Microphase Separation Transformation in Bio-Based Benzoxazine/Phenolic/PEO-b-PCL Diblock Copolymer Mixtures Induced by Transesterification Reaction

Abstract

Herein, we synthesized a difunctionalized bio-based vanillin azine monomer (4,4′-((1E,1′E)-hydrazine-1,2-diylidenebis(methaneylylidene))bis(2-methoxyphenol), VBAZ-2OH) via a Schiff base reaction between vanillin and hydrazine monohydride. Subsequently, we successfully prepared 2,2′-(((1E,1′E)-hydrazine-1,2-diylidenebis(methaneylylidene))bis(8-methoxy-2H-benzo[e][1,3]oxazine-6,3(4H)-diyl))bis(ethan-1-ol), VBAZ-BZ-2OH), which contains oxazine units, through a Mannich condensation reaction of VBAZ-2OH with ethanolamine and paraformaldehyde in 1,4-dioxane as the solvent. The chemical structures of these two monomers (VBAZ-2OH and VBAZ-BZ-2OH) were characterized by using NMR and FTIR analyses. Our study aimed to investigate the transesterification reactions by blending different VBAZ-BZ-2OH/phenolic resin (BP) compositions with a PEO₁₁₂-b-PCL₉₉ (EC) diblock copolymer to form various BP/EC blends. These blends exhibit competitive hydrogen-bonding interaction phenomena, which were analyzed using one-dimensional and two-dimensional FTIR analyses. Interestingly, after thermal treatment of BP/EC blends at 150 °C, the ordered self-assembled lamellae or hexagonal packed cylinder structures transform into the disordered micelle or disorder structure in BP/EC blends as a result of the transesterification reaction due to EC becoming miscible with a VBAZ-BZ-2OH monomer, disrupting the ordered self-assembled structure, which was confirmed through TEM and SAXS analyses. The transesterification reaction could easily understand the order–disorder morphological transformation using BP/EC blends to replace thermogravimetric analysis (TGA) coupled with microcomputed gas chromatography (TGA-GC) analyses.