The glass transition temperature (T_g) as an index of chemical conversion for a high‐T_g amine/epoxy system: Chemical and diffusion‐controlled reaction kinetics

Abstract

SynopsisThe glass transition temperature ( Tg) is used as a parameter to monitor the isothermal cure of a tetrafunctional aromatic diamine and a difunctional aromatic epoxy system. There is a oneto-one relationship between T, and conversion that is independent of cure temperature, T,,,. Prior to vitrification ( Tg = Tmm), the reaction is only kinetically controlled after vitrification, the reaction becomes diffusion-controlled. Time-temperature shifts of T, vs. In (time) data a t different cure temperatures to form a master curve for the kinetically controlled reaction a t an arbitrary reference temperature yield a single Arrhenius activation energy (15.2 kcal/mole). The master curve and the reaction activation energy are used in calculating iso-T, contours prior to vitrification and also the vitrification contour in the time-temperature-transformation (TTT ) isothermal cure diagram for the system. The chemical kinetics of the reaction is satisfactorily described by a n autocatalyzed reaction mechanism. The overall rate constant of the reaction in both kinetically and diffusioncontrolled regimes is modeled by a combination, in parallel, of the chemical rate constant and the diffusion rate constant. The chemical rate constant has the usual Arrhenius form, whereas the diffusion rate constant is assumed to be given by a modified form of the WLF equation. Results suggest that both primary and secondary amino hydrogens are equally reactive. A theoretical model for calculating Tg as a function of conversion is presented for a network-forming system with one bond-forming reaction. INTRODUCTIONCuring of thermosetting materials generally involves the transformation of low molecular weight liquids to high molecular weight amorphous solids by means of chemical reactions. The curing process is of particular importance in the making of structural composites, coatings, adhesives, and electronic encapsulants. A useful framework for understanding and conceptualizing the changes that occur during cure of a thermosetting system is the isothermal time-temperature-transformation ( TTT ) cures. Such a diagram, schematically shown in Figure 1, displays the states of the material and characterizes the changes in the material during isothermal cure vs. time.'-3 Material states include liquid, sol glass, sol/gel rubber, gel rubber, sol/gel glass, gel glass, and char. The various changes occurring in the material during isothermal cure are characterized by contours of the times to reach the events. Relevant contours could include molecular gelation (corresponding, for the simplest systems, to the unique conversion at the molecular gel point), macroscopic gelation (not Journal of Applied Polymer Science, Vol. 41, 2885-2929 shown in Fig. 1 ) (corresponding, e.g., to an iso-viscosity state), vitrification (corresponding to the glass transition temperature, Tg, rising to the cure temperature, T,,,,) , devitrification (corresponding to Tg decreasing to T,,,, because of thermal degradation), and char formation (corresponding to Tg increasing ...

Viscoelastic properties and residual stresses in polyhedral oligomeric silsesquioxane‐reinforced epoxy matrices

“…These properties are chosen because they impact the residual stresses that would occur in fiber-reinforced composites using an epoxy/POSS nanocomposite as matrix. The tetrafunctional aromatic diamine/DGEBAtype epoxy resin used for the study was selected because its cure kinetics 60 and thermomechanical properties 61,62 have been well documented. We examine the effects of POSS loading using a multifunctional POSS, in which each nanosized cage bears 8-12 epoxide groups via a flexible carbon ether linkage.…”

Estudio cinético del efecto de polifenilsulfona sobre el curado de una resina epoxi/amina mediante calorimetría diferencial de barrido convencional y modulada con temperatura: parte II

“…Como en otros sistemas de reacción de epoxi/aminas [9,19,[26][27][28][29][30][31] , el |C P *| aumenta inicialmente, permanece constante y después disminuye y se nivela. El pequeño aumento inicial en |C P *| está asociado a un incremento en el número de estados configuracionales o un incremento de los modos vibracionales durante el crecimiento de las cadenas lineales, mientras que la zona nivelada corresponde al proceso de curado en el cual la red polimérica crece.…”

“…Dependencia del flujo de calor con el tiempo del curado isotérmico, medido con TMDSC, para la resina pura y para los sistemas semi-IPN con 5, 10 y 15 phr de PPSU a las temperaturas de curado de 10, 180 y 200 °C. la formación de una estructura entrecruzada, ocurre una disminución en el número de estados configuracionales, lo cual induce una disminución en |C P *|, esto es atribuido a los cambios irreversibles que ocurren cuando el material pasa del estado líquido a un sólido vítreo, lo cual está relacionado con la vitrificación del sistema durante el curado [27][28][29][30][31][32][33] . El punto de inflexión de las curvas de la Figura 4 corresponde a un tiempo de relajación característico, llamado tiempo de vitrificación, t v , donde el número de estados configuracionales disminuye considerablemente y surge una abrupta disminución en la capacidad calorífica del sistema.…”

Curing behaviour of epoxy resins with a diamine bearing an azobenzene group

“…There exists a one-to-one relationship between T g and conversion, independent of curing temperature [14,15,17,25,26]. In our work, in order to determine the glass transition temperature as a function of fractional conversion, samples have been cured in a DSC cell at 130°C for various periods of time.…”

“…During vitrification in the curing reaction diffusion appears [13]. The overall rate constant is assumed to be a combination of the chemical and diffusion rate constants [14]. The glass transition temperature of a curing system increases with increasing conversion during the reaction, regardless of the cure path, and can be a practical parameter for investigation of the cure process [15].…”