Micro- and Macrophase Separation in Phenolic Resol Resin/PEO-PPO-PEO Block Copolymer Blends: Effect of Hydrogen-Bonded PEO Length

We demonstrate microphase-separated thermosets based on blends of phenolic resol resin and poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO), i. e., so-called Pluronics. Three triblock copolymers are used (PE 9200, PE 10300 and PE 9400) where the molecular weights of the PPO blocks are nearly equal and the weight fractions of the PEO blocks fPEO are 0.20, 0.30 and, 0.40, respectively. The blends are prepared in a particularly straightforward way using aqueous solutions and thermal crosslinking. Structure formation is characterized using transmission electron microscopy and small-angle X-ray scattering. PPO turns out to be sufficiently repulsive to allow microphase separation in the bulk crosslinked phase and the tendency for macrophase separation upon curing is reduced due to the hydrogen bonding between the PEO and resol. The weight fraction of PEO-PPO-PEO in the present blends has been limited to a relatively small value, i. e., 20 wt.-%, and spherical microphase-separated structure is observed for fPEO = 0.40 with a long period of the order 120 Å. Macrophase separation manifests upon curing if the weight fraction of the PEO blocks is smaller, i. e., fPEO = 0.30 or fPEO = 0.20. In addition, in order to prevent macrophase separation, the molecular weights of PEO blocks and resol resin before curing are of the same order. In that respect, the system behaves qualitatively similar to the corresponding thermoplastic homopolymer/block copolymer blends.