Miscibility and compatibilization of poly(trimethylene terephthalate)/acrylonitrile-butadiene-styrene blends

Poly(trimethylene terephthalate)/acrylonitrile-butadiene-styrene (PTT/ABS) blends were prepared by melt processing with and without epoxy or styrene-butadiene-maleic anhydride copolymer (SBM) as a reactive compatibilizer. The miscibility and compatibilization of the PTT/ABS blends were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), capillary rheometer and scanning electron microscopy (SEM). The existence of two separate composition-dependent glass transition temperatures (T_gs) indicates that PTT is partially miscible with ABS over the entire composition range. In the presence of the compatibilizer, both the cold crystallization and glass transition temperatures of the PTT phase shifted to higher temperatures, indicating their compatibilization effects on the blends.The PTT/ABS blends exhibited typical pseudoplastic flow behavior. The rheological behavior of the epoxy compatibilized PTT/ABS blends showed an epoxy content-dependence. In contrast, when the SBM content was increased from 1 wt% to 5 wt%, the shear viscosities of the PTT/ABS blends increased and exhibited much clearer shear thinning behavior at higher shear rates. The SEM micrographs of the epoxy or SBM compatibilized PTT/ABS blends showed a finer morphology and better adhesion between the phases.     

STRUCTURING & RHEOLOGY OF MOLTEN POLYMER/CLAY NANOCOMPOSITES

The evolution and the origin of "solid-like state" in molten polymer/clay nanocomposites are studied. Using polypropylene/clay hybrid (PPCH) with sufficient maleic anhydride modified PP (PP-MA) as compatibilizer, well exfoliation yet solid-like state was achieved after annealing in molten state. Comprehensive linear viscoelasticity and non-linear rheological behaviors together with WAXD and TEM are studied on PPCH at various dispersion stages focusing on time,temperature and deformation dependencies of the "solid-like" state in molten nanocomposites. Based on these, it is revealed that the solid-structure is developed gradually along with annealing through the stages of inter-layer expansion by PP-MA,the diffusion and association of exfoliated silicate platelets, the formation of band/chain structure and, finally, a percolated clay associated network, which is responsible for the melt rigidity or solid-like state. The network will be broken down by melt frozen/crystallization and weakened at large shear or strong flow and, even more surprisingly, may be disrupted by using trace amount of silane coupling agent which may block the edge interaction of platelets. The solid-like structure causes characteristic non-linear rheological behaviors, e.g. residual stress after step shear, abnormal huge stress overshoots in step flows and, most remarkably, the negative first normal stress functions in steady shear or step flows. The rheological and structural arguments challenge the existing models of strengthened entangled polymer network by tethered polymer chains connecting clay particles or by chains in confined melts or frictional interaction among tactoids. A scheme of percolated networking of associated clay platelets, which may in band form of edge connecting exfoliated platelets, is suggested to explain previous experimental results.     

PRELIMINARY INVESTIGATION ON THE MISCIBILITY OF ISOTACTIC POLYPROPYLENE （iPP） AND SYNDIOTACTIC POLYPROPYLENE （sPP） BLENDS

Experimental miscibility studies were performed on different compositions of iPP/sPP blends, where sPP has a low syndiotacticity （[rrrr] = 81%）. Combining optical microscopy, rheology, and solid state NMR spectroscopy, the miscibility of the blends was investigated at different scales in the traditionally thought to be ＂immiscible＂ iPP/sPP blends. For the composition of iPP/sPP （90/10） blend, it shows to be miscible in the melt, and furthermore, the existence of intermolecular chain interactions between sPP and iPP components was detected in the solid state.     

Association under shear flow in aqueous solutions of pectin

Effects of oscillatory and steady shear flows on intermolecular associations in dilute and semidilute aqueous solutions of pectin in the absence and presence of the hydrogen bond breaking agent urea are reported. A weak oscillatory shear perturbation builds up, depending on polymer concentration, multichain aggregates or networks in the course of time and these association structures are mainly stabilized through hydrogen bonds. The association effect is more pronounced at higher concentrations, and the growth of intermolecular interactions is inhibited by the addition of urea. Steady shear measurements on the pectin-water solutions reveal shear thickening at low shear rates for all the concentrations, except the lowest one, and disruption of intermolecular junctions at high shear rates. In the presence of urea, no shear thickening is detected. The polymer concentration dependence of the viscosity at a low shear rate can be described by a power law η ∼ c^x, with x = 1.9 and 1.4 without and with urea, respectively. When a low constant shear rate is applied to pectin solutions and this monitoring shear rate is interrupted periodically by transitory high shear rates perturbations during a short time, prominent association structures evolve upon return to the monitoring shear rate. This effect is more evident at a lower polymer concentration, and in the presence of urea, the growth of the association complexes is damped. The shear-induced alignment and stretching of polymer chains and the formation of hydrogen-bonded structures are analyzed in the framework of a model, where cooperative zipping of stretched chains play an important role. Viscosity enhancement is found for a semidilute pectin-water solution in the presence of moderate levels of salt addition (NaCl), suggesting that partial screening of electrostatic interactions promotes growth of energetic cross-links.     

Rheological characterization of polysaccharide-surfactant matrices for cosmetic O/W emulsions

Rheometrical techniques can be profitably used for polysaccharide matrices in order to evaluate their suitability for the preparation of stable cosmetic O/W emulsions. In particular, the rheological properties of aqueous scleroglucan systems were investigated under continuous and oscillatory shear conditions in a polymer concentration range (0.2-1.2% w/w) embracing the sol/gel transition. The effects due to the addition of two different surfactants (up to 10% w/w) were examined at constant polymer concentration (0.4% w/w). The selected additives are a nonionic polymeric siliconic surfactant (dimethicone copolyol) and a cationic surfactant (tetradecyltrimethylammonium bromide), respectively. Polysaccharide-surfactant interactions leading to complex formation were detected also through rheology. The combined action of both nonionic and cationic surfactants in the polymer solution was examined at two different surfactant concentration levels (5 and 10% w/w), demonstrating the beneficial effects produced on the mechanical properties of the polymer matrix by the coexistence of both surfactants. Such beneficial effects are confirmed by the stability and rheology shown by the emulsions prepared. In this way, the results point out the good agreement between the rheology of the continuous phase and the final characteristics of the emulsion obtained.     

Rheological studies of diatom encapsulation in silica gel

The mutual influence of the mineral and biological components of a specific bio-hybrid system consisting of diatom cells entrapped in a silica gel was studied by rheological methods. Small amplitude shear stress oscillatory measurements indicate that the culture medium alone has a strong impact on the silica network formation and viscoelastic properties. In contrast, the presence of diatoms does not significantly perturb the sol–gel process, and leads to a moderate change in the gel elasticity. Compression tests show that a large difference exists between the mechanical properties of silica gels and diatom shells, suggesting a limited impact of the gel strength on the diatom survival rate. We also show that the biological activity of entrapped diatoms can modify the structural evolution of the silica gel. This work indicates that rheological methods may be important tools for the optimization of whole cells encapsulation procedures and further confirms that encapsulated diatoms are able to interact with the surrounding silica materials.     

Estimation of the relaxation spectrum from dynamic experiments using Bayesian analysis and a new regularization constraint

The relaxation spectrum is estimated from dynamic experiments using Bayesian analysis and a new regularization constraint. In the Bayesian framework, a probability can be calculated for each estimate of the spectrum. This offers several advantages; (1) an optimal estimate of the relaxation spectrum may be calculated as the mean of a large number of estimates, and (2) reliable errors for the optimal estimate can be provided using the deviation of all estimates from the mean. Furthermore, the Bayesian approach (3) gives an estimate of the overall noise level of the experiment, which is usually an important but unknown parameter for the calculation of relaxation spectra from dynamic experiments by indirect methods (determining the regularization parameter), and finally, (4) the information content in a given set of experimental data can be quantified. The validity of the Bayesian approach is demonstrated using simulated data.     

Rheological transitions in asymmetric colloidal star mixtures

We examine the rheological transition from an arrested to a fluid state for different mixtures of star polymers with varying functionality and size ratios. As a general trend, we find that addition of smaller star polymers in an arrested, concentrated solution of larger ones brings about melting of the large star glass. At the same time, the dependence of the amount of additives needed to melt the glass has a nontrivial dependence on the size ratio. Theoretical analysis, based on effective interactions and Mode Coupling Theory, reproduces the experimental results and helps identify two distinct types of glasses in the composite system. This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006.     

The distinction of viscoelastic phases from entangled wormlike micelles and of densely packed multilamellar vesicles on the basis of rheological measurements

It is shown in this work how two viscoelastic surfactant systems that are both shear thinning but differ in their morphology can be distinguished on the basis of rheological measurements. The measurements were carried out on the novel surfactant system cetyltrimethylammonium 2-hydroxy-1-naphthoate. The phases in this system are produced by mixing cetyltrimethylammonium hydroxide and 2-hydroxy-1-naphthoic acid. With increasing counterion surfactant ratio X, the system has two viscoelastic regions that are separated by a two-phase region. It is shown by cryo-transmission electron microscopy and by small angle neutron scattering that the first viscoelastic region which exists between X=0.5 and X=0.75 contains wormlike micelles, while the second viscoelastic region that exists between X=0.9 and X=1.4 contains multilamellar vesicles. Both phases look alike, are highly viscoelastic, have similar storage modulus values, and are shear thinning. The phases and the properties of the phases for the studied system are very similar as the phases for the system CTA-3-hydroxy-2-naphthoate that has been studied before (see Hassan et al. Langmuir, 12:4350–4357, 1996; Horbaschek et al. J Colloid Interface Sci, 206:439–456, 1998). The two viscoelastic phases show the same shear-thinning behavior, but differ in other rheological results. The phases can most easily be distinguished with the help of normal stress measurements. The wormlike viscoelastic solutions show large normal stresses that give rise to a large Weissenberg effect while the vesicle phases show no Weissenberg effect.