I could not have put it better :
During my research for an article on this blog, I have come across this description of the physics of Ironing:
Clothes are made of polymers, natural or synthetic. The amorphousness/crystallinity of these polymers is characterized by something known as the Glass Transition Temperature (Tg). This property defines how a polymer behaves at a given temperature - soft above Tg and, hard, crystal-like below Tg.
For example, cotton (a carbohydrate polymer of glucose) has a Tg = 225° C, so cotton fabrics keep their shape because the cotton molecules cannot move at room temperature. Water acts as a plasticizer or lubricant between the chains allowing them to move more freely and lowers the glass transition temperature to 20° C. Cotton shirts and blouses thus crease most where they absorb most moisture and are under most pressure - inside the elbows, under the arm pits, where they are tucked into trousers, etc.
Most irons operate between 200-240° C for cotton settings. At this temperature, the polymeric chains of the fabric loosen up and are temporarily "remolded" till you crease them again.
see also :
chemistry of ironing