Anton Santamaría

The effect of chain architecture on sharkskin of metallocene polyethylenes

Dynamic viscoelastic and extrusion capillary results of metallocene based polyethylenes are analyzed. Three samples show very high viscosities at low frequencies and large relaxation times, which is a symptom of the presence of small amounts of long chain branching (LCB). A linear correlation is found between the sharkskin dynamics (periodicity) and a characteristic entanglement-disentanglement time. It is found that this correlation does not hold for samples suspected of LCB.

Rheology of hydroxyethylated starch aqueous systems. Analysis of gel formation

This paper reports dynamic viscoelastic and steady shear measurements of aqueous solutions of two commercial hydroxyethyl ethers of potato starch, allowing us to define three different viscoelastic regions, depending on the polymer concentrations. The fluid-like zone (I) corresponds to a homogeneous solution, free of associations between chains, where the linear viscoelastic model can be applied; the fluid-gel transition zone (II) is associated with the rheology of complex systems which possess intermolecular specific interactions; and the gel-like zone (III) involves a network formation giving rise to a gel. The reversibility of the hydrogels was studied by means of steady/dynamic transient experiments allowing breakdown of the gel network and analysis of rehealing. Comparing the elastic moduli of the starch derivative gels leads us to assume that the functionality depends on the capacity to form hydrogen-bonding associations between polymer chains.

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.

Manufacturing, characterization and thermal conductivity of epoxy and benzoxazine multi-walled carbon nanotube buckypaper composites:

The preparation procedure of epoxy and benzoxazine multi-walled carbon nanotube buckypaper composites is described. The morphology of the buckypaper is characterized by scanning electron microscopy and porosimetry, revealing a non-uniform porous structure. The wetting behavior of epoxy and benzoxazine resins in the buckypaper surface is studied by contact angle measurements. Scanning electron microscopy photographs of the composites obtained by infiltration of the buckypaper with epoxy and benzoxazine resins and subsequent curing reveal that a good impregnation is achieved. Thermal conductivity results reveal higher values for pristine buckypaper than for the composites, which is explained by the extremely small thermal conductance of the nanotube–polymer interface. The thicker resin layer surrounding the nanotubes observed in the buckypaper/epoxy composite justifies its lower conductivity, as compared with the buckypaper/benzoxazine composite.

Basic rheological features of block polyurethane solutions : Entanglements, crystallization, and gelation

Three block polyurethane samples [differing in hard (urethane) to soft (macroglycol) segments ratio] dissolved in 2-butanone, are investigated. Rheological results obtained in the range 10–25 °C led to the hypothesis of an order-disorder transition associated with H bonds being discarded. The critical concentration and the characteristic molecular weight for entanglements, Mc, are estimated approximately from linear viscosity results. Mc decreases as the percent of hard segments in the polyurethane increases, a result which is correlated with short-distance chain parameters such as length and molecular weight per bond and characteristic ratio. Annealing clear solutions at temperatures around 0 °C causes haziness; on heating, a clearing temperature is detected at Tc≈15 °C. This transition coincides with a maximum observed in complex viscosity η* versus temperature curves. These results are probably due to incipient crystallization of soft segments domains. This hypothesis is compatible with differential sc...

Enhancement of the first normal stress coefficient and dynamic moduli during shear thickening of a polymer solution

Viscosity results of a solution of poly(2-hydroxyethyl methacrylamide) in glycerine, obtained from a continuous increase of shear rate and under steady state (constant shear rate) conditions, reveal the existence of a shear thickening region. The analysis of normal stresses shows that the first normal stress coefficient exhibits the same shear-thickening effect as the viscosity. The increase of the solution elasticity during shear thickening is also noticed by an enhancement of the storage modulus, measured in experiments using parallel superposition of oscillations on steady flows, as the shear stress is increased. Thermorheological simplicity, observed in dynamic measurements in absence of steady flow, is broken when shear stresses corresponding to the shear thickening region are applied. The results are explained by the presence of shear-induced hydrogen bonds, which provoke a reinforcement of the plateau modulus and an enlargement of the relaxation time spectrum.

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.

Pressure–volume–temperature results and pressure dependency on the viscosity of three liquid crystalline cellulose derivatives

The combined analyses of pressure–volume–temperature tests and oscillatory flow measurements for three thermotropic liquid crystalline cellulose derivatives, one acetoxypropylcellulose and two hydroxypropylcelluloses, are presented. This combined analysis allows for the determination of the pressure–viscosity coefficient b = ∂lnη0/∂P. It was observed that the activation energy of flow, dT g /dP, the compressibility coefficient and the pressure–viscosity coefficient obtained for the investigated thermotropic systems lie within the range of the values of these parameters reported for conventional polymers. However, the values of the thermal expansion coefficient below T g are considerably lower, reflecting the effect of the liquid crystalline order. The differences found between the three systems are explained considering microstructural factors, such as (average molar substitution), length of branching and hydrogen bonds.

Supramolecular structure, phase behavior and thermo-rheological properties of a poly (L-lactide-co-ε-caprolactone) statistical copolymer.

PLAcoCL samples, both unaged, termed PLAcoCLu, and aged over time, PLAcoCLa, were prepared and analyzed to study the phase structure, morphology, and their evolution under non-quiescent conditions. X- ray diffraction, Differential Scanning Calorimetry and Atomic Force Microscopy were complemented with thermo-rheological measurements to reveal that PLAcoCL evolves over time from a single amorphous metastable state to a 3 phase system, made up of two compositionally different amorphous phases and a crystalline phase. The supramolecular arrangements developed during aging lead to a rheological complex behavior in the PLAcoCLa copolymer: Around Tt=131 °C thermo-rheological complexity and a peculiar chain mobility reduction were observed, but at T>Tt the thermo-rheological response of a homogeneous system was recorded. In comparison with the latter, the PLLA/PCL 70:30 physical blend counterpart showed double amorphous phase behavior at all temperatures, supporting the hypothesis that phase separation in the PLAcoCLa copolymer is caused by the crystallization of polylactide segment blocks during aging.