Robust and Transparent Polyamide 1012 Membranes for Biogas Purification Controlled by the Degree of Hydrogen Bonding Order

The present work aims to design thermal protocols to generate different crystal forms in even–even polyamides. Such structural control can lead to tailoring of their properties. To achieve our objective, we have chosen low molecular weight (Mn) Polyamide 1012 (PA1012) because its crystal forms can be more easily controlled by changing cooling rates after erasing thermal history in comparison with high Mn PA1012. When the chosen PA1012 was slowly cooled after erasing thermal history (e.g., 2 °C/min) to a temperature where it had crystallized until saturation (i.e., 150 °C), the standard α form could then be obtained when the sample was further cooled to room temperature (RT) at any rate. However, if the sample was first cooled at faster cooling rates (>20 °C/min) until 150 °C, two types of different crystal forms could be obtained upon subsequent cooling to RT. Combining self-nucleation (SN) experiments and wide-angle X-ray diffraction (WAXD) results, we found that the formation of locally ordered H-bonds at high temperatures (during crystallization from the melt) directly affected the final room-temperature crystal form. It was found that the lower crystallization degree and smaller crystal size of the less thermodynamically stable γ′ form films possess not only good toughness and elongation at break but also excellent transparency as well as higher gas permeability coefficient with better gas selection properties. Among them, the γ′ form has a broader market prospect by endowing PA1012 films with excellent mechanical properties and gas transport properties to produce transparent membranes for biogas purification.