Stretching a polymer

Wanting an experiment to illustrate the teaching on bonding in polymers, we can strips of a bin back. The hypothesis was that for the same load, double the length of strip will have double the extension because twice as many bonds will be being stretched, like a longer chain of springs. The data was very messy. One problem was that given time the knots started to unravel under tension. Another problem is that the hypothesis is probably wrong, although the results did not give us a consistent pattern so that we could tell. Certainly with rubber bands, the chains are tangled by side branch bonds. This makes it harder to stretch at first. Once untangled, it is easy to stretch them until they are straight and only then do the actual bonds get stretched.