Glass transition dynamics in a compatible blend by two-dimensional solid-state NMR

Spatio-temporal heterogeneities in nanosegregated single-molecule polymeric nanoparticles

The study of the coupling between structural and dynamical heterogeneities in nanostructured systems is essential for the design of hybrid materials with the desired properties. Here, we use atomistic molecular dynamics simulations to closely examine the dynamical heterogeneities in nanostructured single-molecule nanoparticles consisting of mikto-arm star copolymers with poly(ethylene oxide), PEO, and polystyrene, PS, arms. The particles exhibit an internally nanostructured morphology, resembling either "Janus-like" or "patchy-like" morphology when the functionality of the stars varies. The differences in the local environment result in strong intramolecular dynamical heterogeneities. In the proximity of the star core, geometric constraints promote unfavorable PEO:PS contacts that lead to a behavior similar to dynamically asymmetric miscible polymer blends or disordered copolymers. In contrast, further away from the core, the nanosegregation induces segmental dynamics very similar to the one found in the homopolymer star analogues.

Dynamic heterogeneity in fully miscible blends of polystyrene with oligostyrene

Binary blends of polystyrene with oligostyrene are perfectly miscible (χ=0) yet dynamically heterogeneous. This is evidenced by independent probing of the dipole relaxation perpendicular to the backbone by dielectric spectroscopy and molecular dynamics. The self-concentration model with a single intramolecular length scale qualitatively describes the slower segmental dynamics. A quantitative comparison based on MD, however, requires a composition-dependent length scale. The pertinent dynamic length scale that best describes the slow segmental dynamics in miscible blends relates to both intra- and intermolecular contributions.

1H and 13C solution- and solid-state NMR investigation into wax products from the Fischer-Tropsch process

(1)H and (13)C solid- and solution-state NMR have been used to characterise waxes produced in the Fischer-Tropsch reaction, using Co-based catalysts either unpromoted or promoted with approximately 1 wt% of either cerium or rhenium. The aim was to measure average structural information at the submolecular level of the hydrocarbon waxes produced, along with identification of the minor products, such as oxygenates and olefins, which are typically observed in these waxes. A parameter of key interest is the average number of carbon atoms within the hydrocarbon chain (N(C)). A wax prepared using an unpromoted Co/Al(2)O(3) catalyst had N(C)∼20, whilst waxes made using rhenium- or cerium-promoted Co/Al(2)O(3) catalysts were found to have N(C)∼21. All three samples contained small amounts of oxygenates and alkenes. The subtle differences found in the waxes, in particular the minor species produced, demonstrate that the different promoters have different effects during the reaction, with the Re-promoted catalyst producing the fewest by-products. It is shown in (13)C solid-state NMR spectra that for that for longer chain (compared to the lengths of chain in previous studies) waxes that the lack of resolution and the complexities added by the differential cross-polarisation (CP) dynamics mean that it is difficult to accurately determine N(C) from this approach. However the N(C) determined by (13)C CP magic angle spinning NMR is broadly consistent with the more accurate solution approaches used and suggest that the wax characteristics do not change in solution. On this basis an alternative approach for determining N(C) is suggested based on (1)H solution state NMR that provides a higher degree of accuracy of the chain length as well as information on the minor constituents.

Solid state 2H-NMR studies of segmental dynamics in polymer blends

Recent two-dimensional (2D) 2H-NMR studies on nearly ideal mixtures are reviewed. The use of selective deuterium labeling allows the unambiguous observation of the individual segmental dynamics of each component in the blend. 2D exchange spectra provide both the mean motional rates and the motional distributions, which was necessary to discriminate among the disparate explanations of blend behavior. The components exhibit very different mean mobilities and broad mobility distributions near the glass transition. The broad macroscopic glass transition in the blends is attributed to both these two kinds of dynamic heterogeneities. The individual mean motional rates are used to define distinct glass transition temperatures, T(g,i) for each species, i. The separation between the T(g,i),s of the two species increases with the content of the high-T(g) component. The widths of their individual motional distributions also increase with the content of the high-T(g) component. These two effects combine to produce the increase in both the anomalously broad glass transition and the thermorheological complexity of these blends with fraction of the high-T(g) component.