Flexible Packaging for High Pressure Treatments: Delamination Onset and Design Criteria of Multilayer Structures

Multi-layer flexible polymeric films employed for high pressure treatments of food packaging for pasteurization and sterilization frequently display delamination phenomena. This problem limits packaging reliability used for this treatment technology. This contribution is aimed at understanding the delamination phenomena of packaging structures under high pressures. Development of interlaminar stress fields, which promote localized delamination events, is here addressed by considering the case of mechanical failure of bi-layer structures. Analytical models and Finite Element based numerical simulations are exploited to this purpose. The theoretical and numerical results, that highlight the crucial role played by the mismatch of Young moduli and Poisson ratios of the laminated film sheets, are in full agreement with experimental findings on high pressure-treated food multilayer packages realized coupling different polymeric materials (i.e. polypropylene-polyethyleneterephthalate, polypropylene-cast polyamide and polypropylene-bioriented polyamide)

Sorption Thermodynamics of CO2, H2O, and CH3OH in a Glassy Polyetherimide: A Molecular Perspective

In this paper, the sorption thermodynamics of low-molecular-weight penetrants in a glassy polyetherimide, endowed with specific interactions, is addressed by combining an experimental approach based on vibrational spectroscopy with thermodynamics modeling. This modeling approach is based on the extension of equilibrium theories to the out-of-equilibrium glassy state. Specific interactions are accounted for in the framework of a compressible lattice fluid theory. In particular, the sorption of carbon dioxide, water, and methanol is illustrated, exploiting the wealth of information gathered at a molecular level from Fourier-transform infrared (FTIR) spectroscopy to tailor thermodynamics modeling. The investigated penetrants display a different interacting characteristic with respect to the polymer substrate, which reflects itself in the sorption thermodynamics. For the specific case of water, the outcomes from molecular dynamics simulations are compared with the results of the present analysis

Time-resolved FTIR/FTNIR spectroscopy: powerful tools to investigate diffusion processes in polymeric films and membranes

AbstractThe application of time-resolved FTIR spectroscopy to investigate diffusion processes in polymers is described. Two thermosetting systems have been studied: a tetrafunctional epoxy resin cured with an aromatic diamine hardener, and a ternary formulation comprising the above components plus a bismaleimide co-monomer. Spectroscopic monitoring of water diffusion, both in the Mid and in the Near IR frequency ranges, yielded accurate and reproducible kinetic curves from which it was possible to evaluate the absolute parameters of diffusion (diffusivity and activation energy). These were found to compare favourably with the values obtained by conventional gravimetric methods. The molecular interactions between the penetrant molecules and the polymer networks were also investigated and it was shown that, in the system containing the bismaleimide component, the fraction of water molecules hydrogen-bonded to the network decreases significantly

Gas Sorption and Diffusion in Amorphous and Semicrystalline Nanoporous Poly(2,6-dimethyl-1,4-phenylene)oxide

In this contribution is presented an analysis of mass transport properties of low molecular weight compounds in amorphous PPO and in two semicrystalline PPOs obtained by treating with benzene and carbon tetrachloride the amorphous sample. It is found that semicrystalline samples are endowed with larger gas sorption capacity and diffusivity as compared to the amorphous ones: this behavior has been attributed prevalently to the nanoporous nature of the crystalline phases induced by treatment with solvents. In particular, sorption experiments, carried out at 30 °C with methane, carbon dioxide, propane and propylene, have shown that both semicrystalline PPOs display rather interesting features which make them suitable for use as membrane materials in gas separation processes, in view of the relatively high values of solubility and diffusivity. Moreover, these peculiar sorption and mass transport properties have been found to be virtually unaffected by thermal aging: in fact, sorption experiments conducted on amorphous and semicrystalline PPO after treatment at 65 °C for three months showed that sorption and transport properties of aged samples are the same as for the untreated ones. This is an important feature to ensure the stability of performances in membrane applications

Hybridization of Nafion membranes by the infusion of functionalized siloxane precursors

Polysiloxane modified hybrid membranes were prepared by introducing pre-swelled commercial Nafion membrane into a sol-gel precursor solution, consisting of a pre-hydrolyzed mixture of tetraethoxysilane (TEOS) and a mercaptan functionalized organoalkoxysilane. The structure of the polysiloxane network was changed by altering the ratio of the two silane components within the precursor solution. The mercaptosilane modifier was used to provide an additional source of acidic Bronsted sites through the oxidisation of the mercaptan groups to sulfonic acid groups. The physical and chemical properties of the hybrid membranes were examined by TGA, FTIR and SEM analysis. The water vapour sorption and proton conductivity characteristics were evaluated at temperatures up to 70°C and with water activity in the region of 0.4 to 1. It was found that the polysiloxane network alters the water vapour sorption mechanism of the Nafion membrane, resulting in an increase in the equilibrium amount of water absorbed in the middle range of water activity (0.4-0.6). At the same time, the increased water absorption capability produced a concomitant increase in ionic conductivity at low water activities

Topology optimization-guided stiffening of composites realized through Automated Fiber Placement

The paper proposes a mixed strain- and stress-based topology optimization method for designing the ideal geometry of carbon fibers in composite laminates subjected to either applied tractions or prescribed displacements. On the basis of standard micromechanical approaches, analytical elastic solutions for a single cell, assumed to be a Representative Volume Element (RVE), are ad hoc constructed by involving anisotropy induced by fiber orientation and volume fraction, also taking into account inter-laminar stresses and strains. The analytical solutions are then implemented in a Finite Element (FE) custom-made topology optimization-based procedure rewritten to have as output the best curves the reinforcing fibers have to draw in any composite laminate layer to maximize the overall panel stiffness or to minimize the elastic energy. To verify the effectiveness of the proposed strategy, different structures undergoing either in-plane or out-plane boundary conditions have been selected and theoretically investigated, determining the optimal fibers’ maps and showing the related results in comparison to standard sequences of alternate fibers disposition for the same composites. Two optimized panels were at the end actually produced using an innovative Automated Fiber Placement (AFP) machine and consolidating the materials by means of autoclave curing processes, in this way replicating the fiber paths obtained from theoretical outcomes. As a control, two corresponding composite structures were also built without employing the fiber optimization strategy. The panels have been tested in laboratory and the theoretical results have been compared with the experimental findings, showing a very good agreement with our predictions and confirming the capability of the proposed algorithm in suggesting the arrangement of the fibers to obtain enhanced mechanical performances. It is felt that the hybrid analytical-FE topology optimization strategy, in conjunction with the possibilities offered by AFP devices, could pave the way for a new generation of ultra-lightweight composites for aerospace, automotive and many industrial applications

Chemical Vapour Deposition Graphene-PMMA Nanolaminates for Flexible Gas Barrier

Successful ways of fully exploiting the excellent structural and multifunctional performance of graphene and related materials are of great scientific and technological interest. New opportunities are provided by the fabrication of a novel class of nanocomposites with a nanolaminate architecture. In this work, by using the iterative lift-off/float-on process combined with wet depositions, we incorporated cm-size graphene monolayers produced via Chemical Vapour Deposition into a poly (methyl methacrylate) (PMMA) matrix with a controlled, alternate-layered structure. The produced nanolaminate shows a significant improvement in mechanical properties, with enhanced stiffness, strength and toughness, with the addition of only 0.06 vol% of graphene. Furthermore, oxygen and carbon dioxide permeability measurements performed at different relative humidity levels, reveal that the addition of graphene leads to significant reduction of permeability, compared to neat PMMA. Overall, we demonstrate that the produced graphene-PMMA nanolaminate surpasses, in terms of gas barrier properties, the traditional discontinuous graphene-particle composites with a similar filler content. Moreover, we found that the gas permeability through the nanocomposites departs from a monotonic decrease as a function of relative humidity, which is instead evident in the case of the pure PMMA nanolaminate. This work suggests the possible use of Chemical Vapour Deposition graphene-polymer nanolaminates as a flexible gas barrier, thus enlarging the spectrum of applications for this novel material

Molecular Sensing by Nanoporous Crystalline Polymers

Chemical sensors are generally based on the integration of suitable sensitive layers and transducing mechanisms. Although inorganic porous materials can be effective, there is significant interest in the use of polymeric materials because of their easy fabrication process, lower costs and mechanical flexibility. However, porous polymeric absorbents are generally amorphous and hence present poor molecular selectivity and undesired changes of mechanical properties as a consequence of large analyte uptake. In this contribution the structure, properties and some possible applications of sensing polymeric films based on nanoporous crystalline phases, which exhibit all identical nanopores, will be reviewed. The main advantages of crystalline nanoporous polymeric materials with respect to their amorphous counterparts are, besides a higher selectivity, the ability to maintain their physical state as well as geometry, even after large guest uptake (up to 10–15 wt%), and the possibility to control guest diffusivity by controlling the orientation of the host polymeric crystalline phase. The final section of the review also describes the ability of suitable polymeric films to act as chirality sensors, i.e., to sense and memorize the presence of non-racemic volatile organic compounds

SVILUPPO DI ELETTROLITI POLIMERICI PER CELLE A COMBUSTIBILE

Messa a punto di materiali innovativi per celle a combustibile basati su blend e strutture ibride orgqanico/inorganico. Caratterizzaione dell'assorbiment di umidità, del trasporto protonico, del cross-over di metanolo e dei parametri strutturali. Modellazione teorica dei processi di trasporto di massa e protonico