Poly(β-amino ester) networks are being explored for biomedical applications, but they may lack the mechanical properties necessary for long term implantation. The objective of this study is to evaluate the effect of adding methyl methacrylate on networks' mechanical properties under simulated physiological conditions. The networks were synthesized in two parts: (1) a biodegradable crosslinker was formed from a diacrylate and amine, (2) and then varying concentrations of methyl methacrylate were added prior to photopolymerizing the network. Degradation rate, mechanical properties, and glass transition temperature were studied as a function of methyl methacrylate composition. The crosslinking density played a limited role on mechanical properties for these networks, but increasing methyl methacrylate concentration improved the toughness by several orders of magnitude. Under simulated physiological conditions, networks showed increasing toughness or sustained toughness as degradation occurred. This work establishes a method of creating degradable networks with tailorable toughness while undergoing partial degradation.

In this paper we explore the relationship between ethnic fractionalization and social capital. First, we test for time differences in the impact of ethnic fractionalization on social capital using U.S. data from 1990, 1997 and 2005. Subsequently we examine the data for evidence of the conflict, contact and hunker-down theories espoused by Putman in explaining what happens over time when individuals interact with those of differing ethnicities. We find no evidence of heterogeneity in the impact of ethnic fractionalization on social capital over time. In addition we find evidence of the conflict theory and no evidence of hunker-down or contact theories. Our results suggest that as communities become more diverse, there is a tendency for social capital to decline.