In: Chemistry
Cotton cloths are cellulose-based fabrics which can wrinkle, if you cold wash it or simply begin to sweat. That's because these materials are highly absorbent and their polymers are linked by hydrogen bonds. The same bonds that hold together molecules of water. Add moisture to a cotton cloths and H2O will penetrate the regions between the long stringy polymers, bringing the fabric into a condition that resembles another material's plastic phase. As the water evaporates, new hydrogen bonds lock in place any creases that formed when the shirt was wet.
Polyester have a glass transition temperature. Below this temperature, the material is in its "glass" phase, with a crystal-like structure holding its individual polymers, long chains of repeating molecules, in place within the fiber. At temperatures above the glass transition threshold, some of the bonds holding these molecules together break. This allows the polymers to shift in relation to one another and form new bonds as they cool down. When the polyester returns to the glass phase, the altered structure gets locked in place in the form of wrinkles.
Ironing works by loosening the bonds between the long-chain polymer molecules in the fibers of the material. While the molecules are hot, the fibers are straightened by the weight of the iron, and they hold their new shape as they cool. Some fabrics, such as cotton, require the addition of water to loosen the intermolecular bonds.