Single-Step Fabrication of Polymer Nanocomposite Films

Polymer nanocomposites are employed in (micro)electronic, biomedical, structural and optical applications. Their fabrication is challenging due to nanoparticle (filler) agglomeration and settling, increased viscosity of blended solutions and multiple tedious processing steps, just to name a few. Often this leads to an upper limit for filler content, requirements for filler–polymer interfacial chemistry and expensive manufacturing. As a result, novel but simple processes for nanocomposite manufacture that overcome such hurdles are needed. Here, a truly single-step procedure for synthesis of polymer nanocomposite films, structures and patterns at high loadings of nanoparticles (for example, >24 vol %) for a variety of compositions is presented. It is highly versatile with respect to rapid preparation of films possessing multiple layers and filler content gradients even on untreated challenging substrates (paper, glass, polymers). Such composites containing homogeneously dispersed nanoparticles even at high loadings can improve the mechanical strength of hydrogels, load-bearing ability of fragile microstructures, gas permeability in thin barriers, performance of dielectrics and device integration in stretchable electronics.


S1. Nanocomposite films with a low PVA solution concentration (C p =1/128 wt%)
shows SEM cross-section images of a SiO 2 -PVA nanocomposite film prepared in a single-step ( Figure 1) using a low PVA concentration (C p = 1/128 wt %) in the sprayed polymer solution and depositing for t = 4 min. Its average thickness (~370 nm, yellow arrows in Figure S1a) is as expected slightly thinner but similar to that of films prepared for same deposition duration where polymer solutions with higher C p (1/16 and 1/32 wt %) are used ( Figure 2). The planar surface seen in Figures S1a is identical to that obtained with higher C p ( Figure 2). Regardless of the low C p of the sprayed polymer solution, the SiO 2 nanofiller (bright spots) is homogeneously distributed within the PVA matrix ( Figure S1b) also in agreement with what is obtained in films where higher C p is used ( Figure 2). Such films are expected to exhibit a filler content that is greater than 25 vol% as this is obtained already when more PVA (C p =1/32 wt %, Figure 2) is added. Such high filler loading with this small nanoparticle size (d SiO2 = 20 nm) is difficult to obtain [1] especially when homogeneity is demanded. The flexibility to add such high filler content significantly enhances the attractiveness of this already rapid and single-step fabrication. Figure S1. SEM cross-section images of a SiO 2 -PVA nanocomposite film prepared with a polymer solution containing C p = 1/128 wt % PVA and depositing for t = 4min on a glass substrate. Bright spots in a and b correspond to SiO 2 filler within the PVA matrix (light grey). Yellow arrows in a indicate the film thickness.

S2. Pure SiO 2 nanoparticle deposition
High versatility of single-step nanocomposite fabrication enables the preparation of pure nanoparticle films by spraying a polymer-free solution (C p = 0 wt %) during nanoparticle deposition. Figure S2 shows SEM cross-section images of such a film at different magnifications prepared with SiO 2 nanoparticles (t = 4 min, C f = 0.25 M). These films exhibit a rougher topography ( Figure S2a) due to nanoparticle agglomerates protruding from its surface. Such a protrusion is enlarged in Figure S2c Figure S1) where individual nanoparticles are seen throughout the entire film cross-section. The compact film morphology also differs substantially from the highly porous ones obtained during regular flame aerosol deposition [2]. Figure S2. SEM cross-section images of a pure SiO 2 nanoparticle film prepared with a polymer-free PVA solution (C p = 0 wt %) and depositing for t = 4 min. The topography is much rougher (a) due to protrusions (c) from its surface. The compact film morphology can be seen in b. The yellow arrows in a and b indicate the film thickness.

S3. Theoretical nanocomposite film thickness
Changing the filler content in SiO 2 -PVA nanocomposites by only varying the polymer solution concentration C p (i.e. identical flame synthesis precursor concentration C f and deposition duration t) leads to a constant total amount of deposited/incorporated nanoparticles (i.e. m SiO2 = constant) but unavoidably alters the resulting film thickness (higher filler loading for lower C p of polymer solution). •min -1 ) is determined, one is able to quickly predict the filler loading solely from the nanocomposite film thickness and deposition duration t.

S4. Filler-free polymer deposition
Single-step fabrication of polymer nanocomposites (Figure 1) is capable of achieving agglomerate-free films with extreme variability of filler content. It is easily tuned by (1) increasing polymer solution concentration (C p ), (2) its feed-rate or (3) the rate at which nanoparticles are synthesized. On the one extreme, filler-free polymers can be prepared by either employing a particle-free flame or eliminating the flame altogether. Figures S4 shows SEM cross-section images of pure PVA films prepared on glass substrates with a particle-free flame. The deposition duration t is identical (2 min) for all three images whereas the polymer solution concentration C p is varied between 1/8 ( Figure S4a), 1/16 ( Figure S4b) and 1/32 wt % ( Figure S4c). Figure S4. Filler-free PVA films prepared with a polymer solution concentration C p =1/8 (a), 1/16 (b) and 1/32 wt % (c) exhibit an uneven surface topography. The yellow arrows indicate the strong variation in film thickness. The deposition duration (t = 2 min) is same for all films. The scale bar shown in c is identical for all images.
In contrast to nanocomposites with filler ( Figure 2-4, Figures S1), these exhibit a less planar surface topography for all C p . In fact, there even are areas where PVA does not cover the substrate. More planar film surface obtained with nanoparticle addition (Figure 2) stems from the surface wetting induced by nanoparticles [3]. Nevertheless, optimization of the fabrication by adjusting the substrate temperature or eliminating the particle-free flame may