Effect of Noncrystallizable Comonomers in Polyethylene on Crystallization, Semicrystalline Morphology, Intracrystalline Dynamics, and Linear Mechanical Properties

An extended SAXS quantitative model incorporating a diffuse interface was developed in this study to analyze the SAXS data of polyethylene and its copolymers with varying comonomer contents. The structural characterization was performed on one high-density polyethylene (HDPE) and three ethylene copolymers with similar molecular weights and distributions. Results showed that while copolymers have a larger diffuse interface than HDPE, this parameter remains unaffected by the comonomer content or isothermal crystallization temperature. Time-resolved SAXS analysis revealed distinct structural characteristics: HDPE as a typical crystal-mobile polymer undergoes lamellar thickening during primary crystallization, while increased comonomer incorporation suppresses lamellar thickening and broadens amorphous layer distribution, tuning the morphology of the material to that of crystal-fixed ones. Dynamic mechanical measurements confirmed that with increasing comonomer content the α_c-relaxation is more and more suppressed, limiting the chain mobility in the crystals. Temperature-dependent AFM illustrates the effect of this suppression on morphology, with thick lamellae forming at high temperatures and thinner ones filling gaps at lower temperatures. This study underscores the strong link between short-chain branching, microstructure, and macroscopic properties of polyethylene-based materials.