Crystallization Behavior and Dielectric Response of Melt Crystallized PLLA/TiO₂ Bio Nanocomposites

In this work, melt crystallized PLLA / TiO₂ bionanocomposites, with concentration of TiO₂ ranging from 1-10%wt, are produced via twinscrew extrusion melt blending. Their crystalline structure and crystallization behavior are studied by means of X-rays Diffraction (XRD) and Polarizing Optical Microscopy (POM) and their dielectric response via Broadband Dielectric Spectroscopy (BDS). XRD results suggest transformation of the α΄-crystal form of PLLA into the more stable α-form at elevated crystallization temperatures. According to POM observations all samples are fully crystallized for crystallization times of 1h. The growth size of PLLA spherulites increases linearly with time, while the growth rate is essentially independent of nanofiller content. BDS analyses reveal the presence of three relaxation processes in PLLA/TiO₂ nanocomposites, at temperatures ranging from 20°C to 140°C. These mechanisms are attributed to α–relaxation of the polylactide matrix, the Maxwell–Wagner–Sillars (MWS) interfacial polarization effect and the dc conductivity effect (CR), in terms of increasing temperature. The dynamics of all three relaxation mechanisms and the fragility index of the PLLA matrix appear independent of filler loading.

Keywords: Bionanocomposites; Poly(L-)lactic acid; Titanium dioxide; Polarizing optical microscopy; Dielectric spectroscopy; Crystallization behavior