Abstract
Transparent polyimides (PI) films with outstanding overall performance are attractive for next generation optoelectronic and microelectronic applications. Semi-alicyclic PIs derived from alicyclic dianhydrides and aromatic diamines have proved effective to prepare transparent PIs with high transmittance. To optimize the combined properties of semi-alicyclic PIs, incorporating bulky trifluoromethyl groups into the backbones is regarded as a powerful tool. However, the lack of fundamental understanding of structure–property relationships of fluorinated semi-alicyclic PIs constrains the design and engineering of advanced films for such challenging applications. Herein, a series of semi-alicyclic PIs derived from alicyclic dianhydrides and trifluoromethyl-containing aromatic diamines was synthesized by solution polycondensation at high temperature. The effects of alicyclic structures and bulky trifluoromethyl groups on thermal, dielectric and optical properties of PIs were investigated systematically. These PI films had excellent solubility, low water absorption and good mechanical property. They showed high heat resistance with Tg in the range of 294–390 °C. It is noted that tensile strength and thermal stability were greatly affected by the rigid linkages and alicyclic moieties, respectively. These films exhibited obviously low refractive indices and significantly reduced dielectric constants from 2.61 to 2.76, together with low optical birefringence and dielectric anisotropy. Highly transparent films exhibited cutoff wavelength even as low as 298 nm and transmittance at 500 nm over 85%, displaying almost colorless appearance with yellowness index (b*) below 4.2. The remarkable optical improvement should be mainly ascribed to both weak electron-accepting alicyclic units and bulky electron-withdrawing trifluoromethyl or sulfone groups. The present work provides an effective strategy to design molecular structures of optically transparent PIs for a trade-off between solution-processability, low water uptake, good toughness, high heat resistance, low dielectric constant and excellent optical transparency.
Highlights
With the development of electronics, microelectronics and large-scale integrated circuits, flexible substrates have attracted considerable interest due to their promising applications in next-generationPolymers 2020, 12, 1532; doi:10.3390/polym12071532 www.mdpi.com/journal/polymersPolymers 2020, 12, 1532 displays, diverse flexible electronic devices and aerospace industry [1,2,3]
As shown in Scheme 1, five soluble semi-alicyclic PIs were prepared by solution polycondensation with equimolar alicyclic dianhydrides and aromatic diamines containing trifluoromethyl
In view of relatively low chemical reactivity of alicyclic dianhydride, the polymerization from PAA to PI is generally recommended at high temperature to synthesize semi-alicyclic PIs with high molecular weight [38,39]
Summary
With the development of electronics, microelectronics and large-scale integrated circuits, flexible substrates have attracted considerable interest due to their promising applications in next-generation. It is found that these semi-alicyclic PIs with fluorine content lower than 10 wt% (by introducing fluorine atoms (-F)) have outstanding solubility and high heat resistance, but they show inferior toughness and relatively higher yellow indices These results indicated that incorporating trifluoromethyl groups into diamine moieties is a more effective design strategy to improve the transparency and coloration of semi-alicyclic PIs. in terms of molecular design of intrinsic low dielectric. The synergistic effects of alicyclic structures and fluorinated groups on the performance of semi-alicyclic PIs, together with solubility, mechanical, thermal, dielectric, optical transparency and other properties were investigated systematically Their aggregation of polymer chains was evaluated by Wide angle X–ray diffractometry, and molecular orbital energies of model compounds were determined by density functional theory (DFT) calculations to clarify the effects of molecular structures on charge transfer interactions. Fundamental insights into structure–property relationships of semi-alicyclic PIs containing trifluoromethyl groups observed in this study offer guidance for the design of flexible transparent PIs for next-generation optoelectronic applications
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