Polyimide films derived from representative PMDA/ODA were prepared with thickness ranging from 5 μm to 25 μm,and the effect of aggregation structure on thermal expansion behavior along different directions was studied.Both in-plane and out-of-plane linear thermal expansion(CTE//and CTE⊥)were respectively characterized by thermal mechanical analysis and FT-near-IR interference method.Volumetric and anisotropic behavior of thermal expansion were also investigated.With increasing film thickness,CTE//gradually increased from 32.2 ppm/℃to 46.1 ppm/℃while CTE⊥ decreased from 149.7 ppm/℃to 128.2 ppm/℃.Volumetric thermal expansion of polyimide films was less sensitive to the varied thickness,but anisotropy of thermal expansion was reduced.Polyimide film of 5 μm thickness showed large birefringence,indicating more considerable in-plane chain orientation anisotropy.Besides,molecular chains were more densely packed along in-plane direction when film thickness increased,while became loosely stacked in the out-of-plane direction.In contrast to the enhanced lateral chain packing for thicker films,higher vertical chain packing order was found in thinner films.The variation of aggregation structure during thermal expansion procedure was analyzed by temperature-dependent WAXD.It is proved that thermal expansion behavior of thinner films could be largely attributed to molecular chain packing,whereas that may be influenced by many factors for thicker films in addition to the effect of chain packing.The results revealed that thermal expansion of films with thickness variation is closely related to molecular chain orientation and packing,which is associated with both chemistry and morphological structure of polyimide.

Key Laboratory of Science and Technology on High-tech Polymer Materials,Institute of Chemistry,Chinese Academy of Sciences,Beijing 100190,China;School of Chemical Sciences,University of Chinese Academy of Sciences,Beijing 100049,China

[1]Chang-Ou Wang;Lei Zhai;Song Mo;Yi Liu;Meng-Yan Gao;Yan Jia;Min-Hui He;Lin Fan-.Effect of Aggregation Structure on Thermal Expansion Behavior of Polyimide Films with Different Thickness)[J].高分子科学（英文版）,2022(12):1651-1661

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