M. E. Muñoz

Thermorheological analysis of PVC blends

The effect of temperature on dynamic viscoelastic measurements of miscible poly (vinyl chloride) (PVC)/ethylene-vinyl acetate–carbon monoxide terpolymer (EVA-CO) and immiscible PVC/high-density polyethylene (HDPE) and PVC/chlorinated polyethylene (CPE) molten blends is discussed. PVC plasticized with di(2 ethyl hexyl) phthalate (PVC/DOP) and CaCO3 filled HDPE (HDPE/CaCO3) are also considered for comparison purposes. Thermorheological complexity is analyzed using two time–temperature superposition methods: double logarithmic plots of storage modulus, G′, vs. loss modulus, G″, and loss tangent, tan δ, vs. complex modulus, G*, plots. Both methods reveal that miscible PVC/EVA-CO and PVC/DOP systems are thermorheologically complex, which is explained by the capacity of PVC to form microdomains or crystallites during mixing and following cooling of the blends. For immiscible PVC/HDPE and PVC/CPE blends the results of log G′ vs. log G″ show temperature independence. However, when tan δ vs. log G* plots are used, the immiscible blends are shown to be thermorheologically complex, indicating that the morphology observed by microscopy and constitued by a PVC phase dispersed in a HDPE or CPE matrix, is reflected by this rheological technique.

Rheology of rodrun liquid‐crystalline polymers

The rheological characterization of two commercial thermotropic liquid crystalline polymers based on poly(ethylene terephthalate) (PET) and para-hydroxybenzoic acid (PHB) is carried out. The thermal transitions determined by DMTA are explained by the random character of these copolyesters, in comparison with nonrandom copolyesters synthesized by Jackson and Kuhfuss. The evolution of the dynamic viscoelastic functions with time in the nematic state is concave in shape for the 20%PET/80%PHB copolymer, a result that leads us to treat this system as a suspension of solid spheres ( unmolten crystals ) where the volume fraction of crystals increases with time according to an Avrami equation. The response of 40%PET/60%PHB copolymer is similar to a chemical or physical gelation and the hypothesis that the polydomain structure gives rise to a network is considered. Continuous flow, time-independent viscosity results reveal the existence of a three-region flow curve for 40%PET/60%PHB copolymer, but a Newtonian zone followed by a shear thinning region for 20%PET/ 80%PHB sample. At high temperatures the isotropization of the samples leads to a very strong decrease of the activation energy of flow, which becomes zero for 40%PET/ 60%PHB.

Viscoplastic behaviour and gelation of the solutions of a thermotropic copolyester in m-cresol

Abstract The rheological properties of solutions of thermotropic 60% p -hydroxybenzoic acid-40% PET copolyester in m -cresol, in the range 2.5 to 45% of polymer concentration, are studied by means of rotational viscometry. Viscoplastic behaviour is observed at all concentrations and temperatures. The plots of the apparent yield stress against polymer concentration present maxima at 15% and minima at 25% of polymer. Above this concentration gelation takes place and the viscoelastic properties of the gels are studied by dynamic viscoelastic measurements. A model based on the inversion from isotropic continuous phase to anisotropic continuous phase is proposed to justify the results obtained. This model is supported by the fact that anisotropic micro-domains are observed by polarizing microscopy. The nature of the micro-domains is studied comparing the FT i.r. spectra of the solutions, the filtered solutions and the precipitate deposited in the filter.

Dynamic viscoelastic measurements of pvc gels

Abstract This paper deals with dynamic viscoelastic measurements on poly(vinyl chloride)/dioctyl phthalate gels. The curves of the storage modulus, G′, vs temperature (at a frequency of 0.1 Hz) show a characteristic gel-like plateau at low temperatures. Using the theory of rubber elasticity, the molecular weights between the associations that constitute the structure of the gel, M c , are obtained for different concentrations and for samples of different molecular weight. It is seen that there is an effect of molecular weight on M e , which is not the normal behaviour for other polymer gels. The frequency dependence of G′ and G″ is studied at various temperatures for the range 10−2 Hz to 102 Hz; the slope of G′ vs frequency increases from zero, at gel behaviour temperatures, to values tending to 2, at temperatures above the fusion of the gel and low frequencies. On the other hand the loss tangents show minima at intermediate frequencies, a feature characteristic of entangled polymer systems. We have also carried out complementary measurements on a steady rotation viscometer, the results being in accordance with the values of dynamic complex viscosity.

Steady and transient shear flow properties of poly(styrene-B-butadiene)-dioctyl phthalate systems

Abstract The viscosity, stress overshoot effect and stress relaxation after cessation of flow of an SBS block copolymer, dissolved in dioctyl phthalate, have been studied at various temperatures and concentrations. Analysis of these functions reveals that there is not a complex flow behaviour, as in SBS melts, and that the mechanism of flow is the same for the whole range of temperatures and concentrations considered.