Direct Evidence of Polar Ordering and Investigation on Cytophilic Properties of Pyroelectrified Polymer Films by Optical Second Harmonic Generation Analysis

Pyroelectrification (PE) has been proposed as a low-cost electrode-free tool for orienting and aligning dipole molecules in polymer layers, by means of the electric field produced by a pyroelectric crystal under appropriate thermal stimulations. Probing and assessing the coherent polarization arrangement is of fundamental importance for the process control and optimization but is also a challenging task. In fact, the probing operation must be noninvasive and avoid any disarrangement the dipoles and thus without interfering with the resulting surface charges. Here we show that the PE-induced polar ordering can be probed in situ by an electrode-free analysis based on the measurement of the intensity of the second-harmonic optical wave generated by the polymer film. In fact, the results show a substantial enhancement of the second-order susceptibility of the polymer layer, caused by the PE-induced dipoles alignment. Moreover, thanks to this approach, it is demonstrated the ability of PE process to polarize polymer layers even in the "noncontact configuration", i.e., when a dielectric spacer (glass substrate) is placed between the pyroelectric crystal and the polymer film.