(663f) Pressure-Induced Melting and Crystallization of Poly(epsilon caprolactone) in Dense Carbon Dioxide

A new experimental approach has been developed to investigate the melting and crystallization dynamics in semi-crystalline polymers that are exposed to dense carbon dioxide. The approach involves placing a melt pressed film of the polymer with thicknesses in the range from 100 to 300 µm in between two microscope glass plates, and then putting the sandwich inside a mold which serves as a holding plate. The mold is machined to have a large circular opening that allows full observation of the film placed in the mold.   The assembly with the sample with its hold plate is then placed inside a variable- volume high pressure view- cell equipped with sapphire observation windows.  The cell is charged with carbon dioxide and the pressure is adjusted using a pressure generator. The system incorporates capabilities to monitor the transmitted light intensity in real time with a dedicated computer.  At a given temperature, pressure scans are carried out in both the increasing and then decreasing directions of pressure while recording the transmitted light intensities. The transmitted light intensity vs pressure plots provide the details of the crystallization process during pressure reduction, and the melting process during pressure increase.  Carrying out these at a number of different temperatures permits the generation of thermodynamic P-T phase diagrams that provide the crystallization and melting boundaries.  The results will be discussed for selected polyesters, focusing mostly on Poly (ɛ-caprolactone) (Mw = 87 K and PDI = 1.67) over a temperature range from 25 – 45 ^oC and over a pressure range up to about 50 MPa.