Miscibility and morphology of thermosetting polymer blends of novolac resin with poly(ethylene oxide)

Polymer blends of novolac resin and poly(ethylene oxide) (PEO) were prepared by solution casting from N,N-dimethylformamide (DNIF). The miscibility and morphology of the blends before and after curing were investigated by optical microscopy differential scanning calorimetry (d.s.c.) and Fourier transform infrared (FTi.r.) spectroscopy. It was found that PEO is miscible with uncured novolac over the entire composition range, as shown by the existence of a single composition-dependence glass transition temperature (T g ). FTi.r. studies revealed that hydrogen bonding interactions exist between the hydroxyl groups of novolac and the ether oxygens of PEO. The relative amount and the average strength of the hydrogen bonds in the blends were higher than those in the pure novolac resin. The curing with 15 wt% hexamine (HMTA) (relative to novolac content) resulted in the disappearance of a detectable T g in both the neat novolac and the novolac-rich blends, due to the reduced mobility of the novolac chain segments. An analysis of the reduction in T m and crystallization rate with increasing novolac content revealed that the HMTA-cured blends remained completely miscible. After curing with HMTA, considerable hydrogen bonding interaction between the components still existed, which is the driving force for the miscibility of the HMTA-cured blends. The relative amount and the average strength of hydrogen bonds in the cured blends were lower than those in the uncured blends.