Effects of Short-Chain Branching on Morphology and Free Volume in Polyethylene under Hydrogen Pressurization Studied by ¹ H, ¹³ C, and ¹²⁹ Xe NMR

The effects of short-chain branching (SCB) on phase distribution, free volume formation, and structural stability in polyethylene under hydrogen pressurization were investigated using medium- and high-density polyethylene (MDPE and HDPE) samples (MDPE-M, MDPE-I, HDPE-G, and HDPE-D). Comprehensive nuclear magnetic resonance (NMR) techniques, including liquid-state ¹H/¹³C NMR and solid-state ¹H/¹²⁹Xe NMR under 250 psi hydrogen and xenon atmospheres, respectively, were employed to characterize SCB content, semicrystalline phase distribution, chain mobility, and free volume. SCB levels ranged from 1.94 to 2.66 branches per 1000 carbon atoms, with a free volume range of 0.297-0.533 cm³/g and a consistent free volume element diameter of ∼0.63 nm. Hexene branches correlated positively with free volume formation, whereas longer branches, such as heptene and octene, suppressed it. Increased SCB content and SCB length reduced crystallinity and chain mobility. Results support a mechanistic model in which randomly distributed SCBs enhance chain stiffness and hydrogen permeability into crystalline regions. In contrast, a lower SCB content allows greater chain flexibility and contraction under hydrogen pressure, limiting gas penetration into the crystalline phase. Hydrogen accumulation in the amorphous phase contributes to crystalline stabilization through resistive stress, yielding a higher failure strain strength under hydrogen pressure. These effects are attributed to hydrogen's smaller kinetic diameter, enabling access to pore networks inaccessible to air, such as nitrogen and oxygen.