Biaxial deformation of polyamide-6: Assessment of orientation by means of infrared trichroism

Infrared spectroscopy was used to study the evolution of structure in films of polyamide-6 drawn on a Cellier tenter frame laboratory tester under conditions of simultaneous equibiaxial stretching and planar uniaxial stretching. The "tilted film" method was used to obtain trichroic spectra corresponding to the machine, transverse. and normal directions, as well the "structural factor" spectrum. From these it was possible to obtain information on the molecular orientation and the evolution of the crystalline structure. The starting films, prepared by melt casting from an extruder on a chilled roll, contained predominantly the mesomorphic P form. The structural factor spectra confirmed that strain-induced transformation into the a form occurred upon drawing, and that the amount of a form increased with the extent of drawing. The trichroic spectra showed that the molecular orientation was localized mainly, but not exclusively, in the a form. Orientation functions could be determined for both the molecular chain axis and the normal to the hydrogen-bonded sheets. For both the equibiaxial and planar uniaxial films, these sheets were found to be strongly oriented parallel to the plane of the film, with the degree of orientation increasing with overall draw ratio. For the biaxial samples, the molecular chain orientation was found to be equibiaxial, as expected. Mechanical test results indicated that the chains are evenly distributed in the film plane rather than showing a preference for the two orthogonal draw directions. For the planar uniaxial samples, the chain orientation was predominantly in the draw direction, but some degree of orientation in the transverse direction was also observed. The variation of orientation functions with draw ratio suggested that the a structure evolves in two stages, the first involving chain orientation in the draw direction and the second involving rotation of the sheets into the plane of the film.