## 1 Introduction  
Polypropylene (PP) is a commonly used plastic material composed of semi-crystalline polymer and exhibits easy processing, chemical resistance, high rigidity, light weight, and a reasonable, relatively low cost [1,2]. However, PP has some disadvantages, such as a large shrinkage ratio, great brittleness at low temperature, and low impact resistance, and thus its development is limited. There are two effective methods of improving the low impact resistance. One method is to modify the PP copolymer by adding ethylene polypropylene rubber (EPR) or multiple-side-chained ethylene groups to enhance the impact resistance of PP-based blends. The other method is to blend elastomer with PP, obtaining synergistic impact strength [3].  
The blending of polymers has gained much attention in both industrial and academic fields. A single specific polymer, or more than one polymer, can be added to polymer blends in order to acquire certain properties. Thermoplastic polyurethane (TPU) is a polymer composed of both a hard segment (HS) and a soft segment (SS). An HS consists of urethane groups, while a SS consists of a polyol that includes ester groups or ether groups. TPU is featured as having an elasticity of rubber as well as high mechanical properties of plastic materials [4; 5; 6]. Therefore, using TPU effectively improves the mechanical properties and impact strength [7].  
[PERSON] et al. indicated that TPU/PP blends with 80 wt% TPU and 20 wt% PP resulted in poor mechanical properties, and the tensile strength, elongation at break, and Young's modulus were 17.4 MPa, 7.0%, and 645.3 MPa, respectively. These results can be attributed to the immiscibility and incompatibility between PP and TPU, which led to an inhomogeneous phase with poor interaction [8]. [PERSON] et al. examined the mechanical properties of TPU/PP blends with various contents of TPU (10-90 wt%) and found that a U-shaped mechanical property curve formed when TPU content increased. The test results indicated that the tensile strength of blends was significantly dependent on the blending ratios. Moreover, due to a weak phase interaction and the disruption of TPU interchange hydrogen bonding, a minimum tensile strength was revealed with respect to blends with 60 wt% TPU and thus led to the propagation of cracks at the weak phase. In addition, the impact strength was obviously improved by a higher content of TPU, indicating that stress could be effectively transferred by a strong interface [9]. Therefore, the blending ratio of TPU and PP played a key role in the mechanical properties of the blends, such as tensile strength, elongation at break, Young's modulus, and impact strength.  
In this study, PP and modified impact-resistant polypropylene (MPP), the former of which is a homopolymer and the latter of which is an impact copolymer, were used as the polymer matrix. MPP was expected to reduce the amount of TPU since TPU is an expensive polymer material. In most references, the blending ratios were set at 80:20, 70:30, and 50:50 for PP and TPU, and the mechanical properties of the PP/TPU blends were further discussed. Our results indicated that the tensile properties of PP/TPU blends decreased as more TPU was added. Some problems resulted from the larger intrinsic structure of TPU and from interphase interaction and transformations [8], so the proportion of TPU was set at 10 wt%. In the meantime, the pure PP and pure MPP were used as control groups, and the effects of using TPU on the blending morphology, melting behavior, crystalline behavior, and mechanical properties of polymer blends were investigated.