Our study was focused on the presupposition that morphology control in immiscible polymer blend could give rise to reinforcement in composites. To investigate the effects of shear and elongational flow in polymer processing, observation of the mechanical properties and the morphology of the polypropylene/polycaprolactone (PP/PCL) blend system was performed. PP/PCL sheets were fabricated by means of a single-screw extruder equipped with a slit-type die to which high shear and elongational stresses were applied. For the sake of comparison, a second series of composites of identical composition was compression molded with a hot-press machine that transmits lower shear and elongational stresses. The results indicate that the extruded sheets have better mechanical properties than those of the compression-molded sheets, a result attributed to the generation of in situ dispersed long fiber minor phases and cocontinuous phases in the extruded composites. The differences in the crystallization behavior of the fibrous and spherically shaped components were indicated clearly by DSC curves. A PP crystalline peak indicative of in situ PP fiber formation is conspicuous around 980 cm(-1) (PP crystalline band) in the FTIR spectrum. (C) 2003 Wiley Periodicals, Inc.