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SCIENCE CHINA Materials, Volume 62 , Issue 12 : 1921-1933(2019) https://doi.org/10.1007/s40843-019-9580-y

Effect of ketyl radical on the structure and performance of holographic polymer/liquid-crystal composites

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  • ReceivedJun 27, 2019
  • AcceptedAug 7, 2019
  • PublishedSep 2, 2019

Abstract

Holographic polymer/liquid-crystal composites, which are periodically ordered materials with alternative polymer-rich and liquid-crystal-rich phases, have drawn increasing interest due to their unique capabilities of reconstructing colored three-dimensional (3D) images and enabling the electro-optic response. They are formed via photopolymerization induced phase separation upon exposure to laser interference patterns, where a fast photopolymerization is required to facilitate the holographic patterning. Yet, the fast photopolymerization generally leads to depressed phase separation and it remains challenging to boost the holographic performance via kinetics control. Herein, we disclose that the ketyl radical inhibition is able to significantly boost the phase separation and holographic performance by preventing the proliferated diffusion of initiating radicals from the constructive to the destructive regions. Dramatically depressed phase separation is caused when converting the inhibiting ketyl radical to a new initiating radical, indicating the significance of ketyl radical inhibition when designing high performance holographic polymer composites.


Funded by

This work was financially supported by the National Natural Science Foundation of China(NSFC,51433002,51773073)

the HUST peak boarding program

the National Science Foundation(NSF)

the Fundamental Research Funds for the Central Universities(2019kfyRCPY089)


Acknowledgment

We thank the financial supports from the National Natural Science Foundation of China (51433002 and 51773073), HUST peak boarding program, the National Science Foundation (NSF) of Hubei Scientific Committee (2016CFA001) and the Fundamental Research Funds for the Central Universities (2019kfyRCPY089). We also thank the technical assistance from HUST Analytical & Testing Center.


Interest statement

The authors declare no competing financial interest.


Contributions statement

Li MD, Xie X and Peng H gave the direction of the experiments; Zhao X and Sun S conducted the experiments together; Zhao Y participated in the discussion; Liao RZ obtained the DFT calculation results; Liao Y gave suggestions to the experiments and revised the manuscript with Peng H.


Author information

Xiaoyu Zhao received his Master’s degree from Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences in 2016. He is now a PhD candidate at Huazhong University of Science and Technology (HUST) under the supervision of Prof. Xiaolin Xie and Prof. Haiyan Peng. His current interest focuses on photopolymerization mechanism and applications.


Ming-De Li obtained his PhD degree at the University of Hong Kong in 2012. Then he conducted his postdoctoral research at the University of California, Berkeley and The University of Hong Kong. Now, he is a professor at Shantou University. His current interest is in the ultrafast laser spectroscopies.


Haiyan Peng received his PhD degree from HUST in 2014. He visited the University of Colorado Boulder from 2012 to 2014, sponsored by the CSC. Then he did research as an Assistant Professor at Guangzhou Institute of Advanced Technology, Chinese Academy of Sciences, and conducted postdoctoral research at City University of Hong Kong. He has been an Associate Professor at HUST since 2016.


Supplement

Supplementary information

Supporting data are available in the online version of the paper.


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  • Figure 1

    Ground state absorptions of KCD, NPG and TA in acetonitrile. NPG was recrystallized before use.

  • Scheme 1

    Schematic illustration on the conversion of the ketyl radical with an inhibition function to a new initiating radical and the corresponding ordered structures.

  • Scheme 2

    Chemical structures of KCD, NPG, TA, 6361-100, EHA and DMAA.

  • Figure 2

    (a–c) Nanosecond transient absorption of pure KCD (175 μmol L−1) in toluene upon a 400 nm pulse laser excitation. (d) Comparison between the experimental spectra (black line) and TD-DFT calculations (red and blue lines). The calculated spectra are scaled by 1.23 times with a half-width of 700 cm−1.

  • Scheme 3

    Schematic illustration on the diffraction efficiency measurement.

  • Figure 3

    (a–c) Nanosecond transient absorption when irradiating the KCD/NPG “photoinitibitor” (175 μmol L−1 for each in toluene) by a 400 nm pulse laser. (d) Spectra comparison between the experimental result (black line) and TD-DFT calculation (red line). The calculated spectrum is scaled by 1.23 times with a half-width of 700 cm–1.

  • Scheme 4

    Simplified structures of KCD and the radical coupling product of ketyl radical for calculations.

  • Figure 4

    (a) Chemical structures of radicals 1, 2 and 3. (b) Experimental and simulated EPR signals of the KCD/NPG “photoinitibitor” upon visible light irradiation.

  • Scheme 5

    Proposed radical diffusion during holographic photopolymerization.

  • Figure 5

    Comparison between the experimental and calculated absorptions of KCD and KCD’ (the radical coupling product). DMF is the solvent during characterization. All alkyl chains are replaced with the methyl group during DFT calculations to save the computation resource. The calculated spectra are scaled by 1.23 times with a half-width of 2500 cm−1.

  • Figure 6

    (a–c) Nanosecond transient absorption when irradiating the KCD/NPG/TA mixture (175 μmol L−1 for each in toluene) by a 400 nm pulse laser. (d) The transitent decay of KCD/NPG and KCD/NPG/TA mixtures at 528 nm. Pseduo-biexponential fitting is implemented to give the lifetime.

  • Figure 7

    (a) Chemical structures of radicals 3 and 4. (b) Experimental and simulated EPR signals of the KCD/NPG/TA system.

  • Figure 8

    Polymerization rate (a) and modulus (b) of the holographic mixture against the irradiation time upon a 460 nm light irradiation (3.0 mW cm−2). The concentration of KCD is 6 mmol L−1, while that of NPG and TA are 60 mmol L−1, respectively.

  • Figure 9

    (a) Electro-optic response of holographic polymer composites with the LC fabricated with the KCD/NPG “photoinitibitor” and KCD/NPG/TA system. SEM (b) and AFM (c) images of the holographic polymer composites with the KCD/NPG “photoinitibitor” (left) and KCD/NPG/TA system (right) after removing the LC. The dark holes in the SEM images and dark channels in the AFM images represent the orginal location of the LC. AFM scanning is hard to give the hole morphology because of the limited tip resolution. The grating ordered structure depth in the AFM images is 128.1±5.9 and 60.4±1.9 nm, respectively. The grating pitch is measured to be 840±10 and 830±40 nm from the SEM and AFM images, respectively, which is slightly smaller than the predesigned (889 nm).

  • Figure 10

    Holographic images reconstructued with the KCD/NPG “photoinitibitor” (a) and KCD/NPG/TA system (b). The images are viewed at the identical angle.

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