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How does spandex allow sportswear and swimsuits to stretch?

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man-made fibre Figure 1: Three common polymer structures. The linear, branched, and network architectures are represented (from top), respectively, by high-density polyethylene (HDPE), low-density polyethylene (LDPE), and phenol formaldehyde (PF). The chemical structure and molecular structure of highlighted regions are also shown.

Spandex allows sportswear and swimsuits to stretch because it is a segmented polyurethane fibre composed of alternating rigid and flexible segments.1

These segments function in a specific way to provide elasticity:
* Flexible segments: These act like springs that connect the rigid segments, allowing the fibre to be stretched to great lengths—often 500–610 percent beyond its original size—without breaking.21
* Rigid segments: These segments tend to aggregate, providing the material with its characteristic stretch-resistance and strength.1

Because of this unique molecular structure, spandex can be stretched repeatedly and will quickly return to its original length. In textile applications, it is frequently used as a core for yarns or covered with other fibres like nylon to balance its high elasticity with a comfortable feel for garments such as swimsuits and sportswear.231