Molecular Dynamics and Structure of Poly(Methyl Methacrylate) Chains Grafted from Barium Titanate Nanoparticles

Abstract

Core-shell nanocomposites comprising barium titanate, BaTiO₃ (BTO), and poly(methyl methacrylate) (PMMA) chains grafted from its surface with varied grafting densities were prepared. BTO nanocrystals are high-k inorganic materials, and the obtained nanocomposites exhibit enhanced dielectric permittivity, as compared to neat PMMA, and a relatively low level of loss tangent in a wide range of frequencies. The impact of the molecular dynamics, structure, and interactions of the BTO surface on the polymer chains was investigated. The nanocomposites were characterized by broadband dielectric and vibrational spectroscopies (IR and Raman), transmission electron microscopy, differential scanning calorimetry, and nuclear magnetic resonance. The presence of ceramic nanoparticles in core-shell composites slowed down the segmental dynamic of PMMA chains, increased glass transition temperature, and concurrently increased the thermal stability of the organic part. It was also evidenced that, in addition to segmental dynamics, local β relaxation was affected. The grafting density influenced the self-organization and interactions within the PMMA phase, affecting the organization on a smaller size scale of polymeric chains. This was explained by the interaction of the exposed surface of nanoparticles with polymer chains.

Keywords: dielectric properties; molecular dynamics; nanocomposites; polymer brushes.

Figure 1

3D map (frequency–temperature dependences of dielectric loss (ε”)) for BTO-g-PMMA (0.43). The solid red lines, drawn as a guide for eyes only, indicate relaxation processes.

Figure 2

Frequency dependence of real part of dielectric permittivity (a) and loss tangent (b) for BTO-g-PMMA nanocomposites determined at 20 °C.

Figure 3

Frequency dependence of imaginary part of dielectric permittivity (a) and its normalized curves (b) for BTO-g-PMMA core–shell composites collected at 80 °C.

Figure 4

Arrhenius plot for the core–shell BTO-g-PMMA composites performed from BDS experiments. Data for PMMA were added for comparison.

Figure 5

Comparison of the ¹³C CP/MAS NMR spectra recorded for BTO-g-PMMA (0.18)—(green), BTO-g-PMMA (0.43)—(blue), BTO-g-PMMA (1.25)—(red) and neat PMMA—(black). Signals marked with asterisks indicate rotational sidebands.

Figure 6

Deconvoluted NMR data and individual Lorentzian components half-width analysis.

Figure 7

ATR FTIR spectra of BaTiO₃, PMMA, and PMMA-g-BTO composites.

Figure 8

ATR FT-IR spectra of PMMA-g-BTO nanocomposites at the regions of C=O and C–O–C stretching vibrations. All spectra were normalized to the intensity (signal amplitude) of the line with a maximum at 750 cm⁻¹. Inset in the middle part of the chart shows changes in the positions of the characteristic line maxima vs. BTO percentage.

Figure 9

Raman spectra of BTO-g-PMMA nanocomposites at some selected regions.