Rheology and extrusion foaming of chain-branched poly(lactic acid)Mihai, Mihaela; Huneault, Michel A.; Favis, Basil D.Contact us / Contactez nous: nparc.cisti@nrc-cnrc.gc.ca. In this study, the effect of macromolecular chainbranching on poly(lactic acid) (PLA) rheology, crystallization, and extrusion foaming was investigated. Two PLA grades, an amorphous and a semi-crystalline one, were branched using a multifunctional styrene-acrylicepoxy copolymer. The branching of PLA and its foaming were achieved in one-step extrusion process. Carbon dioxide (CO 2 ), in concentration up to 9%, was used as expansion agent to obtain foams from the two PLA branched using chain-extender contents up to 2%. The foams were investigated with respect to their shear and elongational behavior, crystallinity, morphology, and density. The addition of the chain-extender led to an increase in complex viscosity, elasticity, elongational viscosity, and in the manifestation of the strain-hardening phenomena. Low-density foams were obtained at 5-9% CO 2 for semi-crystalline PLA and only at 9% CO 2 in the case of the amorphous PLA. Differences in foaming behavior were attributed to crystallites formation during the foaming process. The rheological and structural changes associated with PLA chain-extension lowered the achieved crystallinity but slightly improved the foamability at low CO 2 content. POLYM. ENG. SCI.,

The size of the minor phase in an immiscible polymer blend can have a significant effect on properties such as the impact strength. Few studies, however, have quantitatively considered the parameters controlling the size of the dispersed phase. In this paper, light and scanning electron microscopy have been used to examine the size of the minor phase in polypropylene/polycarbonate blends after melt processing. The size was examined as a function of both the viscosity ratio (p) and the torque ratio (TR). The viscosity ratio is studied in the regions of p > 1 and p < 1. p has a marked effect on the morphology of the dispersed phase with the phase size increasing by a factor of 3 to 4 times from p = 4.5 to p = 17.3. Reduction in the size of the minor phase was achieved below p = 1 with the minimum particle size occurring at p ≃ 0.15. The results for these systems indicate the presence of upper and lower limits of p beyond which deformation becomes difficult. This is shown to be similar in some respects to the behavior of Newtonian fluids in shear flow, although the upper limit extends beyond that observed in the Newtonian fluid studies.

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This article investigates the crystallinity development in cellular poly(lactic acid) (PLA) and the effect of the achieved crystalline content on its properties and microstructure. Carbon dioxide (CO2) in its supercritical state was used as the expansion agent for three different grades of PLA that differed in terms of L‐lactic acid content. Cellular PLA was produced on a twin‐screw extrusion line using capillary dies of various diameters. The obtained crystalline contents were measured by differential scanning calorimetry and X‐ray diffraction techniques. The morphology of the cellular structures was examined using scanning electron microscopy. The crystallinity developed on expansion depended on L‐lactic acid content, on supercritical CO2 concentration, polymer flow rate, and die diameter. Cellular PLA, with densities as low as 30 kg/m3, was obtained under the most favorable conditions. It was shown that the crystallinity development in PLA enhances its cellular structure formation and enables the fabrication of quality cellular materials at lower CO2 concentration. The presence of PLA crystallites within expanded cell walls leads to a peculiar 2D‐cavitation phenomena observed only in the cell walls of semicrystalline foams. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

SynopsisIn this study an immiscible blend of polypropylene (PP) and polycarbonate (PC) was examined.Experiments were carried out in an internal mixer and the objective was to examine the influence of the time of mixing, the speed of rotation of the rotors as well as the effect of time under quiescent conditions on the size and size distribution of the minor phase. The morphology was studied by light and electron microscopy and the photographs were quantitatively analyzed by semiautomatic image analysis. The effect of the time of mixing was studied for up to 20 min at three viscosity ratios for PC dispersed in PP. The most significant particle size deformation and disintegration processes take place within the first 2 min of mixing. After 2 min very little reduction in the size of the minor phase is observed up to 20 min mixing time, although some phase size/time dependence was observed at an intermediate viscosity ratio. The influence of rotor speed was also studied and the results indicate that this blend is insensitive to changes in the speed of rotation of the rotor over a three-to fourfold range. The effect of time at 250°C under quiescent conditions was studied at 7% and 23% composition (volume fraction) for PP dispersed in PC and also for PC in PP. Coalescence coarsening was observed only for 23% PC in PP where the particle size was found to increase by 75% after approximately 90 min at temperature.

Three different categories of blend interfaces are examined systematically in order to isolate the role of the interface in the development of cocontinuous morphologies during melt mixing. They are Type I, compatible binary blends based on high-density polyethylene (HDPE)/styrene-ethylene-butylenestyrene (SEBS) and HDPE/styrene-ethylene-butylene (SEB); Type II, an incompatible binary system comprised of HDPE/polystyrene (PS); and Type III, compatible ternary systems comprised of HDPE/PS compatibilized by SEBS in one case and by SEB in another. The Type I and Type III systems represent conventional approaches to preparing blend systems of low interfacial tension. The cocontinuous morphology is analyzed using three techniques: microscopy/image analysis, solvent extraction/gravimetric analysis, and BET characterization of surface area and pore size. A mechanism for the formation of dualphase continuity based on droplet and fiber lifetimes during melt mixing has been proposed. For the Type I compatible binary systems continuity development and microstructural features are dominated by thread-thread coalescence. In the Type II incompatible HDPE/PS binary system and Type III compatibilized ternary systems, continuity development and microstructural features are controlled by droplet-droplet coalescence and reduced droplet-droplet coalescence, respectively. In the latter case, the generation of fresh interface during droplet deformation results in a system that is only partially emulsified. The relative presence of fibers or droplets during dynamic mixing is analyzed quantitatively using a matrix dissolution/image analysis technique. A thread frequency ratio (TFR) is proposed as a basic general parameter to classify the relative presence of fibers to droplets during mixing and hence the type of continuity development for a given system.