at a heating rate extrapolated to 0 u00b0 k min u00b0 k 66 t m1 is the width of the first polyethylene melting endotherm u00b0 k u03b4h 0 is the melting enthalpy of u2014 ch 2 u2014 ch 2 u2014 taken as 8200 j mol the multi filament yarns of the invention are comprised of filaments having a u201c parameter of intrachain cooperativity of the melting process u201d v of at least 535 preferably at least 545 more preferably at least 555 and most preferably from 545 to 1100 a synchrotron is used as a source of high intensity x radiation the synchrotron x radiation is monochromatized and collimated a single filament is withdrawn from the yarn to be examined and is placed in the monochromatized and collimated x ray beam the x radiation scattered by the filament is detected by electronic or photographic means with the filament at room temperature u02dc 23 u00b0 c and under no external load the position and intensity of the 002 reflection of the orthorhombic polyethylene crystals are recorded if upon scanning across the 002 reflection the slope of scattered intensity versus scattering angle changes from positive to negative twice i e if two peaks are seen in the 002 reflection then two orthorhombic crystalline phases exist within the fiber the following examples are presented to provide a more complete understanding of the invention the specific techniques conditions materials proportions and reported data set forth to illustrate the principles of the invention are exemplary and should not be construed as.