Peter Van Puyvelde

Rheology and morphology of compatibilized polymer blends

Research into the compatibilization of immiscible polymer blends has, during the past few years, begun to focus on the role of block co-polymers (bcp) on various morphological processes. Considerable advances have been made, from both a theoretical and an experimental point of view, in relating the presence of compatibilizers to structure development during flow.

Interfacial elasticity and coalescence suppression in compatibilized polymer blends

Shear-induced coalescence was studied in immiscible blends of polydimethylsiloxane (PDMS) and polyisoprene (PI) with a droplet-matrix morphology, using both rheology and scanning electron microscopy. Dynamic moduli of the blends compatibilized with different amounts of a PDMS–PI diblock were measured. The experimental results indicate that the blend response is characterized by two relaxation mechanisms. The general Palierne model with an interfacial shear modulus was used to analyze the data, since this model can describe the dynamic response of polymer blends in which interfacial tension gradients induce an extra relaxation mechanism besides droplet relaxation. Scanning electron microscopy was used to investigate the droplet size evolution in the blends during coalescence. For systems with a high amount of compatibilizer, it is shown that coalescence is completely suppressed under the conditions studied here.

Effect of compatibilization on the breakup of polymeric drops in shear flow

A block copolymer may be added as a compatibilizer during polymer processing in order to promote intimate mixing of thermodynamically immiscible homopolymers. The action of this compatibilizer can only partially be attributed to its effect on the interfacial tension between the immiscible homopolymers. Here the additional contributions of the compatibilizer are directly probed by measuring the capillary number during coalescence experiments. Model blends consisting of polyisobutylene (PIB) and polydimethylsiloxane (PDMS), compatibilized with various amounts of a PIB–PDMS diblock copolymer, are used for this purpose. The mean capillary number of the droplets is determined from the mechanical frequency response of the blends. With increasing amounts of compatibilizer, a systematic increase in steady shear capillary number is seen, to values well above the critical capillary number for droplet breakup of uncompatibilized systems. This indicates that a simple decrease in interfacial tension is not the only effect of adding the compatibilizer to these immiscible blends. Past simulations suggest that these results are associated with gradients in interfacial tension (Marangoni stresses) induced by the gradients of compatibilizer concentration due to flow. Direct evidence of the presence of such interfacial tension gradients along the surface of compatibilized drops was obtained by optical microscopy.A block copolymer may be added as a compatibilizer during polymer processing in order to promote intimate mixing of thermodynamically immiscible homopolymers. The action of this compatibilizer can only partially be attributed to its effect on the interfacial tension between the immiscible homopolymers. Here the additional contributions of the compatibilizer are directly probed by measuring the capillary number during coalescence experiments. Model blends consisting of polyisobutylene (PIB) and polydimethylsiloxane (PDMS), compatibilized with various amounts of a PIB–PDMS diblock copolymer, are used for this purpose. The mean capillary number of the droplets is determined from the mechanical frequency response of the blends. With increasing amounts of compatibilizer, a systematic increase in steady shear capillary number is seen, to values well above the critical capillary number for droplet breakup of uncompatibilized systems. This indicates that a simple decrease in interfacial tension is not the only ef...

The effect of block copolymer architecture on the coalescence and interfacial elasticity in compatibilized polymer blends

The effect of block copolymer architecture on the suppression of droplet coalescence and on the interfacial elasticity was studied in immiscible blends of polydimethylsiloxane (PDMS) and polyisoprene (PI) with a droplet-matrix morphology. The PDMS-PI diblock copolymers used in this study to compatibilize the blends differ in molecular weight and degree of asymmetry of the blocks. The general Palierne model with an interfacial shear modulus was used to analyze the dynamic measurements performed after different shear histories. It was shown that the coalescence suppression is more effective when the amount of compatibilizer increases and when the overall molecular weight of the block copolymer increases. When comparing the coalescence behavior of a blend and the inverse blend, it was shown that coalescence is suppressed more when the longest block of the block copolymer is located in the matrix. The interfacial relaxation time increases with molecular weight of the blocks for symmetric block copolymers. Asy...

Effect of confinement on the steady-state behavior of single droplets during shear flow

The effect of geometrical confinement on the deformation and orientation of single droplets during steady-state shear flow is investigated microscopically in a counterrotating device. The model system consists of poly(dimethyl siloxane) droplets of varying sizes and viscosities in a poly(isobutylene) matrix. The experimental results are first compared with the predictions of the model by Maffettone and Minale [J. Non-Newtonian Fluid Mech. 78, 227–241 (1998)] for bulk flow. For all viscosity ratios, deviations from the Maffettone and Minale model start to occur at a droplet diameter to gap spacing ratio of the order of 0.4. The droplet deformation increases and the droplets orient more towards the flow direction as a consequence of confinement. At low viscosity ratios, the deviations remain small, whereas at high viscosity ratios, larger deviations from bulk behavior are observed. The observations are also compared with the theory of Shapira and Haber [Int. J. Multiphase Flow 16, 305–321 (1990)] which incl...

Breakup of filaments in blends during simple shear flow

In this paper the breakup of droplets under shear in polymer blends is studied by means of linear conservative dichroism and small angle light scattering. More specifically breakup of long fibrils by interfacial instabilities is considered. Measurements are performed on dilute model systems containing nearly Newtonian components in transient flows that involve a sudden increase or decrease in shear rate. The experimental results are used to evaluate the Khakhar and Ottino theory [Khakhar and Ottino (1987)]. In this theory breakup times are calculated starting from the onset of the instability. It is demonstrated that the scaling derived from the Khakhar and Ottino theory also holds for the more readily accessible total breakup time, calculated from the onset of shear flow in a startup flow. The development of interfacial disturbances is studied in a flow history, which consists of generating fibrils by suddenly applying a shear rate followed by a sudden drop in shear rate during which the breakup process ...

Simultaneous Synchrotron WAXD and Fast Scanning (Chip) Calorimetry: On the (Isothermal) Crystallization of HDPE and PA11 at High Supercoolings and Cooling Rates up to 200 °C s−1

An experimental setup, making use of a Flash DSC 1 prototype, is presented in which materials can be studied simultaneously by fast scanning calorimetry (FSC) and synchrotron wide angle X-ray diffraction (WAXD). Accumulation of multiple, identical measurements results in high quality, millisecond WAXD patterns. Patterns at every degree during the crystallization and melting of high density polyethylene at FSC typical scanning rates from 20 up to 200 °C s(-1) are discussed in terms of the temperature and scanning rate dependent material crystallinities and crystal densities. Interestingly, the combined approach reveals FSC thermal lag issues, for which can be corrected. For polyamide 11, isothermal solidification at high supercooling yields a mesomorphic phase in less than a second, whereas at very low supercooling crystals are obtained. At intermediate supercooling, mixtures of mesomorphic and crystalline material are generated at a ratio proportional to the supercooling. This ratio is constant over the isothermal solidification time.

Effect of confinement and viscosity ratio on the dynamics of single droplets during transient shear flow

The deformation and orientation of droplets during transient shear flow is studied in a counterrotating device using microscopy. The effect of the degree of confinement and viscosity ratio is systematically investigated. The system consists of polydimethylsiloxane droplets of varying sizes and viscosities dispersed in a polyisobutylene matrix. The observations are compared with the predictions of an adapted version of the Maffettone and Minale model [Maffettone, and Minale, J. Non-Newtonian Fluid Mech. 78, 227–241 (1998)] which includes confinement effects [Minale, Rheol. Acta 47, 667–675 (2008)]. For flow start-up at low capillary numbers, the deformation of confined droplets and their orientation towards the flow direction are increased with respect to the unconfined situation for all viscosity ratios under investigation. The confined model results for start-up and the experimental data at low capillary numbers are in good agreement both showing similar monotonous transients. At high degrees of confinem...

Suspension-like hardening behavior of HDPE and time-hardening superposition

The rheology of solidifying high-density polyethylene (HDPE) is investigated. Experiments on an HDPE were performed with a novel RheoDSC device. Results agree quantitatively with simulations for a suspension of elastic spheres in a viscoelastic matrix except for very low values of space filling (<5%), indicating that the rheological behavior of the crystallizing melt in the frequency range investigated is purely suspension like. The hardening behavior of the material is characterized in two different ways; a normalized rheological function and a time-hardening superposition (THS) master curve of rheological properties. An improvement is proposed to the procedure for performing THS that was previously used in the literature. Based on this procedure, a novel method for predicting the rheological properties of crystallizing melts is presented.

Regioselective synthesis of renewable bisphenols from 2,3-pentanedione and their application as plasticizers

2,3-Pentanedione (2,3-PD), a bio-based chemical derived from lactic acid, has the potential to serve as a precursor for the synthesis of novel bisphenols. We developed a solvent-free catalytic strategy for the condensation of phenol with 2,3-PD by using acid catalysts at temperatures ranging from 323 to 373 K. Various soluble and solid acids exhibit high activity, while a high chemoselectivity to bisphenol requires a high phenol to 2,3-PD molar ratio. Bisphenol yields as high as 84% are for instance reported in an excess of phenol in the presence of Nafion NR50. Recycling of the Nafion catalyst after washing with ethanol at room temperature is demonstrated. The regioselectivity in the bisphenol fraction is influenced by the acid strength. A clear trend is presented in which the regioselectivity towards the desired p,p′-isomers increases with increasing acid strength, showing p,p′/o,p′-isomer ratios as high as 100. A tentative mechanism is discussed based on the ionic versus non-ionic pathway. The purified 2,3-PD-derived p,p′-bisphenols are assessed as plasticizers for polyethylene terephthalate (PET), showing promising properties similar to that of the reference bisphenol A, but with a broader processing window due to the lower melting point and higher thermal stability under an inert atmosphere.