Addressing the challenges of modeling the scattering from bottlebrush polymers in solution

Small‐angle scattering measurements of complex macromolecules in solution are used to establish relationships between chemical structure and conformational properties. Interpretation of the scattering data requires an inverse approach where a model is chosen and the simulated scattering intensity from that model is iterated to match the experimental scattering intensity. This raises challenges in the case where the model is an imperfect approximation of the underlying structure, or where there are significant correlations between model parameters. We examine three bottlebrush polymers (consisting of polynorbornene backbone and polystyrene side chains) in a good solvent using a model commonly applied to this class of polymers: the flexible cylinder model. Applying a series of constrained Monte‐Carlo Markov Chain analyses demonstrates the severity of the correlations between key parameters and the presence of multiple close minima in the goodness of fit space. We demonstrate that a shape‐agnostic model can fit the scattering with significantly reduced parameter correlations and less potential for complex, multimodal parameter spaces. We provide recommendations to improve the analysis of complex macromolecules in solution, highlighting the value of Bayesian methods. This approach provides richer information for understanding parameter sensitivity compared to methods which produce a single, best fit

Addressing the challenges of modeling the scattering from bottlebrush polymers in solution

Small‐angle scattering measurements of complex macromolecules in solution are used to establish relationships between chemical structure and conformational properties. Interpretation of the scattering data requires an inverse approach where a model is chosen and the simulated scattering intensity from that model is iterated to match the experimental scattering intensity. This raises challenges in the case where the model is an imperfect approximation of the underlying structure, or where there are significant correlations between model parameters. We examine three bottlebrush polymers (consisting of polynorbornene backbone and polystyrene side chains) in a good solvent using a model commonly applied to this class of polymers: the flexible cylinder model. Applying a series of constrained Monte‐Carlo Markov Chain analyses demonstrates the severity of the correlations between key parameters and the presence of multiple close minima in the goodness of fit space. We demonstrate that a shape‐agnostic model can fit the scattering with significantly reduced parameter correlations and less potential for complex, multimodal parameter spaces. We provide recommendations to improve the analysis of complex macromolecules in solution, highlighting the value of Bayesian methods. This approach provides richer information for understanding parameter sensitivity compared to methods which produce a single, best fit

Concentration Dependence of the Size and Symmetry of a Bottlebrush Polymer in a Good Solvent

Bottlebrush polymers consist of a linear backbone with densely grafted side chains which impact the rigidity of the molecule. The persistence length of the bottlebrush backbone in solution is influenced by both the intrinsic structure of the polymer and the local environment, such as the solvent quality and concentration. Increasing the concentration reduces the overall size of the molecule because of the reduction in backbone stiffness. In this study, we map out the size of a bottlebrush polymer as a function of concentration for a single backbone length. Small-angle neutron scattering measurements are conducted on a polynorbornene-based bottlebrush with polystyrene side chains in a good solvent. The data are fit using a model which provides both the long and short axis radius of gyration (R_(g,2) and R_(g,1), respectively), providing a measure for how the conformation changes as a function of concentration. At low concentrations, a highly anisotropic structure is observed (R_(g,2)/R_(g,1) ≈ 4), becoming more isotropic at higher concentrations (R_(g,2)/R_(g,1) ≈ 1.5). The concentration scaling for both R_(g,2) and the overall Rg is evaluated and compared with predictions in the literature. Coarse-grained molecular dynamics simulations were also conducted to probe the impact of concentration on bottlebrush conformation, showing qualitative agreement with the experimental results

Concentration Dependence of the Size and Symmetry of a Bottlebrush Polymer in a Good Solvent

Bottlebrush polymers consist of a linear backbone with densely grafted side chains which impact the rigidity of the molecule. The persistence length of the bottlebrush backbone in solution is influenced by both the intrinsic structure of the polymer and the local environment, such as the solvent quality and concentration. Increasing the concentration reduces the overall size of the molecule because of the reduction in backbone stiffness. In this study, we map out the size of a bottlebrush polymer as a function of concentration for a single backbone length. Small-angle neutron scattering measurements are conducted on a polynorbornene-based bottlebrush with polystyrene side chains in a good solvent. The data are fit using a model which provides both the long and short axis radius of gyration (R_(g,2) and R_(g,1), respectively), providing a measure for how the conformation changes as a function of concentration. At low concentrations, a highly anisotropic structure is observed (R_(g,2)/R_(g,1) ≈ 4), becoming more isotropic at higher concentrations (R_(g,2)/R_(g,1) ≈ 1.5). The concentration scaling for both R_(g,2) and the overall Rg is evaluated and compared with predictions in the literature. Coarse-grained molecular dynamics simulations were also conducted to probe the impact of concentration on bottlebrush conformation, showing qualitative agreement with the experimental results

Confinement and Processing Can Alter the Morphology and Periodicity of Bottlebrush Block Copolymers in Thin Films

Bottlebrush block copolymers (BBCPs) are intriguing architectural variations on linear BCPs with highly tunable structure. Confinement can have a significant impact on polymer assembly, giving rise to changes in morphology, assembly kinetics, and properties like the glass transition. Given that confinement leads to significant changes in the persistence length of bottlebrush homopolymers, it is reasonable to expect that BBCPs will see significant changes in their structure and periodicity relative to the bulk morphology. Understanding how confinement influences assembly will be important for designing BBCPs for thin film applications including membranes, integrated photonic structures, and potentially BCP lithography. In order to study the effects of confinement on BBCP conformation and morphology, a blade coating was used to prepare films with continuous variation in film thickness. Unlike thin films of linear BCPs, islands/holes were not observed, and instead mixtures of parallel and perpendicular morphologies emerge after annealing. The lamellar periodicity (L₀) of the morphologies is found to be thickness dependent, increasing L₀ with decreasing film thickness for blade coated films. Films coated out of tetrahydrofuran (THF) resulted in a single well-defined lamellar periodicity, verified through atomic force microscopy (AFM) and grazing incidence small-angle X-ray scattering (GISAXS), which increases dramatically from the bulk value (30.6 nm) and continues to increase as the film thickness decreases. The largest observed L₀ was 65.5 nm, and this closely approaches the estimated upper limit of 67 nm corresponding to a fully extended backbone in a bilayer arrangement. Films coated out of propylene glycol methyl ether acetate (PGMEA) resulted in a mixture of perpendicular lamellae and a smaller, likely cylindrical morphology. The lamellar portion of the film shows the same thickness dependence as the lamellae observed in the THF coated films. The scaling of the lamellar L₀ with respect to film thickness follows predictions for confined semiflexible polymers with weak excluded volume interactions and can be related to models for confinement of DNA. Spin coated films shows the same reduction in periodicity, although at very different film thicknesses. This result suggests that the material has shallow free-energy barriers to transitioning between different L₀ and morphologies, a property that could be taken advantage of for patterning diverse structures with a single material

Economic Feasibility of a New Method to Estimate Mortality in Crisis-Affected and Resource-Poor Settings

INTRODUCTION: Mortality data provide essential evidence on the health status of populations in crisis-affected and resource-poor settings and to guide and assess relief operations. Retrospective surveys are commonly used to collect mortality data in such populations, but require substantial resources and have important methodological limitations. We evaluated the feasibility of an alternative method for rapidly quantifying mortality (the informant method). The study objective was to assess the economic feasibility of the informant method. METHODS: The informant method captures deaths through an exhaustive search for all deaths occurring in a population over a defined and recent recall period, using key community informants and next-of-kin of decedents. Between July and October 2008, we implemented and evaluated the informant method in: Kabul, Afghanistan; Mae La camp for Karen refugees, Thai-Burma border; Chiradzulu District, Malawi; and Lugufu and Mtabila refugee camps, Tanzania. We documented the time and cost inputs for the informant method in each site, and compared these with projections for hypothetical retrospective mortality surveys implemented in the same site with a 6 month recall period and with a 30 day recall period. FINDINGS: The informant method was estimated to require an average of 29% less time inputs and 33% less monetary inputs across all four study sites when compared with retrospective surveys with a 6 month recall period, and 88% less time inputs and 86% less monetary inputs when compared with retrospective surveys with a 1 month recall period. Verbal autopsy questionnaires were feasible and efficient, constituting only 4% of total person-time for the informant method's implementation in Chiradzulu District. CONCLUSIONS: The informant method requires fewer resources and incurs less respondent burden. The method's generally impressive feasibility and the near real-time mortality data it provides warrant further work to develop the method given the importance of mortality measurement in such settings

Self-Assembly of ABC Bottlebrush Triblock Terpolymers with Evidence for Looped Backbone Conformations

Bottlebrush block copolymers offer rich opportunities for the design of complex hierarchical materials. As consequences of the densely grafted molecular architecture, bottlebrush polymers can adopt highly extended backbone conformations and exhibit unique physical properties. A recent report has described the unusual phase behavior of ABC bottlebrush triblock terpolymers bearing grafted poly(dl-lactide) (PLA), polystyrene (PS), and poly(ethylene oxide) (PEO) blocks (LSO). In this work, a combination of resonant soft X-ray reflectivity (RSoXR), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and self-consistent field theory (SCFT) was used to provide insight into the phase behavior of LSO and underlying backbone chain conformations. Consistent with SCFT calculations, RSoXR measurements confirm a unique mesoscopic ACBC domain connectivity and decreasing lamellar periods (d0) with increasing backbone length of the PEO block. RSoXR and NEXAFS demonstrate an additional unusual feature of brush LSO thin films: when the overall film thickness is ∼3.25d0, the film–air interface is majority PS (>80%). Because PS is the midblock, the triblocks must adopt looping configurations at the surface, despite the preference for the backbone to be extended. This result is supported by backbone concentrations calculated through SCFT, which suggest that looping midblocks are present throughout the film. Collectively, this work provides evidence for the flexibility of the bottlebrush backbone and the consequences of low-χ block copolymer design. We propose that PEO blocks localize at the PS/PLA domain interfaces to screen the highest χ contacts in the system, driving the formation of loops. These insights introduce a potential route to overcome the intrinsic penalties to interfacial curvature imposed by the bottlebrush architecture, enabling the design of unique self-assembled materials

FACTORS ASSOCIATED WITH TB/HIV CO-INFECTION AMONG DRUG SENSITIVE TUBERCULOSIS PATIENTS MANAGED IN A SECONDARY HEALTH FACILITY IN LAGOS, NIGERIA

Background: This study assessed factors associated with TB/HIV co-infection among TB patients managed in a secondary health facility in Lagos Nigeria. Materials and Methods: A retrospective review of treatment cards of patients seen at a secondary referral hospital between January 1 2014 and December 31 2014 was conducted. Treatment outcomes and factors associated with TB/HIV co-infection were assessed. Results: Of the 334 records of patients reviewed, the proportion of patients with TB/HIV co-infection was 21.6%. The odds of having TB/HIV co-infection was 2.7 times higher among patients above 40 years than patients less than 25 years (AOR 2.7 95% CI 1.1 – 6.5, p =0.030). In addition, the odds of having TB/HIV co-infection was 3.3 higher among extrapulmonary TB cases (AOR 3.3; 95% CI 1.2 – 9.5; p = 0.026) and 2.1 times higher among retreated patients (AOR 2.1; 95% CI 1.1 – 3.9; p = 0.017) than pulmonary TB and new patients respectively. The chance of having TB/HIV co-infection was 2.7-fold more in patients with poor treatment outcomes than patients with treatment success (AOR 2.7; 95%CI 1.3 – 5.4; p =0.006). Conclusion: TB/HIV co-infection rate was high in the study area. There is need to put measures in place to improve treatment outcomes of TB/HIV co-infected patients

Nomenclature- and Database-Compatible Names for the Two Ebola Virus Variants that Emerged in Guinea and the Democratic Republic of the Congo in 2014

In 2014, Ebola virus (EBOV) was identified as the etiological agent of a large and still expanding outbreak of Ebola virus disease (EVD) in West Africa and a much more confined EVD outbreak in Middle Africa. Epidemiological and evolutionary analyses confirmed that all cases of both outbreaks are connected to a single introduction each of EBOV into human populations and that both outbreaks are not directly connected. Coding-complete genomic sequence analyses of isolates revealed that the two outbreaks were caused by two novel EBOV variants, and initial clinical observations suggest that neither of them should be considered strains. Here we present consensus decisions on naming for both variants (West Africa: “Makona”, Middle Africa: “Lomela”) and provide database-compatible full, shortened, and abbreviated names that are in line with recently established filovirus sub-species nomenclatures