Flory introduced the concept of excluded volume to polymers. (Werner Kuhn had already invented the term in 1934 for molecules in general.) "Excluded volume" refers to the idea that one part of a long chain molecule can not occupy space that is already occupied by another part of the same molecule. Excluded volume causes the ends of a polymer chain in a solution to be further apart (on average) than they would be were there no excluded volume. The recognition that excluded volume was an important factor in analyzing long-chain molecules in solutions provided an important conceptual breakthrough. Excluded volume explained several puzzling experimental results of that time. It also led to the concept of the theta point, the set of conditions at which an experiment can be conducted that causes the excluded volume effect to be neutralized. At the theta point, the chain reverts to ideal chain characteristics – the long-range interactions coming from the excluded volume are eliminated. This allows experimenters to more easily measure short-range features such as structural geometry, bond rotation potentials, and steric interactions between near-neighboring groups. Flory taught another advantage of performing the experiment at the theta point: the chain dimension in polymer melts would have the size computed for a chain in ideal solution. This works because excluded volume interactions are neutralized at the theta point.
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