We acknowledge helpful contributions from N. J. Lin, J. Sun, L. M. Pakstis, and S. Lin-Gibson (NIST). This work was supported by NIST and NIH/NIBIB R21 EB006497-01. Polymers were synthesized at Rutgers University supported by “RESBIO” – the Integrated Resource for Polymeric Biomaterials (NIH Grant P41 EB001046). This article, a contribution of the National Institute of Standards and Technology, is not subject to US copyright. Certain equipment and instruments or materials are identified in the paper to adequately specify the experimental details. Such identification does not imply recommendation by the National Institute of Standards and Technology, nor does it imply the materials are necessarily the best available for the purpose.