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<p>The development of high-performance transparent substrates is critical for next-generation flexible electronic devices. Herein, we designed two novel meta-substituted diamines incorporating trifluoromethyl (—CF<sub>3</sub>) and methyl (—CH<sub>3</sub>) groups to synthesize colorless copolyimide (CPI) films <i>via</i> copolymerization with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)/3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA). The combination of meta-substituted architecture and substituents enables the simultaneous attainment of an ultralow dielectric constant (<i>D</i><sub>k</sub>) and high transparency. The meta-substitution geometry and electronic effects of —CF<sub>3</sub>/—CH<sub>3</sub> effectively suppressed charge-transfer complex (CTC) formation, expanded fractional free volume (FFV), and restricted <i>π</i>-electron conjugation, as validated by DFT calculations and wide-angle X-ray diffraction (WAXD) analysis. The optimized CPI film (PIA<sub>1</sub>-6FDA/BPDA(10/0)) achieved outstanding transmittance (<i>T</i><sub>450</sub>=88.15%), ultralow dielectric constant (<i>D</i><sub>k</sub>=2.08 at 1 kHz), and minimal dielectric loss (<i>D</i><sub>f</sub>=0.0012), while maintaining robust thermal stability (<i>T</i><sub>d5%</sub>&gt;523 °C) and mechanical strength (<i>σ</i> = 87.5 MPa). This work establishes a molecular engineering strategy to concurrently enhance the optical and dielectric properties, positioning meta-substituted CPIs as promising candidates for transparent flexible devices.</p>

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Achieving Enhanced Optical Transparency and Low-dielectric Properties in Meta-substituted Copolyimides for Flexible Substrates

  • 期刊论文

  • 发表日期：

    2026年1月16日

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  • Zi-Yang Liu,
  • Yan-Yu Gao,
  • Cheng Wang,
  • Qian-Qian Yu,
  • Zhi-Long Zhang,
  • Lin-Ge Wang

摘要

The development of high-performance transparent substrates is critical for next-generation flexible electronic devices. Herein, we designed two novel meta-substituted diamines incorporating trifluoromethyl (—CF3) and methyl (—CH3) groups to synthesize colorless copolyimide (CPI) films via copolymerization with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)/3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA). The combination of meta-substituted architecture and substituents enables the simultaneous attainment of an ultralow dielectric constant (Dk) and high transparency. The meta-substitution geometry and electronic effects of —CF3/—CH3 effectively suppressed charge-transfer complex (CTC) formation, expanded fractional free volume (FFV), and restricted π-electron conjugation, as validated by DFT calculations and wide-angle X-ray diffraction (WAXD) analysis. The optimized CPI film (PIA1-6FDA/BPDA(10/0)) achieved outstanding transmittance (T450=88.15%), ultralow dielectric constant (Dk=2.08 at 1 kHz), and minimal dielectric loss (Df=0.0012), while maintaining robust thermal stability (Td5%>523 °C) and mechanical strength (σ = 87.5 MPa). This work establishes a molecular engineering strategy to concurrently enhance the optical and dielectric properties, positioning meta-substituted CPIs as promising candidates for transparent flexible devices.