To remove polyimide coating from our Simplus™ brand or Zero™ or Controlled Flow™ capillaries for CE without using heat, it is best to use fuming sulfuric acid at 100°C.
You may want to heat the acid on a hot plate in a small shallow plate, using a fume hood and all the safety equipment needed. Then dip the capillary into the hot liquid. After a brief amount of time in the liquid, the polyimide should be able to be wiped away with a clean, soft cloth. Be careful not to apply too much pressure to the capillary.
Best practices in this case may call for practicing with capillary parts before you attempt on a coated capillary.