The influence of water released from particles in epoxy-based nanocomposites

Recent studies have hypothesized that the reinforcing effects of boehmite nanoparticles (BNPs) in polymer nanocomposites (PNCs) are partly related to the particles themselves and partly to the water released from the BNP during curing. In this work, PNCs made from dried BNP (dBNP) with concentrations up to 15 wt% are investigated to differentiate particle and water related effects. The observed trend of the storage modulus in dynamic mechanical thermal analysis measurements was found to be independent of the drying procedure. Stiffness maps from intermodulation atomic force microscopy showed that dBNP leads to a stiffening of the interphase surrounding the particles compared with the unaffected epoxy matrix, while a softer interphase was reported for PNCs with as received BNP. A slight decrease in the glass transition temperature was observed by broadband dielectric spectroscopy related to a lowered crosslink density due to the particles. A significantly higher decrease was reported for PNCs with BNP, attributed to water influencing the curing process. In conclusion, the stiffening of PNC with BNP is related to the particles themselves, while the release of water causes the formation of a soft interphase in the vicinity of the particles and a significant decrease in crosslink density. The reinforcing effects of boehmite nanoparticles (BNPs) in anhydride cured epoxy nanocomposites are strongly dependent on the natural water content of BNPs. This is due to the influence of water on the curing mechanism and the formation of a soft interphase. Here, nanocomposites of dried BNPs are studied by macroscopic methods (dynamic mechanical thermal analysis) and interphase sensitive methods (diffuse reflectance infrared Fourier transform spectroscopy, broadband dielectric spectroscopy and high-resolution force spectroscopy). The aim is to show the difference in the formation of the matrix network and a stiff interphase and thus to assess the influence of water by exclusion. image