Influence of structural modification on the properties of poly(amide–imide)s

In this study, the influence of length of flexible groups on the properties of poly(amide–imide)s (PAIs), three-model polymers (poly(amide–imide)-4-aminobutyric acid, poly(amide–imide)-6-aminocaproic acid, and poly(amide–imide)-11-aminoundecanoic acid) possessing different flexible methylene units ((CH ₂ ) ₃ , (CH ₂ ) ₅ , and (CH ₂ ) ₁₀ ) in the main chain were designed. With increasing the number of methylene units, it is found that the tensile strength of PAIs decreased from 75 MPa to 55 MPa; meanwhile, the elongation at break increased from 6% to 15%. On the other hand, the glass transition temperature decreased from 207°C to 112°C; fortunately, the starting decomposition temperature kept almost same with a high point around 400°C. Furthermore, the PAI with (CH ₂ ) ₁₀ unit in the main chain is a semicrystalline polymer, while the one with (CH ₂ ) ₅ or (CH ₂ ) ₃ unit is an amorphous material. In other words, the length of the flexible chain in the polymer backbone not only plays an important role in mechanical and thermal performances but also affects their phase transition. These findings highlight the important role of structural modification in high-performance polymers and may help in the further development of novel PAIs for their potential applications in advanced technology.