The 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, 1999
Approaching the glass-transition by polymerizing, cooling and compressing
GALLONE, GIUSEPPE CARMINE;CAPACCIOLI, SIMONE;ROLLA, PIERANGELO
1999-01-01
Abstract
The 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, 1999I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
