Effect of Chemical Structure and Preparation Process on the Aggregation Structure and Properties of Polyimide Film Ying Wang, Yang Yang, Zhenxing Jia, Jiaqiang Qin, Yi Gu College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, People’s Republic of China Correspondence to: J. Qin (E-mail: jqqin@scu.edu.cn) ABSTRACT: In this article, polyimide (PI) films were fabricated via the three-step method including the reactions of condensation poly- merization, chemical imidization, and thermal imidization. In comparison with the conventional two-step method to produce PI films, there was an additional step in the present method, i.e., chemical imidization. The aim of chemical imidization was to get PI intermedi- ates with different pre-imidization degree (pre-ID). And PI component in PI intermediates acted as in-situ rigid-rod segments and induced orientation in the films of PI intermediates. Then the orientations of molecular chains were preserved in the following thermal imidization, and caused the difference in aggregation structure and property of the final PI films. The test results indicated that the orderly degree of molecular chains and mechanical properties of PI films increased with pre-ID increasing. Furthermore, this tendency was much more obvious for more rigid backbone structure. V C 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 000: 000–000, 2012 KEYWORDS: polyimides; films; crystallization; mechanical properties; thermal properties Received 3 March 2012; accepted 17 May 2012; published online DOI: 10.1002/app.38068 INTRODUCTION Aromatic polyimides (PIs) have been widely used in technology fields due to their excellent thermal stability, good dimensional stability, outstanding mechanical properties, and high resistance to chemicals and radiations. 1–4 PI molecular chains consist of abundant imide and aromatic rings. Besides of common Van der Waals force, there are other interactions among the molecu- lar chains, such as charge transfer interaction, preferred layer packing and mixed layer packing. 5 The insoluble and infusible characteristics of PIs resulted from the above factors make them difficult to process. So far, most of PI films are prepared from the conventional two-step method, and the derived products are almost amorphous without special treatment. 6 Thus, the me- chanical properties were not high enough to meet the demand of high-performance PI films in high-tech applications. Orderly structures such as orientation and crystallization can dramatically enhance the mechanical and thermal properties, and decrease linear coefficients of thermal expansion (CTE) of polymer materials. Thermal cure of a slightly drawn film of pol- y(amic acid) (PAA) showed marked spontaneous orientation to- ward the stretching direction. 7 Parallel to the draw direction the modulus of PMDA-ODA PI films increased, while the modulus decreased perpendicular to the draw direction. 8 Kochi et al. 9 prepared a high-modulus and high-strength PI material by a technique of thermal imidization after cold drawing of PAA films. The studies 10–12 on the spontaneous molecular orienta- tion of PI showed CTE decreased almost linearly with an increase in the degree of in-plane orientation. In order to increase the degree of crystallinity, researchers designed the backbone structure of PI in two ways: to increase the mobility of molecular chains and the amount of rigid molecular chains. Since molecular chains could move freely in solution, PI particles obtained from imidization in solution were highly crystalline. 13 It has been found that introduction ether linkage or carbonyl group into diamine could improve the mobility of molecular chains and increase the orderly degree. 14–16 However, the flexible groups of PI main chains give rise to decreasing of mechanical properties. To enhance the degree of crystallinity and mechanical properties of PI simultaneously, some rigid-rod segments which could easily induce the formation of crystalline structure were introduced to the backbone, and found the crystallinity, in-plane orientation, hardness and modulus of the PI films increased with increasing contents of rigid-rod segments. 17–19 PAA converts to PI by thermal or chemical imidization, and the intermediate in imidization is a copolymer composed of amic- acid and imide, called copolyamic acid-imide(PAA-PI). 20 According to the previous papers, 20–22 the mechanical properties of PI materials can be enhanced with the precursor (PAA) Additional Supporting Information may be found in the online version of this article. V C 2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2012, DOI: 10.1002/APP.38068 1