J. E. Puig

Understanding thixotropic and antithixotropic behavior of viscoelastic micellar solutions and liquid crystalline dispersions. I. The model

Abstract A simple model consisting of the Upper Convected Maxwell constitutive equation and a kinetic equation for destruction and construction of structure, first proposed by Fredrickson in 1970, is used here to reproduce the complex rheological behavior of viscoelastic systems that also exhibit thixotropy and rheopexy under shear flow. The model requires five parameters that have physical significance and that can be estimated from rheological measurements. Several steady and unsteady flow situations were analyzed with the model. The model predicts creep behavior, stress relaxation and the presence of thixotropic loops when the sample is subjected to transient stress cycles. Such behavior has been observed with surfactant-based solutions and dispersions. The role of the characteristic time for structure built up, λ, in the extent and shape of the thixotropic loops is demonstrated.

Flexural, impact and compressive properties of a rigid-thermoplastic matrix/cellulose fiber reinforced composites

Abstract The mechanical behavior of rigid-thermoplastic matrix–cellulose fiber reinforced composites is investigated. These materials exhibit a brittle behavior, that is, they possess a high modulus and therefore, a very limited amount of deformation to fracture. Such behavior makes the characterization of interfacial properties difficult to evaluate. In this paper, the flexural, impact and compressive behavior of cellulose fibers reinforced polymeric matrices, such as poly(methyl methacrylate) (PMMA) and poly(styrene- co -acrylonitrile) (SAN) are investigated and special attention is given to the effect of fiber surface treatment on the effective properties. The flexural strength of the composites remains constant when the fiber is grafted with PMMA and a brittle interface is formed around the cellulose fibers, regardless of fiber content. This behavior is not observed in composites reinforced with untreated cellulose fibers or poly(butyl acrylate)-grafted fibers, which show a low flexural strength at higher fiber contents. In the case of impact loading, the presence of an elastomeric type material, in this case, poly(butyl acrylate)-grafted cellulose fibers seems to provide an alternative mechanism for energy dissipation in the composite, thus, showing a better impact behavior than the composites with the other fiber surface treatments. The impact behavior seems to be improved by the mechanical properties of the cellulose fibers. Specifically, the low elastic modulus of the cellulose fiber resulting from the attack of the grafting process, contributes to decrease the rigidity of the composite. Using the fact that glassy materials such as PMMA and SAN are able to yield when they are loaded under compression, even at room temperature, compression experiments were performed using two different sample geometries. The first was the typical rectangular sample and the other was the hourglass shaped specimen. Due to the compressive state of stress induced at the middle portion of the last specimen, a better indication of fiber matrix adhesion could be evaluated as compared to the other sample geometry that induced mixed stress component signs in the test area. The fiber–matrix interfacial properties are assessed from scanning electron microscope photographs.

On the shear banding flow of elongated micellar solutions

Abstract Under steady shear flow, elongated micellar solutions show shear stress saturation above a critical shear rate due to the formation of shear bands that result in non-homogeneous flow. Long transients and oscillations accompany this stress plateau. When measurements are done with a controlled stress rheometer, frequently a metastable branch is observed. At higher shear rates, a second upturn is observed above a second critical shear rate, which indicates that homogeneous flow is recovered. Here, a model consisting of the codeformational Maxwell constitutive equation coupled to a kinetic equation to account for the breaking and reformation of the micelles is presented to reproduce the features described above in steady shear flow. The model also predicts a second metastable branch and long transients at higher shear rates and the existence of an inflexion point in stress-shear rate plots above which no shear banding behavior is detected.

Dynamics of worm-like micelles: the Cox-Merz rule

Abstract The viscoelastic behaviour of worm-like micelles in small-amplitude oscillatory, steady simple shear and uniaxial extensional flows are analyzed with a model that couples the Oldroyd-B constitutive equation with a kinetic equation that accounts for the structural changes induced by the flow. In some cases, the constitutive equation predicts a viscoelastic behaviour that is consistent with the Cox–Merz rule. Departures from this rule are also predicted. Experimental data obtained for two worm-like micellar systems indicate that in these solutions, the Cox–Merz rule is not usually followed, in agreement with the predictions of our model. In uniaxial extensional flow, the model predicts a strain hardening in the extensional viscosity at low extensional rates and a strain-thinning at high extensional rates.

Swelling kinetics of poly(acrylamide)/poly(mono‐n‐alkyl itaconates) hydrogels

Hydrogels of mono-n-alkyl itaconate/N-acrylamide have been synthesised. The swelling process at three different pH values (acid, neutral and basic) has been studied. The experimental data indicate that our hydrogels follow second-order swelling kinetics. According to this, the kinetic constant, K∞, and the swelling capacity at equilibrium, W∞, have been calculated. The influence of the solvent pH and the molar mass of the mono-n-alkyl itaconate monomeric unit has been analysed. It seems that the general balance between the hydrogen bonding and the hydrophobic interactions regulates the swelling process of these hydrogels.

Mechanical properties of acrylate-grafted henequen cellulose fibers and their application in composites

Abstract The properties of cellulose fiber and PMMA- or PBA-grafted cellulose fibers are investigated as a function of the initiator (ceric-ammonium nitrate) concentration and the amount of grafted polymer onto cellulose fiber. The molecular weight of cellulose decreases while the crystallinity increases with an increment of initiator concentration because of the partial degradation of the amorphous zone of the fibers exposed to the oxidation by the initiator. This results in a reduction of the elastic modulus and tensile strength at high initiator concentrations. Degradation of cellulose is partially inhibited during the grafting process and, therefore, the effect of initiator on the mechanical properties is less notorious in the grafted cellulose fiber. The grafting of PMMA or PBA on the fiber results in lower mechanical properties than those of the ungrafted cellulose fiber. The reduction of the elastic modulus is independent of the amount of grafted PMMA or PBA, but the tensile strength decreases with the PBA content on the PBA-grafted fiber. Either the grafted or the ungrafted cellulose fibers improve the mechanical properties of plasticized PVC composites, and the best results are obtained for PMMA-grafted cellulose fibers because of the better fiber–matrix adhesion. The Halpin–Tsai equation seems to better agree with the experimental data when there is a good fiber–matrix adhesion. In contrast, for poor fiber–matrix adhesion the experimental data has a better agreement with the parallel arrangement equation.

Preparation and characterization of henequen cellulose grafted with methyl methacrylate and its application in composites

The grafting polymerization of methyl methacrylate (MMA) and cellulose from henequen (Agave fourcroydes) is investigated as a function of the initiator concentration (cerium-and-ammonium nitrate) and the monomer/cellulose ratio. The formation of cellulose-g-PMMA is confirmed by IR spectroscopy, DSC, and TGA. Both the initiator concentration and the MMA/cellulose ratio have a strong influence over the grafting parameters and over the molecular weight of the grafted PMMA. A higher initiator concentration and a lower monomer/cellulose ratio result in a lower molecular weight of the grafted polymer. Increasing the amount and the molecular weight of the grafted PMMA increases the compatibility of the fibers with SAN and PVC, as demonstrated by a mechanical test and scanning electron microscopy. SAN and PVC composites made with grafted cellulose exhibit higher flexural and tensile moduli, respectively, than those produced with the ungrafted fibers. Both moduli increase as the amount of reinforcement increases.

Comparison of oil-soluble and water-soluble initiation of styrene polymerization in a three-component microemulsion

The polymerization of styrene in three-component oil-in-water microemulsions made with the cationic surfactant dodecyltrimethylammonium bromide is studied by dilatometry and quasielastic light scattering as a function of type and concentration of initiator. Fast polymerization rates, high conversions, and high molecular weight polymers are achieved with both oil-soluble (AIBN) and water-soluble (potassium persulfate) initiators. The rate of polymerization shows initiation and termination intervals, but no constant-rate interval is observed. Stable monodisperse microlatexes are obtained with both types of initiators. For both AIBN and potassium persulfate, polystyrene molecular weight is proportional to initiator concentration [I]−0.4 and particle radii decrease as [I]−0.2. Polymerization initiation occurs in or at the microemulsion droplets, and polymer particles grow by recruiting monomer and surfactant from uninitiated swollen micelles.

Theophylline release from poly(acrylic acid-co-acrylamide) hydrogels

Abstract Hydrogels are hydrophilic polymers that swell to an equilibrium volume in the presence of water but preserve their shape. The dynamic swelling behavior of poly(acrylic acid-co-acrylamide) copolymers, at 37°C, was investigated. It was observed that the liberation mechanism was non-Fickian. Diffusion coefficients according to Ficḱs law as a function of concentration of N,N′-methylenebisacrylamide and results on the effects of crosslink density on the release of theophylline are reported.

Phase behavior of the Pluronic P103/water system in the dilute and semi-dilute regimes

The detailed temperature-composition phase diagram of the P103/water system in the dilute and semidilute regions is reported here using density and ultrasound velocity measurements, differential scanning calorimetry (DSC), rheometry and dynamic (DLS) and static light scattering (SLS). These techniques allow a precise determination of the critical micellar temperature (CMT), the sphere-to-rod micellar transition temperature (GMT) and the cloud point temperature (CPT) as a function of concentration. DLS and SLS measurements were employed to gain information on unimers and aggregate sizes and on the transition from spherical-to-rod micelles.