Rheological and Dielectric Analysis of Polydopamine-Modified Boron Nitride/Polyethylene Oxide Nanocomposites

Hexagonal boron nitride (h-BN) nanoparticles are of high interest in nanocomposites due to their outstanding mechanical and thermal stability, however due to its chemical inertness/stability, h-BN is not capable of forming favorable interfacial interactions with a polymer matrix, which consequently affects dispersion quality of h-BN nanoparticles. Non-covalent surface modification has shown promise in both improving dispersion quality of h-BN nanoparticles, and improving the interfacial properties between h-BN nanoparticles and the polymer matrix. Polydopamine (PDA), an adhesive protein, was used to non-covalently modify the surface of h-BN nanoparticles to create nanocomposites with improved interfacial interactions and dispersion quality. Both pristine h-BN nanoparticles (PBN) and h-BN nanoparticles modified by PDA (SBN) were incorporated into a polyethylene oxide (PEO) matrix towards bio-friendly polymer nanocomposites.

PDA modification improved SBN-PEO interactions. Enhanced interfacial interactions in PEO/SBN nanocomposites provided a significant increase in G’ and G” than that of PEO/PBN nanocomposites, with strong concentration dependence. This increase was noticeable even at the lowest SBN loading level (0.5 wt%). The enhanced interfacial interactions seemed to have great impacts on the roles of hydrodynamic reinforcement effects of BNs in PEO melt. Relaxation behaviors revealed that PBN nanoparticles produced faster relaxation and better elastic energy storage capability, likely due to the hydrodynamic reinforcement effects and plasticizing effects of PBN. Yet, stronger interactions as a result of PDA modification restricted the mobility of PEO chains in PEO/SBN nanocomposites, and seemingly overpowered the hydrodynamic reinforcement effects.

Dielectric testing revealed very different behaviors between nanocomposites made with PBN nanoparticles (PEO/PBN) and SBN nanoparticles (PEO/SBN), respectively. Dielectric constants (ε’) of PEO/PBN nanocomposites at all PBN concentrations varied only slightly from that of pure PEO, contrary to PEO/SBN nanocomposites, which produced an SBN concentration-dependent ε’. PEO/0.5wt%SBN nanocomposite obtained an improved ε’ from that of pure PEO, implying that the PEO-BN interfacial interactions were improved as a result of surface modification via PDA, and those improved interactions likely contributed positively to interfacial polarization. As SBN concentration increased, ε’ decreased, with PEO/20wt%SBN nanocomposite producing the lowest ε’. In comparison, PEO/20wt%SBN nanocomposite produced a much lower dielectric constant than PEO/20wt%PBN nanocomposite, indicating that at high SBN loading levels, the improved PEO-SBN interaction restricted dielectric polarization. This possible restriction is believed to be related to the suppressed α-relaxation behavior of PEO. While PEO/PBN nanocomposites again showed only slight variation in α-relaxation, PEO/SBN nanocomposites showed slower α-relaxation as SBN concentration increased, with the slowest α-relaxation observed in PEO/20wt%SBN nanocomposite.