According to the invention, a method for the plastic deformation of a polymer is provided, which is characterized in that the polymer is treated with electromagnetic radiation having a wavelength in the range from 0.8 to 100 μm with simultaneous action of pressure and shearing and thermal energy. By means of the method according to the invention, it is also possible to plastically deform polymers which have to date not been accessible to plastic deformation, such as chitin and in particular cellulose.