Atomistic Modeling of a New Thermoplastic Polyimide in the Amorphous State: Structure and Energetics

• Published in: Macromolecules 1995-10, Vol.28 (22), p.7454-7460
• Main Authors: Zhang, Renshi, Mattice, Wayne L
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma00126a026

Atom-based molecular modeling is conducted for the study of the bulk properties of a recently developed semicrystalline polyimide of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) and 2,2-dimethyl-1,3-(4-aminophenoxy)propane (DMDA) in the amorphous phase at the experimental density of room temperature. This polyimide is abbreviated as PI-2. A short-time high-temperature molecular dynamics procedure is performed in order to relax the PI-2 structures. The energy of each structure is subsequently minimized using molecular mechanics to form the final structures for our study. Ten different amorphous structures were constructed in order to determine the precision of our study by inspecting the standard deviations. Electrostatic interactions are explicitly considered by assignment of proper partial charges to individual atoms, and the Ewald summation method is employed for evaluation of the nonbonded interactions. The radial distribution functions of the atoms appear to show no long-range order, assuring the amorphous nature of the built structures. The correlation between the phenyl rings in the systems suggests that there is local order in the range of 3-4.5A between the rigid ring units due to the van der Waals interactions. The conformational statistics of the polymer backbone in the amorphous structure are compared with those of a PI-2 single chain in vacuum at 300K, obtained from an earlier study. The differences indicate the effect of packing at bulk density. The calculation of the dimensionless chracteristic ratio of the end-to-end distance suggests that the PI-2 chain in the bulk amorphous phase is less extended than a PI-2 single chain in vacuum at 300K. For a single chain in vacuum, C sub infinity =6.4, while for the bulk structures, C sub infinity =5.0. This reduction in C sub infinity arises almost entirely from the appearance of two new rotational isomeric states at a pair of equivalent bonds in the repeat unit. The Hildebrand solubility parameter of amorphous PI-2 calculated from the cohesive energy density yields a value of approx11.3 (cal/cm exp 3 ) exp 1/2 .