The Effect of Convection on a Propagating Front with a Liquid Product: Comparison of Theory and Experiments

This work is devoted to the investigation of propagating polymerization fronts converting a liquid monomer into a liquid polymer. We consider a simplified mathematical model which consists of the heat equation and equation for the depth of conversion for one-step chemical reaction and of the Navier-Stokes equations under the Boussinesq approximation. We fulfill the linear stability analysis of the stationary propagating front and find conditions of convective and thermal instabilities. We show that convection can occur not only for ascending fronts but also for descending fronts. Though in the latter case the exothermic chemical reaction heats the cold monomer from above, the instability appears and can be explained by the interaction of chemical reaction with hydrodynamics. Hydrodynamics changes also conditions of the thermal instability. The front propagating upwards becomes less stable than without convection, the front propagating downwards more stable. The theoretical results are compared with experiments. The experimentally measured stability boundary for polymerization of benzyl acrylate in dimethyl formamide is well approximated by the theoretical stability boundary. (C) 1998 American Institute of Physics

Crystallization and melting studies of branched isotactic polypropylenes

Crystallization, melting and structural studies were conducted on iostactic polypropylenes treated with varying dosages of electron beam radiation and an untreated iPP. Through FTIR methods, all specimens were found to be greater than 99% isotactic. Crystallization and melting studies were performed using light depolarizing microscopy (LDM) and other melting experiments were conducted using differential scanning calorimetry (DSC). Structural studies were conducted by use of a wide-angle x-ray diffractometer (WAXD). Through isothermal crystallization studies it was found that at the highest supercoolings all specimens had approximately the same half-time of crystallization values, t½, attributed to increased nucleation by increased supercooling. At higher temperatures of crystallization, Tc, it was observed that t½ varied for the specimens. This was attributed to the effects of branching on primary nucleation and to the size of the spherulites. All specimens were observed to nucleate in the heterogeneous mode, meaning that nuclei density stayed constant throughout the isothermal crystallization process. Average spherulite growth geometry (Avrami) exponent, n, values were in the range of 2.2 and 2.5. These low values were a consequence of the amount of branching and stereoregularity of the polymer chains and secondary crystallization. The spherulite growth rates, k, for all the samples decreased with decreasing supercooling, resulting from the decrease in the number of nuclei forming into spherulites. Through x-ray studies the predominant crystal form was found to be of the α modification, with some β and γ modifications observed. No structural changes at the crystal lattice level were detected. The degree of crystallization was seen to decrease as a result of increased branching in the treated specimens and attributed to thermal degradation in the untreated one. From the DSC endotherms small melting peaks in the range of 140 °C to approximately 145 °C was noticed in some of the specimens and attributed to the β modification as a consequence of nucleating agent(s) and stresses induced during sample film preparations. The equilibrium melting points taken from the highest peak and the return to baseline of the endothermic curves showed that the treated samples had lower points than the untreated one. This was due to branching and degradation from the irradiation process. The melting ranges of the treated specimens were shifted to lower values as compared to the untreated specimen, as a consequence of branching and degradation The temperature ranges for the irradiated specimens were broader than the melt range of the untreated sample The α peak also showed broadening as a result of branching

Influencing Factors on the Velocity and Temperature of Propagating Fronts in Acrylate Composites

Thermal frontal polymerization is a type of polymerization in which a localized reaction zone propagates through an unstirred system. It is incumbent upon the production and transport of heat produced as a result of the exothermic reaction associated with free-radical polymerization. First discovered in the 1970s, frontal polymerization has been since utilized to produce a variety of different materials, utilizing a variety of different chemistries. The temperature of the propagating front and the velocity at which it propagates can be influenced via chemical or physical means. We show that through careful selection of monomers and control of the concentration of double bonds in a system, that increasing the functionality of the monomer can increase the velocity of a propagating front. We have also shown that residual water in the monomer can effectively lower the front velocity and temperature via heat loss due to vaporization. It was also shown that secondary functional groups present in certain monomers can act as chain-transfer agents. This slows the propagation of the front. We have also tested influencing the fronts’ velocity and temperature with fillers and conductive elements. The use of powdered fillers with high thermal diffusivity and thermal conductivity can lead to more efficient transport of heat through a system. As heat is transported more efficiently the front can propagate much faster with less heat. Using powdered fillers can become expensive in real world applications so composites were studied that had a continuous conductive element embedded through the length of the composite. In our study copper was used. Copper strips were shown to increase the velocity of the front without changing the front temperature by conducting heat ahead of the propagating front

Phase-field-crystal models for condensed matter dynamics on atomic length and diffusive time scales: an overview

Here, we review the basic concepts and applications of the phase-field-crystal (PFC) method, which is one of the latest simulation methodologies in materials science for problems, where atomic- and microscales are tightly coupled. The PFC method operates on atomic length and diffusive time scales, and thus constitutes a computationally efficient alternative to molecular simulation methods. Its intense development in materials science started fairly recently following the work by Elder et al. [Phys. Rev. Lett. 88 (2002), p. 245701]. Since these initial studies, dynamical density functional theory and thermodynamic concepts have been linked to the PFC approach to serve as further theoretical fundaments for the latter. In this review, we summarize these methodological development steps as well as the most important applications of the PFC method with a special focus on the interaction of development steps taken in hard and soft matter physics, respectively. Doing so, we hope to present today's state of the art in PFC modelling as well as the potential, which might still arise from this method in physics and materials science in the nearby future.Comment: 95 pages, 48 figure

DOUBLE-NETWORK MATERIALS VIA FRONTAL POLYMERIZATION & SUPERCRITICAL CO2 PROCESSING

This dissertation presents work focused on producing materials in non-equilibrium states by taking advantage of novel processing techniques. First, epoxy-based resins which can undergo radically promoted, cationic, thermal, frontal polymerization are investigated for their potential use as adhesives. These resins are found to be capable of sustaining propagating polymerization fronts between several different substrate materials, resulting in high levels of adhesion in some cases. In addition, a similar frontal resin was developed that can undergo sequential gelation and frontal polymerization. The gels are formed by radically crosslinking acrylate monomers within the epoxy resin. These gels can then be manipulated, and subsequently frontally polymerized into cross-linked glassy materials. This resin system shows a high degree of stability at room temperature in both the liquid and gelled state. A model was developed to describe the temperature and propagation rate of the front. The resin was also used to produce gelled carbon fiber composites which can be crosslinked with global heating, or frontal polymerization, forming cured composites. Supercritical CO2 (scCO2) was investigated to produce double-network materials from polyamide 6 (PA6) substrates. The scCO2 was used, in combination with methanol, as a solvent to diffuse and polymerize styrene within the amorphous regions of PA6. These PA6/Polystyrene double-network materials are found to have significantly different mechanical and sintering properties compared to PA6. It was also found that the PA6 and Polystyrene are not just trapped via molecular entanglements, but are covalently bound, as demonstrated by changes in solubility of the double-network material with varying polystyrene content. Finally, the same scCO2 method was used to diffuse and polymerize mixtures of blocked isocyanate and hydroxy functional methacrylate monomers within PA6. This was performed in an attempt to create a blend with latent functionality that could crosslink under sintering conditions used in selective laser sintering (SLS). It was found that, while deblocking of the isocyanates is evident at temperatures below the melting temperature of the double-network materials, no crosslinking is evident. However, when the material was held above its melting temperature, an increase in modulus and temperature at failure suggest that some degree of crosslinking may have occurred

Ultrastable glasses : new perspectives for an old problem

Altres ajuts: the ICN2 was funded by the CERCA programme / Generalitat de Catalunya.Ultrastable glasses (mostly prepared from the vapor phase under optimized deposition conditions) represent a unique class of materials with low enthalpies and high kinetic stabilities. These highly stable and dense glasses show unique physicochemical properties, such as high thermal stability, improved mechanical properties or anomalous transitions into the supercooled liquid, offering unprecedented opportunities to understand many aspects of the glassy state. Their improved properties with respect to liquid-cooled glasses also open new prospects to their use in applications where liquid-cooled glasses failed or where not considered as usable materials. In this review article we summarize the state of the art of vapor-deposited (and other) ultrastable glasses with a focus on the mechanism of equilibration, the transformation to the liquid state and the low temperature properties. The review contains information on organic, metallic, polymeric and chalcogenide glasses and an updated list with relevant properties of all materials known today to form a stable glass

Curing of epoxy resin DER-331by Hexakis (4-acetamidophenoxy) cyclotriphosphazene and properties of the prepared composition

The method of optical wedge revealed that the optimum temperature for compatibility of hexakis(4-acetamidophenoxy)cyclotriphosphazene (ACP) and DER-331 epoxy resin is in the range of 220–260◦C. The interdiffusion time of components at these temperatures is about 30 min. The TGA and differential scanning calorimetry (DSC) methods revealed the curing temperature of 280◦C for thiscomposition. IRspectroscopyconfirmedthatthereactionbetweentheresinandACPiscompleted within 10 mi