Characterization and properties of polyimide films prepared in different monomer ratios by vapor deposited polymerization
Authors: Hou Kun  , Zhang Zhan-Wen  , Huang Yong  , Wei Jian-Jun  ,   Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China;,   Research Center of Laser Fusion, CAEP, P. O. Box 919-987, Mianyang 621900, China
Published: December 2020

The improvement of mechanical and thermal properties of the polyimide (PI) capsule, which is one of the most important ignition capsules in inertial confinement fusion experiment, has great significance to realize the ignition. PI capsules can be prepared by solution method and vapor deposited polymerization (VDP) method. In comparison with the traditional solution method, the polyimide film prepared by the method of vapor deposition has the characteristics of controllable thickness, uniformity, and better surface roughness. At the same time, it can be deposited on the surface of complex structures. Because of its excellent properties, VDP has great advantages in the preparation of PI films and capsules. The difference between the capsule and the film prepared by VDP is mainly caused from the geometric size and the substrate. By adjusting the monomer content, the performance of PI film can be improved, and it is important for enhancing the performance of PI capsules. In this paper, pyromellitic dianhydride (PMDA) and oxydianiline (ODA) are used to prepare poly amic acid (PAA) films in different monomer ratios by vapor deposited polymerization. The evaporation temperature of ODA is 95 ℃, and that of the PMDA may be 120 ℃, 123 ℃, 124 ℃ or 126 ℃ respectively. FT-IR spectra measurement shows that the absorbance of PMDA becomes stronger with increasing evaporation temperature. After heat treatment, the excessive PMDA is evaporated again. There exists only PI in the final product. But the structure, elastic modulus, and hardness of the PI thin films are influenced by the existence of excessive monomer in the progenitor. With the increase of PMDA evaporation temperature, the stretching vibration peaks of C-N bond intensity will be 72.1%, 91.6%, 69.2% and 63.5%, compared with the vibration peaks of benzene ring; this indicates that the molecular weight of PI is reduced by the imbalance of the composition. The XRD curve shows that the film with composition close to it has a higher crystallization degree. FT-IR and XRD curvs indicate that the presence of excess monomer could inhibit further growth of the molecular chain, resulting in the decrease of molecular weight. The elastic modulus and hardness of polyimide film are measured by the nano-indentation tester, and the thermal stability is analyzed by a thermogravimetric analyzer. Samples of low molecular weight show low elastic modulus and hardness, while the thermal stability becomes worse. The temperatures of different samples with 5% molecular weight are 487 ℃, 524 ℃, 542 ℃ and 533 ℃ respectively, showing that increase of molecular weight is beneficial to the improvement of thermal stability of the films. Scanning electron microscopy images show that the polyimide film has a layered structure, the samples having composition close to each other show better surface roughness, being in good agreement with the molecular growth theory of polyimide.