Reaction-Induced Phase Separation and Morphology Evolution of Benzoxazine/Epoxy/Imidazole Ternary Blends

Abstract

Introducing multiphase structures into benzoxazine (BOZ)/epoxy resins (ER) blends via reaction-induced phase separation has proved to be promising strategy for improving their toughness. However, due to the limited contrast between two phases, little information is known about the phase morphological evolutions, a fundamental but vital issue to rational design and preparation of blends with different phase morphologies in a controllable manner. Here we addressed this problem by amplifying the difference of polymerization activity (PA) between BOZ and ER by synthesizing a low reactive phenol-3,3-diethyl-4,4′-diaminodiphenyl methane based benzoxazine (MOEA-BOZ) monomer. Results indicated that the PA of ER was higher than that of BOZ. The use of less reactive MOEA-BOZs significantly enlarged their PA difference with ER, and thus increased the extent of phase separation and improved the phase contrast. Phase morphologies varied with the content of ER. As for the phase morphological evolution, a rapid phase separation could occur in the initial homogeneous blends with the polymerization of ER, and the phase morphology gradually evolved with the increase in ER conversion until the ER was used up. The polymerization of ER is not only the driving-force for the phase separation, but also the main factor influencing the phase morphologies.

Analysis of the Chemical Distribution of Self-Assembled Microdomains with the Selective Localization of Amine-Functionalized Graphene Nanoplatelets by Optical Photothermal Infrared Microspectroscopy

“…The chemical distribution is consistent with the blend with A-GNP, indicating that the phase structure could be constructed without the introduction of A-GNP, while the incorporation of A-GNP highlights the phase structure constructed in this epoxy blend, enabling the separated phases to be observed without the necessity of an etching process, which is normally the method used to study the phase morphology of the phaseseparated TS/TS blend. 21,28 With the combination of the chemical mapping on both blends, it could be confirmed that the A-GNPs were selectively located in the domain constructed by the AEPIP-epoxy building blocks during the curing process.…”

“…As shown in Figure 4d, the contrast between the two phases is ambiguous and the extent of phase separation is too low to be detected by the optical images, which is one of the main obstacles impeding the investigation of the phase behavior of TS/TS blended systems. 21 However, the chemical difference is clearly evident in the chemical maps shown in Figure 5e,f. The chemical distribution is consistent with the blend with A-GNP, indicating that the phase structure could be constructed without the introduction of A-GNP, while the incorporation of A-GNP highlights the phase structure constructed in this epoxy blend, enabling the separated phases to be observed without the necessity of an etching process, which is normally the method used to study the phase morphology of the phaseseparated TS/TS blend.…”

“…For further conformation, a similar blend lacking A-GNP was fabricated. As shown in Figure d, the contrast between the two phases is ambiguous and the extent of phase separation is too low to be detected by the optical images, which is one of the main obstacles impeding the investigation of the phase behavior of TS/TS blended systems . However, the chemical difference is clearly evident in the chemical maps shown in Figure e,f.…”

“…So far, in addition to the morphological analysis, such as AFM, SEM, TEM, and optical microscopy, which were the imaging techniques being relied on for decades for studying the phase behavior of TS blends, the O-PTIR technique reported here offers a novel and powerful approach for having a better understanding of the phase behavior of complex thermoset systems reinforced by nanofillers, highlighting the importance of accessing the chemical information to explain the phase structure and properties via O-PTIR spectroscopy. Furthermore, identifying the composition of the different micronanodomains for investigating the underlying mechanism provides a probability to design the materials with desired properties in a controllable manner, 21 thus unlocking the potential application of these materials. However, due to the complexity of thermoset systems, for example, the mechanism of the selective localization of A-GNP in AEPIP-rich could not be confirmed yet.…”

“…Wang et al 20 fabricated benzoxazine/bismaleimide blends with bicontinuous phase structures via an imidazole catalyst and found that both the phase separation and phase morphology were mainly determined by the viscosity of blends. Yue et al 21 investigated the PIPS behavior in benzoxazine/epoxy systems and the results showed that it was possible to effect the sequential polymerization of the epoxy resin and the benzoxazine either by increasing the initial molecular weight of the epoxy component or by adding imidazole as the catalyst, resulting in a phase-separated structure. Despite much progress in understanding the structure, properties, and phase behavior of TS/TS blends, these studies relied heavily on imaging and thermal analysis techniques, while direct evidence about the distribution of each component at the molecular level was lacking.…”