Molecular Dynamic Analysis of Intramolecular Mobility in Polyimides

Molecular dynamics models of five polyimides (PIs) based on commercially available dianhydrides PMDA, BTDA, ODPA, 6FDA, and BPADA and 1,4-bis-[2′-trifluoromethyl 4′-(4″-aminophenyl)phenoxy] 2,5-di-t-butylbenzene (FMTBDA) were constructed. Intramolecular mobility of these polyimides was investigated via analysis of the distribution of torsion angles values, torsion angle fluctuation amplitude and rotational mobility of aromatic rings. It was demonstrated that two phenyl rings in diamine connected by С–С bond, are forming quite rigid fragments with predominantly uniplanar position of the rings, while two phenyl rings connected by C–N bonds (between diamine and di-anhydride) allow greater variety of conformations. Nevertheless, the imide cycle of the dianhydride fragment conjugated to the diphenyl fragment of diamine tend to form a plane. The mobility of the polymers being investigated ascends in the order: BPADA-FMTBDA < 6FDA-FMTBDA ≅ ODPA-FMTBDA ≅ BTDA-FMTBDA < PMDA-FMTBDA. This trend agrees well enough with previously obtained estimates for their fractional free volume (FFV). Trajectories for the pairs of dihedral angles were estimated, showing that these mainly contain the jumps between the minima of the rotation energy diagrams. Lower rotational mobility of the di-tert-butylbenzene fragment in all the polymers was observed, which is due to the moment of inertia of the bulky tert-butyl groups and due to the restriction of the mobility in a dense amorphous polymer matrix because of these groups.