Real-Time Micro-Raman Measurements on Stressed Polyethylene Fibers. 1. Strain Rate Effects and Molecular Stress Redistribution

The molecular deformation of ultrahigh-molecular-weight polyethylene monofilaments was monitored in real time by remote laser Raman microscopy (ReRaM). A shift to lower wavenumbers of the symmetric and asymmetric C−C stretching mode bands, was observed in agreement with previously published data. The stress or strain dependence of the Raman wavenumbers as a function of strain rate was also examined at room temperature. Furthermore, the broadening of the Raman bands as a function of applied stress or strain was investigated and compared with the wavenumber shift in each case. The Raman band shift for the asymmetric and symmetric C−C stretching modes per stress or strain was found to be only marginally affected for strain rates ranging from 10 -4 to 10 -3 s -1 . The onset of the observed dramatic decrease of the Raman wavenumbers as a function of stress or strain was directly related to the existence of two yield points. The first “yield” point at relatively low applied strain/stress is thought to be associated with the plastic flow of the amorphous part. The second “yield” point at high strain/stress is possibly associated with the initiation of microcracking, which leads to the ultimate failure of the fiber. Finally, the corresponding band broadening indicated the presence of a highly nonuniform stress field acting upon the molecules of the crystalline fraction that give rise to the broad symmetric C−C and asymmetric stretching modes. A number of structural models have been reviewed in the light of the Raman data presented here.