Dynamic viscoelastic properties of poly(vinyl chloride) (PVC)/bis(2-ethylhexyl) phthalate (DOP) gels with different molecular weights and various polymer concentrations have been studied as a function of temperature. These PVC gels exhibited an elastic solid at room temperature and gradually became liquid (sol) with increasing temperature. The sol−gel transition took place at a critical gel temperature at which the scaling law of G‘(ω) ∼ G‘‘(ω) ∝ ω n held, allowing an accurate determination of the critical gel temperature by means of the frequency independence of the loss tangent. The scaling exponent n obtained was 0.75, independent of temperature as well as molecular weight. This is in good agreement with the previous results observed at 40 °C and suggests the formation of a similar fractal structure of the PVC gels. Furthermore, the molecular structure of the gel junctions was analyzed using the modified Eldridge−Ferry procedure developed by Tanaka and Nishinari. The junction structure was found to be almost independent of molecular weight and gelation temperature, giving a support to the similarity of the fractal structure in the PVC gels.
Nonlinear rheology was examined for a critical gel of poly(vinyl chloride) in dioctyl phthalate (PVC/DOP). PVC crystal domains worked as the cross-linking domains in this physical gel. In the linear regime, the gel exhibited power-law dependence of storage and loss moduli on frequency ω, G‘(ω) ∝ G‘‘(ω) ∝ ω n with n ≅ 0.7. In stress relaxation experiments the gel exhibited nonlinear damping of the relaxation modulus G(t,γ) with increasing step-strain γ, but the magnitude of damping was much smaller than that for homogeneous homopolymer liquids. In addition, for the gel, the G(t,γ) data for the largest strain examined (γ = 5) were in close agreement with the linear G(t) measured after imposition of this strain. On start-up of shear flow at a rate γ, the viscosity growth function η+(t,γ) of the gel followed the linear η+,L(t) at a short time scale, deviated downward from η+,L(t) but still increased gradually at an intermediate time scale, exhibited an apparent plateau (pseudo-steady-flow behavior) over a long time scale and finally decreased with further increases of time. These features of η+(t,γ) and G(t,γ) were discussed in relation to flow/strain-induced changes of the fractal structure in the gel.
Three blends were prepared from a high molecular weight of poly(vinyl chloride) (PVC) (Mw ) 173 000, Mw/Mn ) 2.0) and a low molecular weight PVC (Mw ) 39 400, Mw/Mn ) 1.7). Dynamic viscoelastic properties of these PVC blends in bis(2-ethylhexyl) phthalate (DOP) were measured at 40 °C as a function of polymer concentration, and the effect of long chains on gelation has been studied. The scaling exponent n at the gel point was found to be constant ()0.75), independent of molecular weight and molecular weight distribution. The critical concentration cg for the sol-gel transition still followed the relation cg ∝ Mw -1 , which was unchangeable with the molecular weight distribution and was also in good agreement with the previous results. As a result, cg was well expressed by a mixing rule, 1/cg ) w1/cg1 + w2/cg2, where wi is the weight fraction of the component polymer i. The gel strength Sg at the gel point did not obey the relation Sg ∝ Mw -1 , but scaled as Sg ∝ Mz -1 to show the effect of long chains on gelation. In the postgel state, the gel elasticity determined by the quasi-equilibrium modulus Ge still followed the scaling law, Ge ∝ z , where is the relative distance to the gel point and z ) 2.6 for the (PVC blend)/DOP samples, but the Ge values at the same were observed to be dominated by the long chains of PVC.
Rheo-dielectric behavior was examined for suspensions of two types of carbon black (CB) particles in varnish. For one type of CB particles having plain carbon surfaces, an agglomerated network was fully developed to exhibit lowfrequency elasticity (under small strains) and high electrical conductance. This conductance decreased significantly under flow, reflecting flow-induced rupture of the network structure. This rheo-dielectric change well corresponded to the pseudo-plastic, non-Newtonian behavior of the viscosity η. In contrast, for the other type of CB particles having oxidized carbon surface, only fragmented aggregates were formed. Correspondingly, this CB exhibited just moderate rheo-dielectric changes and weak nonlinearities in its η. These results demonstrated the usefulness of the rheo-dielectric method for detecting the connectivity of the network composed of conductive CB particles.
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