¹³C NMR Investigation of Molecular Chain Diffusion in Semicrystalline Polyethylene. 1. Effect of Cross-Linking

Abstract

Solid-state molecular chain diffusion in linear high-density polyethylene (HDPE) is established as the dominant mechanism for the crystalline 13C longitudinal relaxation at 60 °C, confirming previous work of Schmidt-Rohr and Spiess. A progressive saturation NMR experiment was undertaken on several samples of HDPE, where different degrees of cross-linking were introduced by electron irradiation. A decrease in the rate of recovery of the crystalline signal was observed with an increase in cross-link density, as measured by gel fraction. As irradiation forms cross-links on the fold surfaces of the lamellae, this behavior cannot be explained through a conventional dipolar spin−lattice mechanism or 13C spin diffusion. Transport of magnetization via chain diffusion to the amorphous region, where it experiences an efficient relaxation process, is consistent with the relaxation data and the Overhauser enhancement. As the gel fraction increased from zero to 87%, the effective diffusion coefficients almost halved from the nonirradiated sample (0.033 nm2·s-1) to 0.018 nm2·s-1.

Anomalous diffusion of chains in semicrystalline ethylene polymers

“…2, the 13 C nuclear Overhauser enhancement factor ͑NOEF͒ for orthorhombic phase is yielded as NOEF= 1 + = 1.9, which is comparable to that obtained in PE. 22,23 As indicated previously, a large NOEF in the orthorhombic phase is an important experimental evidence for interphase chain diffusion. 22,23 Similarly, the large NOE observed in the present orthorhombic phase in EVA is also indicative of the chain diffusion between the crystalline and amorphous/interfacial regions.…”

“…21 In the lamellar orthorhombic crystalline regions, the chains take rigid all-trans conformation, and make 180°flip motion around their chain axes, accompanied by simultaneous translational hopping along the chain axis. [22][23][24][25][26] Hopping along the chain axis leads to the long-range chain diffusion between crystalline and amorphous/interfacial regions. The longrange chain diffusion was demonstrated by two-dimensional NMR spectrum and by nuclear Overhauser enhancement ͑NOE͒ measurements.…”

“…The longrange chain diffusion was demonstrated by two-dimensional NMR spectrum and by nuclear Overhauser enhancement ͑NOE͒ measurements. [22][23][24][25][26] Moreover, this long-distance chain diffusion is also responsible for the nonexponential decay behavior of the 13 C longitudinal magnetization: [22][23][24][25][26] the chain segments inside the crystallite migrate into the interfacial/amorphous regions where the 13 C longitudinal magnetization decays rapidly via an intrinsic spin-lattice relaxation ͑ 13 C spin-lattice relaxation time T 1 of amorphous/ interfacial regions is very short͒.…”

Morphological Behaviour of Thermoplastic Polyurethanes During Repeated Deformation

“…In support of this hypothesis, NMR studies by Schmidt-Rohr and Spiess, [46] Robertson et al [47] and Miyoshi et al [48] presented evidence of chain diffusion through polymer crystals. Moreover, van der Schuur and Gaymans [49] recently reported considerable differences in the mechanical behaviour of segmented block copolymers with similar compositions but containing different amounts of covalent crosslinks.…”

Diffusion of D2O in archaeological wood measured by 1-D NMR profiles

“…(1) using a nonlinear iterative algorithm [17]. However, at short diffusion times, the solution of the diffusion equation can be simplified:…”