Authors:Libby Yoerg, M. Ellen Matthews, Lakshmi Kaza, Naullage Indika Perera, David W. Ball, John Moran, and Alan T. Riga
, including melting point (for crystalline materials) and glass transition (for amorphous materials). Dielectric analysis (DEA) reveals increasing amorphicity and molecular rearrangement depicted by the increased organization of the dielectricrelaxation
On the basis of a qualitative theory the phonon spectrum is obtained. Two- and three phonon interactions are examined. The theoretical results are applied for the analysis of the experimental data: for heat conduction, dielectric relaxation, heat capacity.
The calorimetric glass transition and dielectric dynamics of -relaxation in propylene glycol (PG) and its five oligomers (polypropylene glycol, PPG) have been investigated by the modulated differential scanning calorimetry (MDSC) and the broadband dielectric spectroscopy. From the temperature dependence of heat capacity of PPGs, it is clarified that the glass transition temperature (Tg) and the glass transition region are affected by the heating rate. The kinetic changes of PG and PPGs near Tg strongly depend on the underlying heating rate. With increasing the molecular mass of PPGs, the fragility derived from the relaxation time against temperature also increases. The PG monomer is stronger than its oligomers, PPGs, because of the larger number density of the —OH end group which tends to construct the intermolecular network structure. Adam-Gibbs (AG) theory could still hold for MDSC results due to the fact that the dielectric relaxation time can be related to the configurational entropy.
Dielectric relaxation and thermal transitions in β-PVDF have been investigated by Thermo-Stimulated Current spectroscopy and
Differential Scanning Calorimetry respectively. A comparative study of spectra and thermograms has been performed.
The relaxation mode associated with the glass transition of the true amorphous phase is characterized by relaxation times
obeying a compensation law due to cooperative molecular movements. A conformationally disordered structure is proposed for
β-PVDF to explain thermal events occurring around 60°C. Ageing of ferroelectric properties of β-PVDF has been associated with
cooperative molecular movements liberated largely below the melting point.
Confinement of the glass-forming regions in the nanometer range influences the α-relaxation which is associated with the glass transition. These effects were investigated for semicrystalline poly(ethylene terephthalate) by dielectric spectroscopy and differential scanning calorimetry. The results are discussed within the concept of cooperative length, i.e. the characteristic length of the cooperative process of glass transition. Both experiments showed a dependence of the glass transition on the mean thickness of the amorphous layers. For the dielectric relaxation, the loss maximum was found to shift to higher temperatures with decreasing thickness of the amorphous layers, but no differences were observed in the curve shape for the differently crystallized samples. For the calorimetric measurements, in contrast, there was no correlation for the glass transition temperature, whereas the curve shape did correlate with the layer thickness of the mobile amorphous fraction. From the structure parameters, a characteristic length of approximately (2.5±1) nm was estimated for the unconfined glass relaxation (transition).
Authors:Mohan T. Hosamani, Narasimha H. Ayachit, and D. K. Deshpande
dielectricrelaxation in relation to temperature of a variety of polar substances is reported in the literature, such studies still seem to be warranted on some other molecules, especially, heterocyclic compounds which are least studied, chosen in the present
Authors:Ana Brás, María Viciosa, Madalena Dionísio, and J. Mano
A detailed dielectric characterization
of the relaxation modes found in a poly(L-lactic
acid), PLLA, film containing 0.4 mass% of water is provided. The sub-glass
relaxation process is a superposition of two processes, one highly influenced
by water with activation energy of 50 kJ mol–1,
and another one, with longer relaxation times and lower intensity having activation
energy of 38 kJ mol–1. Dried PLLA exhibits
an abnormally broad secondary β-relaxation that probably corresponds to
the superposition of multiple processes. Upon water sorption the strength
of the more mobile process significantly increases being shifted to lower
temperatures which allows the detection of the underlying process. The glass
transition relaxation process is deviated to higher frequencies almost one
decade due to the water plasticizing effect. The reported results show that
small quantities of water may have a profound impact in the relaxational features
in PLLA, which should be taken in account when considering the properties
and performance of this system.
The relaxation of electric field-induced polar orientation in a side-chain-bearing liquid-crystalline polysiloxane was measured
by means of thermally stimulated depolarization currents. Different relaxation mechanisms were identified and characterized:
the glass transition cooperative relaxation exhibits compensation behaviour. On the other hand, lowerTg and upperTg discharges were observed and their molecular nature is discussed.
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.
Authors:C. Bacharan, C. Dessaux, A. Bernès, and C. Lacabanne
Thermally Stimulated Current (TSC) spectrometry has been applied to the characterization of polymeric materials. The study
of a series of amorphous polymers having different physical structures has shown that the compensation parameters are independent
of physical aging; contrarily, the activation enthalpy distribution reflects the evolution of the heterogeneity of the amorphous
In copolymers, TSC allows us to identify segregated amorphous phases. In semi-crystalline polymers, with semi-rigid chains,
we have shown the existence of an amorphous crystalline interphase characterized by a plateau in the temperature distribution
of activation enthalpy.