高温热处理对聚酰胺-酰亚胺薄膜超低热膨胀行为的影响

Effect of High Temperature Annealing on Thermal Expansion Behavior of Poly(amide-imide) Films with Ultralow Coefficient of Thermal Expansion

采用刚性结构的二酐与含酰胺结构的二胺设计制备了具有高耐热稳定性的聚酰胺-酰亚胺薄膜,系统研究了高温热处理对薄膜热膨胀行为的影响.结果表明,薄膜经高温热处理后玻璃化转变温度(T_g)有较明显提高,并在30~400℃宽温域内具有超低热膨胀特性;随着热处理温度提高至400℃,薄膜的CTE由负值逐渐趋近于0.利用双折射、傅里叶红外变换光谱、广角X射线衍射(WAXRD)和X射线散射(WAXS)等方法,对聚合物聚集态结构的变化规律进行考察发现,随着热处理温度升高,薄膜的面内分子链取向程度增大,链间氢键相互作用增强,且分子链堆积更加紧密;当热处理温度达到425℃时,由于膜厚方向上分子链间距离增大导致薄膜呈较明显热收缩.研究结果表明,通过分子结构和制备工艺可实现薄膜热膨胀性能的有效调控,为制备兼具高耐热和超低CTE的聚合物薄膜提供了新思路.

A kind of representative poly(amide-imide)(PAI) films derived from 1,2,4,5-benzenetetracarboxylic dianhydride(PMDA) and amide-containing diamine i.e. N,N’-(1,4-phenylene)bis(4-aminobenzamide)(PABA)were prepared via thermal imidization, and then annealed at various high temperatures including 350, 375, 400 and 425 ℃, respectively. With the increase of annealing temperature, the heat-resistance of PAI films were improved with higher T_g values, whereas their thermal decomposition stabilities were reduced to some extent especially when annealed above 400 ℃. All of these PAI films exhibited ultralow thermal expansion with negative coefficient of thermal expansion(CTE) values from –6.87 ppm/℃ to –3.84 ppm/℃ even in a wide temperature range of 30 – 400 ℃. It was noted that the CTE values of PAI films were increased to around zero as annealing temperature elevated. The annealing effect on aggregation structures and thermal expansion behavior was further investigated by birefringence(Δn), FTIR, WAXRD and WAXS. The birefringence of PAI films was extraordinarily larger than that of aromatic polyimide films, indicating that PAI molecular chains were more oriented in the in-plane direction. Their Δn values ranged from 0.2438 to 0.2621 as annealing temperature increased from 350 ℃ to 425 ℃. The hydrogen bonding interactions were proved to be maintained even at high temperature as the main reason for the dimension stabilities of PAI films. It was also found that annealing at high temperature could contribute to the enhanced intermolecular interactions. In addition, the intermolecular chain distance of PAI films was observed to be reduced with the increasing temperature, suggesting that molecular chains were packed more densely. Furthermore, the interchain distance in the film thickness direction was more affected by annealing with large variation than that of in-plane direction. PAI-425 film showed significantly negative thermal expansion mainly because of its expanding out-of-plane interchain distance. Based on high temperature annealing, the relationship between thermal expansion behavior and aggregation structures of PAI films was established to be used for the regulation and control of thermal expansion. It provided a new strategy to prepare heat-resistant polymer films with ultralow CTE values by the structure design and high temperature annealing.