The presence of minima at x≈0.08 is ascribable to the fact that the crosslinking density between six-membered rings in the glass reaches a maximum at
this composition. We show that the anharmonic parameters strongly correlate with the fragility metrics.
We employ temperature
modulated DSC (TMDSC) to determine the dependence of the fictive temperature
on cooling rate for a series of polychlorinated biphenyls (PCB). From the
slopes of semi-logarithmic plots of cooling rate vs.
fictive temperature, the latter normalized by the fictive temperature for
an arbitrary cooling rate, we determine the enthalpic fragilities. Despite
significant differences in glass transition temperature and chemical structure
(specifically chlorine content), the PCB have the same fragility. The value
of the fragility determined using TMDSC is consistent with the fragility previously
determined using dielectric relaxation spectroscopy.
The concept of fragility has been used widely to characterize the temperature dependence of the viscosity of glass forming
materials. However, the physical background that determines the degree of fragility is still not well understood. In the present
study an expression for the fragility is derived based on a simple model of the melt. According to the model, the fragility
is determined by the relaxation of structural units that form the melt, and is described in terms of the bond strength (E0), coordination number (Z0), and their fluctuations (ΔE and ΔZ). It is shown that a strong system is characterized by large value of total bond strength (Z0E0) and small value of its fluctuation ((ΔZ)2(ΔE)2). On the other hand, a fragile system is characterized by small value of total bond strength and large value of its fluctuation.
Authors:C. Angell, J. Green, K. Ito, P. Lucas, and B. Richards
In this paper we introduce two key notions related to understanding the glassy state problem. One is the notion of the excitation profile for an amorphous system, and the other is the notion of the simple glassformer. The attributes of the latter may be used, in quite different ways, to calculate and characterize the former. The excitation profile itself directly reflects the combined phonon/configuron density of states, which in turn determines the liquid fragility. In effect, we are examining the equivalent, for liquids, of the low temperature Einstein-Debye regime for solids though, in the liquid heat capacity case, there is no equivalent of the Dulong/Pettit classical limit for solids.To quantify these notions we apply simple calorimetric methods in a novel manner. First we use DTA techniques to define some glass-forming systems that are molecularly simpler than any described before, including cases which are 80 mol% CS2, or 100% S2Cl2. We then use the same data to obtain the fragility of these simple systems by a new approach, the 'reduced glass transition width' method. This method will be justified using data on a wider variety of well characterized glassformers, for which the unambiguous F1/2 fragility measures are available. We also describe a new DTA method for obtaining F1/2 fragilities in a single scan. We draw surprising conclusions about the fragility of the simplest molecular glassformers, the mixed LJ glasses, which have been much studied by molecular dynamics computer simulation.These ideas are then applied to a different kind of simple glass — one whose thermodynamics is dominated by breaking and making of covalent bonds — for which case the excitation profile can be straight-forwardly modeled. Comparisons with the profile obtained from computer studies of the molecularly simple glasses are made, and the differences in profiles implied for strong vs. fragile systems are discussed. The origin of fragility in the relation between the vibrational and configurational densities of states is discussed, and the conditions under which high fragility can convert to a first order liquid-liquid transition, is outlined.
Thermal behaviour of the glass series (100-x)[50ZnO-10B2O3-40P2O5]·xSb2O3 (x=0-42 mol%) and (100-y)[60ZnO-10B2O3-30P2O5]·ySb2O3 (y=0-28 mol%) was investigated by DSC and TMA. The addition of Sb2O3 results in a decrease of the glass transition temperature and crystallization temperature in both compositional series. All
glasses crystallize on heating in the temperature range of 522–632°C. Thermal expansion coefficient of the glasses monotonously
increases with increasing Sb2O3 content in both series and varies within the range of 6.6–11.7 ppm °C−1. From changes of thermal capacity within the glass transition region it was concluded that with increasing Sb2O3 content the ‘fragility’ of the studied glasses increases.
We investigated the features of the glass transition relaxation of two room temperature ionic liquids using DSC. An important
observation was that the heat capacity jump, that is the signature of the glass transition relaxation, shows a particularly
strong value in this type of new and promising materials, candidates for a range of applications. This suggests a high degree
of molecular mobility in the supercooled liquid state. The study of the influence of the heating rate on the temperature location
of the glass transition signal, allowed the determination of the activation energy at the glass transition temperature, and
the calculation of the fragility index of these two ionic glass-formers. It was concluded that this kind of materials belong
to the class of relatively strong glass-forming systems.
Applied Technology Council 1996: Seismic Evaluation and Retrofit of Concrete Buildings. Report No.ATC-40, Redwood City, California, 1996
Baballëku M 2006: Fragility of Typified Educational System Facilities in Albania
Kraus , A. , Roth , H.-P. and Kirchgessner , M. ( 1997a ): Supplementation with vitamin C, vitamin E or β-carotene influences osmotic fragility and oxidative damage of erythrocytes of zincdeficient rats . J. Nutr. 127 , 1290 – 1296
The thermal behaviour
of 2- and 4-biphenylmethanol were studied by differential scanning calorimetry
(DSC). It was found that the 2-isomer shows a relatively strong resistance
to crystallisation, and that it easily vitrifies on cooling. Oppositely, 4-biphenylmethanol
readily crystallizes on cooling. The slow molecular mobility of 2-biphenylmethanol
in the amorphous solid state was studied by DSC and by thermally stimulated
depolarisation currents (TSDC). Both techniques indicate that 2-biphenylmethanol
is a relatively strong glass-former, with a fragility index of ~50 in
the Angell's scale.
Authors:Gabriella Csóka, A. Gelencsér, Dorottya Kiss, Eszter Pásztor, I. Klebovich, and Romána Zelkó
The present study aimed to apply fragility index (m) of polymers in the determination of the optimal amount of plasticizer in polymer films. The fragility index of different Eudragit polymers (RS, RL, EPO) was assessed by differential scanning calorimerty (DSC), applying the Arrhenius connection (logq–1/Tg). The fragility of Eudragit EPO films proved to be the highest, while in the case of RS and RL, the increase of the alkyl-chain length caused the increase of fragility. Studying the effect of plasticizer (triethyl citrate, TEC) on the m value of Eudragit RL and RS films, a near linear reduction of the fragility index could be observed between 5–30% TEC concentration, but above 30%, this value leveled out to constant.