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Comment on "Decrease in the configurational entropy during a melt's polymerization" [Chem. Phys. 305 (2004) 231]
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Pressure dependence of structural relaxation times in terms of the Adam-Gibbs model
No full textA new equation describing the behavior of the structural relaxation lime, tau (T,P), as a function of both pressure and temperature, is discussed. This equation has been derived from the Adam-Gibbs theory by writing the configurational entropy, S-c, in terms of the excess thermal heat capacity and of the molar thermal expansion. Consequently, the parameters introduced in the expression are directly related to specific physical properties of the material, such as the thermal expansion coefficient alpha and the isothermal bulk modulus K-o. At a fixed pressure, for low pressures, the found equation reduces to a Vogel-Fulcher-Tammann equation of tau versus temperature with the fragility parameter independent from pressure. The equation for tau (T,P) was successfully tested directly by fitting the dielectric relaxation time data for two isothermal and one isobaric measurements on diglycidyl ether of bisphenol-A, carried out in previous experiments. The parameters estimated by the best fit were in reasonable agreement with the Values determined from the known physical properties of the material. Finally, the expression for the change versus pressure of the temperatures at which the same value of tau (max) is obtained (e.g., the change versus pressure of the glass transition temperature) agrees with several expressions previously proposed in the literature to provide a phenomenological description of the observed phenomena
Approaching the glass-transition by polymerizing, cooling and compressing
No full textThe vitrification of a glass-former can be driven by different physical-chemical processes: the usual cooling, the compression and the chemical polymerization reaction. The phenomenology of these three vitrification processes applied to a common epoxy glass-former, the diglycidyl-ether of bisphenol-A (DGEBA), has been described and analyzed by measuring the changes occurring in some relevant dielectric parameters, such as time and strength of the various relaxation processes, when the glassy state is approached by any of these ways.
When the epoxy system was cooled, the main relaxation time increased according to a Vogel-Fulcher (VF) or, equivalently, to a William-Landel-Ferry (WLF) law, while the secondary relaxation time reflected an activated behavior of the corresponding relaxation process. At the same time, the main relaxation strength increased linearly with the reciprocal temperature while the secondary one decreased showing a change of slope just at the glass transition temperature [1].
The variable pressure measurements revealed that the pressure dependence of the main relaxation time in DGEBA is better described by a second order polynomial function rather than a VF-like function. The perfect scaling observed between couples of isobaric and isothermal spectra with the same value of the main relaxation time, suggests that both temperature and pressure play an important role in controlling the dielectric response [2]. Also with respect to pressure, the relaxation strengths showed linear trends, as it was found with respect to the reciprocal temperature.
Two different polymerization reactions , leading to a linear (DGEBA-butylamine) and a crosslinked (DGEBA-etylenediamine) molecular structure, respectively, were analyzed [1]. The changes occurring in both molecular structures and density produced an increase of the main relaxation time with the conversion obeying to a WLF-like law, while the secondary relaxation time had an Arrhenius-like behavior. In this context the relaxation strengths appear likely to reflect the disappearance (appearance) of the reagents (products) of the polymerization reaction.
The relaxation characteristics when the glassy state is approached by the three different ways were compared and discussed to establish the relative influence of temperature, volume and molecular structure on the vitrification phenomenon.
References
[1] R.Casalini, S.Corezzi, D.Fioretto, A.Livi, P.A.Rolla, Chem. Phys. Lett., 258, 470 (1996)
[2] S.Corezzi, M.Lucchesi, P.A.Rolla, S.Capaccioli, G.Gallone, M.Paluch, submitted to Philos. Mag. B, 199
New developments in MacroInorganics. The thermodynamics of basic polymers
No full textThe protonation and complex formation of some basic polymers have been studied in aqueous solution by potentiometric, calorimetric, spectrophotometric and viscosimetric techniques. Specific methods for the treatment of either ‘sharp’ or ‘apparent’ thermodynamic functions in polyelectrolyte bearing one or two basic groups in the repeating unit have been developed. The results are related to structure features and presence of various types of chemical function besides the aminic function
Splitting Between Main and Secondary Relaxations in Mono-, Di-, and Tri-Epoxy Compounds
No full textWideband dielectric spectroscopy (10^2 - 2•10^10 Hz) was used to study the dynamics of mono-, di-, and tri-epoxide compounds from below to above the glass transition temperature, Tg. Dielectric spectra above Tg revealed the existence of two relaxations, a structural and a secondary process, which merge at the splitting temperature Ts, located some tenths of degrees above Tg. A d.c. conductivity contribution is also present. The glass transition phenomenon markedly affects the temperature dependence of both the dielectric strength and the low frequency slope of the secondary process. The prediction of the Stokes-Einstein-Debye (SED) model was verified for mono- and di-epoxide, while a fractional power law (FSED) replaces the SED relation in tri-epoxide for T<Ts. Moreover, a transition temperature TB~Ts between two different Vogel-Fulcher regimes was recognised in all systems. The overall picture of the dynamics of the systems is enriched and very recent ideas on the splitting between main and secondary relaxations are confirmed. Finally, the triepoxy compound shows an additional relaxation, which is masked by the conductivity contribution and it is slower than the structural one. Such relaxation seems to be related to the conductivity and it is found to follow the FSED law
Copper(II) complex formation properties of a basic polymer containing SO2 groups in the main chain
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