Strain Hardening in Polymer Glasses: Limitations of Network Models

Simulations are used to examine the microscopic origins of strain hardening in polymer glasses. While traditional entropic network models can be fit to the total stress, their underlying assumptions are inconsistent with simulation results. There is a substantial energetic contribution to the stress that rises rapidly as segments between entanglements are pulled taut. The thermal component of stress is less sensitive to entanglements, mostly irreversible, and directly related to the rate of local plastic arrangements. Entangled and unentangled chains show the same strain hardening when plotted against the microscopic chain orientation rather than the macroscopic strain.Comment: 4 pages, 3 figure

Kidney Patients’ Intention to Receive a Deceased Donor Transplant: Development of Stage of Change, Decisional Balance and Self-efficacy Measures

In order to sustain life, patients whose kidneys fail must receive dialysis or obtain a transplant. This study reports on the development and validation of measures of Stage of Change, Decisional Balance and Self-efficacy based on the Transtheoretical Model (TTM) to assess patients’ readiness to receive a deceased donor transplant. We surveyed 293 transplant-eligible kidney patients about their deceased donation readiness. Exploratory and confirmatory analyses for all measures demonstrated factor structures similar to previous application of the TTM to other health behaviors, excellent model fit and good internal and external validity. These brief, reliable instruments with good psychometric properties can guide the development of improved, individually-tailored transplant education for patients

Unified Description of Aging and Rate Effects in Yield of Glassy Solids

The competing effects of slow structural relaxations (aging) and deformation at constant strain rate on the shear yield stress $\tau^y$ of simple model glasses are examined using molecular simulations. At long times, aging leads to a logarithmic increase in density and $\tau^y$. The yield stress also rises logarithmically with rate, but shows a sharp transition in slope at a rate that decreases with increasing age. We present a simple phenomenological model that includes both intrinsic rate dependence and the change in properties with the total age of the system at yield. As predicted by the model, all data for each temperature collapse onto a universal curve.Comment: 4 pages, 3 figure

Anticoagulation: a practical guide for strabismus surgeons.

An increasing number of surgical strabismus patients are taking oral anticoagulant and antiplatelet agents, with more diverse mechanisms of action than those used in the past. The decision as to whether to continue these drugs throughout the perioperative period is difficult and must be based on the balance between hemorrhagic and thrombotic risk. To help guide strabismus surgeons with clinical management in these cases, we review potential hemorrhagic complications of strabismus surgery and examine the use of anticoagulant and antiplatelet drugs during the perioperative period. Surgical strategies that might help minimize intraoperative hemorrhage in patients on anticoagulant therapy are also discussed

Jamming under tension in polymer crazes

Molecular dynamics simulations are used to study a unique expanded jammed state. Tension transforms many glassy polymers from a dense glass to a network of fibrils and voids called a craze. Entanglements between polymers and interchain friction jam the system after a fixed increase in volume. As in dense jammed systems, the distribution of forces is exponential, but they are tensile rather than compressive. The broad distribution of forces has important implications for fibril breakdown and the ultimate strength of crazes.Comment: 4 pages, 4 figure

Measuring the processes of change for increasing blood donation in black adults

Background: Blacks have significantly lower blood donation rates than whites. Many views, experiences, and behaviors associated with blood donation are unique to black culture. Evidence suggests that culturally tailored health promotion programs help with increasing black blood donation. To be effective, tailored interventions should be based on valid and reliable measures. The Transtheoretical Model\u27s (TTM) Processes of Change (POC) construct provides an assessment of participants\u27 covert and overt activities and experiences in blood donation. This study describes development and validation of POC for increasing blood donation tailored to blacks. Student Design and Methods: Cross‐sectional measure development with online survey dissemination was used in 566 blacks in the Northeastern United States. Factor analytic structural modeling procedures were used to examine validity of the POC measure. Blood donation POC were examined in participants representing a range of blood donation history and intentions (nondonors, sometimes donors, regular donors) based on an established algorithm. Results: Confirmatory analyses replicated the theoretically expected structure of POC scales which is a 10‐factor, fully correlated best‐fit model. Expected POC patterns by Stages of Change based on theoretical and empirical predictions were confirmed. The range of effect sizes for 10 POC were η2 = 0.04 to 0.25, indicating that TTM POC are strong strategies in blood donation decision making for blacks and can be applied to interventions to increase blood donation for a minority population. Conclusion: POC measure was internally and externally valid in a sample of blacks. Interventions can utilize the POC measure to guide stage‐matched interventions to encourage use of relevant experiential and behavioral strategies to increase blood donation

Development and Validation of a Socioeconomic Kidney Transplant Derailers Index

Background: Socioeconomic barriers can prevent successful kidney transplant (KT) but are difficult to measure efficiently in clinical settings. We created and validated an individual-level, single score Kidney Transplant Derailers Index (KTDI) and assessed its association with waitlisting and living donor KT (LDKT) rates. Methods: The dataset included 733 patients presenting for KT evaluation in a transplant center in California. Exploratory factor analysis was used to identify socioeconomic barriers to KT (derailers) to include in the index. Potential KT derailers included health insurance, employment, financial insecurity, educational attainment, perception of neighborhood safety, access to a vehicle, having a washer/dryer, and quality of social support. Validity was tested with associations between KTDI scores and the following: (1) the Area Deprivation Index (ADI) and (2) time to KT waitlisting and LDKT. Results: Nine derailers were retained, omitting only social support level from the original set. The KTDI was scored by summing the number of derailers endorsed (mean: 3.0; range: 0–9). Black patients had higher estimated KTDI scores than other patient groups (versus White patients, 3.8 versus 2.1; P \u3c 0.001, effect size = 0.81). In addition, the KTDI was associated with the ADI (γ = 0.70, SE = 0.07; P \u3c 0.001). Finally, in comparison to the lower tertile, patients in the upper and middle KTDI tertiles had lower hazard of waitlisting (upper tertile hazard ratio [HR]: 0.34, 95% confidence interval [CI]: 0.25-0.45; middle tertile HR: 0.54, 95% CI: 0.40-0.72) and receiving an LDKT (upper tertile HR: 0.15, 95% CI: 0.08-0.30; middle tertile HR: 0.35, 95% CI: 0.20-0.62). These associations remained significant when adjusting for the ADI and other patient characteristics. Conclusions: The KTDI is a valid indicator of socioeconomic barriers to KT for individual patients that can be used to identify patients at risk for not receiving a KT

Invariance of measures to understand decision-making for pursuing living donor kidney transplant

Living donor kidney transplant is the ideal treatment option for end-stage renal disease; however, the decision to pursue living donor kidney transplant is complex and challenging. Measurement invariance of living donor kidney transplant Decisional Balance and Self-Efficacy across gender (male/female), race (Black/White), and education level (no college/college or higher) were examined using a sequential approach. Full strict invariance was found for Decisional Balance and Self-Efficacy for gender and partial strict invariance was found for Decisional Balance and Self-Efficacy across race and education level. This information will inform tailored feedback based on these constructs in future intervention studies targeting behavior change among specific demographic subgroups

Contact and Friction of Nano-Asperities: Effects of Adsorbed Monolayers

Molecular dynamics simulations are used to study contact between a rigid, nonadhesive, spherical tip with radius of order 30nm and a flat elastic substrate covered with a fluid monolayer of adsorbed chain molecules. Previous studies of bare surfaces showed that the atomic scale deviations from a sphere that are present on any tip constructed from discrete atoms lead to significant deviations from continuum theory and dramatic variability in friction forces. Introducing an adsorbed monolayer leads to larger deviations from continuum theory, but decreases the variations between tips with different atomic structure. Although the film is fluid, it remains in the contact and behaves qualitatively like a thin elastic coating except for certain tips at high loads. Measures of the contact area based on the moments or outer limits of the pressure distribution and on counting contacting atoms are compared. The number of tip atoms making contact in a time interval grows as a power of the interval when the film is present and logarithmically with the interval for bare surfaces. Friction is measured by displacing the tip at a constant velocity or pulling the tip with a spring. Both static and kinetic friction rise linearly with load at small loads. Transitions in the state of the film lead to nonlinear behavior at large loads. The friction is less clearly correlated with contact area than load.Comment: RevTex4, 17 pages, 13 figure

AVIAN SPECIATION IN THE PANTEPUI: THE CASE OF THE RORAIMAN ANTBIRD (PERCNOSTOLA [SCHISTOCICHLA] ‘‘LEUCOSTIGMA’’ SATURATA)

This is the published version. Copyright Central Ornithology Publication OfficeWe document the first records for Guyana of Roraiman Antbird (Percnostola [Schistocichla] “leucostigma” saturata), an endemic of the tepui highlands of southeastern Venezuela, northern Brazil and western Guyana. This form is well differentiated from nominate leucostigma (Spot-winged Antbird) of the Guianan lowlands in morphology, vocalizations, and genetics, and replaces it both altitudinally and ecologically. The two taxa are distributed parapatrically on the continuously forested northeastern slopes of the eastern tepuis, and they almost certainly come into contact, yet there is no evidence of intermediacy. We recommend that saturata be treated as a distinct species of Percnostola, and consider its' origin in the light of various models of speciation in the tepuis