Dynamic viscoelasticity of a series of poly(4-n-alkylstyrene)s and their alkyl chain length dependence

Dynamic viscoelastic measurements were performed for a series of poly(4-n-alkylstyrene)s with six different n-alkyl side chains, that is, methyl, ethyl, propyl, butyl, hexyl and octyl groups. Based on the time-temperature superposition principle, storage modulus G′, loss modulus G″ and loss tangent tan δ were shifted with horizontal shift factor a_T and vertical shift factor b_T, giving well superposed master curves for all the polymers. With increase of the number of carbon atoms, plateau modulus G_N ⁰, which corresponds to G′ value at minimum tan δ, decreases, while entanglement molecular weight M_e(=ρRT/G_N ⁰) increases, where ρ is the polymer density, R is the gas constant and T is the absolute temperature. Degree of polymerization at onset of entanglement, N_e, which defined as M_e/M₀, where M₀ denotes the molecular weight of each monomer, also increases with increase of n-alkyl side chain length. Packing length estimated from the empirical equation by Fetters also increases with increase of n-alkyl lengths. These results are presumably due to large polymer chain thickness caused directly by substituents on phenyl rings of styrene units.