Juan J. Peña

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.

The effect of a miscible and an immiscible polymeric modifier on the mechanical and rheological properties of PVC

Abstract PVC/copoly(ester-urethane) (Baymod PU®) and PVC/EVA copolymer blends have been analysed from the point of view of the effect of miscibility on the mechanical and rheological properties. The FTIR and NMR analysis of Baymod PU modifier leads to the conclusion that its miscibility with PVC (determined by dynamic mechanical analysis) is due to the presence of 31.9% poly(e-caprolactone) and 38.4% adipates in the main chain of the copoly(ester-urethane) polymer. However the PVC/EVA system considered in this work results in an immiscible blend, due to the low content of vinyl acetate, 33 wt%. The comparison of the mechanical and rheological properties of both blends with those of a PVC plasticised with a low molecular weight common plasticiser (PVC/DOP system), reveals that compatibility is not a determinant point in what some ultimate properties are concerned. The molecular weight (related to the viscosity of pure polymers, which is similar for both polymeric modifiers) seems to play a more important role than the miscibility. The stress–strain curves of both PVC/copoly(ester-urethane) (PU) miscible blends and PVC/EVA immiscible blends resemble each other and are very different with respect to PVC/DOP curves. The PVCs plasticised with DOP show a considerably higher elongation at break and a lower Young’s modulus than PVC mixed with polymeric modifiers. Miscible systems, PVC/PU and PVC/DOP, show similar values of the stress at break, slightly higher than those of immiscible PVC/EVA blends. The results of the latter two-phase system are well fitted to a two-parameter equivalent box model developed by Kolarik, considering a value very close to zero for the parameter, which accounts for the adhesion in between the phases. The reduction of the viscosity, induced by PU and EVA polymeric modifiers, with respect to that of pure PVC, is remarkable at shear rates such as those involved in calendering and milling, but is less significant at shear rates above 1000 s −1 which correspond to injection moulding processes. A better performance is obtained with DOP plasticiser at such high shear rates. For immiscible PVC/EVA blends the variation of the viscosity with composition has been adjusted using an adaptation of the Kolarik model, being this a novel approach to relate the viscous and mechanical behaviour of immiscible but in some way compatible blends [1] . As expected miscible PVC/PU blends viscosity data follow a free volume additivity model.

Viscoelastic measurements on main-chain thermotropics

Abstract Rheological properties, determined by torsion experiments, of three main-chain thermotropics are investigated. Stable, time-independent measurements are obtained in two of these polymers, and the corresponding dynamic viscoelastic and first normal difference results are analysed. In the nematic state both storage and loss moduli scale as G′, G″ ∝ ω 1 2 , and no shear modulus associated with an entanglement network is detected. Recoverable compliances J0e, which are determined from steady-state N1 values, are several orders of magnitude higher than those of isotropic polymers. For one of the polymers, the characteristic molecular weight between entanglements, Mc, is determined from Newtonian viscosities of isotropic solutions. Although the molecular weight of our sample is above Mc, viscoelastic measurements at temperatures corresponding to the nematic state show no vestige of the entanglement network

Physical properties of blends of a liquid crystalline copolyester with poly(butylene terephthalate)

Abstract Physical properties, including rheology, dimensional stability, dynamic viscoelasticity and differential scanning calorimetry (d.s.c.) of blends of thermotropic 60% p -hydroxybenzoic acid-40% poly(ethylene terephthalate) P(HBA-ETP) copolyester with poly(butylene terephthalate) PBT are studied. The viscosity of the blend is not decreased by the addition of a small quantity of P(HBA-ETP) which contradicts the general trend observed in the literature for blends of liquid crystalline polymers. In our case the viscosity decreases only for blends of ⩾40% P(HBA-ETP). Dimensional stability (shrinkage) measurements, however, reveal more sensitivity to the influence of small quantities of P(HBA-ETP), e.g. 5P(HBA-ETP)/95PBT blends show very good dimensional stability, similar to that of P(HBA-ETP) samples. The shrinkage results are related to d.s.c. measurements and we consider that PHBA crystallites act to lock the retraction in the case of blends. We suggest that the presence of PBT favours the formation of crystals in P(HBA-ETP), when both polymers are blended. The heterogeneous morphology of P(HBA-ETP) copolymer is manifested by the existence of various relaxations detected by dynamic viscoelastic measurements. Compatibility between PBT and the PET-rich phase of the liquid crystalline copolyester is deduced from dynamic viscoelastic results.

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.

Critical temperature in PVC melts detected by rheological measurements

Abstract The shear flow of PVC is analyzed under conditions similar to those of industrial processing. The existence of two mechanisms of flow is confirmed by the presence of two activation energies of flow and by change with temperature of the exponent “n” in the equation σ 21 = K γ n . The influence of molecular weight on the critical temperature T c , separating these different flow regions is considered.

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.

Elastic behaviour of polyvinyl chloride melts

Abstract A study has been made of the elastic behaviour of Polyvinylchloride commercial samples at high shear stresses, under similar conditions to those used in the processing industry, by means of an extrusion capillary rheometer. The influence in the die swell of variables as molecular weight, temperature and shear rate has been studied. Results are in agreement with general literature data. Smooth extrudates are obtained only in the zone of intermediate shear rates. In this zone it is observed that the die swell increases with shear rate.

Physical features of blends based in a liquid crystalline polymer: the effect of the mixing devices

Abstract Rheology, viscoelastic measurements and morphological aspects of blends of a commercial liquid crystalline polymer (LCP) and a polyarylate of bisphenol A (PAr) are presented. Viscoelastic results reveal incompatibility between the components and in the case of 60 LCP/40 PAr blends the physical properties are different, depending on the mixing method. This is attributed to the relationship between rheology, morphology and properties. The influence of the composition and the draw ratio on the storage tensile modulus is also studied.