Direction-dependent elastic properties and phononic behavior of PMMA/BaTiO3 nanocomposite thin films

Abstract

Determination of the anisotropic mechanical properties of nanostructured hybrid films is of great importance to improve fabrication and to enable reliable utility. Here, we employ spontaneous Brillouin light spectroscopy to record the phononic dispersion relation along the two symmetry directions in a supported PMMA (poly(methylmethacrylate))-BaTiO hybrid superlattice (SL) with a lattice constant of about 140 nm. Several dispersive elastic modes are resolved for in-plane wave propagation, whereas along the periodicity direction the SL opens a wide propagation stop band for hypersonic phonons and near UV photons both centered at about 280 nm. A thorough theoretical analysis based on the finite element method quantitatively captures the band diagrams along the two main symmetry directions, helps identify the large density mismatch effect on the unexpectedly low sound phase velocity, and reveals significant anisotropy of the SL elastic tensor. Phonon propagation is a sensitive index of the structure, density, and the mechanical moduli of nanocomposite films.

Elastic Modulus and Thermal Conductivity of Thiolene/TiO2 Nanocomposites

“…The X-ray diffraction pattern shown in Figure S2 , however, confirms that the TiO 2 nanoparticles are indeed crystalline, without pores inside. We note that in a previous report on the sound velocity of PMMA/BaTiO 3 nanocomposites, 9 a c L, eff trend in better agreement with the Wood’s law’s prediction is observed. If ρ TiO 2 is fixed at 3900 kg/m 3 and c L, TiO 2 at 8880 m/s, the trend of the fitted curve deviates from the experimental data but shows a similar trend with the results for the PMMA/BaTiO 3 nanocomposites.…”

“…If ρ TiO 2 is fixed at 3900 kg/m 3 and c L, TiO 2 at 8880 m/s, the trend of the fitted curve deviates from the experimental data but shows a similar trend with the results for the PMMA/BaTiO 3 nanocomposites. 9 The surprisingly low ρ TiO 2 , required to represent the experimental c L, eff , could be attributed to the specific interfacial effects (i.e., covalent cross-linking) in this polymer nanocomposite. In Figure 3 b, we show the variation of the effective elastic longitudinal modulus with the TiO 2 wt %.…”

Robustness of elastic properties in polymer nanocomposite films examined over the full volume fraction range

“…S1 ). Refractive index values were found to be n = 1.56 (50 wt%) and n = 1.41 (100 wt%) at 532 nm, not far from our previous value for PMMA-BaTiO 3 superlattices 3 .…”

“…Polymer nanocomposites are of current interest due to the possibility of tuning material properties by varying size and volume fractions of included nanoparticles 1 , 2 . In particular, the elastic modulus is an important property for applications and knowledge of the component moduli is not sufficient for predicting it, effective medium theories being often unreliable 3 , 4 . Thus, the combination of the two materials can sometimes exceed the bulk behavior of the constituents, owing to a synergistic interplay between the two components.…”

“…From our previous study 3 , we know that nanoparticle films can display lower sound velocities than their bulk constituent material, depending on the contact stiffness between the nanoparticles 4 , 14 . This was unexpected behavior, since polymer nanocomposites are generally stiffer than their constituent polymers 15 – 17 .…”