Differences in Lamellae-Thickening Mechanism in Nascent and Solution-Crystallized Ultrahigh-Molecular-Weight Poly(ethylene-<i>alt</i>-CO)

The differences in the lamellae thickening mechanisms with temperature in nascent ultrahigh molecular weight poly(ethylene-alt-CO) (UHMWPK-E) and the corresponding solution-crystallized film are investigated. The nascent UHMWPK-E, synthesized using a Pd-based single-site catalytic system, essentially consists of single-isolated crystals, where chains having a molar mass approaching 5 million g/mol fold back and forth in the lamellae thickness of 6 nm. The same nascent polymer, when crystallized from solution, is likely to form stacked crystals, resulting in a solution-crystallized film. Even though the molecular characteristics of the nascent and the solution-crystallized samples are the same, there are significant differences in the lamellae-thickening mechanisms upon heating. The nascent single crystals of UHMWPK-E, achieved through the synthesis protocol, exhibit restricted primary thickening that requires cooperative motion of the whole chain after the solid–solid phase transition (from high-density orthorhombic to low-density orthorhombic phase), followed by melting and recrystallization at the onset of melting temperature. However, the stacked crystals in the solution-crystallized film demonstrate secondary thickening, facilitated by localized mobility between the adjacent stacked crystals, followed by melt recrystallization. This difference in lamellar thickening mechanisms has been systematically investigated using transmission electron microscopy, small-angle X-ray scattering/wide-angle X-ray diffraction, solid-state NMR, and dynamic mechanical thermal analysis. These findings highlight the structural evolution pathways in UHMWPK-E under thermal conditions prior to melting and emphasize the influence of initial crystalline morphology on thickening mechanisms in the same polymer.