โ Scribed by Stephan A. Lehmann; A. Donald Meltzer; Hans Wolfgang Spiess
No coin nor oath required. For personal study only.
The microphase separation (MPS) in polyureas based on methylene diphenyl diisocyanate (MDI) hard segment, diethyltoluenediamine chain extender, and amino-terminated polypropylene glycol soft segment prepared by reaction injection molding (RIM) was studied by advanced solid-state NMR spectroscopy. Incomplete microphase separation leads to the presence of mobilized hard segments dispersed in the soft segment domains as well as immobilized soft segments residing in the hard domains. This is detected by 1 H-NMR spectra recorded under spinning at the magic angle (MAS) as well as two-dimensional wide-line separation (WISE) NMR spectra. The sizes of the various domains as well as the interfaces between them are quantified by spin diffusion measurements. In this way the impact of annealing, method of polymerization, and hard segment content on MPS is studied. Whereas annealing at temperatures up to 170ะC results in improving the MPS, major changes are observed after annealing at higher temperatures (190ะC), where the system changes from ''soft-inhard'' to ''hard-in-soft'' behavior. The MPS decreases with increasing hard segment content. The highest MPS is observed for solution polymerized samples. The various NMR experiments clearly reveal the nonequilibrium nature of RIM systems.
๐ SIMILAR VOLUMES
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A new type of crystal of glycylglycine (GlyGly) hydrate was crystallized from an aqueous solution, and the structure of the crystal has been determined by x-ray diffraction. The crystal is monoclinic, and the space group is C2/c, with the cell constants of a ร 15.941(2) A ห, b ร 4.774(2) A ห, c ร 19