Electron paramagnetic resonance studies of molecular fracture in oriented polymers

Abstract

It is often assumed that the strength limiting process in the fracture of semicrystalline polymers is the rupture of tie molecules connecting the crystalline regions along the fiber axis. The strain and rupture behavior of a hydrocarbon chain is calculated by using a Morse potential and published force constants. The rupture stress is calculated to be 1630 kg/mm² on a chain of 19 Ų cross section at a strain of 13%. The model used assumes that the molecules are held rigidly in crystalline regions which occupy approximately 2/3 of the fibril long period as taken from published x-ray data. EPR techniques were used to take quantitative measurements of the number of fractured molecules versus the applied stress and strain in Nylon 6 fibers. These measurements were aided by stabilizing the free radical signal with chloranil as an additive in laboratory prepared fibers. The results obtained from other stabilizers, hydroquinone, duroquinone, and crystal violet, are described. The comparison of theory and experiment implies that in the low stress region (up to 60% of ultimate strength) the molecules are rigidly held which allows one to calculate the distribution of tie molecules having lengths close to ultimate elongation. The observed break in the rate of radical production versus stress and the decreasing modulus of the fiber at high applied stress indicates that the crystalline regions are breaking down and a large fraction of the tie molecules are slipping instead of breaking in this region.