The morphology changes of PB due to different reaction conditions supported the evolution of pore characteristics. Besides, there were many small solid particles on the surface of PB, which was due to the accumulation of ash in poplar with the increase of pyrolysis temperatures. Fig. 6 (b) showed the SEM images of the impact fractured surfaces of the PB/HDPE composites. As expected, the molten HDPE had infiltrated in to the pores of PB particles in the composites, and a physical/mechanical interlocking [41] was formed from Fig. 6 (b), which is so different from fiber-polymers composites: the fibers particles are distributed in matrix and covered by it [42,43]. SEM image of PB-50 wt% showed a smooth and uniform interface, which revealed a good dispersion, and this is the cause for enhanced flexural strength and the best of the composites with 50 wt% PB content. HDPE not only embedded into the pores of PB but also held the PB particles tightly as the binder. It should also be noted from SEM image of PB-70 wt % that although interlocking structure could be visually observed, the amount of HDPE embedded in the pores of PB was very small, which meant that the interlocking of composites and bonding effect of HDPE were greatly weakened. Thus, the mechanical strength of PB/HDPE composites decreased sharply with higher concentrations of PB i.e. 70 wt% amount of PB (Fig. 5).