Abstract
After blending the triblock copolymer, poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-b-PPO-b-PEO) with novolac-type phenolic resin, Fourier transform infrared spectroscopy revealed that the ether groups of the PEO block were stronger hydrogen bond acceptors for the OH groups of phenolic resin than were the ether groups of the PPO block. Thermal curing with hexamethylenetetramine as the curing agent resulted in the triblock copolymer being incorporated into the phenolic resin, forming a nanostructure through a mechanism involving reaction-induced microphase separation. Mild pyrolysis conditions led to the removal of the PEO-b-PPO-b-PEO triblock copolymer and formation of mesoporous phenolic resin. This approach provided a variety of composition-dependent nanostructures, including disordered wormlike, body-centered-cubic spherical and disorder micelles. The regular mesoporous novolac-type phenolic resin was formed only at a phenolic content of 40–60 wt %, the result of an intriguing balance of hydrogen bonding interactions among the phenolic resin and the PEO and PPO segments of the triblock copolymer.
Highlights
Mesoporous materials having controllable pore sizes are attractive to both the academic and industrial communities, because of their various applications in the fields of filtering, separating, sensing, catalysis and controlled drug release [1,2,3,4,5]
We have reported the formation of long-range-ordered cylinder and gyroid nanostructures of mesoporous phenolic resin templated by poly(ethylene oxide-b-ε-caprolactone) (PEO-b-PCL) [11,12]
The higher value of Tg (−27 °C) presumably arose from the phenolic/PEO phase formed through intermolecular hydrogen bonding between the OH groups of the phenolic resin and the ether groups of PEO; the lower value (−62 °C) presumably arose from the PPO block segment
Summary
Mesoporous materials having controllable pore sizes are attractive to both the academic and industrial communities, because of their various applications in the fields of filtering, separating, sensing, catalysis and controlled drug release [1,2,3,4,5]. Zheng et al reported the formation of long-range-ordered nanostructures in phenolic thermosets after curing novolac and the diblock copolymer poly(styrene-b-ethylene oxide) (PS-b-PEO) with HMTA [33]. Long-range-ordered nanostructures can be formed after mixing uncured phenolic resins with block copolymers featuring a block that forms sufficiently strong hydrogen bonds with the phenolic OH groups, such that curing of the phenolic resin will preserve the self-assembled structure without undergoing macroscopic phase separation. Ikkala et al reported the blending of resol-type phenolic resins with PEO-PPO-PEO triblock copolymers of three different molecular weights, namely EO8PO47EO8, EO17PO56EO17 and EO21PO47EO21 [43]; they observed only disordered micelle structures in these phenolic/block copolymer blends Novolac, another popular type of phenolic resin, has been difficult to use in the preparation of ordered mesoporous polymers and carbons. We describe the phase behavior and hydrogen bonding interactions of phenolic nanostructures, which we investigated using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM)
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