Slow motion in [ring-fluoro]polycarbonate by CODEX

Molecular basis of secondary relaxation in stiff-chain glassy polymers

Recent progress in establishing local order in polycarbonate-like glasses using rotational echo double resonance and centerband-only detection of exchange solid-state nuclear magnetic resonance (NMR) has stimulated a renewed attempt to connect molecular motion within glassy polymers and the mechanical properties of the glass. We have in fact established a correlation between molecular motion characterized by NMR and the mechanical secondary relaxation (tan δ) for nine polycarbonate-like glasses. All of the NMR and mechanical data are for T ≪ Tg. The resulting structural insights suggest that the chains of these polymers are simultaneously both Flory random coils and Vol'kenstein bundles. The cooperative motions of groups of bundles can be described qualitatively by a variety of constrained-kinetics models of the glass. All of the models share a common trait for large-amplitude motion: an exponential increase in the time required for an inter-bundle dilation event with a linear increase in bundle group size. This dependence and a locally ordered Vol'kenstein bundle lead to an understanding of the surprising 60° (K) shift of tan δ to higher temperature for ring-fluoro-polycarbonate relative to that of polycarbonate by the apparently minor substitution of a fluorine for a hydrogen on every fourth ring.

Investigation of slow molecular dynamics using R-CODEX

A solid state NMR experiment is introduced for probing motions on the millisecond time scale, based on dephasing and refocusing (1)H-(13)C or (1)H-(15)N dipolar couplings. The method is related to the previously described Centerband-Only Detection of Exchange or CODEX experiment. The use of an R-type dipolar recoupling sequence takes advantage of the strong (1)H-(13)C or (1)H-(15)N dipolar coupling, while suppressing the effect of (1)H-(1)H homonuclear coupling. This approach paves the way to detect both the correlation time and reorientational angle of the dynamics in fully protonated samples. The performance of this pulse sequence is demonstrated using imidazole methyl sulfonate.

Chain packing in polycarbonate glasses

Chain packing in homogeneous blends of carbonate (13)C-labeled bisphenol A polycarbonate with either (i) CF(3)-labeled bisphenol A polycarbonate or (ii) ring-F-labeled bisphenol A polycarbonate has been characterized using (13)C{(19)F} rotational-echo double-resonance (REDOR) nuclear magnetic resonance. In both blends, the (13)C observed spin was at high concentration, and the (19)F dephasing or probe spin was at low concentration. In this situation, an analysis in terms of a distribution of isolated heteronuclear pairs of spins is valid. Nearest-neighbor separation of (13)C and (19)F labels was determined by accurately mapping the initial dipolar evolution using a shifted-pulse version of REDOR. Based on the results of this experiment, the average distance from a ring-fluorine to the nearest (13)C=O is more than 1.2 A greater than the corresponding CF(3)-(13)C=O distance. Next-nearest and more-distant-neighbor separations of labels were measured in a 416-rotor-cycle constant-time version of REDOR for both blends. Statistically significant local order was established for the nearest-neighbor labels in the methyl-labeled blend. These interchain packing results are in qualitative agreement with predictions based on coarse-grained simulations of a specially adapted model for bisphenol A polycarbonate. The model itself has been previously used to determine static and dynamic properties of polycarbonate with results in good agreement with those from rheological and neutron scattering experiments.

Staphylococcus aureus peptidoglycan tertiary structure from carbon-13 spin diffusion

The cell-wall peptidoglycan of Staphylococcus aureus is a heterogeneous, highly cross-linked polymer of unknown tertiary structure. We have partially characterized this structure by measuring spin diffusion from (13)C labels in pentaglycyl cross-linking segments to natural-abundance (13)C in the surrounding intact cell walls. The measurements were performed using a version of centerband-only detection of exchange (CODEX). The cell walls were isolated from S. aureus grown in media containing [1-(13)C]glycine. The CODEX spin diffusion rates established that the pentaglycyl bridge of one peptidoglycan repeat unit of S. aureus is within 5 A of the glycan chain of another repeat unit. This surprising proximity is interpreted in terms of a model for the peptidoglycan lattice in which all peptide stems in a plane perpendicular to the glycan mainchain are parallel to one another.