Why are raw elastomers so tough at room temperature ?

Good question, and a good attempt on your part to hazard a guess at an answer. Keep it up and you will go far.

People have attempted to arrive at universal viscoelasticity curves for polymers, much like you are trying to do, but have failed. While most addition polymers have an all carbon backbone, the moities along the change have a huge impact on the thermo/mechanical properties. They not only influence the glass transition temperture, but they also influence the ability of the polymers to slide past each other at melt temperatures.

Regarding the molecular weights you have posted, the viscosity of a polymer (assuming that the polymer chains are long enough to be entangled) is proportional to the 3.4 power of the molecular weight. In other words, a 10% increase in molecular weight can increase the viscosity be nearly 40%.

You also need to consider that the molecular weight of the monomers for the polymers you are looking at are all different so that the degrees of polymerization are different even if the molecular weight of the polymers are exactly the same. I.e., it takes 960 styrene monomers to make a chain of 100,000 Daltons, but you need 1468 isoprene monomers to make a natural rubber chain of the same mass. Since the C-C backbone bonds are the same length, the NR chain is 53% longer.

Lastly, NR can crystallize, although it is a slow reaction and you probably are not looking at a rubber with a high degree of crystallinity. The toughness that you observe is most likely due to the other causes mentioned above.

Ferry's book, The Viscoelastic Properties of Polymers, would be something to look at for further reading, although I think the entire book would be too advanced for you at this point.

John

Good question, and a good attempt on your part to hazard a guess at an answer. Keep it up and you will go far.

People have attempted to arrive at universal viscoelasticity curves for polymers, much like you are trying to do, but have failed. While most addition polymers have an all carbon backbone, the moities along the change have a huge impact on the thermo/mechanical properties. They not only influence the glass transition temperture, but they also influence the ability of the polymers to slide past each other at melt temperatures.

Regarding the molecular weights you have posted, the viscosity of a polymer (assuming that the polymer chains are long enough to be entangled) is proportional to the 3.4 power of the molecular weight. In other words, a 10% increase in molecular weight can increase the viscosity be nearly 40%.

You also need to consider that the molecular weight of the monomers for the polymers you are looking at are all different so that the degrees of polymerization are different even if the molecular weight of the polymers are exactly the same. I.e., it takes 960 styrene monomers to make a chain of 100,000 Daltons, but you need 1468 isoprene monomers to make a natural rubber chain of the same mass. Since the C-C backbone bonds are the same length, the NR chain is 53% longer.

Lastly, NR can crystallize, although it is a slow reaction and you probably are not looking at a rubber with a high degree of crystallinity. The toughness that you observe is most likely due to the other causes mentioned above.

Ferry's book, The Viscoelastic Properties of Polymers, would be something to look at for further reading, although I think the entire book would be too advanced for you at this point.

John

I do not want to establish a law, I just would like to have a basic understanding of a strange difference :

- PS is viscous and plastic at Tg + 30°C, and liquid at Tg + 70°C

- NR and SBR are elastic at Tg + 70°C

- and at the same time, the molar masses are approximately ordered like : M(SBR) < M(PS) < M(NR)

I would have expected the properies to be also ordered like : "SBR < PS < NR". And this is not the case ! Don't you find it strange ?

You proposed me to count the number of carbons in the backbone. Typical values are approximately : PS : 8000 SBR: 15000 NR : 60000

OK, this goes in the right direction, but does not explain the difference either. From these values, I would expect SBR to have properties "between" PS and NR. However, SBR is by far closer to NR than to PS.

The molar masses of the polymers I consider are above their critical masses, so the 3.4 power law should be valid.

However, this law is about the viscosity of the melt. Can something be implied on the elastic properties of the rubber ?

Your line of thought is exactly the same as those who have tried to establish such a "law". They have failed and can rationalize why they have failed. The basic assumption, that you only need to look at (T - Tg) to understand materials properties, is faulty.

It's actually about the zero-shear viscosity. The point I was trying to make was that small differences in molecular weight can have a huge impact on the rheology.

In my earlier post I also mentioned "viscoelasticity". Polymers have both viscosity and elasticity at the same time. While you can devise experiments to determine the separate the contributions of each, the real world is a messy combination of the two. The elasticity comes from crosslinks, but the toughness comes from the difficulty of deformation, i.e., flow. It is not a permanent flow, as the elasticity dominates and it recovers, but it is still flow.

John