Origin, extent, and effect of molecular stress concentration in fibrillar structures

Electron spin resonance and infrared spectroscopy investigations of the deformation of oriented polymers have given insight into the loading and rupture of chain segments. Whereas there is agreement on the phenomenology of chain scission, fiber deformation, and stress relaxation, quite controversial interpretations have been given of the observed phenomena. Particularly disputed have been the extent of interfibrillar slip, the effect of chain scission on macroscopic stress‐strain behavior, and the cause of microcrack formation. Dealing with polyamide‐6 fibers it is shown that the microfibrils do not unload through slippage and that the lateral rigidity of the crystal blocks within the sandwich‐structured microfibrils must be large enough so as to permit the build‐up of large stress concentrations. The extent of molecular stress concentration is calculated. The qualitative agreement between the slopes of stress‐strain and free radical‐strain curves is analyzed. Unless this agreement is coincidental, it can be sought in an amplification effect accompanying the rapid release of stored elastic energy. The ensuing local temperature rise will facilitate the extension of kinked chains tinder annihilation of kinks. This is practically equivalent to unloading those chains without breaking them. The formation of submicrocracks is not related to the breakage of chains. The direct influence of submicrocracks as potential stress concentrators is weak and ineffective with regard to accelerating chain scission.