Synthesis and characterization of Thermally Rearranged (TR) polymers: influence of ortho-positioned functional groups of polyimide precursors on TR process and gas transport properties
作者:Ruilan Guo、David F. Sanders、Zachary P. Smith、Benny D. Freeman、Donald R. Paul、James E. McGrath
DOI:10.1039/c2ta00799a
日期:——
Aromatic polyimides bearing various ortho-functional groups (i.e., acetate group and pivalic acetate group) were prepared via acetylation of a poly(hydroxyimide) containing ortho-positioned hydroxy groups using acetic anhydride or pivalic anhydride. The completeness of acetylation was confirmed by 1H NMR and FTIR. Chemically derivatized polyimides were used as precursors for an imide-to-benzoxazole thermal rearrangement (TR) process. The influence of various ortho-functionalities on the TR process and gas transport properties of the resulting TR polymers was studied. Differing from the –OH groups in a poly(hydroxyimide), the acetate groups of acetylated polyimide precursors degrade at elevated temperatures, and the degradation process interplays with imide-to-benzoxazole conversion. The acidic degradation product, as detected by 1H NMR, is suspected to have some catalytic effect on the TR process, which along with the protecting function of the acetate groups, resulted in a lower onset TR conversion temperature, the ability to conduct the TR process in air, and a higher TR conversion level. Gas permeation properties greatly depend on the ortho-functionality of polyimide precursors as well. The precursor films containing larger functional groups are much more permeable with comparable gas selectivities. Similarly, the resulting TR polymers formed from polyimides with larger leaving groups also showed much higher gas permeabilities despite similar degrees of TR conversion. The incorporation of bulkier functional groups in the TR precursors provides an effective way to significantly improve the gas transport performance, particularly the gas permeabilities of both the polyimide precursors and the resulting TR polymers.
通过使用乙酸酐或新戊酸酐对含有邻位羟基的聚羟基酰亚胺进行乙酰化,制备了带有各种邻位官能团(即乙酸酯基团和新戊酸乙酸酯基团)的芳香族聚酰亚胺。通过1H NMR和FTIR证实乙酰化的完全性。化学衍生的聚酰亚胺被用作酰亚胺到苯并恶唑热重排(TR)过程的前体。研究了各种邻位官能团对 TR 过程和所得 TR 聚合物气体传输性能的影响。与聚(羟基酰亚胺)中的-OH基团不同,乙酰化聚酰亚胺前体的乙酸酯基团在高温下降解,并且降解过程与酰亚胺到苯并恶唑的转化相互作用。通过 1H NMR 检测,酸性降解产物被怀疑对 TR 过程具有一定的催化作用,这与乙酸酯基团的保护功能一起,导致较低的 TR 转化起始温度、进行 TR 过程的能力在空气中,以及更高的TR转换水平。气体渗透性能也很大程度上取决于聚酰亚胺前体的邻位官能团。含有较大官能团的前体膜在具有相当的气体选择性的情况下具有更高的渗透性。类似地,由具有较大离去基团的聚酰亚胺形成的TR聚合物也表现出更高的气体渗透性,尽管TR转化程度相似。在TR前体中引入较大的官能团提供了一种显着改善气体传输性能的有效方法,特别是聚酰亚胺前体和所得TR聚合物的气体渗透性。