Predicting the plateau modulus from molecular parameters of conjugated polymers

The relationship between Kuhn length b, packing length p, and plateau modulus G_N^° initially proposed by Graessley and Edwards and experimentally investigated by Everaers, while well-studied for flexible and stiff polymers, has a large gap in experimental data between the flexible and stiff regimes. This gap prevents the validation of theoretical models of the crossover between flexible and stiff polymers and therefore, the prediction of mechanical properties from chain structure of 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 cross-over region. Using small angle neutron scattering (SANS), static light scattering (SLS), and oscillatory shear rheology along with the freely rotating chain model we have shown that nine non-crystalline conjugated polymers and three aromatic polymers not only populate a large part of this gap, but that they follow the proposed relationship between b, p, and G_N^° as well. We have also experimentally validated the freely rotating chain model for various conjugated polymers using SANS and SLS and find good agreement in Kuhn length values from 1.7 nm in aromatic polymers up to 23.2 nm in conjugated polymers.