Effects of molecular weight on the drawing and the draw efficiency of nylon-6

Tensile drawing was carried out on nylon-6 films of different viscosity average molecular weights (Mv=77 and 440×103) and crystalline forms (α- and γ-forms). The effects of Mv on the deformation behavior and the draw efficiency were discussed on the basis of the stress/strain curves recorded upon hot drawing, and wide-angle and small-angle X-ray diffraction, birefringence and tensile properties for the draw ratio (DR) series. The draw stress at a given strain and the fracture stress upon hot drawing were both higher for the higher Mv than for the low Mv, in either the α- or the γ-form sample. For a given Mv, the draw stress was lower for the γ-form than one with the α-form and the fracture stress was independent of the crystal forms, leading the former crystalline form to have a higher drawability than the latter, as previously reported. The Mv and the crystal form of predrawn samples had no effect on the crystalline chain orientation function (fc) measured as a function of DR. In contrast, the draw efficiency evaluated from the amorphous chain orientation function (fa) vs. DR and the tensile modulus and strength vs. DR was significantly higher for the higher Mv than for the low Mv. These drawing characteristics were discussed in terms of the entanglement effect and the deformability of the two crystal forms. The maximum achieved tensile modulus of 7.7 GPa and a strength of 760 MPa were thus obtained by the straight tensile drawing of the higher Mv sample that was initially a γ-form.