Monitoring and optimization of the synthesis process of the holographic doped photopolymers
Cao Liangcai, Wu Shenghan, He Zehao, Li Yaoyao, Jin Guofan     
State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University, Beijing 100084, China

Overview: The photopolymer has the advantages of high sensitivity, high resolution, low cost, simple fabrication and good optical performance. The photopolymer has the disadvantages of high shrinkage and low diffraction efficiency, which limit its applications. The doped photopolymer is an improved holographic material compared to the pure photopolymer because it has lower shrinkage and higher diffraction efficiency. The synthesis process of the doped photopolymer has a significant impact on its properties. The traditional optimization method for the synthesis process of the doped photopolymer depends on experimental parameters and experimental experience. A method for quantitative monitoring and optimization of the synthesis process by absorption spectrum is presented in this paper.

Different substances have different compositions and structures. The absorption spectra of various substances are also different. Therefore, qualitative or quantitative analysis of substances could be carried out according to their absorption spectra. The sample used in this work is nanoparticle doped polymethyl methacrylate (PMMA) photopolymer. The single wavelength electromagnetic wave is obtained by a white light source through the spectroscopic system. After passing through the sample at a certain distance, the single-wavelength electromagnetic wave enters the detector. By comparing the light intensity of transmitted single-wavelength electromagnetic wave with the original intensity, the absorption intensity of the sample under the specific wavelength could be obtained. By measuring the absorption intensities under various wavelengths, the absorption spectra of the sample could be obtained.

The pre-polymerization process is very important for the synthesis of the photopolymer. However, there is no quantitative monitoring technology for the process of pre-polymerization. Because the pre-polymerization process results in strong changes in absorption spectrum, the pre-polymerization process could be monitored by the proposed absorption spectrum method. With the absorption spectrum method, the degree and the rate of the pre-polymerization could be monitored and expressed by absorption intensity. More experimental conditions of pre-polymerization could also be determined. Thus, the synthesis process of doped photopolymer can be optimized.

This optimization method can not only be applied to the preparation of doped PMMA photopolymer, but also has important value for the preparation of other materials synthesized by polymerization. It also provides an effective way to fabricate stable doped photopolymers.

Supported by National Natural Science Foundation of China (61775117) and Solid State Laser Technology Key Laboratory Fund (9140C010102150C04017)