Degradation of polydimethylsiloxane Si3(CH3)6O3?

[dawin]

In a discussion I had recently, a topic was raised concerning some polymer degradation. Under high heat flow conditions and an oxygen environment, what is the plausibility of a cross-linked polydimethylsiloxane polymer degrading into hexamethylcyclotrisiloxane?

I believe it is unlikely, and a colleague noted that it is unlikely a lower energy compound such as cross-linked Si(CH3)2O would degrade into a "higher energy" compound such as HMTS. I'm no chemist, what makes HTMS higher energy -- is it the bonds present, cyclical nature of the compound, both, neither?

 

[alxm]

Well the bonds and groups present are the same, so the comparison here is pretty straightforward.

Offhand, HMTS likely has higher energy due to its ring structure. It has a more constrained geometry, so the bonds are more strained and have higher energy because of that. (in general: "ring strain")

On the other hand, PDMS is a larger molecule and as such has lower entropy. That means that HMTS should have lower energy and so be the more stable form at higher temperatures.
So I don't think it's unlikely to happen at all at higher temperatures. I'd expect PDMS to pyrolyse into HMTS to some extent at higher temperatures, at least in an oxygen-free environment (otherwise you might just have complete or partial combustion).
To take a wild guess, I think you'd see pyrolysis starting to occur maybe somewhere between 150-200 C or so.

 

[dawin]

Well the bonds and groups present are the same, so the comparison here is pretty straightforward.

Offhand, HMTS likely has higher energy due to its ring structure. It has a more constrained geometry, so the bonds are more strained and have higher energy because of that. (in general: "ring strain")

On the other hand, PDMS is a larger molecule and as such has lower entropy. That means that HMTS should have lower energy and so be the more stable form at higher temperatures.
So I don't think it's unlikely to happen at all at higher temperatures. I'd expect PDMS to pyrolyse into HMTS to some extent at higher temperatures, at least in an oxygen-free environment (otherwise you might just have complete or partial combustion).
To take a wild guess, I think you'd see pyrolysis starting to occur maybe somewhere between 150-200 C or so.

Thanks for your response. I haven't studied chemistry for a few years and am mostly self-educated re: o-chem; I'm trying to understand this statement:

On the other hand, PDMS is a larger molecule and as such has lower entropy. That means that HMTS should have lower energy and so be the more stable form at higher temperatures.

Is this implying that HMTS would tend to form in absence of oxygen due to... Gibbs free energy? Is this a conservation of energy, increasing entropy deal or is that that completely off base?

 

[alxm]

Is this implying that HMTS would tend to form in absence of oxygen due to... Gibbs free energy? Is this a conservation of energy, increasing entropy deal or is that that completely off base?

Yes, Gibbs free energy. If you recall, ΔG = ΔH - TΔS

Now, HMTS is higher energy in terms of the enthalpy ΔH (due to the ring strain), but the reaction entropy ΔS is certainly positive, because HMTS is a smaller molecule. (and all else being equal, a smaller molecule has higher entropy) So with enough temperature the entropy term is going to win out, and you'll have a negative Gibbs' free energy of reaction.

In fact, just googling "polydimethylsiloxane pyrolysis", the first link turned up is http://pubs.acs.org/doi/abs/10.1021/ac50058a020" , which indeed shows that HMTS is formed when heating PDMS. They did use a very high temperature in that case (max temp 980 C), but given that their experiment formed HMTS instantly, I think it's safe to say the minimum temperature at which it's formed is quite a bit lower. OTOH I'm not a polymer chemist, so my guesses might be off. But it certainly doesn't require temperatures that high in general. (they were after all looking for general pyrolysis products and not just HMTS)

 

[Nemus]

It is an increase in entropy and therefore also a lowering of Gibbs free energy.
G=H-TS. A few long polymer chains forming random smaller fragments. Probably your cyclic trimer too, among others.
This is not the exact reversal of the polymerization because that is driven by the addition of high energy anions that open the rings.