Determination of the molecular weight between cross-links for different ambers: Viscoelastic measurements of the rubbery plateau*

While much is known about the chemistry of amber, there are, to our knowledge, no quantitative estimates of the molecular weight between cross-links or quantitative estimates of the amount of cross-linking. In the present study we provide new data that advances our understanding of ancient ambers through an investigation of the network structure based on measurements of the rubbery modulus. Then, using rubber elasticity theory, we provide semiquantitative insights into the network structure. The work provides direct mechanical evidence of the chemically cross-linked structure of a Burmese amber, a Dominican amber, a Fushun amber, and two Baltic ambers by quantifying the molecular weight between cross-links (Formula presented.). The values of (Formula presented.) of the studied ambers ranges from 1800 to 3000 g/mol, and the number of monomers between two cross-link junctions is estimated to be between 6 and 10.