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Dynamics and structure of semicrystalline polymers as characterized by NMR, and their relationship to macroscopic properties

Weiguo Hu, University of Massachusetts Amherst


In this dissertation the 180°-flip motion and morphology in polyethylene we studied by solid-state NMR. An explanation of the varying (ultra)drawability of semicrystalline polymers in terms of αc-relaxation chain mobility is proposed. Crystalline domains composed of “polyethylene-like” chains in soil organic matter are detected and characterized. In high-density polyethylene (HDPE) and in ultra-high molecular-weight polyethylene (UHMWPE) fibers, the 180° chain-flip motions are observed in terms of changes in the 13C-13C dipolar couplings. In the HDPE sample, the motion is observed directly via two-dimensional exchange spectroscopy, stimulated-echo decays, and 1D lineshape changes. The data yield an activation energy of 122 ± 10 kJ/mol. In the fibers, the narrowing of natural-abundance 13C-13C dipolar satellites indicates a flip rate of 1000/s at 360 K. An explanation of the varying (ultra)drawability of semicrystalline polymers is proposed, based on NMR evidence of αc-relaxation-associated helical jumps and chain diffusion through the crystallites of “α c-mobile” polymers. The chain motions provide a mechanism by which these polymers can be ultradrawn to draw ratios >30, without melting. Other semicrystalline polymers that lack a crystalline α-relaxation only have draw ratios of <14. The structure of ultradrawn UHMWPE fibers is investigated by solid-state NMR. A crystallinity of (88 ± 2)% was determined by traditional 1H NMR and a new adaptation of the 13C NMR direct-polarization method. 1H spin diffusion yields amorphous domain sizes of 10 ± 5 nm, and crystalline regions of 100 ± 50 nm diameter. A second, highly mobile, amorphous phase, making up (0.8 ± 0.2)% of the sample, was detected by 1H NMR. The fraction of partially mobile, oriented interfacial material or tie-molecules in the fiber was found to be ∼5%. Crystalline domains composed of poly(methylene) chains have been unambiguously detected by solid-state NMR and wide angle X-ray scattering in several samples of soil organic matter, including surface soil (peat), humic acids from surface soil and young coal, and humins. The crystallite thickness is deduced from the NMR-detected melting range and 1H spin diffusion experiments. The crystallites are expected to have long residence times in the soil and may be related to the formation of petroleum.

Subject Area

Materials science|Polymers|Biogeochemistry

Recommended Citation

Hu, Weiguo, "Dynamics and structure of semicrystalline polymers as characterized by NMR, and their relationship to macroscopic properties" (1999). Doctoral Dissertations Available from Proquest. AAI9950164.