Effect of cooling rate and frequency on the calorimetric measurement of the glass transition

Curing and Molecular Dynamics Simulation of MXene/Phenolic Epoxy Composites with Different Amine Curing Agent Systems

Herein, the curing kinetics and the glass transition temperature (T_g) of MXene/phenolic epoxy composites with two curing agents, i.e., 4,4-diaminodiphenyl sulfone (DDS) and dicyandiamine (DICY), are systematically investigated using experimental characterization, mathematical modeling and molecular dynamics simulations. The effect of MXene content on an epoxy resin/amine curing agent system is also studied. These results reveal that the MXene/epoxy composites with both curing agent systems conform to the SB(m,n) two-parameter autocatalytic model. The addition of MXene accelerated the curing of the epoxy composite and increased the T_g by about 20 K. In addition, molecular dynamics were used to simulate the T_g of the cross-linked MXene/epoxy composites and to analyze microstructural features such as the free volume fraction (FFV). The simulation results show that the introduction of MXene improves the T_g and FFV of the simulated system. This is because the introduction of MXene restricts the movement of the epoxy/curing agent system. The conclusions are in good agreement with the experimental results.

Molecular mobility and relaxation process of isolated lignin studied by multifrequency calorimetric experiments

The glass transition of lignin has been studied by multifrequency calorimetric measurements in order to highlight the morphological changes and the dynamic aspects associated to this relaxation process. Influences of water sorption and thermal annealing on molecular mobility have been considered. Additional investigations by thermogravimetry, infra-red spectroscopy and rheometry have been performed to corroborate the claims. The relaxation process of annealed lignin shows a different behaviour as the consequence of micro-structural modifications of lignin. These are explained by redistribution of secondary bonds as well as formation of new interunit linkages. Concerning the dynamic aspects, apparent activation energy, E, and sizes of cooperatively rearranging region, V(crr), have been evaluated respectively from the frequency dependence and heat capacity measurements of the glass transition. Compared to dried lignin, both E and V(crr) significantly decrease in a water-sorbed matrix indicating that the three-dimensional structure presents a higher mobility and is less confined.