The reactivities of different aromatic diamines, i.e. 4,4'-diaminodiphenyl sulfone (DDS), 4,4'-methylenedianiline (MDA),4,6-diethylaniline] (MDEA), and 4,4'-methylenebis [3chloro-2,6-diethylaniline] (MCDEA), cured with an epoxy prepolymer, the diglycidyl ether of Bisphenol A (DGEBA), were compared and correlated to their chemical structures. The reactivities of the diamines are in the following order: MDA > MDEA > DDS > MCDEA. Although MDA, MDEA, and MCDEA exhibit very different reactivities, the ratio of the reactivity of secondary to primary amine hydrogens (ki/ki) is found to be the same r = 0.65. On the other hand, the reactivity ratio of DDS is found to be r = 0.45. This lower value may be the result of the lower number of stable conformations afforded by the -SO2group in comparison to the -CH2group. No temperature dependence was observed. The DGEBA-DDS and DGEBA-MCDEA kinetics were fully characterized as regards conversions and Tg measurements with time at different temperatures. Relationships between the Tg and x were validated for both systems. Kinetics were modeled throughout the whole range of cure. For the DGEBA-MCDEA system, etherification reactions were found to occur and were then integrated into the kinetic model. The effect of diffusion control was incorporated by modifying the overall rate constant according to the Rabinovitch model. The Adam-Gibbs theory was preferred to express the temperature dependence of the diffusion rate constant. The kinetic models have been satisfactorily extended to the isothermal cure of an epoxy with a binary mixture of diamines.

(7600) Mar del Plata, Argentina SynopsisThe curing reaction of a commercial bisphenol A diglycidyl ether (BADGE) with ethylenediamine (EDA) was studied by differential scanning calorimetry. Different kinetic expressions were found with isothermal (low temperature rafige) and dynamic (high temperature range) runs. Two competitive mechanisms are shown to be present: an autocatalytic one (activation energy E = 14 kcal/mol) and a noncatalytic path characterized by a second-order reaction with E = 24.5 kcal/mol. At low temperatures both mechanisms took place simultaneously, showing a significant decrease in the reaction rate after the gel point. At high temperatures only the noncatalytic reaction was present, without showing a noticeable rate decrease in the rubber region. Also, a third-order dependence of the glass transition temperature on reaction extent is shown.

High-molar-mass silsesquioxanes (SSQO) based on (3-glycidoxypropyl)trimethoxysilane (GPMS) and (3-methacryloxypropyl)trimethoxysilane (MPMS) were synthesized. The hydrolytic condensation of GPMS was performed using HCOOH (0.1 N) as catalyst, keeping the molar ratio H 2O/Si ) 3. A first step was performed in tetrahydrofuran (THF) at 50°C, followed by a second step in diglycidyl ether of bisphenol A (DGEBA), where temperature was increased in steps up to 140°C. The hydrolytic condensation of MPMS was performed in bulk with HCOOH 98%, at T ) 50 or 70°C, using molar ratios of HCOOH/Si ) 3 or 6. Homogeneous solutions were obtained for both silanes. The reaction was followed by size exclusion chromatography (SEC), and final products were characterized by matrix-assisted ultraviolet laser desorption/ionization time-of-flight mass spectrometry (UV-MALDI-TOF MS), FTIR, and 1 H and 29 Si NMR. Molar-mass distributions showed the presence of clusters corresponding to products formed in different generations. With the aid of UV-MALDI-TOF MS, the different species present in every cluster could be identified for one of the silsesquioxanes derived from MPMS. During the initial stage of the hydrolytic condensation, species with 7-12 Si atoms were produced. They mainly consisted of incompletely condensed polyhedra (species with 1-3 OH per molecule) and ladder-type structures (species with 4 OH per molecule). Species with more OH groups were condensed with a higher probability, giving place to a second generation of products. This process accounts for the presence of a cluster of species with 14-24 Si atoms and the enrichment of the first cluster in the more condensed structures (T 7(OH), T8(OH)2, and T9(OH)). Third and fourth generations of condensation products were also present. Structures of different species may be depicted as combinations of incompletely condensed polyhedra with ladder fragments.