Chain dynamics of halogenated precision polyethylenes in different crystal polymorphs

Abstract

We investigate the crystallinity and chain dynamics of two crystalline forms I and II of bromine-substituted polyethylene, where bromine is substituted on every 21st backbone carbon −[(CH2)20–CHBr]–. Form I exhibits an all-trans planar conformation, while form II adopts a zigzag, nonplanar, herringbone-like structure with gauche conformations around the CHBr group. Static 1H NMR revealed that form II has a slightly higher crystallinity compared to form I, illustrating minor crystallinity differences despite their distinct crystal structures. Both forms, however, achieve much lower crystallinity relative to typical polyethylene (PE), highlighting the pronounced effect of low Br substitution. This chain alteration also impedes chain diffusion on the time scale relevant for lamellar growth, a phenomenon commonly observed in semicrystalline PE. 13C–1H dipolar MAS NMR experiments indicate that form II’s gauche conformers stiffen adjacent CH2 chains, leading to lower motion amplitude in the crystalline chain stem compared to form I, which displays more uniform vibrational motion amplitude. Additionally, 13C T1-relaxation measurements reveal atypically short and distributed crystal-related T1 values in both forms. By using 1H crystal-phase filtering and spin diffusion into the near-surface crystalline stems, we detect the CH2 in the semicrystalline interphase to be biased toward short T1, highlighting faster local mobility down to the ns time scale close to the fold surface as compared to the core.