Measurement and interpretation of nonlinear mechanical energy losses in high polymer fibers

Dynamic mechanical loss measurements were made on fibers at large tensile strains which caused nonlinear viscoelastic behavior. Measurements on fibers from polyethylene, polypropylene, nylon 66, nylon 4 and an experimental polymer led to seven energy loss peaks for each sample in the temperature range of 120–350°K. The peaks were evenly spaced in temperature at intervals of 30–35°K. rather than at unequal temperature intervals of approximately 100–150°K. normally observed under conditions of linear viscoelastic behavior. In every case, the array of evenly spaced peaks occurred only at temperatures below the glass transition temperature. The temperatures at the energy loss peaks were virtually independent of crystallinity and molecular orientation and were interpreted in terms of polymer molecular structure. The data could be explained only by a single mechanism, common to all polymers, which could operate in a quantized manner, e. g., diffusional motion of molecular chain segments. To account for the constant temperature spacing between peaks of a given sample, it was necessary to assume that the rate controlling step in the energy loss process is the return of a displaced segment to equilibrium. Calculations from the experimental data indicated that peaks at higher temperatures stem from displaced molecular segments which experience high potential energy barriers and which have to be excited to higher skeletal vibrational energies to overcome the barrier. Precedence for this interpretation is provided by Tanaka and Ishida, who have associated molecular vibrations with the well‐known β loss peaks in polymers.