Re: Re: F1M Cat 1 record upped again

From: <leop_at_lyradev.com>
Date: 17 Mar 2014 13:06:05 -0700

No, rubber does not have a negative thermal coefficient of expansion. Rubber's volume increases with an increase in temperature. The effect (heating when pulled, cooling when relaxed; or pulling more when heated and less when cooled called the Gough–Joule effect) is seen is only when the rubber is under a strain (stretched). When under no stress such as a tension force from pulling), rubber is internally just a bunch of long chains that are all kinked up and are not aligned in any particular direction. However, when stretched, the chains start to align in the direction of the tension. Because of the cross linked internal structure of the polymer (the kinks in simple terms), the rubber molecular chains try to "re-kink" when heated as some of the cross-links were broken or strained during the stretch. This causes the rubber to seem to have a "negative" coefficient of expansion but the apparent decrease in volume caused by the change in length in the tension direction is more than compensated for by the increase in the cross section (area).

 There has been some dispute on the Internet about rubber having a negative TCoE and one college physics demonstration gives the negative TCoE explanation. But the measured physical properties contradict that view. There are materials with negative thermal coefficients of expansion. Water just above freezing is one as are some glass-like materials (often used to make telescope mirrors). And, yes, there are some polymers with negative TCoE's in some limited range of conditions (like for water). Finally, when rubber is stretched and the chains align, some molecules link up with other nearby molecular chains other than themselves. Then, the rubber has less tendency to re-kink and pull back but rather wants to stay at the stretched length. Unfortunately, this linking to other parallel chains weakens the rubber (links in the direction of the tension are replaced by links perpendicular to the tension direction) and the rubber strand can break. Many of us have experienced just this!
 

Received on Mon Mar 17 2014 - 13:06:05 CET