Mechanical relaxation of single crystal mats of polyethylene in a crystalline dispersion region

Stress-relaxation measurements have been conducted over the temperature range 15°–105°C on mats of single crystals of two linear polyethylene fractions. The single crystals were grown isothermally at several different temperatures. The relaxation modulus was observed to be strain dependent, indicating that the single crystal mats exhibited nonlinearity.

In spite of this appearance of nonlinearity, it was found possible, when the relaxation modulus was extrapolated to zero strain by an appropriate method, to obtain correct relaxation spectra for the mats of single crystals prepared by isothermal crystallization. This spectrum was then used to calculate the dynamic viscoelastic functions and, for the unannealed sample crystallized at 80°C, good agreement was found between experimental results and calculated ones. Two annealed samples showed multiple absorptions and, under these circumstances, strict application of the method of reduced variables for time and temperature was impractical.

The effect of molecular weight on the intensity of the relaxation spectrum was investigated. It was found that the single crystal with the higher molecular weight showed an increased spectrum intensity.

Observations were also made of the effect of increased lamellar thickness on both the relaxation spectra and the dynamic complex moduli and the results obtained on the fractions studied were compared with prior studies in whole polymer.