In general, as temperature increases, the viscosity of liquid solution decreases and it should exhibit more liquid behaviour. But the case of PDMS is quite opposite. On increasing the temperature, liquid PDMS gets cured (and turns into solid PDMS exhibiting more of a solid/gel like behaviour). Can anyone explain the reason behind this? Is it the same scenario with Alginate also?
Some pdms formulations are intended to cure at high temperatures using Pt catalysis or a free radical mechanism. However, I have also seen a silicone oil bath solidify. We were using free radical initiators, so I assumed this was the cause.
Hi,
1. As you know, a PDMS kit has two components: the monomer and the cross-linker. The cross-linker is required to polymerize the monomer into long chains so that the final PDMS structure as we see is obtained. Temperature is a factor that controls the rate at which this polymerization occurs. At higher temperatures (for eg. 100 deg C), polymerization completes in 15 minutes. On the other hand, at lower temperatures (for eg. 25 deg C), it takes nearly 30 hours to cure.
2.In case one did not mix the curing agent and the monomer, I believe that the monomer will not solidify at higher temperatures. In this condition, as you said, the viscosity will reduce. In other words, polymerization cannot happen without the cross-linker irrespective of the temperature.
3. Alginate is a hydrogel. I have not worked with it. But I read that there are different gelling methods for it, thermal gelation being one of them. The most common method of gelation requires a cross-linker.
"Alginate is not inherently thermo-sensitive" [Section 3.1.3 of Lee and Mooney, "Alginate: properties and biomedical application", Prog. Polym. Sci., 2012].
I hope this helps.
Regards,
Bhuvana.
Thanks a lot for your answer Bhuvana. It was very informative and useful for my research. Thanks once again!
Actually PDMS has 3 at least different meanings. In general it means polydimethylsiloxan, which may refer to:
1. a type of silicone oil, polymeryzed siloxane, which has nothing to do with further polymeryzation or cross-linking (it just remains a viscous liquid, regardless of temperature, unless you just freeze it). Viscosities of these oils range from 4 mPa s to 10000 and more, depending on the molecular length (degree of polymeryzation), and they decrease with increasing temperature (as in almost all known liquids)
2. a two component kit comprising of (i) viscous oil similar to the case 1., but not the same (it must have special terminating groups to enable further polymeryzing and cross linking) and (ii) hardener, also known as "curing agent", activating the process of cross-linking when mixed with the component (i). The process of cross-linking and polymeryzation runs faster in elevated temperatures, but it will complete even in the room temperature, taking much longer time to achieve the same final state (regardless of temperature this process of curing is irreversible - you cannot go back to the liquid state!)
3. the flexible solid (elastomer) resulting from polymeryzation/cross-linking of the mentioned kit.
The case of alginate gel is completely different story - it follows thermally induced gelation, which is reversible and does not lead to any chemical reaction. There are many similar substances which gelify with decreasing temperatures, there are also examples of liquids which become gels in elevated temperature and come back to the liquid state when cooled down (e.g. methylcellulose).
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