(246a) Glass Transition Behavior of Poly(methyl methacrylate) in Compressed Carbon Dioxide Revisited

Poly(methyl methacrylate) is historically one of the model polymers that has been investigated for its behavior in supercritical fluids and is known to display a retrograde T_g behavior in the presence of carbon dioxide. The retrograde glass transition behavior exhibited by PMMA in compressed CO₂ has been studied using high-pressure volumetric, thermal, sorption, creep compliance, chromatographic, quartz crystal resonator, and spectroscopic measurement techniques. Currently there is a wide variability of the data reported in the literature, and specifically a noted absence of retrograde behavior in some publications. This talk will present recent results of a study that was undertaken to bring some clarity to potential causes of this variability in the literature. The polymer molecular weight, whether the thermal transition is approached by increasing temperature or pressure, and the heating or the pressurization rate during assessment all play a role in shifting the observed depressed glass transition, yet such information is not provided in the literature in any systematic manner.

The thermophysical behavior of two samples of PMMA with different molecular weights {Sample 1: M_w = 89,700; PDI = 2.02; T_g(℃) = 109 | Sample 2: M_w = 457,000; PDI = 1.51; T_g (℃) = 124}were assessed using High-Pressure Torsional Braid Analysis (HP-TBA).

HP-TBA is a recent technique developed in our laboratory. Briefly, it consists of a polymer impregnated fiber-glass braid attached to an inertial mass which is housed in a high-pressure chamber. An externally positioned electromagnet is used to induce oscillations of the inertial mass. The decay of the oscillations, which follows a damped sine wave, is monitored and characterized. The frequency of oscillations and the change in amplitude are related to the rigidity and mechanical loss of the polymer. The technique allows the assessment of the relative rigidity and mechanical damping as a function of temperature at a given pressure, or as a function of pressure at a given temperature.

The measurements were conducted in two different modes, one involving increasing temperature at constant pressure and the other involving increasing pressure at constant temperature. Constant-pressure HP-TBA temperature scans were conducted with both the Low and High MW PMMA samples in carbon dioxide at 30, 50, 75, 100, and 200 bar. Two additional scans at higher pressures of 150 and 220 bar were conducted with the High MW PMMA sample. Constant-temperature pressure scans were conducted with both the Low and High MW PMMA sample starting from 20 bar until a transition was observed in the damping peak in real-time.

The Low MW PMMA system displays classic retrograde behavior which was assessed by HP-TBA scans by holding temperature constant and increasing pressure and by holding pressure constant and increasing temperature. High MW PMMA does not display classic retrograde behavior; instead, the system displays a plateau-like region at higher pressures. This investigation into the glass transition behavior of PMMA has shown that the measurement of the depression in the thermal behavior of a polymer material in the presence of dense and/or compressed fluids is nontrivial.