The importance of stretching rate in achieving true stress relaxation in the elasto-capillary thinning of dilute solutions

This work focuses on inferring the molecular state of the polymer chain required to induce elasto-capillary stress relaxation and the accurate measure of the polymer relaxation time in uniaxial stretching of dilute polymer solutions. This work is facilitated by the discovery that constant velocity applied at early times leads to initial constant extension rate before reaching the Rayleigh-Plateau instability. Such constant rate experiments are used to correlate initial stretching kinematics with the thinning dynamics in the elasto-capillary Regime. We show that there is a minimum initial strain-rate required to induce rate independent elastic effects. Below the minimum extension rate, insufficient stretching of the chain is observed before capillary instability, such that the polymer stress is comparable to the capillary stress at long times and true stress relaxation is not achieved. Above the minimum strain-rate, the chain reaches a critical stretch before instability, such that during the unstable filament thinning the polymer stress is significantly larger than the capillary stress and true stress relaxation is observed. Using a single relaxation mode Oldroyd-B model, we show that the the minimum strain rate leads to a required initial stretch of the chain before reaching the Rayleigh Plateau limit. Along with the accurate measure of relaxation time, this work introduces a characteristic dimensionless group, called the stretchability factor, that can be used to quantitatively compare different materials based on the overall material deformation/kinematic behavior, not just the relaxation time. Overall, these results demonstrate a useful methodology to study the stretching of dilute solutions using a constant velocity stretching scheme.Comment: 27 pages, 9 figure

Formulation of a Model Resin System for Benchmarking Processing-Property Relationships in High-Performance Photo 3D Printing Applications.

A well-defined resin system is needed to serve as a benchmark for 3D printing of high-performance composites. This work describes the design and characterization of such a system that takes into account processability and performance considerations. The Grunberg–Nissan model for resin viscosity and the Fox equation for polymer Tg were used to determine proper monomer ratios. The target viscosity of the resin was below 500 cP, and the target final Tg of the cured polymer was 150 °C based on tan-δ peak from dynamic mechanical analysis. A tri-component model resin system, termed DA-2 resin, was determined and fully characterized. The printed polymer exhibited good thermal properties and high mechanical strength after post-cure, but has a comparatively low fracture toughness. The model resin will be used in additive manufacturing of fiber reinforced composite materials as well as for understanding the fundamental processing–property relationships in light-based 3D printing

Effect of extrusion conditions and postextrusion techniques on the morphology and thermal/mechanical properties of polycaprolactone/clay nanocomposites

The effect of extrusion conditions on the performance of polycaprolactone /organo-modified clay nanocomposites was studied. It was demonstrated that the extrusion parameters have negligible effect on the molecular weight of polycaprolactone, on the morphology of the nanocomposites and on the final thermal/mechanical properties of the materials. This result was a consequence of the previous optimization of both polymer/clay compatibility and clay processing stability. Finally, the molten-polycaprolactone/clay mixtures were post-processed by different techniques submitting the mixtures to extensional flow. Clay platelets alignment was observed as a function of the extensional flow intensity which further improved the mechanical properties of the nanocomposites.Fil: Ludueña, Leandro Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigación en Ciencia y Tecnología de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingenieria; ArgentinaFil: Kenny, J. M.. Università di Perugia; ItaliaFil: Vazquez, Analia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Houssay. Instituto D/tec.y Cs.de la Ing.;Fil: Alvarez, Vera Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigación en Ciencia y Tecnología de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingenieria; Argentin