(169e) High-Pressure Torsional Braid Analysis of Polymers in Carbon Dioxide: A New Twist in Polymer Characterization Under Pressure

Understanding the mechanical and thermal properties of polymers exposed to compressed gases is of importance in polymer modifications such as foaming with carbon dioxide or nitrogen as physical blowing agents. In foaming, if the modulus of the polymer + carbon dioxide solution is too high bubble nucleation and growth are limited and likewise if the modulus of the solution is too low the gas escapes the polymer during depressurization.

High-Pressure Torsional Braid Analysis (HP-TBA) is a recently developed technique for the assessment of the thermal behavior and the changes in relative modulus of polymer materials exposed to compressed or supercritical fluids.¹ The HP-TBA is a torsional pendulum that consists of a polymer impregnated fiber glass braid housed inside a high-pressure vessel. The braid has a pendulum mass on the end that is externally rotated and the damped sinusoidal oscillations of the pendulum are recorded over time and can be fitted to the equation y = exp(-αt)sin(ωt). Measurements can be made with respect to temperature or pressure by holding one of the variables (P or T) constant. From the damped oscillations the damping coefficient (α), frequency (ω), and the period of the oscillations (2Ï€/ω) can be determined; these parameters are linked to the storage modulus, loss modulus, and mechanical damping. Carbon dioxide acts a diluent in most polymer materials – providing chain mobility and therefore lowering the observed thermal transition temperature (T_g or T_m). The modulus, or rigidity, of the polymer determines the nature of the damped oscillations, and as the polymer approaches T_g or T_m a sharp decrease in modulus can be observed in addition to a peak in the mechanical damping. In this presentation, the unique experimental system, and the recent results with amorphous, semicrystalline, and rubbery polymers will be discussed.

1. Erdogan Kiran, John C. Hassler, High-Pressure Torsional Braid Analysis (HP-TBA): A new technique for assessment of thermal transitions and changes in moduli of polymer exposed to supercritical or compressed fluids, The Journal of Supercritical Fluids 143 (2019) 223-231