Bottom-up Solution Synthesis of Graphene Nanoribbons with Precisely Engineered Nanopores

The incorporation of nanopores into graphene nanostructures has been demonstrated as an efficient tool in tuning their band gaps and electronic structures. However, precisely embedding the uniform nanopores into graphene nanoribbons (GNRs) at the atomic level remains underdeveloped especially for in-solution synthesis due to the lack of efficient synthetic strategies. Herein we report the first case of solution-synthesized porous GNR (pGNR) with a fully conjugated backbone via the efficient Scholl reaction of tailor-made polyphenylene precursor (P1) bearing pre-installed hexagonal nanopores. The resultant pGNR features periodic subnanometer pores with a uniform diameter of 0.6 nm and an adjacent-pores-distance of 1.7 nm. To solidify our design strategy, two porous model compounds (1 a, 1 b) containing the same pore size as the shortcuts of pGNR, are successfully synthesized. The chemical structure and photophysical properties of pGNR are investigated by various spectroscopic analyses. Notably, the embedded periodic nanopores largely reduce the π-conjugation degree and alleviate the inter-ribbon π–π interactions, compared to the nonporous GNRs with similar widths, affording pGNR with a notably enlarged band gap and enhanced liquid-phase processability

NMR experiments and molecular dynamics simulations of the segmental dynamics of polystyrene

abstractWe have performed NMR spin?lattice relaxation experiments and molecular dynamics (MD) computer simulations on atactic polystyrene (a-PS). The segmental correlation times of three different molecular weight a-PS (Mn = 1600, 2100, 10 900 g/mol) were extracted from NMR by measuring the 2H spin?lattice relaxation times (T1) over a broad temperature range (390?510 K). MD simulations of an a-PS melt of molecular weight 2200 g/mol were carried out at 475, 500, and 535 K. Comparisons between experiments and simulations show that the MD simulations reproduce both the shape of the P2(t) orientation autocorrelation function and its temperature dependence, while the simulated segmental correlation times are slower than experimental results by a factor of 1.8. If the simulations are rescaled by this factor, they reproduce both the experimental T1 values and the slight difference in dynamics between the backbone and side group of PS

Modeling of Differences Between Body Core and Forehead Temperatures Measured by Infrared Thermometers

In recent years, outbreaks of highly contagious diseases, like the Ebola virus, have motivated vigorous efforts to screen travelers entering the United States, especially at airports. Screening involves monitoring the body temperature of entering travelers, and blocking entry of those showing a fever, indicating a potential infection. Typically, screening is performed using commercially available non-contact infrared thermometers (NCITs). These thermometers require specific use protocols (e.g., working distances) to provide accurate results, which may not be followed by inspectors reluctant to approach potentially contagious travelers. Furthermore, the NCITs’ accuracy is based on an assumption that the NCIT readings from a forehead will predict the body core temperatures using a simple common one-size-fits-all correction offset. Unfortunately, the temperature detected on the forehead surface by an NCIT may not represent the true body core temperature, due to the changing conditions of the external environment and/or surface conditions of the forehead skin. It is not clear whether the correction factor is able to adjust to the thermal environment, or whether the surface condition of the forehead, including sweat and skin tone, affects the NCIT readings. Before a clinical study is conducted to understand the differences between the forehead temperatures and the body core temperatures, a computational model to simulate temperature distribution inside and on the surface of the body is a cost-effective way to identify factors that influence the temperatures and to study the reasons for their deviations. The objectives of this study were to 1) develop a numerical whole-body model and perform computational heat transfer simulations of different body geometries and 2) perform parametric studies to evaluate the effect of environmental factors, such as air temperature and heat transfer coefficient, on the differences between the forehead temperature and body core temperature. This data can be used to evaluate correction factors or needed to use the measured forehead temperature to predict the body core temperature.</jats:p

Warm water pathways, transports, and transformations in the northwestern North Atlantic and their modification by cold air outbreaks

This paper presents a numerical study of the warm water pathways, transports, and water mass transformation in the Newfoundland Basin region and an investigation of the rectification effects of a series of cold air outbreaks (CAOs) on the above processes. An initial mean state simulation was compared with observations and showed good agreement. Then, repeated CAO events were explicitly included in the surface fluxes and illustrated the following rectification effects. The thermal regime of the entire baroclinic layer was impacted, as the thermocline deepened and the temperature anomaly was noticeable down to ∼500 m. Different mechanisms were responsible for the propagation of the temperature anomaly at different stages. During the onset of the CAO, vertical diffusion propagated the temperature anomaly downward near the surface, then vertical advection further propagated the anomaly downward between CAO events to about 500 m depth. With CAOs, the North Atlantic Current carried more warm water, and its pathway in the Northwest Corner shifted towards the southeast. The volume-averaged mean and eddy kinetic energy increased by 30% and 20%, respectively, and the water mass transformation rate in the Newfoundland basin was doubled. The Gulf Stream carried more heat, but heat transport to the eastern basin decreased owing to the increase in heat release to the atmosphere

NMR Investigation of Segmental Dynamics in Disordered Styrene−Isoprene Tetrablock Copolymers

13C and 2H NMR relaxation time measurements were performed on four linear styrene−isoprene tetrablock copolymers (SISI) of overall molecular weight about 12 000 g/mol in order to characterize the segmental dynamics of both components. The two isoprene blocks are equal in length as are the two styrene blocks. A wide range of temperatures (from 285 to 530 K) and several compositions (volume fraction of styrene:  23%, 42%, 60%, and 80%) were investigated. Previous small-angle neutron scattering (SANS) results by Lodge et al. showed that these materials are homogeneous on length scales much larger than the radius of gyration. The segmental correlation times for both components in SISI were extracted by fitting the experimental data to the modified Kohlrausch−Williams−Watts (mKWW) orientation autocorrelation function and Vogel−Tammann−Fulcher (VTF) temperature dependence. While previous measurements found that the global dynamics in this system are quite homogeneous, these NMR measurements show that the segmental relaxation times of styrene and isoprene segments differ by orders of magnitude. At high temperatures, the dynamics of styrene segments are about 10 times slower than that of isoprene segments; this can be attributed to the difference in the intrinsic mobility of the two types of segments. The segmental dynamics of 5% polyisoprene (PI) tracers in one tetrablock matrix are very similar to that of the matrix isoprene segments

Comparison of the Composition and Temperature Dependences of Segmental and Terminal Dynamics in Polybutadiene/Poly(vinyl ethylene) Blends

13C NMR relaxation measurements were performed on two series of miscible polymer blends:  entangled polybutadiene (PB) in perdeuterated oligomeric poly(vinyl ethylene) and entangled poly(vinyl ethylene) (PVE) in perdeuterated oligomeric polybutadiene. A wide range of temperatures (∼ Tg + 50 K to Tg + 200 K) and several compositions (weight fractions of 0, 0.25, 0.5, 0.75, 1) were investigated. The segmental correlation times for the entangled components in both series of blends were extracted and compared to the corresponding terminal dynamics reported by Yang et al. The terminal dynamics have a stronger composition dependence than the segmental dynamics in PB/PVE blends. In contrast, a previous study of polyisoprene/poly(vinyl ethylene) blends revealed that the segmental and terminal dynamics exhibit equivalent dependences on the temperature and composition in that system. The Lodge/McLeish model satisfactorily fits the segmental correlation times in PB/PVE blends, but with the fit parameter φself different than the model prediction. We also found that the segmental dynamics of cis and trans units in the polybutadiene homopolymer have slightly different temperature dependences

Segmental Dynamics of Dilute Polystyrene Chains in Miscible Blends and Solutions

2H NMR relaxation time measurements have been used to investigate the segmental dynamics of polystyrene chains in six miscible polymer blends in the limit of low polystyrene concentration. These data are combined with previously reported results for the segmental dynamics of dilute polystyrene chains in four solvents. Polystyrene dynamics in these 10 hosts are strongly correlated with the glass transition temperature Tg of the host, but the dilute chain dynamics are not slaved to the host matrix dynamics. The Lodge−McLeish model, using a single value for the self-concentration φself = 0.35, reasonably describes polystyrene segmental dynamics in all 10 hosts and also describes previously reported dynamics of dilute polyisoprene chains in four miscible blends. The dilute polystyrene segmental dynamics can be more accurately described if φself is allowed to depend on blending partner, with the resulting variation of φself from 0.14 to 0.48