From flexible to mesoporous polybenzoxazine resins templated by poly(ethylene oxide-b-ε-caprolactone) copolymer through reaction induced microphase separation mechanism

When we blended the diblock copolymer poly(ethylene oxide-b-epsilon-caprolactone) (PEO-b-PCL) with the monomer (3-phenyl-3,4-dihydro-2H-1,3-benzoxazin-6-yl) methanol (PA-OH), Fourier transform infrared (FTIR) spectroscopy revealed that the ether groups of the PEO block were stronger hydrogen-bond acceptors for the OH group of PA-OH than were the C=O groups of the PCL block. Thermal curing resulted in the block copolymer being incorporated into the polybenzoxazine resin, forming cylindrical, wormlike, and disordered spherical nanostructures through a mechanism involving reaction-induced microphase separation, as evidenced using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Wormlike micelles of PCL, formed as the second phase in polybenzoxazine at a high aspect ratio, appearing to possess the optimal length scale to result in greater toughness. Mild pyrolysis conditions led to removal of the PEO-b-PCL diblock copolymer and formation of mesoporous polybenzoxazines. This approach also provided composition-dependent nanostructures that were similar to the structures of the non-pyrolyzed samples. Regular mesoporous polybenzoxazine resin was formed only when the polybenzoxazine content was 40-70 wt%. Intriguingly, the structures were affected not only by the balance between the contents of the polybenzoxazine and the diblock copolymer but also by the curing temperature and process.