气相沉积法制备聚酰亚胺薄膜不同单体配比的表征及其性能影响

聚酰亚胺微球作为惯性约束聚变的重要候选靶丸之一,其力学性能和热学性能的提高对于实现聚变点火有重要意义.本文以均苯四甲酸二酐和二氨基二苯醚为原料,采用气相沉积法制备了不同单体配比的聚酰胺酸薄膜,研究了制备聚酰胺酸薄膜过程中不同单体蒸发温度对薄膜组成的影响,并对聚酰胺酸薄膜进行热环化处理.采用红外光谱仪分析了聚酰胺酸薄膜和聚酰亚胺薄膜的组成,结果表明:随着二酐蒸发温度的增加,聚酰胺酸薄膜中过量二酐单体的红外吸收振动特征峰(1780 cm~(-1),1850 cm~(-1)强度增加,单体配比由二胺过量到二酐二胺配比均衡再到二酐过量.热环化处理的过程中,薄膜中过量单体会再次蒸发,其红外图谱显示最终产物只有聚酰亚胺,但单体配比接近样品聚酰亚胺红外特征振动峰(1380 cm~(-1))强度更大.X射线衍射图谱显示配比接近的样品具有更高的晶化程度,说明过量单体的存在抑制了聚酰胺酸分子链的生长,造成分子量降低.采用纳米压痕仪和热重分析仪分别测量了聚酰亚胺薄膜的弹性模量和硬度以及失重曲线,结果表明分子量的降低会造成聚酰亚胺薄膜的弹性模量和硬度的降低,同时热稳定性也变差.扫描电子显微镜图像显示聚酰亚胺薄膜呈层状结构,单体配比接近的样品表面状况更好,这一点与聚酰亚胺分子的生长规律相符合.

Characterization and properties of polyimide films prepared in different monomer ratios by vapor deposited polymerization

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.