There are two possibilities, dissolved gas or too high temperature. In the first case try to remove dissolved gaseous impurities at decreased pressure just before polymerization. Other possibility, if the temperature inside Your disc is too high to styrene or organic stuff boils, try to perform polymerization at lower temperature. What type of initiator do You use? May be try to add less. Start polymerization at lower temperature. If You use peroxide, they will form radicals at much lower temperature when some tertiary amine is present, or try ionic initiators…

There are two possibilities, dissolved gas or too high temperature. In the first case try to remove dissolved gaseous impurities at decreased pressure just before polymerization. Other possibility, if the temperature inside Your disc is too high to styrene or organic stuff boils, try to perform polymerization at lower temperature. What type of initiator do You use? May be try to add less. Start polymerization at lower temperature. If You use peroxide, they will form radicals at much lower temperature when some tertiary amine is present, or try ionic initiators…

What about the physical stance of the container? Does it full at the beginning, which i mean do you using the whole volume?  Does it shake during the reaction or the mixing rate; this may produce the bubbles even if there is no dissolved gas. I agree with Tomasz and further the mixing rate and the reactant volume/container volume should be taken into account too. The dissolved gas can be removed with vacuum after freezing the sample. This is safer than ultrasonic agitation in your situation. And finally the heating does it homogeneous enough to prevent from the local high temperatures? 

Thanks Dr. Tomasz Ciach

I am using Azobisisobutyronitrile(AIBN) as initiator and I kept the temperature below 80 C. I am thinking the dissolved gases may be the problem and I have used purging with argon for few mins before heat treatment but it didn't work. may be I need to work on removing the dissolved gases better.

Thanks Kerem for your suggestions. My container is more than half empty and I used ultrasonication and vortex mixture to mix it well but once it is inside oven for heat treatment I don't touch it. I will try your idea of vacuum after freezing the sample.

 You are welcome Sunil :) Since you specify the initiator.  You are surely aware of the side product during the initiation. It produces nitrogen gas. Reducing the amount will help you to reduce but  to achieve zero bubble formation so had to change it with another. Peroxides also used for initiation. 

i suggest yopu change your initiator since it produces nitrogent gas, or use anti-foaming agent but adding it at high % it may affect your polymer properties.

and make sure that your vessel pressure is high enough to supress and bubble formation

Sunil, try to look for another initiator. Peroxides, especially liquid, are good for transparent – optical systems. I would suggest cumen hydroperoxide (liquid). Liquid initiator easily mixes with the system You use, 1 - 2 % should be enough.. Don’t use ultrasonic, normal gentle stirring is enough and will not introduce bubbles.

Thank you all for very important answers to my questions. I will work on your suggestions and see if it works.

Another problem related to bubble formation is related to density change. Due to the Trommsdorff effect while sudden density change (typical of FRP) the polymer if subject to strong shrinkage while it looses its flow properties. This, determines void bubble formation. In order to avoid this problem you can try to apply pressure on your system during the polymerisation.