High-Pressure Torsional Braid Analysis for Assessments of Melting and Crystallization Temperatures of Polymers in Carbon Dioxide

Torsional braid analysis is a technique that involves an inertial mass suspended from a polymer-impregnated glass braid that is subjected to an initial oscillation. This results in a decaying oscillation that is describable by a sine or cosine wave. Comparison of the frequency and period of oscillations over a range of temperatures allows for the assessment of changes in the relative rigidity and mechanical loss which helps identify the thermophysical transitions such as the glass transition, melting, or crystallization temperatures. As transition temperatures are approached, mechanical damping goes through a maximum, and is accompanied with a significant change in relative rigidity. The changes in the mechanical damping and relative rigidity are then used to assign the transition temperatures.

High-pressure torsional braid analysis (HP-TBA) is a modification of this technique to carry out these analyses in a compressed fluid environment. Analyses can be carried out by conducting temperature scans at a given pressure, or pressure scans at a given temperature.

In the temperature scan mode, the system is pressurized with carbon dioxide to the desired target pressure using an ISCO syringe pump. Pressure is kept constant throughout the temperature scan while the temperature is increased in a step-wise controlled rate. Every 80 seconds oscillations are initiated, and data are recorded at the prevailing temperature. After the heating scan is completed, the system is cooled, while again recording data.

In this poster, we will show the results of recent evaluations carried out with a glassy (poly(methyl methacrylate)), and rubbery (poly(ethylene-co-vinyl acetate-co-carbon monoxide) (EVACO) polymer in compressed carbon dioxide. Polymer impregnated braids were prepared from 5 wt % solutions of PMMA in acetone, and EVACO in 1,2,4-trichlorobenzene. The impregnated braids were dried for a day in open air in a hood and then in a vacuum oven for an additional day to remove the excess solvent before installing in the HP-TBA system. After installing the braid, the system was fully purged with CO₂ prior to pressurization. For each polymer, transition temperatures were then determined during temperature-scans at selected CO₂ pressures. The poster will show the results in terms of the shifts in the transition temperatures as a function of carbon dioxide pressure.