after crystallizing and drawing the fiber spruiell et al do not expose the nucleated fiber to any heat which is necessary to impart the very best shrinkage properties therefore the shrinkage of their fibers was similar to conventional polypropylene fibers without a nucleating agent additive of particular interest and which has been determined to be of primary importance in the production of such inventive low shrink polypropylene fibers is the discovery that at the very least the presence of nucleating agent within heat set polypropylene fibers as discussed herein provides high long period measurements for the crystalline lamellae of the polypropylene itself this discovery is best explained by the following polymers when crystallized from a melt under dynamic temperature and stress conditions first supercool and then crystallize with the crystallization rate dependent on the number of nucleation sites and the growth rate of the polymer which are both in turn related to the thermal and mechanical working that the polymer is subjected to as it cools these processes are particularly complex in a normal fiber drawing line the results of this complex crystallization however can be measured using small angle x ray scattering saxs with the measured saxs long period representative of an average crystallization temperature a higher saxs long period corresponds to thicker lamellae which are the plate like polymer crystals characteristic of semi crystalline polymers like pp and which is evidenced by a saxs peak centered at a lower scattering angle than for comparative unnucleated polypropylene tape fibers.