Comparison of Crystalline Phase Transitions in Fluorniated vs Nonfluorinated Parylene Thin Films

Abstract

Thermally induced morphological changes in thin (1−5 μm) films of poly(α,α,α‘,α‘-tetrafluoro-p-xylylene, (C6H4CF2)n) (Parylene-F or PPX-F) are characterized using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and thermal stress measurements. A reversible crystalline phase transition is observed between 360 and 400 °C, which to our knowledge has not been reported previously. The transition is accompanied by an increase of the in-plane tensile stress of the film, which is attributed to contraction of the in-plane polymer structure during the transition to the high-temperature crystalline form. The stress and thermal behavior are qualitatively similar to those occurring in the more extensively studied nonfluorinated material, poly(p-xylylene, (C6H4CH2)n) (Parylene-N or PPX-N), which undergoes a shift in stress during the β1−β2 transition (270−300 °C).