Real-time crystallization and melting of poly(n-octadecyl methacrylate) induced by temperature and pressure. A dielectric spectroscopy investigation

We have studied the crystallization and melting of poly(n-octadecyl methacrylate) (PNODMA) by performing temperature and pressure jump experiments from the melt to the final semicrystalline state. The evolution of the crystallization process is studied in real time by recording the changes in the dielectric spectrum. The single α process in the melt upon crystallization looses intensity and a new slower α´ process appears, reflecting the dynamics of the restricted amorphous phase. The effect of pressure on increasing the crystallization temperature is stronger than the increase of the melt glass transition temperature, i.e. dT_c/dP≈2.3T_g ^m/dP. The isothermal/isobaric crystallization experiments induced by T and P jumps give rise, respectively, to stretched exponential and to typical nucleation and growth kinetics of the Avrami type. The different crystallization kinetics induced by T and P reflect the `paradoxical´ situation of increasing mobility with increasing pressure. From the equivalence of the dynamics we concluded that undercooling of 1 K corresponds to an overpressure of 20 MPa, i.e. δT/δP≈50 WGPa