Molecular changes on drawing isotopic blends of polyethylene and ethylene copolymers: 1. Static and time-resolved sans studies

Selective deuteration along a polyethylene chain: Differentiating conformation segment by segment

To improve the circularity and performance of polyolefin materials, recent innovations have enabled the synthesis of polyolefins with new structural features such as cleavable breakpoints, functional chain ends, and unique comonomers. As new polyolefin structures become synthetically accessible, fundamental understanding of the effects of structural features on polymer (re)processing and mechanical performance is increasingly important. While bulk material properties are readily measured through conventional thermal or mechanical techniques, selective measurement of local material properties near structural defects is a major characterization challenge. Here, we synthesized a series of polyethylenes with selectively deuterated segments using a polyhomologation approach and employed vibrational spectroscopy to evaluate crystallization and melting of chain segments near features of interest (e.g., end groups, chain centers, and mid-chain structural defects). Chain-end functionality and defects were observed to strongly influence crystallinity of adjacent deuterated chain segments. Additionally, chain-end crystallinity was observed to have different molar mass dependence than mid-chain crystallinity. The synthesis and spectroscopy techniques demonstrated here can be applied to range of previously inaccessible deuterated polyethylene structures to provide direct insight into local crystallization behavior.

Structural changes and chain radius of gyration in cold-drawn polyethylene after annealing: small- and wide-angle X-ray scattering and small-angle neutron scattering studies

Samples made from linear polyethylene were drawn at room temperature and subsequently annealed at high temperatures below the melting point. The structural changes of the crystalline lamellae and lamellar superstructures as well as the single chain radius of gyration were studied by means of combined small- and wide-angle X-ray scattering and small-angle neutron scattering (SANS). After drawing, the polymeric chain segments in the crystalline phase are preferentially oriented along the drawing direction with a high degree of orientation whereas the lamellae in the samples are found to be slightly sheared exhibiting oblique surfaces as evidenced by X-ray scattering. SANS indicates that the chains are highly elongated along the drawing direction. Annealing the deformed samples at temperatures where the mechanical alpha-process of polyethylene is active leads to a thickening of both crystalline lamellae and amorphous layers. The chains in the crystalline phase retain their high degree of orientation after annealing while the lamellae are sheared to a larger extent. In addition, there is also lateral growth of the crystalline lamellae during high-temperature annealing. Despite the structural changes of the crystalline and amorphous regions, there is no evidence for global chain relaxation. The global anisotropic shape of the chains is preserved even after prolonged annealing at high temperatures. The results indicate that the mobility of polyethylene chains-as seen, e.g., by 13C NMR-is a local phenomenon. The results also yield new insight into mechanical properties of drawn PE, especially regarding stress relaxation and creep mechanisms.

Mixed crystal infrared spectroscopy of uniaxially drawn polyethylene films

Blends of fully deuterated polyethylene with unlabeled polymer have been studied using Fourier transform infrared (FT-IR) spectroscopy. Melt quenched samples were drawn at 1 mm min(-1), clamped, and cooled with liquid nitrogen. The CD(2) bending vibration is sensitive to the local molecular arrangement. Changes in band components were related to (1) crystallization of amorphous polymer; (2) the untwisting of spherulite ribbons perpendicular to the draw direction; and (3) coarse slip. Measurement of remaining doublet splittings enabled estimates of surviving crystallite block sizes to be made. In addition, CH(2) wagging and rocking bands showed evidence of increasingly defective crystals and of a Martensitic transformation to the monoclinic crystal form.