834 research outputs found

    Rhéocinétique lors de la polymérisation avec séparation de phase dans un systÚme thermoplastique/thermodurcissable.

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    4 p.Pour les matĂ©riaux polymĂšres, la combinaison de l'Ă©coulement et de la rĂ©action chimique dans un outillage de transformation permet de gĂ©nĂ©rer in situ des structures ou des morphologies particuliĂšres Ă  l'origine de nouvelles propriĂ©tĂ©s. Le contrĂŽle du procĂ©dĂ© requiert alors la connaissance de l'Ă©volution de la rhĂ©ologie du systĂšme pendant la rĂ©action chimique (rhĂ©ocinĂ©tique). Le travail exposĂ© s'attache Ă  dĂ©crire l'Ă©volution des modules dynamiques d'un systĂšme complexe thermoplastique/thermodurcissable (polystyrĂšne/Ă©poxy-amine) lors de la polymĂ©risation du thermodurcissable. Les prĂ©curseurs du rĂ©seau sont initialement miscibles Ă  haute tempĂ©rature mais une sĂ©paration de phase sous forme de nodules sphĂ©riques intervient en cours de polymĂ©risation. Des mesures microcalorimĂ©triques de l'Ă©volution de la transition vitreuse du thermoplastique et des observations en microscopie de la sĂ©paration de phase ont Ă©tĂ© rĂ©alisĂ©es. L'Ă©volution des modules du systĂšme complexe au cours du temps a Ă©tĂ© suivie en rhĂ©omĂ©trie dynamique. Sur cette base expĂ©rimentale, un modĂšle rhĂ©ocinĂ©tique prĂ©dictif des modules de perte et de conservation a Ă©tĂ© dĂ©veloppĂ©. Il intĂšgre les phĂ©nomĂšnes de sĂ©paration de phase, de variation de la tempĂ©rature de transition vitreuse, de dilution des enchevĂȘtrements de la phase TP dans le cadre d'un modĂšle d'Ă©mulsion gĂ©nĂ©ralisĂ© Ă  partir des modules dynamiques du TP et du TD en cours de polymĂ©risation. La gĂ©nĂ©ralisation du modĂšle d'Ă©mulsion est rĂ©alisĂ©e en prenant en compte les interactions entre les particules de phase dispersĂ©e au delĂ  du seuil de percolation mĂ©canique sous la forme d'une loi de mĂ©lange

    Thermomechanical properties and shape-memory behavior of bisphenol a diacrylate-based shape-memory polymers

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    A series of acrylate-based shape-memory materials are synthesized from bisphenol A diacrylate monomers as crosslinking agents. Networks are synthesized by keeping constant the content of bisphenol A-based crosslinking agent and systematically varying the content ratio of different monofunctional chain builder monomers. The implications of the structure of bisphenol A-based monomers and the chemical structure and content of monofunctional monomers on thermomechanical properties are discussed. Thermomechanical properties are analyzed using dynamic mechanical analyses and mechanical properties are studied at room temperature and at the onset of the glass transition temperature. Shape-memory performances under isothermal and transient temperature conditions are also carried out. Tensile tests show excellent values of stress at break up to 45 and 15 MPa at room and high temperature, respectively. The measurements show excellent shape recovery and shape fixity ratios, ˜95% and 97%, respectively. These materials also show very high recovery velocities under transient temperature conditions, up to 24% min-1, and very short recovery times, up to 1.5 s, under isothermal conditions in a water bath. The results confirm that networks synthesized from bisphenol A crosslinkers are promising shape-memory materials.Postprint (author's final draft

    Investigation of the effect of double-walled carbon nanotubes on the curing reaction kinetics and shear flow of an epoxy resin

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    In this article, the effect of combined temperature-concentration and shear rate conditions on the rheology of double-walled carbon nanotubes (DWCNTs)/RTM6-Epoxy suspension was investigated to determine the optimum processing conditions. The rheological behavior and cure kinetics of this nanocomposite are presented. Cure kinetics analysis of the epoxy resin and the epoxy resin filled with DWCNTs was performed using Differential Scanning Calorimeter (DSC) and parameters of the kinetics model were compared. The DWCNTs have an acceleration effect on the reaction rate of the epoxy resin but no significant effect is noted on the glass transition temperature of the epoxy resin. This study reveals that the effect of shear-thinning is more pronounced at high temperatures when DWCNTs content is increased. In addition, the steady shear flow exhibits a thermally activated property above 60°C whereas the polymer fluid viscosity is influenced by the free volume and cooperative effects when the temperature is below 60°C

    Capturing the Uncapturable: The Relationship between Universities and Copyright through the Lens of the Audio-Visual Lecture Capture Policies

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    The COVID-19 pandemic has consolidated a double move. On the one hand, universities are becoming increasingly aware of the strategic value of copyright. On the other hand, the necessity to embrace distance education is making universities realise that there is a wealth of issues that go beyond ownership of research outputs and reprography rights. Understanding the role of universities as copyright subjects today requires understanding the rise of the ‘platformisation’ of learning, which this chapter does by analysing the audio-visual lecture capture policies of the largest universities in the United Kingdom, Ireland, Italy and France. The rules on lecture capture represent a meaningful entry point to investigate strengths and weaknesses of the universities’ approach to the creation and use of protected content online. Through this lens, it is possible to reflect on both the underlying exacerbated power imbalance between universities and teachers, and the diverging approach towards copyright law across the European higher education landscape. First, the chapter considers whether the selected universities had a specific policy on lecture capture, and, if so, what it covered, and where the default rule sat in the continuum between opt-out and opt-in. Then, it investigates issues of ownership of the lecture recordings, including the incorporated slides and other materials as well as the performance rights. Ownership rules significantly affect who can exploit the recordings and under which conditions, thus raising delicate and timely questions on the management and commercialisation of recorded lectures, the protection of moral rights and the retention of economic ones. Despite stark differences between the selected countries’ approach, the clear trend towards an increased expectation that teachers have to record their lectures epitomises the digital dispossession that is inherent to the platformisation of education. Nonetheless, the oft-forgotten rights in performances can still play a role in pursuing a fairer balance between the competing interests at play

    Copyright and Remote Teaching in the Time of COVID-19: A Study of Contractual Terms and Conditions of Selected Online Services

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    The spread of the COVID-19 virus forced educational institutions to transition to online education. This contribution analyses, through the lens of copyright law, the terms and conditions of some selected online services used to deliver remote teaching. The study highlights the most problematic terms and their detrimental effects on remote teaching by focusing on copyright ownership, liability, and content moderation

    Capturing the Uncapturable: The Relationship between Universities and Copyright through the Lens of the Audio-Visual Lecture Capture Policies

    Get PDF
    The COVID-19 pandemic has consolidated a double move. On the one hand, universities are becoming increasingly aware of the strategic value of copyright. On the other hand, the necessity to embrace distance education is making universities realise that there is a wealth of issues that go beyond ownership of research outputs and reprography rights. Understanding the role of universities as copyright subjects today requires understanding the rise of the ‘platformisation’ of learning, which this chapter does by analysing the audio-visual lecture capture policies of the largest universities in the United Kingdom, Ireland, Italy and France. The rules on lecture capture represent a meaningful entry point to investigate strengths and weaknesses of the universities’ approach to the creation and use of protected content online. Through this lens, it is possible to reflect on both the underlying exacerbated power imbalance between universities and teachers, and the diverging approach towards copyright law across the European higher education landscape. First, the chapter considers whether the selected universities had a specific policy on lecture capture, and, if so, what it covered, and where the default rule sat in the continuum between opt-out and opt-in. Then, it investigates issues of ownership of the lecture recordings, including the incorporated slides and other materials as well as the performance rights. Ownership rules significantly affect who can exploit the recordings and under which conditions, thus raising delicate and timely questions on the management and commercialisation of recorded lectures, the protection of moral rights and the retention of economic ones. Despite stark differences between the selected countries’ approach, the clear trend towards an increased expectation that teachers have to record their lectures epitomises the digital dispossession that is inherent to the platformisation of education. Nonetheless, the oft-forgotten rights in performances can still play a role in pursuing a fairer balance between the competing interests at play.Output Status: Forthcomin

    Vitrification during the Isothermal Cure of Thermosets: Comparison of Theoretical Simulations with Temperature-Modulated DSC and Dielectric Analysis

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    Vitrification during the isothermal cure of a thermoset, which is monitored by dynamic techniques such as temperature-modulated differential scanning calorimetry (TMDSC) or dielectric analysis (DEA), is analyzed in terms of its dependence on frequency. A simulation was used to obtain the vitrification time as a function of frequency, considering it as the time when the (frequency-dependent) glass transition temperature, Tg, of the curing system reaches the cure temperature. Simulations were carried out at different cure temperatures. Other parameters, such as the exponents (reaction orders) in the Kamal equation, l in the DiBenedetto equation (controlling the dependence of Tg on the degree of cure), and the activation energy for the frequency dependence of Tg, were also considered. The results are compared with those obtained experimentally by a TMDSC technique at low frequencies and by DEA at high frequencies. From the simulations it is found that the vitrification time decreases nonlinearly with log(frequency) in the low frequency range but approaches a linear dependence at high frequencies, in agreement with experimental data.Peer ReviewedPostprin
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