From Spherical Micelles to Hexagonally Packed Cylinders: The Cure Cycle Determines Nanostructures Generated in Block Copolymer/Epoxy Blends

Block copolymer (BCP)/epoxy blends have been intensively investigated during the past decade. Macro-phase separation of the BCP in the cured thermoset is avoided by selecting one block that is initially immiscible or that phase separates early in the polymerization and another block that remains miscible up to high conversions. But the quality of the thermoset as a solvent of the miscible block varies along the cure cycle with both conversion and temperature. It shifts from a good solvent to a poor solvent, and eventually to a nonsolvent, by increasing conversion mainly due to the increase in the average molar mass before gelation and the cross-link density after gelation. It also changes with temperature due to the corresponding variation of the interaction parameter. Therefore, for a constant BCP concentration different nanostructures might be accessed and fixed by changing the cure cycle. This can be of interest to modulate final properties of the material (e.g., toughness, transparency, etc.). The selected system to prove this concept was a solution of 20 wt % PS-b-PMMA (M-n = 67 100, Phi(PS) = 0.69) in a stoichiometric mixture of diglycidyl ether of bisphenol A (DGEBA) and 4,4'-methylenebis(2,6-diethylaniline) (MDEA). Generated nanostructures varied with the selected cure cycle from a dispersion of spherical micelles to a dual morphology consisting of domains of hexagonally packed cylinders and regions with a dispersion of spherical micelles. This produced changes in transparency and in dynamic-mechanical properties of the resulting nanocomposites.