Tailoring of Polyimide Properties by Structural Modifications, Copolymerization and Nanocomposites Synthesis

Abstract

Aromatic polyimides are materials of scientific and commercial interest due to their high thermal stability, excellent mechanical strength and stiffness at elevated temperatures, chemicals and radiation resistance etc. Owing to such worthy combination of aforementioned properties, these polymers have broad spectrum of applications in modern industries. These exceptional properties of polyimides are mainly dependent on their chemical structures hence, can be attuned as required. The prime objective of the dissertation was to tailor the properties of polyimides. The task was accomplished by implementing three different methodologies: i) structural modifications of monomers (diamine/dianhydride), ii) copolymerization, and iii) nanocomposite engineering. With respect to structural modifications, eight new diamine monomers were synthesized successfully having systematic and comparable variations in their structures. Their design involved the considerations of structure rigidity (having different contents of aromatic and aliphatic moieties) and catenation (different positioning of amino groups). These synthesized diamines were condensed with three different dianhydrides for the development of twenty four novel polyimides. Another series comprising of fifteen polyimides was prepared by using five structurally related commercial diamines to explore the influence of different side or pendant groups on the properties of resulting polyimides. The dianhydrides used in the project were selected to study the impact of different bridging groups, with respect to tailoring of properties. The new copolyimides were synthesized by simultaneous reaction of two different diamines (one of the synthesized diamine + 4,4ʹ-methylenedianiline) with the same dianhydride. The polyimide nanocomposites were engineered via incorporating Al2O3 and ZnO nanoparticles within the polyimide matrix at different loading levels. The spectroscopic techniques like FTIR and NMR (1H, 13C) were used for structural elucidations of dinitro compounds, diamine monomers, polyimides and copolyimides. The single crystal X-ray diffraction analysis of the synthesized dinitro and diamine compounds validated their proposed structures from the spectroscopic data and provided valuable information about their spatial orientations and inter/intra-molecular attractions. The incorporation of nanoparticles (Al2O3 and ZnO) inside the polyimide matrix was verified by WAXRD analysis while SEM and TEM microscopic techniques disclosed their homogenous distribution throughout the matrix. The properties of polyimides, copolyimides and polyimide nanocomposites were evaluated by dynamic and isothermal TGA (nitrogen and air atmospheres), DMTA (dynamic mechanical thermal analysis) and WAXRD studies. Polyimides displayed significantly high thermal stability as their decomposition started around 400 °C and experienced a relatively small weight loss up to the temperatures of 450-500 °C. Isothermal TGA revealed that the synthesized polyimides displayed substantially high thermal endurance at 400 °C i.e. capable to withstand elevated temperatures for long time. Their thermal performance was affected considerably as a function of monomer architect. The thermal stability/endurance of polyimides was also tailored by changing catenation of amino functionality and substituting different side or pendant groups in the diamine monomers. The nature of bridging group in the dianhydrides influenced the thermal behavior of obtained polyimides. The incorporation of 4,4ʹ-methylenedianiline (MDA) as co-monomer within the backbone of polyimides (copolymerization) resulted in substantial enhancement of their thermal stability and high temperature durability. The nanocomposites engineering also modified the properties of polyimide. The glass transition temperatures of polyimides exhibited an increasing trend with the methyl substitution on the benzene rings of MDA or replacement of its methylene hydrogens with -CF3 and 9-fluorenylidene moieties. However, it was decreased upon ethyl exchange on the benzene rings. The polyimides displayed storage modulus (Eʹ) above one GPa up to the temperature around 250 °C. The Al2O3 and ZnO nanoparticles increased the glass transition temperature of the polyimide matrix in concentration dependent manner up to the optimum level. WAXRD analysis revealed the semicrystalline to amorphous morphology of polyimides depending upon the monomer structure. The polyimides derived from 3,3ʹ,4,4ʹ-benzophenonetetracarboxylic dianhydride (BTDA) and para-catenated diamines displayed semi-crystalline behavior. However, it was decreased upon copolymerization and changed to amorphous while going from: para to meta/ortho or BTDA to ODPA/6FDA analogues (ODPA = 4,4ʹoxydiphthalic anhydride and 6FDA = 4,4'-hexafluoroisopropylidenebisphthalic anhydride).