Isothermal and Gradient Foaming of Polycaprolactone in Supercritical Carbon Dioxide

Polycaprolactone (PCL) is a polymer of biomedical importance. Scaffolds based on PCL are of great significance in tissue engineering applications. Thus, there is much interest in foaming polycaprolactone foams using benign physical blowing agents such as carbon dioxide. In this poster, we will present the foaming behavior, pore size, pore distributions, and bulk foam density of polycaprolactone exposed to carbon dioxide at 100 bar and varying temperatures. These foaming conditions were determined based on the melting temperature reduction as displayed in the changes in the rigidity of the PCL in carbon dioxide. The changes in rigidity are assessed with High Pressure Torsional Braid Analysis (HP-TBA), a technique that allows for determination of T_m and T_c from the assessment of changes in relative rigidity and mechanical damping as functions of temperature or pressure.

The foamed polymers were prepared in gradient and isothermal foaming cells. The gradient foaming cell exposes a strip of polycaprolactone to a range of temperatures and a uniform set pressure. The temperature is controlled at one end of the cell, allowing the development of a gradient temperature distribution within the cell. In the isothermal cell, the temperature is controlled to ensure a uniform temperature throughout. Either cell was then pressurized with carbon dioxide, using an ISCO pump to maintain the set pressure. After an hour of equilibration under the target pressure, the cells were rapidly depressurized to induce pore nucleation and growth. The foams were then freeze-fractured in liquid nitrogen and then sputter coated with gold and characterized for pore morphology with SEM. Pore size distributions were assessed using ImageJ software. The bulk foam densities were also determined by measuring the mass of the sample in air as well as submerged in water.

The foaming experiments were first carried out in the gradient cell over a temperature gradient of 55-43°C, 45-37°C, 40-33°C, and 35-29°C at 100 bar, and then in isothermal cell at 30, 35, 37 and 40°C, at 100 bar. The results of foaming will be presented in terms of bulk foam density and SEM images and compared with the rigidity reduction path in carbon dioxide. We will also provide the carbon dioxide sorption levels during the saturation stage in carbon dioxide prior to depressurization which were determined using magnetic suspension balance (MSB).