Effect of poly(amic diethyl ester) precursor on residual stress behavior of aromatic polyimides

The influence of poly(amic diethyl ester) (PAE) and poly(amic acid) (PAA) precursors on the residual stress behavior of polyimide thin films was investigated. Rodlike poly(p-phenylene pymellitimide) (PMDA-PDA), pseudo-rodlike poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), semiflexible poly(4,4-oxydiphenylene pyromellitimide), and poly(4,4-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA) were used. For polyimides prepared from PAE precursor, residual stress of fully cured films at 400 °C were 13.4 MPa for the PMDA-PDA, 19.5 MPa for BPDA-PDA, 30.3 MPa for PMDA-ODA, and 39.5 MPa for BPDA-ODA. For polyimides prepared from PAA precursor, residual stress after fully cured at 400 °C was -5.1 MPa for the PMDA-PDA, 7.0 MPa for BPDA-PDA, 29.4 MPa for PMDA-ODA, and 40.1 MPa for BPDA-ODA. The effect of different size of precursor on the residual stress behavior was significantly high for rodlike polyimides having high chain rigidity and a high intermolecular packing order, but relatively low for the semi-flexible polyimides with relatively low chain rigidity. The degree of molecular order for the rigid structure polyimides, PMDA-PDA and BPDA-PDA, was hindered by the bulky ethyl ester group evaporated during thermal imidization. However, the effect of the bulky ethyl ester group was not significant for the flexible structure polyimides, PMDA-ODA and BPDA-ODA. Residual stress was in decreasing order BPDA-ODA<PMDA-ODA<BPDA-PDA<PMDA-PDA polyimide regardless of the precursor origins. These results indicate that the residual stress behavior in the polyimides depending on precursor origin is significantly related to the morphological structure as well as the chain mobility of the polyimide during thermal imidization.