(743c) In Situ-Polymerized CNT/Polyimide Nanocomposites: Effect of Reaction Stoichiometry On the Glass Transition Properties of the Nanocomposites

Polyimides (PIs) are one of the most important classes of high-performance polymers. Due to their outstanding electrical, thermal and high-temperature mechanical properties, polyimide materials have been widely used in industry as high-temperature insulators, dielectrics, coatings, adhesives, and matrices for high-performance composites. Yet for many applications, for example, in advanced microelectronics and aerospace technologies, further improvements of the properties are demanded. Since the discovery of carbon nanotubes (CNTs) in 1991, a great amount of research has been devoted to the study and development of CNT/polymer nanocomposite materials with tailored/improved properties. CNTs possess many of the desired structural, chemical, electrical and thermal properties. Therefore, CNT/PI nanocomposites have attracted great interest of researchers, because such materials are anticipated to further improve the properties of the already-useful PI materials for various applications. Unfortunately, CNTs are typically very difficult to disperse in polymeric media, and CNT/polymer composites are reported to have a tendency to undergo phase separation over time. To achieve well-dispersed CNT/PI nanocomposites, several approaches have been demonstrated, including the surface functionalization of CNTs with chemical groups compatible with PI, the pre-imidization mixing between poly(amic acid) (PAA) and CNT in solution, and the post-imidization melt processing. These methods involve mixing of CNTs with either of the PAA (i.e., before imidization) or PI polymers. To obtain better dispersed CNT particles, the in situ polymerization technique using CNTs surface-functionalized with reactive chemical groups has been proposed. In this method, the monomers are polymerized in the presence of the reactive (i.e., polymerizable) CNTs, and the CNT fillers become chemically connected to the polymer matrix as a result of the polymerization reaction. In most prior examples in the literature demonstrating CNT/PI composites prepared via the in situ polymerization process, the role that the reaction stoichiometry (i.e., the ratio between the functional groups) plays in determining the molecular weight and thus the properties (such as the glass transition property) of the polycondensation product has yet not been carefully examined. The present work aims to address this issue. For this study, a model PI system was synthesized by polycondensation of 4,4'-Oxydiphthalic anhydride (ODPA) and 4,4'-Oxydianiline(ODA). CNT/PI nanocomposites were prepared using COOH-functionalized multiwall CNTs. The glass transition temperature (T_g) properties of the CNT/PI nanocomposites prepared under various functional group ratios were examined. The T_g results showed an unambiguous correlation with the molecular weight of the matrix polymer, demonstrating that the functional group ratio is indeed an important control parameter that needs to be optimized for obtaining the best possible performance of an in situ polymerized CNT/PI nanocomposite. Quantitative details regarding how the in situ-polymerized CNT fillers impact the glass transition properties of the matrix polymers will be discussed in the presentation.