Enhanced Crystallization of Bisphenol A Polycarbonate in Thin and Ultrathin Films by Supercritical Carbon Dioxide

The crystallization behavior of thin bisphenol A polycarbonate (PC) films after treatment in supercritical CO2 (ScCO2) was investigated by using polarized optical microscopy (POM) and atomic force microscopy (AFM). Experimental results indicated that the crystallization ability in thin PC film of 259 nm thick was higher than that in the bulk in a much wider temperature range, and the crystallization window was further broadened when the thickness of samples decreased. The 15 nm film crystallized under 20 MPa CO2 at 60 degrees C, i.e., more than 90 K below the glass transition temperature of the bulk at ambient pressure, while the 259 nm film remained amorphous under the same treatment condition. The results further revealed that crystalline morphology was affected by the CO2 treatment condition and film thickness. And the 7 nm film dewetted the substrate in the treatment at 20 MPa/60 degrees C instead of crystallization. It was indicated that chain mobility of the polymer was strongly increased in ScCO2 when the film thickness was decreased to the scale of radius of gyration (ca. 6 nm) of the polymer. A modified three-layer model was proposed to explain these findings by introducing the effect of CO2 adsorption. The excess CO2 adsorbed at the free surface and polymer/substrate interface enlarged portions of these two layers and enhanced the polymer mobility therein, which took effect in thin films with thickness from hundreds down to several nanometers.