Enhancing Mechanical Properties and Crystallization Kinetics of Isotactic Polypropylene by Boronic Acid-Cross-Linked Additives

We have found that isotactic polypropylene (iPP) significantly increases its mechanical properties by the addition of boronic acid-cross-linked iPP. The cross-linked iPP, bearing multiple boronic acids per polymer chain, was successfully prepared by the copolymerization of propylene and a boranediamine comonomer and the following treatment with acid. The melting point and crystallinity of the iPP did not show any changes by blending the cross-linked PP, whereas the crystallization rate was significantly accelerated and the fraction of b-axis oriented crystallite increased. These tendencies were similar to the effect of polar nucleating agent, indicating that boroxine functionality in the cross-linked polymer acts as a nucleating agent. SAXS measurement showed the presence of both lamellae of iPP crystal and network structure from the cross-linked PP, proposing that the presence of network structure contributed to the improvement of mechanical property. Our developed cross-linked iPP shows great promise as an additive to restore the mechanical properties lost during PP recycling.