Elastomeres are \"defined\" as: \"linear-chain polymers with widely spaced cross

Question

Elastomeres are "defined" as:

"linear-chain polymers with widely spaced cross-links attaching each molecule to its neighbours"

Now I found sentences (talking about glass transition):

"This means that at room temperature the secondary bonds are melted and the molecules can slide relative to each other with ease. Were it not for the cross-links, the material would be a viscous liquid, but the cross-links give it a degree of mechanical stability."

So, what exactly are these bonds that melt? and which ones do not? An example would be nice. Chemistry is not my strongest field so be rigorous.

The quotations are from cellular solids by gibson and ashby.

Explanation / Answer

This is really a chemistry question, but I don't think there is a chemistry SE at the moment.

Anyhow, there may not be clearly defined secondary bonds. If you take silicone (dimethicone) it has a glass transition temperature about -125C. There aren't any specific bonds involved: it's just due to the Van der Waals forces that you get in any solid.

When you have polymers with polar groups, e.g. polymethylmethacrylate you can get hydrogen bonding between the polymer chains. These are more what you'd describe as a "secondary bond" and are indeed broken by rising temperature. You generally find hydrogen bonded polymers have much higher glass transition temperatures than polymers that cannot form hydrogen bonds.

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