Question: Why Do Clothes Wrinkle?

Answer: Heat and water cause wrinkles. Heat breaks the bonds holding polymers in place within the fibers of a fabric. When the bonds are broken, the fibers are less rigid with respect to each other, so they can shift into new positions. As the fabric cools, new bonds form, locking the fibers into a new shape. This is both how ironing gets wrinkles out of your clothes and why letting clothes cool in a heap fresh from the dryer will instill wrinkles.

Not all fabrics are equally susceptible to this type of wrinkling. Nylon, wool, and polyester all have a glass transition temperature, or temperature below which the polymer molecules are almost crystalline in structure and above which the material is more fluid, or glassy.

Water is the key culprit behind wrinkling of cellulose-based fabrics, such as cotton, linen, and rayon. The polymers in these fabrics are linked by hydrogen bonds, which are the same bonds that hold together molecules of water. Absorbent fabrics allow water molecules to penetrate the areas between the polymer chains, permitting the formation of new hydrogen bonds.

Reference

http://chemistry.about.com/od/howthingsworkfaqs/f/clotheswrinkle.htm