Frédéric Prochazka

Cocontinuity in immiscible polymer blends: A gel approach

Rheological properties of poly(ethylene oxide)/poly(vinylidenefluoride)-(hexafluoropropylene) immiscible polymer blends were studied in the low frequency domain. According to the blend composition, two critical compositions were determined by rheology. These compositions exhibit a power law relaxation spectrum with a critical exponent Δ. This observation leads us to compare the evolution of the morphology in a binary blend with the process of gelation. The supermolecular structure at the limit of the cocontinuity zone is comparable to the chemical or physical incipient network obtained during the gelation. This point is discussed in term of self-similar superstructure and fractal dimension.

Dynamic mechanical properties of linear and cross-linked polyurethane

Linear polyurethane melts were prepared by a polycondensation reaction of poly(oxypropylene) (POP) diol with a diisocyanate. Covalently cross-linked gels were obtained using three-armed star POP triol. The glass transition temperature and the viscoelastic properties were investigated as a function of the molar mass of the POP precursors. The variation of Tg is dominated by the density of urethane links. The loss peak of the shear modulus at high frequencies or low temperatures broadens with increasing density of urethane links. The gel modulus of end-linked POP triol decreases linearly with increasing molar mass of the precursors. The loss shear modulus of end-linked POP triol has a power law frequency dependence at low frequencies. The exponent of the power law dependence decreases with increasing molar mass of the precursors. Gels formed with POP triol with molar mass larger than 6 kg/mol show the effect of entanglements at intermediate frequencies.

Crosslinking in the melt of EVA using tetrafunctional silane: gel time from capillary rheometry

Crosslinking of ethylene vinyl acetate (EVA) copolymers can be performed in the melt by use of a transesterification reaction using tetrapropoxysilane as a crosslinking agent and dibutyl tin oxide as a catalyst [Polymer 43 (2002) 6085]. Whether the kinetics of the reaction can be suitably controlled is an important point that must be answered before any processing method such as extrusion or injection molding is considered. This investigation was carried out in a capillary rheometer. Various stoichiometric ratio and processing temperatures were studied. Viscosity was measured during the reaction and the evolution of the flow curves with reaction time was calculated. Gel point under shear conditions was considered to be achieved at the appearance of a yield stress on the flow curves. It was shown that an excess of propoxy groups slows down the reaction kinetics but until 120 °C, the gelling time is longer than 8 minutes which is long enough for processing with conventional techniques.

Slow dynamics in gels

Abstract Shear modulus data are presented for a polyurethane system at different stages of the gelation process. Data were collected over a wide frequency domain covering both the α relaxation and the internal mode relaxation. At the gel point, both the loss modulus G″ and the storage modulus G′ have a power law frequency dependence over four decades with an exponent 0.70 close to the value predicted by percolation theory assuming Rouse dynamics. After the gel point G′ exhibits a gel modulus which increases with increasing reaction extent, while G″ continues to have a power law behaviour at low frequencies with an exponent that decreases initially to 0.5 and then increases to 0.7 for the fully grown gels. The presence of slow dynamical processes in gels is visible in most reported dynamic mechanical measurements on different types of gels and is not expected by the standard model of polymeric gels. A number of possible explanations are mentioned, but we conclude that the slow dynamics in gels are at present an unexplained experimental observation.

Antimicrobial Activity of Nisin and Natamycin Incorporated Sodium Caseinate Extrusion-Blown Films: A Comparative Study with Heat-Pressed/Solution Cast Films

Antimicrobial edible films based on sodium caseinate, glycerol, and 2 food preservatives (nisin or natamycin) were prepared by classical thermomechanical processes. Food preservatives were compounded (at 65 °C for 2.5 min) with sodium caseinate in a twin-screw extruder. Anti-Listeria activity assays revealed a partial inactivation of nisin following compounding. Thermoplastic pellets containing food preservatives were then used to manufacture films either by blown-film extrusion process or by heat-press. After 24 h of incubation on agar plates, the diameters of K. rhizophila growth inhibition zones around nisin-incorporated films prepared by solution casting (control), extrusion blowing or heat pressing at 80 °C for 7 min of nisin-containing pellets were 15.5 ± 0.9, 9.8 ± 0.2, and 8.6 ± 1.0 mm, respectively. Since heat-pressing for 7 min at 80 °C of nisin-incorporated pellets did not further inactivate nisin, this indicates that nisin inactivation during extrusion-blowing was limited. Moreover, the lower diameter of the K. rhizophila growth inhibition zone around films prepared with nisin-containing pellets compared to that observed around films directly prepared by solution casting confirms that nisin inactivation mainly occurred during the compounding step. Natamycin-containing thermoplastic films inhibited Aspergillus niger growth; however, by contrast with nisin-containing films, heat-pressed films had higher inhibition zone diameters than blown films, therefore suggesting a partial inactivation of natamycin during extrusion-blowing.

Phase inversion and co-continuity domain in immiscible polyethylene/polystyrene blends

Abstract Blends of polystyrene and high-density or linear low-density polyethylene have been prepared in an internal mixer and studied in a wide range of compositions. Phase inversion compositions have been determined using selective extraction and scanning electron microscopy. It appears that phase inversion can occur in a domain of compositions rather than at a single point. The existing models of phase inversion are not complete enough to explain the entire phenomenon, and percolation of each component may be considered to describe the formation of co-continuity.