Effect of thickness on the crystallinity and Curie transition behavior in P(VDF/TrFE) (72/28) copolymer thin films using FTIR-transmission spectroscopy

The effect of thickness on the ferroelectric behavior in spin-cast poly(vinylidene fluoride/trifluoroethylene) (P(VDF/TrFE)) (72/28 mol%) random copolymer films with varying degree of crystallinity was analyzed qualitatively and quantitatively using Fourier transform infrared-transmission spectroscopy (FTIR-TS) data. From the qualitative analysis of raw IR data, the critical thickness of crystallization was observed near 130 nm, below which the films exhibited significant changes in crystalline peak (1292 cm^-1) absorption. Using factor analysis (FA) of their raw IR data, we have isolated the pure crystalline and amorphous phase spectra, and also quantitatively calculated their degree of crystallinity as a function of thickness. FA of the mixture spectra obtained at varying temperatures during a heating-cooling cycle (30 °C → 140 °C → 30 °C) was conducted to isolate the pure spectra of the low-temperature (ferroelectric) and high-temperature (paraelectric) crystalline phases and to determine the content of each phase quantitatively as functions of thickness and temperature. Curie transition temperature during heating (T_c^↑) and cooling (T_c^↓) decreased and increased, respectively, resulting in reduced temperature range between T_c^↑ ↔ T_c^↓ with decreasing film thickness used in our study and the quantitative data obtained from FA were quite comparable with the qualitative data obtained from raw IR analysis.