Molecular Weight Dependence of Polymersome Membrane Elasticity and Stability

Vesicles prepared in water from a series of diblock copolymers and termed "polymersomes" are physically characterized. With increasing molecular weight $\bar{M}_n$, the hydrophobic core thickness $d$ for the self-assembled bilayers of polyethyleneoxide - polybutadiene (PEO-PBD) increases up to 20 $nm$ - considerably greater than any previously studied lipid system. The mechanical responses of these membranes, specifically, the area elastic modulus $K_a$ and maximal areal strain $\alpha_c$ are measured by micromanipulation. As expected for interface-dominated elasticity, $K_a$ ($\simeq$ 100 $pN/nm$) is found to be independent of $\bar{M}_n$. Related mean-field ideas also predict a limiting value for $\alpha_c$ which is universal and about 10-fold above that typical of lipids. Experiments indeed show $\alpha_c$ generally increases with $\bar{M}_n$, coming close to the theoretical limit before stress relaxation is opposed by what might be chain entanglements at the highest $\bar{M}_n$. The results highlight the interfacial limits of self-assemblies at the nano-scale.Comment: 16 pages, 5 figures, and 1 tabl

The Covid-19 Pandemic and the Consumer Staples Sector: A Test of Market Efficiency

On Wednesday March 11, 2020, the World Health Organization (WHO) declared the covid 19 outbreak a global pandemic (Cucinotta, 2020). How efficient is the consumer staples market reaction to the announcement of a global pandemic? The purpose of this study is to examine the risk adjusted returns on and around the pandemic announcement to test the semi-strong form market efficiency hypothesis using the standard event study methodology in the finance literature. Will returns in the consumer staples industry show larger than expected gains on and surrounding the WHO’s COVID-19 pandemic announcement? The finance literature offers little evidence supporting the link between pandemics and the stock market. The announcement of a pandemic by the WHO should significantly and quickly affect the market. According to Eugene Fama (1970), if the market is semi-strong form efficient, all public information is immediately factored into the market and no investor can use this information to achieve an above normal return when adjusted for risk. To study this relationship, data for the S&P 500 and 10 consumer staples sector firms were collected for the event period surrounding the announcement of the COVID-19 pandemic. This study tests the global pandemic announcement for semi-strong form market efficiency (Bacon &Howell 2021). Evidence here supports the expected positive signal associated with the sample of consumer staples sector firms in reaction to the announcement of the pandemic. Likewise, the study results support the semi-strong form efficient market hypothesis and suggest the possibility of trading on this information up to 15 days prior to the announcement consistent with the behavioral finance literature (Bacon & Howell 2021)