Synthesis and characterization of processable aromatic polyimides and their initial evaluation as promising biomaterials

Abstract

Aromatic polyimides are one of the successful classes of synthetic polymers featuring high thermal stability, thermooxidative stability, low dielectric constants, excellent mechanical properties and chemical and solvent resistance, which are some of the reasons for the immense scientific and commercial interest in aromatic polyimides [1, 2]. Due to these superior properties, polyimides enjoy a wide range of application in different fields such as biomaterials, adhesives, gas separation membranes, composite matrices, coating and foams [3–6]. The prospects of synthesis and studies of extended rod-like or rigid aromatic polyimides have been extended due to their superior qualities. These include thermal stability, high modulus and high strength fibers, with low thermal expansion coefficient for packing in microelectronic applications [7]. However, the rigid, rod-like polyimides cannot be used as engineering materials as their poor solubility and processability hinders them to react below their decomposition temperature [8]. Various research efforts have been carried out to synthesize soluble polyimides in fully imidized form in such a way that their excellent properties are not compromised [9–11]. One of these investigations was the introduction of flexible segments [e.g. -O-, -SO2-, -CH2-, -C(CF3)2and –NHCO-] into the polymer chain. These groups provide kinks between the rigid phenyl rings in the backbone, and these lead to enhanced solubility of the polymer [12–17]. The replacement of symmetrical aromatic rings by unsymmetrical ones led to the reduction in crystallinity [18], while introduction of bulky lateral substituents (such as t-butyl, phenyl and adamantly groups) decreases the close-packing in the polymer backbone thus enhancing solubility of the polymer [19–22]. Besides the above applications, another striking reason for particular attention towards aromatic polyimides is that their thermal properties bear a close resemblance to a class of copolyimides called poly(amide-imide)s. Furthermore, the inclusion of an amide group into the polyimide backbone increases its processability, solubility and moldability [23]. In addition to this, poly(amide-imide)s have shown outstanding hydrolytic stability, excellent resistance to high temperature and promising balance of other physical and chemical properties [24]. Aromatic polymers with arylsulfone linkages are generally amorphous, have higher chain flexibility, lower glass transition temperatures and better tractability as compared to their corresponding polymers without these groups in the repeat units. The enhanced solubility and lower glass transition temperatures are credited to the flexible linkages that provide a polymer chain with a lower energy for internal rotation. Different disciplines such as sciences, medicine, material sciences and engineering functioned together to form the field of biomaterials [25]. The use of polymers as biomaterial is also known to the world. Biomedical polymers need to be biocompatible to avoid adverse biological effects to the nearby tissues and degradation of the ionic biological environment if exposed for a longer period [26]. G. Waris :H. M. Siddiqi (*) : Z. Akhtar Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan e-mail: humaira_siddiqi@yahoo.com