Stable polypropylene, PP - low molecular weight modifier systems were prepared in order to analyze the influence of the physical state of the amorphous phase on thermo-mechanical properties. The modifier was introduced into solidified material by diffusion to the amorphous phase regions. The change of state of stress of the transmitters connecting adjacent crystals caused by substantial increase of interlamellar distance was observed. The new state of stress of the molecular network of the amorphous phase induced respective change of the stress state of the crystalline component. As the effect the tensile yield stress value was substantially reduced, also the stress caused a decrease of melting temperature of PP crystals. The above proposed mechanism served to explain the changes of thermo-mechanical properties of the blends with identical composition prepared by blending in a molten state. During the solidification of the blend, the modifier molecules were exuded out of the growing crystals. The physical state of the molecular network of the amorphous phase was changed in a similar way as for the systems with infused modifier into previously solidified polymer matrix. The changes of mechanical properties were additionally correlated with the observed change of the melting temperature of the crystals. (C) 2015 Elsevier Ltd. All rights reserved.