A significant increase in the melting temperature of knotted fibers of poly(vinylidene fluoride) (PVDF) was detected by differential scanning calorimetry. The melting peak partially returned to the original peak after the fibers were unknotted. Knotted PVDF fibers were observed with an optical microscope at crossed-nicol conditions. The knotted portions of the fibers showed birefringence even above the melting temperature of the fibers before knotting. The dependence of the physical properties of PVDF under applied stress was estimated in order to investigate the influence of knotting. The fracture temperature of PVDF fibers increased with applied stress below 1 MPa and decreased above 10 MPa because the applied stress increased the melting temperature of PVDF crystals, but strong stress mechanically broke the fibers. The X-ray diffraction patterns of the PVDF fibers under different stress were divided into the peaks of - and -phase crystals and amorphous. The peak area of the -phase crystal increased and that of the amorphous decreased with applied stress. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013