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Comment on "Decrease in the configurational entropy during a melt's polymerization" [Chem. Phys. 305 (2004) 231]
No full textUV-Curing of Acrylic Formulations by Means of Polymeric Photoinitiators with the Active 2,6-Dimethylbenzoylphosphine Oxide Moieties Pendant from a Tetramethylene Side Chain
No full textPolymeric photoinitiators (PIs) bearing the 2,6-dimethylbenzoylphosphine oxide moiety pendant from flexible tetramethylene oxide side chains were synthesised by reacting their polymeric precursors, based on 4-(4'-acryloyloxy-1'-butoxy)-2,6-dimethylbenzoic acid (ABDBA), with diphenylmethylphosphinite. The resulting 4-(4'-acryloyloxy-1'-butoxy)-2,6-dimethylbenzoylphosphine oxide (ABDBPO) homo- and copolymers with butyl acrylate (BA), poly(ABDBPO) and poly(ABDBPO-co-BA), were employed in UV-curing experiments using model clear-coating (hexandiol diacrylate/butyl acrylate, HDDA/BA) and adhesive acrylic formulations. The photoinitiated radical polymerisations were followed by microwave dielectrometry, which allows real-time monitoring of the main kinetic parameters such as induction time, t(0), polymerisation rate, and absolute monomer conversion. The low molecular weight 2,4,6-trimethylbenzoylphospine oxide (TMBPO) and the couple ethyl 4-dimetllylaminobenzoate/isopropyl thioxantone (EPD/TTX) were also used as reference acylphosphine oxide (APO) and nonAPO PIs in parallel experiments. The kinetic data from the HDDA/BA system were compared with analogous literature data on acrylic PI with the APO moiety directly attached to the backbone. The presence of the flexible spacer does not significantly affect the initiation efficiency of the APO, whereas the initial viscosity of the medium resulting from the presence of macromer resins in the adhesive formulations markedly reduces both polymerisation rate and final conversion
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
Hopping charge transport in conducting polymers studied by d.c. conduction and dielectric response analysis
No full textThe role of primitive relaxation in the dynamics of aqueous mixtures, nano-confined water and hydrated proteins RID A-8503-2012
No full textThe relaxation scenario in aqueous systems, such as mixtures of water with hydrophilic solutes, nano-confined water and hydrated biomolecules, has been shown to exhibit general features, in spite of the huge differences in structure, chemical composition and complexity. Dynamics, in all these systems, invariably shows at least two relaxations: (i) a slower process, related to cooperative and structural motions of water and solute molecules (in the case of mixtures) or related to interfacial processes in the case of confined water and (ii) a faster process, with non-cooperative character originating from water. The latter has properties including timescale and temperature dependence similar or related in all the aqueous systems. This water-specific relaxation can be identified as the primitive relaxation, or the Johari-Goldstein beta-relaxation. The primitive process is the precursor of the many-body relaxation process which increases in length-scale with time until the terminal a-relaxation is reached. Using new experimental data (at atmospheric and high pressure) along with a revision of most of the recent literature on the dynamics of confined water and aqueous mixtures, we show that the two abovementioned relaxation processes are inter-related as evidenced by correlations in their properties. For instance, both relaxation time and dielectric strength of the water-specific relaxation exhibit a crossover from a stronger to a weaker dependence with decreasing T, at the temperature where the slow process attains a very long timescale (>1 ks) and becomes structurally arrested, exactly analogous to that found for beta-relaxation in van der Waals liquids. Moreover, the primitive relaxation of water is shown to play a pivotal role in determining the dynamics of hydrated biomolecules in general, including the "dynamic transition" observed by neutron scattering and Mossbauer spectroscopy. We show that the primitive relaxation of the solvent is responsible for the dynamic transition, even in the case that the solvent is not pure water or an aqueous mixture. (C) 2010 Elsevier B.V. All rights reserved
Effect of the isobaric and isothermal reductions in excess and configurational entropies on glass-forming dynamics
No full textDielectric relaxation times tau, over a broad range of temperatures and pressures for simple molecular glass-forming systems reveal good agreement with the pressure-extended Adam-Gibbs equation derived from the Adam Gibbs model, relating directly the dynamics to the thermodynamics through the configurational entropy S-c, assuming it to be proportional to the excess entropy S-exc of the melt with respect to the crystal. In the present study by making use of both calorimetric and expansivity measurements it was possible to make an accurate determination of the pressure and temperature dependences of S-exc over the whole investigated range, and the expected proportionality between Sc and Sexc was experimentally checked. Results show that different proportionality factors g(P) and g(T) exist between S-c and S-exc in isothermal and isobaric conditions and the ratio g(T)/g(P) of these is nearly 0.7 for all the systems
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