Viable Approach for Forming Uniform Polymer Nanocomposites with Ultrahigh Filler Loading
Authors: Hsieh Chia-Yun , Lau Kenneth K.S. 
Published: February 2020

Nanocomposites have the potential in offering unique material properties and behavior due to synergy between the matrix and nanofiller. Organic-inorganic nanocomposites, such as polymers filled with inorganic nanoparticles, have drawn significant attention with the combination of soft and hard matter, which have found use in a diverse range of applications, including energy capture, energy storage, aviation, aerospace, separations, barrier protection, and biomedicine.

Current methods of making nanocomposites, including sol-gel, solution casting, physical blending and surface-initiated polymerization, often encounter filler aggregation problems, particularly when the amount of nanofiller exceeds 10–15 wt%. The aggregation behavior has been shown through experiments and simulation to cause phase separation that weakens the nanocomposite structure and capabilities. This natural tendency for nanoparticle aggregation has severely limited filler loading and potential material property enhancements.

Here, initiated chemical vapor deposition (iCVD) is introduced as a unique approach for enabling uniform and ultrahigh loading of fillers in polymer nanocomposites. iCVD is a liquid-free polymerization technique that simultaneously grows a polymer thin film on supporting substrates. To achieve uniform and ultrahigh loading, the nanofiller network is first constructed to create a porous 3D nanostructure containing nanovoids in which iCVD can be utilized to fill the pore spaces. With the low pressure, solventless CVD environment, rapid diffusion of monomer and initiator vapor precursors leads to conformal polymer growth throughout the porous network under reaction-limited conditions. This contrasts with current methods in which the nanofiller is physically mixed and compounded with the polymer in solution or as a melt.

The iCVD approach enables ultrahigh loading (>80 wt%) of nanofiller that is well-dispersed. As a result, it opens the door for investigating the properties of polymer nanocomposites under ultrahigh nanofiller loading, an area that has been previously limited by processing challenges. With an appreciable interaction of the polymer and the nanofiller at their interface at such high loading, there will be significant effects on the thermal, mechanical, and electronic properties of the composite material.

For example, one of the first polymers investigated has been poly(glycidol) (PGL). Utilizing a cationic ring opening polymerization of glycidol monomer together with boron trifluoride initiator, iCVD successfully polymerized PGL within the mesopores (10–25 nm) of a network of TiO2 nanoparticles. The PGL-TiO2 nanocomposite showed a remarkable increase of 50–60 °C in the glass transition temperature compared to bulk PGL without any nanofiller. As one of the highest glass transition temperature shifts recorded, this resulted in PGL shifting from a soft, hydrogel-like polymer to a hard glass at room temperature. The glass transition change is attributed to appreciable hydrogen bonding interactions between PGL and surface hydroxyl groups on the TiO2 nanoparticle surfaces.

Here, we will discuss in detail the iCVD approach in forming ultrahigh loading polymer nanocomposites. In particular, the interactions of different polymers with the TiO2 nanoparticle network will be presented, including PGL, poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA), polytetrafluoroethylene (PTFE). The impact on thermal, mechanical, and microstructural properties will be discussed through thermogravimetric analysis (TGA), dynamic scanning calorimetry (DSC), atomic force microscopy (AFM)/nanoindentation, and X-ray diffraction (XRD) results.