Fabrication and Ultraviolet Characterization of Polyimide Film

Panpan Zhang 1, a, Jiaqi Lin 1, 2, b and Wenlong Yang1,c 1College of Applied Science, Harbin University of Science and Technology, Harbin, 150080, China

2Key Laboratory of Engineering Dielectric and Its Application, Ministry of Education, Harbin, 150080

aa851386031b@163.com, bljqi405@163.com, cyangwenlong1983@163.com

Keywords: polyimide, optical properties, microwave Imidization, film

Abstract. A new microwave imidization method was used to prepare polyimide (PI) film in this

paper. FT-IR spectrum and ultraviolet absorption spectrum are measured to study the chemical

structure and optical properties of this film. The FT-IR spectrum shows that the characteristic imide

groups are observed at the peaks of 727.18 cm-1

, 1379.67 cm-1

and 1776.51 cm-1

, which confirmed

imide formation. The ultraviolet absorption spectrum reveals that optical band gap of the PI is about

2.64 eV.

Introduction

Aromatic polyimide (PI) is the most important class of high temperature thermo-stable polymer. In

recent years, PI has received more and more attention in electrical and electronic industries due to its

high thermal and chemical stability, excellent mechanical properties, superior weather resistance, low

thermal expansion coefficient, and its formability [1–4]. Two-step method is a common method

which is used to prepare the polyimide film. And the imidization process from polyamic acid (PAA)

to PI is one of the most important steps. In general, the imidization method can be classified into two

broad categories: thermal imidization and chemical imidization. In addition, thermal imidization is

performed by a high temperature oven, which is time-consuming and expensive.

In this study, the PI film were prepared by a new microwave imidization process, which also can be

considered as one kind of the thermal imidization in some extent. The infrared spectrum and

ultraviolet absorption spectrum of the PI film have been measured, and the results have also been

further explored and discussed.

Experimental

Raw materials. The Pyromellitic dianhydride, PMDA (AR), 3,3',4,4'-benzophenonetetracarboxylic

dianhydride, ODA (AR) were obtained from Sinopharm Chemical Reagent Co., Ltd.

1-Methyl-2-pyrrolidone, DMAC (99.0%) was purchased from Tianjin Guangfu Fine Chemical

Research Institute. The monomers and solvent were used as the raw materials without further

purification. The chemical structures of the monomers used are shown in Fig. 1.

Fig. 1 The chemical structures of monomers

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Synthesis of PI. The synthesis of aromatic polyimide was carried out by reacting the PMDA and

ODA in DMAC solution. The mole ratio between PMDA and ODA were 1.02:1. The synthesis

procedure was as follows. First, ODA was diluted with a DMAC solution in a pre-equilibrated

3-necks round bottom flask with a stirrer at 25°C. After complete dissolution of ODA in DMAC,

PMDA was added into the suspension in six portions to ensure the complete dissolution of the

previous portion. Each portion was added at intervals of 6 min. When PMDA was completely added

and dissolved, the polyamic acid （PAA ）solutions with a solid content of 12 wt% were obtained by

reacting and stirring for 1 h at room temperature. Then the PAA solution was cast onto a cleaned glass

plate for imidization. Here, the imidization was performed using a microwave oven

(MM721NH1-PW, Midea, China) through a five step cure process (low power/20 min, low middle

power/20 min, middle power/20 min, high middle power/20 min, high power/ 20min).On cooling, the

films were stripped off the glass surface by immersion of the glass plate into pure water. The average

thicknesses of the films are 28-30µm in final. The chemical structure of PI is shown in Fig. 2. Finally,

these films were washed using acetone for testing. FT-IR spectrum was performed using a fourier

infrared spectrometer (Avatar 370, Thermo Nicolet Corporation, America). Ultraviolet absorption

spectrum was obtained by double beam UV-VIS spectrophotometer (TU-1901, Persee, China) at

wavelength range 200-900 nm.

Fig. 2 The chemical structures of PI

Results and discussions

Infrared spectral analysis. The chemical structure of PI was measured by FT-IR spectroscopy. The

FT-IR spectrum of PI film is given in Fig.3. As shown in Fig. 3, the characteristic imide groups are

observed at the following peaks: the peaks at 727.18 cm-1

, showing C=O bending, 1379.67 cm-1

,

representing C–N stretching, and 1776.51 cm-1

, indicating C=O asymmetric stretching, confirmed

imide formation.

Fig. 3 FT-IR spectum of PI film

Advanced Materials Research Vols. 821-822 907

Ultraviolet analysis. The ultraviolet absorption spectrum of polyimide films are shown in Fig. 4. It

can be seen from Fig. 4 that the ultraviolet absorption edge of PI film is 469.70 nm. In addition, the

optical band gap is estimated about 2.64 eV [5]. As is known to all, the optical band gap, to a certain

extent, can reflect the electrical band gap which is a key index to measure the insulation performance

of materials. Although the certain equation between the optical band gap and the electrical band gap

is unknown, we consider that the optical band gap can be used to compare the electrical band gaps for

the PI films, which is meaningful in practical application.

Fig. 4 The ultraviolet spectrum of the PI film

Conclusions

A new microwave imidization method has been used to prepare the PI film. FT-IR spectrum and and

ultraviolet absorption spectrum are measured to study the chemical structure and optical properties of

the film. Three characteristic peaks of PI ( 727.18 cm-1

, 1379.67 cm-1

and 1776.51 cm-1

) can be

obtained from FT-IR spectrum, which confirmed imide formation. The ultraviolet absorption edge of

the PI film is 469.70 nm. In addition, the optical band gap is estimated about 2.64 eV.

Acknowledgments

This work was supported by the Scientific Research Fund of Heilongjiang Provincial Education

Department (No.12531143).

References

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