Reactively extruded PVC/PMMA and PVC/PS polymer blends were investigated by 1H and 13C high‐resolution and 13C MAS NMR spectroscopy. To obtain blends, liquid solutions of monomer, initiator, and cross‐linking agent were absorbed into porous polyvinyl chloride (PVC) particles, forming dry blends; subsequently, the dry blends were fed into a twin‐screw extruder at 180°C activating in situ polymerization within the PVC matrix. Polymer and monomer fractions of resultant extruded blends were identified and characterized at the molecular level by nuclear magnetic resonance (NMR) spectroscopy, providing important insights into the microscopic details of the blends. NMR characterization includes: residual monomer content and its dispersion and site heterogeneity within the PVC matrix; effect of initial concentrations of monomer, initiator, and cross‐linker on the final products; and possible occurrences of copolymer grafting. NMR spectroscopy reveals efficient polymerization of methyl methacrylate (MMA) (ca 90% polymethyl methacrylate (PMMA)) and inefficient polymerization of styrene (less than 10% polystyrene (PS) with a significant fraction of unreacted monomer remaining entrapped in the PVC matrix) under the reactive extrusion of PVC/monomer (15 phr monomer) dry blends. Morphologically, the reactively extruded PVC/PMMA forms a single‐phase blend. In contrast, the PVC/PS forms a phase separated blend. Both the in situ polymerization efficiency and the phase behavior of the resulting blends are rationalized in terms of the affinity of the monomers in the initial dry blends, and of the resulting polymers in the final reactively extruded blends, to the PVC. This understanding can guide the design and control of properties of blends obtained through the reactive extrusion process and other in situ polymerization techniques. Copyright © 2007 John Wiley & Sons, Ltd.
1H solution NMR spectroscopy is used synergistically with 3D crystallographic structures to map experimentally significant hydrophobic interactions upon substrate binding in solution under thermodynamic equilibrium. Using saturation transfer difference spectroscopy (STD NMR), a comparison is made between wild‐type xylanase XT6 and its acid/base catalytic mutant E159Q – a non‐active, single‐heteroatom alteration that has been previously utilized to measure binding thermodynamics across a series of xylooligosaccharide–xylanase complexes [Zolotnitsky et al. (2004) Proc Natl Acad Sci USA 101, 11275–11280). In this study, performing STD NMR of one substrate screens binding interactions to two proteins, avoiding many disadvantages inherent to the technique and clearly revealing subtle changes in binding induced upon mutation of the catalytic Glu. To visualize and compare the binding epitopes of xylobiose–xylanase complexes, a ‘SASSY’ plot (saturation difference transfer spectroscopy) is used. Two extraordinarily strong, but previously unrecognized, non‐covalent interactions with H2–5 of xylobiose were observed in the wild‐type enzyme but not in the E159Q mutant. Based on the crystal structure, these interactions were assigned to tryptophan residues at the −1 subsite. The mutant selectively binds only the β–xylobiose anomer. The 1H solution NMR spectrum of a xylotriose–E159Q complex displays non‐uniform broadening of the NMR signals. Differential broadening provides a unique subsite assignment tool based on structural knowledge of face‐to‐face stacking with a conserved tyrosine residue at the +1 subsite. The results obtained herein by substrate‐observed NMR spectroscopy are discussed further in terms of methodological contributions and mechanistic understanding of substrate‐binding adjustments upon a charge change in the E159Q construct.
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