ABSTRACT:The development of in-plane orientation in pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyimide film coated on silicon substrate during thermal imidization was investigated by means of FT-IR spectroscopy. The increase in thermal imidization temperature caused the decrease in the degree of in-plane orientation in spite of the increased chain rigidity. It suggests that the chain immobilization limit the development of the in-plane orientation during thermal imidization. The residual solvent was found to decrease the degree of in-plane orientation probably due to the increase of the chain immobilization rate as indicated by the quantitative measurements of the degree of imidization and the residual solvent content as a function of imidization time at various temperatures.KEY WORDS Pyromellitic Dianhydride-Oxydianiline I Thermal lmidization I Chain Rearrangement I In-Plane Orientation I Residual Solvent I Chain Immobilization IThe chain orientation in polyimide coated on substrate is characterized by the term 'in-plane orientation' which means that the director of polyimide chain is parallel to substrate. 1 -3 Since various anisotropic properties of dielectric constant, refractive index, and coefficient of thermal expansion depend on degree of in-plane orientation,4-8 the development of in-plane orientation during thermal imidization is a matter of concern.Since the in-plane orientation of polymer film coated on substrate is originated from the restricted chain rotation near the substrate, a polymer of higher chain rigidity shows stronger tendency for developing in-plane orientation, when polymers of different rigidity are compared. 9 · 10 However, we previously found in a study on the effect of imidization condition on the in-plane orientation of polyimide that degree of in-plane orientation of polyimide was, in some cases, found to be higher at lower chain rigidity_ll This behavior was explained in terms of the chain immobilization which is caused by the increase of the chain rigidity during thermal imidization. Even though the driving force for developing in-plane orientation would increase as imidization proceeds, the long-range motion of the chain leading to in-plane orientation could be limited by the decreased chain mobility. In this work, we changed the chain immobilization rate for pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyamic acid cast on silicon wafer by varying the imidization temperature and the residual solvent content, and investigated the corresponding changes in the in-plane orientation of polyimide by means of FT-IR spectroscopy to obtain better understanding on the development of in-plane orientation during thermal imidization.
EXPERIMENTAL
Materials and P AA Cast PreparationPyralin® (DuPont), the mixture of the polyamic acid ( 4,4' -oxydiphenylene pyromellitamic acid), N-methylt To whom all correspondence should be addressed. 154 pyrrolidinone (NMP), and petroleum ether, was poured into excess methanol to precipitate the polyamic acid. The precipitates were washed with...