Rigid rod particle like viscoelastic responses of poly(vinylidene fluoride) in N-methylpyrrolidone solution

Abstract

A systematic study was performed on the dynamic viscoelastic properties of N-methylpyrrolidone (NMP) solutions of poly(vinylidene fluoride) (PVDF) samples with a wide range of weight average molar masses ( Mw) from 140 to 3300 kg mol−1 over a temperature range of −24 to 35 °C in the dilute to semidilute concentration ( c) regime in this study. In previous studies under extremely dilute conditions, it was confirmed that PVDF molecules behave as isolated highly elongated rigid rods, and the average particle length (⟨ L⟩) and diameter (⟨ d⟩) for different PVDF samples were precisely determined. The fundamental viscoelastic parameters used to analyze the dynamics of PVDF molecules dissolved in NMP were precisely determined, including the zero-shear shear viscosity ( η0), the steady state compliance ( Je), the average relaxation time ( τw), and the activation energy ( E*v) of τw in this study. E*v increased stepwise above c values corresponding to the reciprocal of the intrinsic viscosity ([ η]−1) for different solutions, independent of Mw, showing that contact or the formation of entanglements between PVDF molecules causes the increase in E*v. Je−1 was found to be proportional to the number density of PVDF molecules (ν =  cNA Mw−1, where NA denotes the Avogadro constant) over the entire investigated v range, irrespective of Mw and the presence of entanglements between PVDF molecules. The reduced specific viscosities, ηsp NA⟨ L⟩3( Mw[ η])−1, were reasonably described as a universal function in the parameter ν⟨ L⟩3 over the entire investigated range, and ηsp NA⟨ L⟩3( Mw[ η])−1 ∝ ( ν⟨ L⟩3)3 was found over the region 102 <  ν⟨ L⟩3 < 103, irrespective of Mw.