CORONA POLING OF POLYMER FILMS

The molecules of a polymer film or polymer of a core replaced fiber can be aligned through a process called corona poling. Corona poling is ideal for non-contact situations where air or nitrogen can be used as an environment. The goal of corona poling is to align molecules within the core-replaced fiber to create an electro-sensitive waveguide. Once correctly poled, the polymer’s index of refraction will change under an electric field. We use PMMA doped with DR1 as the polymer. more on polymers


Corona poling setup includes a sharp needle or thin wire suspended above a larger ground plate. The distance between the wire and the ground plate is approximately 1 cm. Once a high voltage (5-10 kV) is applied to the needle or wire, the surrounding air will be ionized. That air will then collect on any insulator placed between the wire and the ground plate. The charge will continue to build until equilibrium is met between the electro-forces of the plate and the environment. One benefit of corona poling is that if a charge finds a path to the ground plate (through a hole in the sample, for example), other charges will remain on the sample surface maintaining a high poling field. In plate-to-plate contact poling, all the charge may be lost if a path between the two plates exists.

The polymer’s dipole orientation will remain relatively unchanged unless it is heated. Therefore, the poling process involves heating the sample, applying the poling field, and cooling the sample allowing the polymer’s dipoles to solidify while aligned. The heating/cooling rates and poling temperature are all significant in effective corona poling. With PMMA/DR1, a 20 minute ramp up (from room temperature to 100° C) is followed by a 10 minute poling (at 100° C) and then a 40-50 minute cool down while maintaining the poling field. It is important to maintain a field over a slow cool-down. If the cool-down is too quick the molecules will not be able to settle in a uniform pattern, leaving gaps and inconsistencies.


Corona Poling Procedure

PROCESS STEP	TEMPERATURE	TIME
Ramp temperature up	From room temperature to 100° C	20 Minutes
Apply voltage and maintain temperature	100° C	10 Minutes
Maintain voltage and ramp temperature down	From 100° C to room temperature	45 Minutes
Disconnect voltage and retrieve sample	Room temperature	none

Poled material will maintained poled for a length of time depending on type of polymer used. With any polymer, an initial charge will remain on the surface even after the field is turned off. This charge can be expected to dissipate after a short period of time.

1. References
   1. 1) Blaze Labs on Corona Discharge
      
   2. 2) Modeling Corona
      
   3. 3) Corona Discharge-Niels Jonassen
      
   4. 4) Corona Discharge-Answers.com
      
   5. 5) Corona Charging of Polymers-Giacometti, Fedosov, Costa
      
   6. 6) Corona Charging of Polymers-Giacometti, Oliviera
      
   7. 7) Corona Poling of Ferroelectric & NLO Polymers-Fedosov, Sergeeva
      
   8. 8) SHG and Absorbtion of Corona-Poled Polymers-Mortazavi, Knoesen, Kowel
      
   9. 9) Damage of Poled Polymer-Lee, Garner
      

Things to be aware of:

* Corona poling is likely to cause damage to the surface of the polymer. This is due to the "neutral activated species" of ozone -which causes the blue glow- and nitrogen oxides which do not assist in actually poling the material. (Ref #5)

* If the dielectric sample does not cover the plane electrode entirely, most of the current flows not through the sample, but through the uncovered part of the electrode. (Ref #5)