Spandex (as it’s called in North America; Europe knows the same material as “elastane” and the Brits refer to it as “lycra,” but that’s confusing because Lycra® is actually a trade name) is a rigid copolymer of polyurethane and polyurea and is a rubbery material, like polypropylene oxide. These two polymers do not mix well, so tiny domains of each polymer form. It’s this separation (of the hard bits from the soft bits) that gives Spandex its stretchy, yet strong, behavior.