Effect of different monomer precursors with identical functionality on the properties of the polymer network

Thermo-mechanical properties of polymer networks depend on functionality of the monomer precursors -- an association that is frequently exploited in materials science. We use molecular simulations to generate spatial networks from chemically different monomers with identical functionality and show that such networks have several universal graph-theoretical properties as well as near universal Young's modulus. The vitrification temperature is shown to be universal only up to a certain density of the network, as measured by the bond conversion. The latter observation is explained by the fact that monomer's tendency to coil enhances formation of topological holes, which, when accumulated in the network, amount to a percolating cell complex restricting network's mobility. This higher-order percolation occurs late after gelation and is shown to coincide with the onset of brittleness, as indicated by a sudden increase in the glass transition temperature. This phenomenon may signify a new type of phase transition in polymer materials

Acrylate Network Formation by Free-Radical Polymerization Modeled Using Random Graphs

A novel technique is developed to predict the evolving topology of a diacrylate polymer network under photocuring conditions, covering the low-viscous initial state to full transition into polymer gel. The model is based on a new graph theoretical concept being introduced in the framework of population balance equations (PBEs) for monomer states (mPBEs). A trivariate degree distribution that describes the topology of the network locally is obtained from the mPBE, which serves as an input for a directional random graph model. Thus, access is granted to global properties of the acrylate network which include molecular size distribution, distributions of molecules with a specific number of crosslinks/radicals, gelation time/conversion, and gel/sol weight fraction. Furthermore, an analytic criterion for gelation is derived. This criterion connects weight fractions of converted monomers and the transition into the gel regime. Valid results in both sol and gel regimes are obtained by the new model, which is confirmed by a comparison with a “classical” macromolecular PBE model. The model predicts full transition of polymer into gel at very low vinyl conversion (<2%). Typically, this low-conversion network is very sparse, as becomes apparent from the predicted crosslink distribution

Effect of different monomer precursors with identical functionality on the properties of the polymer network

The association between thermo-mechanical properties in polymers and functionality ofmonomer precursors is frequently exploited in the materials science. However, it is notknown if there are more variables beyond monomer functionality that have a similar link.Here, by using simulations to generate spatial networks from chemically different monomerswith identical functionality we show that such networks have universal graph-theoreticalproperties as well as a near-universal elastic modulus. The vitrification temperature wasfound to be universal only up to a certain network density, as measured by the bond con-version. The latter observation is explained by the fact that monomer’s tendency to coilenhances formation of topological holes, which, when accumulated, amount to a percolatingcell complex restricting network’s mobility. This higher-order percolation occurs late aftergelation and is shown to coincide with the onset of brittleness, as indicated by a suddenincrease in the glass transition temperature

Effect of different monomer precursors with identical functionality on the properties of the polymer network

The association between thermo-mechanical properties in polymers and functionality ofmonomer precursors is frequently exploited in the materials science. However, it is notknown if there are more variables beyond monomer functionality that have a similar link.Here, by using simulations to generate spatial networks from chemically different monomerswith identical functionality we show that such networks have universal graph-theoreticalproperties as well as a near-universal elastic modulus. The vitrification temperature wasfound to be universal only up to a certain network density, as measured by the bond con-version. The latter observation is explained by the fact that monomer’s tendency to coilenhances formation of topological holes, which, when accumulated, amount to a percolatingcell complex restricting network’s mobility. This higher-order percolation occurs late aftergelation and is shown to coincide with the onset of brittleness, as indicated by a suddenincrease in the glass transition temperature

Computational modelling of metal soap formation in historical oil paintings: the influence of fatty acid concentration and nucleus geometry on the induced chemo-mechanical damage

Metal soap formation is one of the most wide-spread degradation mechanisms observed in historical oil paintings, affecting works of art from museum collections worldwide. Metal soaps develop from a chemical reaction between metal ions present in the pigments and saturated fatty acids, which are released by the oil binder. The presence of large metal soap crystals inside paint layers or at the paint surface can be detrimental for the visual appearance of artworks. Moreover, metal soaps can possibly trigger mechanical damage, ultimately resulting in flaking of the paint. This paper departs from a recently proposed computational model to predict chemo-mechanical degradation in historical oil paintings, as presented in Eumelen et al. (J Mech Phys Solids 132:103683, 2019). The model describes metal soap formation and growth, which are phenomena that are driven by the diffusion of saturated fatty acids and proceed by a nucleation process from a crystalline nucleus of small size. This results into a chemically-induced strain in the paint, which may promote crack nucleation and propagation. The proposed model is here used to investigate the effects of saturated fatty acid concentration and initial nucleus geometry on the amount of chemo-mechanical damage generated. Numerical simulations show that both factors have a marginal influence on the growth rate of the metal soap crystal, but play a significant role on the extent of fracture induced in the paint

Evidence for the catalytic properties of ultramarine pigment

Ultramarine blue paint layers in oil paintings can be affected by ultramarine ‘disease’ or ‘sickness’: a phenomenon described by a grey appearance and a loss of detail in the artwork. An explanation for this phenomenon is an interaction between the organic binder network and ultramarine pigment, with the pigment acting as a catalyst for the breakdown of the network. This breakdown results in micro-cracks in the paint film, which influences the appearance of the artwork. To investigate the possible catalytic property of ultramarine pigment, a test reaction – the dehydration of methanol to dimethyl ether – was carried out, with and without ultramarine pigment present in a micro-reactor with in-line gas chromatography mass spectrometry. It was observed that a higher yield of dimethyl ether was obtained in the presence of ultramarine pigment, confirming that ultramarine pigment possesses catalytic properties similar to commercial zeolitic silica-alumina catalysts

**Bankstown-Lidcombe Hospital

Abstract: The purpose of this paper is to elaborate criteria by which the principles of curriculum reform can be judged. To this end, the paper presents an overview of standard critiques of medical education and examines the ways medical curriculum reforms have responded to these critiques. The paper then sets out our assessment of these curriculum reforms along three parameters: pedagogy, educational context, and knowledge status. Following on from this evaluation of recent curriculum reforms, the paper puts forward four criteria with which to gauge the adequacy medical curriculum reform. These criteria enable us to question the extent to which new curricula incorporate methods and approaches for ensuring that its substance: overcomes the traditional opposition between clinical and resource dimensions of care; emphasizes that the clinical work needs to be systematized in so far as that it feasible; promotes multi-disciplinary team work, and balances clinical autonomy with accountability to non-clinical stakeholders