Mesomorphic form of isotactic polypropylene in stereodefective polypropylene: Solid mesophase or liquid-crystal like structure

A study of the crystallization of the mesomorphic form of isotactic polypropylene (iPP) in samples of different stereoregularity prepared with metallocene catalysts is presented. Contrary to what claimed in the literature, we have found that the mesomorphic form can be obtained by quenching of the melt at 0 °C also in the case of low stereoregular samples, provided that the samples are kept at 0 °C for long time. The key is the formation of mesomorphic bundles with chains in ordered 3/1 helical conformation of size large enough to be stable and inhibit the crystallization of the α form at room temperature. For stereoirregular samples the concentration of long ordered helical stretches is low and this requires long residence time at 0 °C for the formation of mesomorphic aggregates of size larger than the critical size. This result provides evidence that the mesomorphic aggregates are not formed as a result of a cooperative process driven by entropy, as in the case of liquid crystals, but rather they form via a nucleation and growth mechanism, as in the normal crystallization processes, according to the idea that the mesophase of iPP is a highly defective crystalline form.     

Development of an antimicrobial material based on a nanocomposite cellulose acetate film for active food packaging

Nanocomposites based on biopolymers have been recognised as potential materials for the development of new ecofriendly food packaging. In addition, if these materials incorporate active substances in their structure, the potential applications are much higher. Therefore, this work was oriented to develop nanocomposites with antimicrobial activity based on cellulose acetate (CA), a commercial organoclay Cloisite30B (C30B), thymol (T) as natural antimicrobial component and tri-ethyl citrate (TEC) as plasticiser. Nanocomposites were prepared by a solvent casting method and consisted of 5% (w/w) of C30B, 5% (w/w) of TEC and variable content of T (0%, 0.5% and 2% w/w). To evaluate the effect of C30B into the CA matrix, CA films without this organoclay but with T were also prepared. All nanocomposites showed the intercalation of CA into the organoclay structure; furthermore this intercalation was favoured when 2% (w/w) of T was added to the nanocomposite. In spite of the observed intercalation, the presence of C30B inside the CA matrices increased the opacity of the films significantly. On the other hand, T showed a plasticiser effect on the thermal properties of CA nanocomposites decreasing glass transition, melting temperature and melting enthalpy. The presence of T in CA nanocomposites also allowed the control de Listeria innocua growth when these materials were placed in contact with this Gram-positive bacterium. Interestingly, antimicrobial activity was increased with the presence of C30B. Finally, studies on T release showed that the clay structure inside the CA matrix did not affect its release rate; however, this nanofiller affected the partition coefficient K_P/FS which was higher to CA nanocomposites films than in CA films without organoclay. The results obtained in the present study are really promising to be applied in the manufacture of food packaging materials.     

Development of poly(lactic acid) films with propolis as a source of active compounds: Biodegradability,physical, and functional properties

Active films (AFs) using poly(lactic acid) (PLA) as a polymeric matrix containing various propolis concentrations (5, 8.5, and 13%) as the active agent (AA) were developed using a casting method. The purpose was to determine the effects of the incorporation of AA on the physical properties of the films and to evaluate the antioxidant and antimicrobial activities. Tensile strength and elastic modulus of the AFs decreased relative to the control (PLA without AA). Introducing the active substances from propolis into the PLA also affected its thermal properties (glass transition). Adding AAs to the polymer generated more opacity with a green-yellowish color compared to the control. In addition, AFs exhibited reduced water vapor permeability as the AA concentration increased. Biodegradation assay showed that the AFs degraded faster than the control. AFs exhibited antioxidant activity, which was measured as the ability to scavenge free radicals (2,2-diphenyl-1-picrylhydrazyl and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate)), due to the presence of bioactive compounds (phenolics). Antimicrobial activity was evaluated against Escherichia coli and showed a reduction over 4-log cycles. Therefore, incorporation of propolis is a useful strategy for the development of active packaging with antioxidant and antimicrobial effects, which increase the shelf life of food products. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47090.     

A traditional aboriginal condiment as an antioxidant agent in the development of biodegradable active packaging

In this study, antioxidant food‐packaging materials were developed by the incorporation of merkén, an aboriginal Chilean spice, into a biodegradable polymer matrix, poly(lactic acid). Antioxidant activity assays and the phenolic content of merkén showed interesting results compared to other condiments, such as oregano and parsley. Active materials with merkén at 3 and 5 wt % were successfully obtained by extrusion. Chromatographic analysis revealed the chemical composition of merkén, and its active material was based on interesting flavonoids and organic acids, such as catechin, myricetin, and gallic and ellagic acids. Scanning electron micrographs showed that the merkén particles accumulated in aggregates; this resulted in an increase in the permeability values and a reduction in the crystallinity and mechanical properties of the polymer matrix. Nevertheless, these materials presented interesting antioxidant activities as radical scavengers on aqueous food simulants. The kinetics of release was dependent on the type of food and the nominal content of merkén in the films. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44692.     

Suitability of alternative fatty food simulants to study the effect of thermal and microwave heating on overall migration of plastic packaging

The suitability of alternative fatty food simulants on overall migration determination under thermal treatment and microwave heating were analysed for different plastic materials commonly used for packs subjected to thermal treatment or microwave heating. Elevated temperature and microwave heating increased overall migration. The results demonstrated that polymer–simulant affinity plays a large role in determining the suitability of the alternative simulant. For PA/PE, ethanol 95% represented the best alternative fatty food simulant to reproduce overall migration with olive oil. PVC presented a high overall migration. Microwave heating increased migration up to 45 mg/dm². For PVC, isopropanol was able to reproduce olive oil even after microwave heating. For PP, none of the alternative fatty food simulants were able to reproduce overall migration with olive oil. PP presented high inert behaviour. Microwave heating did not produce a significant increase in overall migration; meanwhile, the time of exposure produced a more significant migration. Copyright © 2004 John Wiley & Sons, Ltd.     

On a possible model of crack propagation in a solid subjected to a cyclic loading

By the use of the Griffith model, the propagation of a crack in a solid subjected to a cyclic loading is studied. It is assumed that all the energy lost per cycle in the Bauschinger process is transferred to the crack to increase its size.The paper is divided in two parts, the first dealing with materials which do not undergo work-hardening and the second with materials subjected to said phenomenon during cyclic loading. Notwithstanding the crude approximation of this model, it is shown that the main features of fatigue phenomena are accounted for.

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Zusammenfassung DurchGebrauch des Griffithschen Models, wind die Fortpflanzung eines Risses in einem zyklischbeladenenfesten Körper studiert. Es wird angenommen, dass alle in einem Zyklus verlorene Energie während des Bauschingerverfahrens zum Riss übergehen wird, um dessen Grösse zu erweiters.Das Werk umfasst zwei Teile. Der erste Teil behandelt Stoffe, die der Bearbeitungsverhärtung nichtausgesetzt siud; and der zweite Tell Stoffe, die dem Verfahren während zyklischer Belastung uaterworfen werden. Trotz dergrossen Annäherung dieses Models, wird gezeigt, dass die Hauptzüge der Zermürphassomeneerklärt werden.

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Résumé L'étude de la propagation d' une fissure dans un solide soumis à des charges cycliques a été entreprise en partant du modèle de Griffith, et d' une hypothèse selon laquelle 1' énergie dissipée au cours de chaque cycle d'hystéresis sert intégralement à accroitre la dimension de la fissure.On considère séparément le cas des materiaux qui ne subissent pas un durcissement en cours de sollicitation cyclique, et le cas des materiaux qui sont sujets à un tel durcissement.Malgré la faiblesse des hypotheses de base, due à lent caractère de grossière approximation, les résultats de l'analyse concordent qualitativement avec les caractéristiques principales du phénomène de fatigue.

     

Application of β-Cyclodextrin/2-Nonanone Inclusion Complex as Active Agent to Design of Antimicrobial Packaging Films for Control of <Emphasis Type="Italic">Botrytis cinerea</Emphasis>

The aim of this study was to develop, characterize and evaluate in vitro the efficacy of active films, based on an inclusion complex formed by β-cyclodextrin, 2-nonanone and two polymer matrices (polylactic acid and low density polyethylene). The different films were characterized by scanning electronic microscopy (SEM), differential scanning calorimetry (DSC), themogravimetric analysis (TGA), optical properties and antimicrobial activity against B. cinerea. The results showed important differences in the parameters evaluated where the level of agglomerates of additives was a key to explain these changes. Finally, microbiological analysis showed high effectiveness in reducing the Botrytis cinerea growth. The active films developed in this study were able to inhibit the growth of phytopathogenic fungus B. cinerea at different experimental conditions. The studied films have potential use for packaging fresh fruit susceptible to biological attack by this fungus.