Why Do Polymers Shrink?

Hello Nems,

After a long time I see a great question: Here is my thinking.

Typically C-H, C-C, C-O, Si-H, Si-C bonds have certain bond energies, bond strengths and bond lengths. When you subject a polymer to so many calories of energy (by subjecting them to heat) and if the heat energy exceeds their bond energy, then it will result in " Chain-Scission " "Shock" and "Distortion" of the chains

As far as i know Polymer molecules strands are elastic, so when a polymer who is formed in someway is heated it tries to return to it original state

I do not actually know but can consider the polymers being more dense than the monomer or monomers from which they react obviously occupy less space. Can they then, when heated, continue shrinking upon further heating?

heating a plastic to 500C can drive any residual monomer into the atmosphere. In addition, you may cause some breakdown of the polymer and this liberated monomer evaporates. This is really the srat of thermal degradation which can occur in thermoset fully crosslinked polymers that do not soften much on heating. The break down may cause the object to shrink. In addition, many plastic have a small amount of a plasticizer added, and this can be driven off as well.

If the plastic gets very brittle you may have also degraded it by random chain breakage and it may become a lot wekaer.

I Agree. If there are any unreacted/unconverted monomers yes that will lead to decomposition which may result in shrinkage and decomposition. Plasticizer decomposition!!! yes that will also lead to shrinkage. But it seems to me chain scission type of thermal degradation is the major factor because the temperatures he mentioned are too high for any polymer. But it could be all of the above

I don't will become weaker, it was proven that by increasing the temperature to up of 1500 deg C in an inert gas the polymers will be converted to graphite which is robus than carbon.

Welcome aboard! Good question, and I think there are several answers, most of which have been provided already. It will depend on the type of polymer. There are thermoset materials and thermosetting materials with very different properties. In many cases, shrinkage is due to shearing of co-polymer bonds (like breaking wooden rungs on a rope ladder will let the ropes move closer together).

I had a discussion with my colleagues and we got to a conclusion that the remaining solvent in the polymer structure evaporates due to high temperature.

Ps. apparently the link for the video was moved to this page

http://www.memsuniverse.com/carbon-mems-technology-c-mems/