Predicting the Plateau Modulus from Molecular Parameters of Conjugated Polymers

The relationship between Kuhn length l(k), Kuhn monomer volume v(0), and plateau modulus G(N)(0), initially proposed by Graessley and Edwards for flexible polymers, and extended by Everaers, has a large gap in experimental data between the flexible and stiff regimes. This gap prevents the prediction of mechanical properties from the chain structure for any polymer in this region. Given the chain architecture, including a semiflexible backbone and side chains, conjugated polymers are an ideal class of material to study this crossover region. Using small angle neutron scattering, oscillatory shear rheology, and the freely rotating chain model, we have shown that 12 polymers with aromatic backbones populate a large part of this gap. We also have shown that a few of these polymers exhibit nematic ordering, which lowers G(N)(0). When fully isotropic, these polymers follow a relationship between l(k), v(0), and G(N)(0), with a simple crossover proposed in terms of the number of Kuhn segments in an entanglement strand N-e.