Impact of Network Architecture and Cross-Link Aggregation on the Crystallization Behavior of a Polypropylene-Based Thermoset Polyurethane

Transforming polyolefin waste to form cross-linked networks is a promising way to access a wide range of properties and create methods of repurposing these materials in new applications. In this work, maleic anhydride-grafted polypropylene (PPgMAH) was hydroxylated to form hydroxylated PP (PPOH) and converted to a processable thermoset polyurethane (PU) with the use of a urethane exchange catalyst. The rheological behavior of the PU transitioned from that of a solid-like network to a viscoelastic liquid as the catalyst concentration increased. At all catalyst concentrations, the material was processable under conditions used in compression molding. Small-angle X-ray scattering demonstrated aggregation of the hard segments containing cross-links in the PP-derived PU, through observation of a correlation peak that persisted upon heating the system above the melting temperature (Tm) of the PP domains. The PP-rich domains underwent crystallization as the system was cooled below Tm, and while the choice of crystallization temperature (Tc) influenced lamellar dimensions (lamellar thickness, and long period), formation of the urethane network had no impact on these parameters. The PU exhibited a lower amorphous content and higher interlamellar spacing as compared to its PPgMAH and PPOH precursors. Despite the higher amorphous content in the PU, PP formed spherulites of similar size and nuclei density across all materials (PU, PPOH, PPgMAH) and choice of Tc. Crystallization kinetics of PP in the network were unchanged from those of the PPgMAH and PPOH, with the crystallization activation energy and nucleation constant being similar. The rate of nucleation and time required for crystal growth were also unchanged. These results thus point to isolated PP-rich domains, which are microphase separated from the urethane cross-links, and undergo crystallization processes consistent with neat PP.