Polymer solidification under pressure and high cooling rates

Polymer solidification under processing conditions is a complex phenomenon in which the kinetics of flow, high thermal gradients and high pressures de termine the product morphology. The study of polymer structure formed under pressure has been mainly made using conventional techniques such as dilato metry and differential scanning calorimetry under isothermal conditions or non isothermal conditions but at cooling rates several orders of magnitude lower than those experienced in industrial processes. A new equipment has b een recently developed and improved to study the crystallization of polypro pylene when subjected to pressure and cooled rapidly. An experimental appar atus essentially constituted of a special injection mould has been employed . Polymer samples can be cooled at a known cooling rate and under a known p ressure. Micro Hardness (MH), Wide angle x-ray diffraction (WAXD), Polarise d Optical Microscopy (POM) and density measurements are then used to charac terize the sample morphology. The results of rapid cooling experiments unde r pressure on an iPP ample display a lower density and a lower density depe ndence on cooling rate for increasing pressure. Micro hardness confirms the trend. A deconvolution technique of WAXD patterns is sued to evaluate the final ph ase content of samples and to assess a crystallization kinetics behavior. P hase distribution results indicate that the decrease of alpha phase wit pre ssure is balanced by an increase of the mesomorphic phase leaving unaffecte d the amorphous phase content. This peculiar behavior can be easily related to a negative influence of pressure on the kinetics of the crystallization of alpha phase.