Polymerization-induced ordering and liquid-crystalline ordered melts of semicrystalline homopolymers with dynamic properties.
Abstract
Melts of semicrystalline homopolymers are classically described as disordered random coils that crystallize into folded-chain lamellar nanocrystals. Here, using ultrahigh-molecular-weight polyethylene (UHMWPE) as a model system, we show that polymerization induces hierarchical ordering that evolves into liquid-crystalline ordered melts, two-dimensional self-assembled mesocrystals, and intelligent, photonic, and dynamic nanocomposites. Upon static melting, an unusual conformationally ordered orthorhombic phase is retained in the melts of less-crosslinked meso-granules and physically intra-crosslinked meso-fibrils of as-polymerized nascent powders up to a universal order-disorder transition temperature (TO-D) of approximately 190 °C, after which it transformed into a disorder globular melt. During dynamic shearing, a less-entangled columnar (hexagonal) liquid-crystalline phase forms at 153-176 °C, followed by chain explosion at TO-D and the formation of temperature-responsive, recoverable, non-equilibrium entangled networks above TO-D. Isothermal crystallization of these ordered melts generates macroscale and microscale 2D-ordered and randomly 3D organized nanofibrillar superstructures, which impart self-toughening behaviour and ultrahigh impact resistance to UHMWPE. In addition, shape-memory and transparent photonic sheets were fabricated through mesoscale 2D self-assembly from preserved, heterogeneously entangled, intercrosslinked ordered melts under pressure. We further developed self-reinforced monomaterial "eplastomers" with tensile responses ranging from elastomeric to plastic-like through complete nanofibrillation of only 5 wt% less-crosslinked mesoparticles in a polyolefin elastomer matrix and reversible exchange between 1D-ordered nanofibrillar networks and disordered 3D mesogranules. Together, these findings deepen the understanding of semicrystalline polymers and provide a foundation for ultrahigh-performance, 3D-printable intelligent and photonic crystals and sustainable monomaterial dynamic nanocomposites based on reversible entangled nanofibrillar networks.