Time and frequency domain dielectric spectroscopy for in-situ and ex-situ determination of amorphous fractions of isothermally cold-crystallized Polylactic acid

The three–phase structure of Polylactic acid (PLA), cold crystallized at two different crystallization temperatures (T C ), is performed by complementary electrical characterization techniques in a wide frequency range. Initially amorphous samples are crystallized from the glassy state at 80 °C / 4 h and 130 °C / 2 h in two different Dielectric Spectroscopy setups i.e., a typical Dielectric Response setup (0.1 Hz-1 MHz) for T C = 80 °C, and an RF Impedance Analyzer (1 MHz to 1 GHz) for T C = 130 °C. The implementation of both setups allows for the crystallization monitoring by continuous acquisition of the dielectric α-relaxation spectra, since the corresponding dielectric loss maxima are located at around 10 4 Hz for T C = 80 °C, and 5 × 10 7 Hz for T C = 130 °C. X-ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) studies reveal the presence of only the α΄-crystal form of PLA at T C = 80 °C, transforming to the more stable α-form at elevated temperatures. Spherulite size and density is monitored by Polarizing Optical Microscopy (POM) and are strongly depended on the crystallization (cold/melt) process for samples crystallized at 130 °C. Determination of the different fractions (Crystalline, Mobile Amorphous and Rigid Amorphous) reveals that samples crystallized at 130 °C exhibit an extra amount of RAF, formed upon cooling below T g , contrary to the case of samples crystallized at lower temperatures. Combined dielectric response studies over nine orders of magnitude (from 0.1 Hz to 1 GHz) are performed systematically, as a function of temperature, time and frequency and a complete analysis of all relaxation dynamics is presented. The implementation of high frequency dielectric response, along with other typically used techniques, is proven to be a powerful tool for the complete characterization of semicrystalline polymers.