Molecular Structure, Chain Dimensions, and Linear Rheology of Poly(4-vinylbiphenyl)

The linear viscoelasticity and molecular structure of a series of poly­(4-vinylbiphenyl) (PVBP) samples with polydispersities ranging from 1.08 to 2.75 are investigated using wide-angle X-ray scattering (WAXS), small-angle neutron scattering (SANS), and rheological measurements. PVBP chain dimensions, such as statistical segment length, persistence length, and packing length, are obtained from SANS measurements. Because of the bulkiness of the biphenyl monomer, PVBP has the lowest chain flexibility and the largest reptation tube diameter among most common thermoplastics, including polystyrene and polyvinyl­cyclohexane. Nevertheless, PVBP follows the universal power law dependence between elastic modulus and packing length (G_N⁰ ∝ p^–3). X-ray diffraction of PVBP consist of two main amorphous peaks: one associated with the interchain (backbone) correlations and a second one arising from phenyl–phenyl correlations. The latter are due to π–π stacking of the phenyl rings, mostly arranged in the T-shaped configuration. Molecular dynamic simulations validate the diffraction peak assignments. The linear rheology of PVBP samples reveals a thermorheologically simple response and a very low value of G_N⁰ (0.065 MPa). This is consequence of the low chain flexibility, which also explains the relatively high entanglement molecular weight value for PVBP (58.3 kg/mol).