(718h) Mesoscopic Structure of Semi-Crystalline Vitrimers: The Remarkable Case of Polyethylene

Polyethylene (PE) is the most common and important synthetic polymer in the world. Linear PE (a thermoplastic) may be converted into a covalently crosslinked network (a thermoset) to greatly enhance its properties critical for high-end applications (e.g., creep and chemical resistance). Crosslinked PE, however, cannot be reshaped or recycled. To both improve its properties and maintain processability, PE may be converted into a vitrimer, a novel class of polymers introduced in 2011 by Leibler et al. Vitrimers – which are permanently crosslinked and insoluble like thermosets, but flow when heated like thermoplastics – contain dynamic links and/or crosslinks that undergo exchange reactions. These covalent crosslinks maintain network connectivity, yet also allow the chain topology to fluctuate. For the first PE vitrimers, dynamic crosslinking occurs between dioxaborolane maleimide (DM) moieties that are grafted and crosslinked during extrusion. These dynamic crosslinks permit the polymer to simultaneously exhibit insolubility, greater melt strength, and even enhanced compatibility with other polymers, while still remaining processable. Although PE vitrimer is robust and versatile, two important questions regarding its morphology remain unanswered: (1) do the dynamic crosslinks disturb PE crystallization and (2) does the incompatibility between the nonpolar PE backbone and polar DM moiety induce micro-phase separation? To address these ambiguities, we systematically studied how conversion of PE to a vitrimer affects both the crystalline and amorphous microstructure. PE vitrimer composition was quantified by Fourier-transform infrared spectroscopy. Based on differential scanning calorimetry measurements, we found that crosslink density had little influence on the percent crystallinity of PE vitrimer, which starkly contrasts trends observed for conventionally crosslinked PE. Instead, the percent crystallinity decreases as the vitrimer gel fraction increases. Using a combination of transmission electron microscopy and synchrotron-sourced X-ray scattering (both wide- and small-angle), we determined that the DM moiety and dynamic crosslink concentrations had minimal effect on the PE crystal unit cell parameters and lamellae thickness, suggesting these components partition completely into the amorphous region. Furthermore, SAXS patterns revealed that this amorphous section contains structural heterogeneities strongly affected by vitrimer composition. Theoretical arguments, however, suggest that there is no micro-phase separated domains of ethylene or DM repeat units within the amorphous phase or above melting temperature. These surprising findings reveal the remarkable physics of vitrimers, justify the need for deeper fundamental understanding of these novel materials, and will direct future vitrimer design.