Shape memory polyimide composites with high storage modulus and high glass transition temperature

Shape memory polymers (SMPs) are smart materials that can be programmed to change shape under external stimuli, whereas the low storage modulus limit the application of them. Herein, carbon fabric (CF) reinforced shape memory polyimide composites (SMPICs) with high storage modulus were manufactured via hot pressing molding process. Firstly, we synthesized one kind of thermoplastic shape memory polyimide (SMPI) with glass transition temperature of 205°C by the two-step high-temperature solution polycondensation. In addition, the triamine was added in the SMPI system as a crosslinking agent to form the thermosetting SMPI with different crosslinking degree. In order to improve the storage modulus of SMPI, the CFs with three layers were embedded in thermosetting SMPI matrix. The storage modulus of the obtained SMPICs was as high as 26 GPa. The glass transition temperature and thermal decomposition temperature of SMPICs were up to 213°C and 505°C, respectively. Moreover, the shape fixation rate and recovery rate of SMPICs were both more than 94%. These SMPICs with high storage modulus is of great significance, proving more application potential in many fields such as aerospace.

Mesopore silica effect on chemical, thermal and tribological properties of polyimide composites

In this experimental study, the effects of mesoporous silica filler content on the chemical, thermal and tribological properties of polyimide composites were investigated. For that purpose, Pi/mesoporous silica composites were produced by in situ polymerization with various mesoporous silicas. After fabrication, thermal stability and chemical characterization were determined using TGA and FTIR. Morphological alterations were monitored with a scanning electron microscope (SEM). Texture structure (pore size and pore volume) were determined by the BJH method. Friction and wear properties were investigated by using a pin-on-disc arrangement. At the end of the study, minor shifts of Pi/mesoporous silica composites were observed. Thermal stability, as well as pore size and pore volume, was decreased with mesoporous silica. The coefficient of friction and specific wear rate decreased with the addition of mesoporous silica. Abrasive wear behaviors were seen for both neat Pi and Pi–Si composites. Hence, this study evidenced that the properties of Pi are influenced by mesoporous dimensions and content of Si employed.

Thermo-oxidative aging and thermal cycling of PETI-340M composites

Polyimide composites (PETI-340M) were fabricated and subjected to high-temperature aging and thermal cycling to evaluate resistance to degradation. Mechanical-degradation mechanisms and kinetics depended on aging temperature. Aging at 232°C resulted in strength loss due to polymer degradation, while intra-tow cracking was the dominant mechanism during aging at 288°C. Composite panels subjected to thermal-cycling fatigue (−54°C to 232°C) retained mechanical properties without microcracking. However, in regions containing pre-existing fabrication-induced defects (primarily voids), intra-tow microcracks were observed after thermal cycling. Unlike some polyimide composites (PMR-15), oxidative aging effects during thermal cycling were negligible. The thermo-oxidative stability and the retention of mechanical performance after thermal cycling indicates potential for long-term, high-temperature structural applications.