Dielectric and thermal properties of Parylene D at high temperature: Thermal instability

Thin and transparent Parylene D (-CH
₂
-C
₆
H
₂
Cl
₂
-CH
₂
-) films have been prepared by chemical vapor deposition (CVD). The impact of the crystallinity transformations and the thermal stability were identified in Parylene D based on the dielectric spectroscopy (DS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). Changes of (ac)-conductivity with temperature (from 240°C to 340°C) and frequency (from 10
^-1
Hz to 10
⁶
Hz) are discussed. Good insulating properties of Parylene D were obtained until operating at high temperatures. Moreover, its dielectric properties were inserted between those of Parylene N and Parylene C making Parylene D interesting to replace the latter for specific applications. However, DSC measurements of these samples show after multiple melting-crystallization cycles the appearance of an instability endothermic peak when the crystallization of the sample is completed. This peak increases and shifts towards lower temperatures as the sample is further thermally treated. At this point, the thermal instability of Parylene D is associated to a crystallographic structure change and not due to a chemical degradation.