Confocal microscopy study of phase morphology evolution in epoxy/polysiloxane thermosets

Mechanism of Morphology Development in HDGEBA/PAMS Hybrid Thermosets: Monte Carlo Simulation and LSCM Study

Reactive combinations of aliphatic epoxy resins and functional polysiloxanes form a class of hybrid thermosetting materials with properties that may come from both the organic and the inorganic phases. The two typically immiscible phases form a suspension whose morphology, composition, and thermal properties vary with curing time. The aim of this research was to elucidate the mechanism by which morphology changed with time and to simulate it through Metropolis-Monte Carlo. The selected system was hydrogenated epoxy (HDGEBA) and a synthetic polyaminosiloxane (PAMS). It was studied by DSC, FTnIR, gel point, viscometry, and in-situ laser scanning confocal microscopy. A mechanism for morphology generation was proposed and simulated, exploring a wide range of values of the "a priori" relevant variables. The essential features were captured by simulations with a reasonable agreement with experimental data. However, the complete process was more complex than the geometrical approach of the simulation. The main deviations that were found and qualitatively explained are: (i) the induction period on the rate of coalescence, and (ii) PAMS-rich domain average size increases faster than predictions.

Phase separation of ternary epoxy/PEI blends with higher molecular weight of tertiary component polysiloxane

A tertiary component with higher molecular weight of epoxy terminated polysiloxane (DMS-E11) was incorporated into the diglycidyl ether of bisphenol-A (DGEBA)/thermoplastic polyetherimide (PEI) blends. In this ternary DGEBA/PEI/DMS-E11 system, 25 or 30 wt% PEI and no more than 20 wt% DMS-E11 were used to ensure the formation of a continuous PEI-rich phase via reaction induced phase separation for optimum mechanical properties of blends. The results of morphology monitoring by OM and TRLS indicated that the addition of DMS-E11 could accelerate phase separation of DGEBA/PEI. Obvious differences were observed by SEM/EDS in the final morphologies of the blends. DMS-E11 localized in the PEI-rich phase continuously while it separated with DGEBA into spherical particles in the DGEBA-rich phase. DMA measurements found that the storage modulus and T_g decreased with DMS-E11 content but were compensated partly by the presence of PEI. The results of tensile tests confirmed the synergistic strengthening for epoxy resin from PEI and DMS-E11.

Effect of higher molecular weight epoxy-terminated polysiloxane DMS-E11 on morphologies and properties of DGEBA/PEI blends.

Molecular Insights into Sequence Distributions and Conformation-Dependent Properties of High-Phenyl Polysiloxanes

The excellent performance and wide applications of phenyl polysiloxanes are largely due to their phenyl units and monomer sequences. However, the relationship between molecular structure and material properties has not been explicitly elucidated. In this work, the sequence distribution and microstructure of random copolymers were quantitatively investigated by means of a molecular dynamics (MD) simulation combined with experimental verification. The results of ²⁹Si NMR showed that the large number of phenyl units not only shortened the length of the dimethyl units, but also significantly increased the proportion of consecutive phenyl units. The simulation results indicated the attraction between adjacent phenyl groups that were effectively strengthened intra- and inter- molecular interactions, which determined the equilibrium population of conformations and the dynamics of conformational transitions. Furthermore, the evolution of bond angle distribution, torsion distribution, and mean-squared displacements (MSD) shed light on the conformational characteristics that induce the unique thermodynamics properties and photophysical behavior of high-phenyl polysiloxanes. Differential scanning calorimetry (DSC), dynamical mechanical analysis (DMA), spectrofluorimetry, and laser scanning confocal microscopy (LSCM) were performed to verify the conclusions drawn from the simulation. Overall, the complementary use of MD simulations and experiments provided a deep molecular insight into structure-property relationships, which will provide theoretical guidance for the rational design and preparation of high-performance siloxanes.