Chemical Vitrification Research Articles

Branched 1,6-Diaminohexane-Derived Aliphatic Polyamine as Curing Agent for Epoxy: Isothermal Cure, Network Structure, and Mechanical Properties

Aliphatic diamines and polyamines are long used as important curing agents for epoxy resins, especially in room-temperature-cure epoxy coatings and adhesives due to their high reactivity and low cost. Herein we systematically evaluate our newly developed liquid branched aliphatic polyamine, N,N,N′,N′-tetra(3-aminopropyl)-1,6-diaminohexane (TADH), as the curing agent for bisphenol A epoxy (DGEBA), emphasizing the isothermal cure reaction, network structure, and mechanical properties. The isothermal curing reaction of DGEBA/TADH at 40, 50 60, and 70 °C is autocatalytic, and we adequately simulate the curing kinetic rate with the extended autocatalytic Kamal model. Further isoconversional kinetic analysis reveals that the effective activation energy changes dramatically as the reaction proceeds, especially for the diffusion-controlled stage due to the chemical vitrification (Tg∞(DSC) = 119 °C). Compared to DGEBA/1,6-diaminohexane (a starting material of TADH), a dynamic mechanical analysis illustrates that t...

Stress effects on the elastic properties of amorphous polymeric materials.

Brillouin light scattering measurements have been used to study the stress induced modification in the elastic properties of two glass forming polymers: polybutadiene and epoxy-amine resin, prototypes of linear and network polymers, respectively. Following the usual thermodynamic path to the glass transition, polybutadiene has been studied as a function of temperature from the liquid well into the glassy phase. In the epoxy resin, the experiments took advantage of the system ability to reach the glass both via the chemical vitrification route, i.e., by increasing the number of covalent bonds among the constituent molecules, as well as via the physical thermal route, i.e., by decreasing the temperature. Independently from the particular way chosen to reach the glassy phase, the measurements reveal the signature of long range tensile stresses development in the glass. The stress presence modifies both the value of the sound velocities and their mutual relationship, so as to break the generalized Cauchy-like relation. In particular, when long range stresses, by improvise sample cracking, are released, the frequency of longitudinal acoustic modes increases more than 10% in polybutadiene and ∼4% in the epoxy resin. The data analysis suggests the presence of at least two different mechanisms acting on different length scales which strongly affect the overall elastic behaviour of the systems: (i) the development of tensile stress acting as a negative pressure and (ii) the development of anisotropy which increases its importance deeper and deeper in the glassy state.

Influence of Al2O3 nanoparticles on the isothermal cure of an epoxy resin

The influence of Al2O3 nanoparticles on the curing of an epoxy thermoset based on diglycidyl ether of bisphenol Awas investigated using temperature-modulated differential scanning calorimetry (TMDSC)and rheology. Diethylene triamine was used as a hardener. TMDSC not only allows for asystematic study of the kinetics of cure but simultaneously gives access to the evolution ofthe specific heat capacities of the thermosets. The technique thus provides insight into theglass transition behaviour of the nanocomposites and hence makes it possible to shed somelight on the interaction between the nanoparticles and the polymer matrix. TheAl2O3 fillers are shown to accelerate the growth of macromolecules upon isothermal curing.Several mechanisms which possibly could be responsible for the acceleration are described.As a result of the faster network growth chemical vitrification occurs at earlier timesin the filled thermosets and the specific reaction heat decreases with increasingnanoparticle concentration. Rheologic measurements of the zero-shear viscosity confirmthe faster growth of the macromolecules in the presence of the nanoparticles.

Physical and chemical vitrification: the role of configurational entropy

Glasses can be formed in numerous ways, involving very different microscopic processes. Here we report a dielectric and photon-correlation study of chemical vitrification, a process where the slowdown in the dynamics of a liquid system is controlled by the irreversible formation of chemical bonds. We find that vitrification dynamics in chemically reactive systems can be explained in terms of their configurational restrictions in the same manner as in stable glass-forming liquids under cooling or compression.

Configurational entropy and dynamics in chemical vitrification

It is common practice to form a glass starting from a liquid in a metastable state, by cooling or compressing the system so as to avoid crystallization (‘physical vitrification’). However, there exist in nature and technology different ways to form a glass. The hardening of natural and synthetic resins and the formation of most of the materials used in engineering plastics and high-performance composites are based on ‘chemical vitrification’, a process involving progressive polymerization of initially liquid monomers via the formation of irreversible chemical bonds. Explaining the similarity observed in the slowing down of the dynamics in physical and chemical vitrification constitutes a challenge to general understanding of the glass transition and may disclose its universal nature. Here we use relaxation data from several techniques to show that the similarity between the dynamic behaviours of physical and chemical glass formers originates in a similar evolution of their configurational restrictions and in a similar dynamics-to-thermodynamics correlation. In particular, we derive a relation between relaxation properties and extent of reaction in step polymerization, in remarkable agreement with experimental results.

Bond-controlled configurational entropy reduction in chemical vitrification.

Glass formation is usually viewed in terms of physical vitrification: a liquid in a metastable state is cooled or compressed so as to avoid crystallization. However, glasses may also be formed by chemical vitrification, a process involving progressive polymerization of the constituent molecules via the formation of irreversible chemical bonds. The formation of most of the materials used in engineering plastics and the hardening of natural and synthetic resins are based on chemical vitrification. Despite the differences in the molecular processes involved in chemical and physical vitrification, surprising similarities are observed in the slowing down of the dynamics and in the thermodynamical properties of the resulting glasses. Explaining such similarities would improve general understanding of the glass transition and may disclose its universal nature. Here we report dielectric and photon-correlation measurements that reveal the origin of the similarity in the dynamical behaviour of physical and chemical glass formers. We find that the evolution of their configurational restrictions proceeds in a similar manner. In particular, we make a connection between the reduction in configurational entropy and the number of chemical bonds, a quantity that can be controlled in experiments.

Segmental Dynamics and Density Fluctuations in Polymer Networks during Chemical Vitrification

Molecular dynamics of network-forming reactive polymers were examined as a function of the advancement of chemical reaction as the materials structure undergoes a transition from liquid to amorphous solid. The accompanying changes in the segmental relaxation time (α process) are a signature of the materials state (within the liquid to amorphous solid spectrum of physical properties). Both broad-band dielectric relaxation spectroscopy (DRS), which probes the α process via dipolar reorientational mobility, and dynamic light scattering (DLS), which probes the α process via density fluctuations, were used to monitor the system under reaction conditions in the reaction bath (to our knowledge, the first study of its kind). An excellent agreement was found between DRS and DLS for the α process characteristic parameters: relaxation time and KWW, the stretched exponential parameter (characterizing the relaxation breadth). That dipole dynamics exhibit the same α relaxation characteristics as time dependent density fluctuations, suggests that the rotational motion observed in a DRS measurement is directly controlled by the behavior of the density domains. It was concluded that the broadening of the α process is due to a general phenomenon: the micro/nanoscale heterogeneous nature of glass formers. The results of the study are discussed in terms of cooperative and local relaxation modes in the growing network.

Unified dielectric description of the dynamics of polymeric systems undergoing either thermal or chemical vitrification

We discuss the behaviour of the dielectric relaxation times of an epoxy-amine system both undergoing a polymerisation reaction and cooled below the glass transition. The relaxation times versus conversion are suitably described by equations which parallel the Vogel-Fulcher and Arrhenius equations, commonly used to describe the behaviour of the relaxation time versus temperature. Under special conditions, the bilinear dependence of the glass-transition temperature on conversion, predicted by both theoretical models and experimental results, is verified and the increase with conversion of the activation energy of the secondary relaxation process is found to be linear.

Glassy Polymers as Matrices for Advanced Composites

This paper is concerned with several fundamental aspects of the glassy polymer structure and properties. One aspect is chemical vitrification, the process whereby the reactants are transformed from the liquid into the glassy state due to chemical reactions. The influence of that transformation on the Young moduli and Tg of formed polymers is considered. This is exemplified by the epoxy cure process. Another aspect is the mechanism of plastic deformation of polymeric glasses and the third one is the isothermal enthalpy and volume relaxations in glassy polymers during physical aging. Choice of these topics is connected with their importance for polymer materials and advanced composite science. Some new experimental results for glassy polymers are introduced. The plastic deformation of glassy polymers is interpreted in the terms of two independent deformation modes. Mode I (low deformation temperature mode) is crystal-like and based on the nucleation and growth under stress of defects which have many features of dislocations. The second (high deformation temperature) Mode II is the diffusion motion of vacancies (holes) due to simultaneous action of stress and temperature. In the simultaneous measurement of volume and enthalpy isothermal relaxation a new type of relaxation asymmetry for sample contraction and expansion was found.