SYNTHESIS AND PROPERTIES OF SEMI-ALICYCLIC POLYIMIDES WITH HIGH REFRACTIVE INDICES AND HIGH TRANSPARENCY

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High refractive index polyimides(PIs) have been widely utilized for the fabrication of microlens for CMOS image sensors(CIS).In the present work,a series of semi-alicyclic Pis were developed from an alicyclic dianhydride,2,3,5-tricarboxycyclopentylacetic acid dianhydride(TCAAH) and two sulfur-containing aromatic diamines,4,4′-thiobis[(p-phenylenesulfanyl) aniline](3SDA) and 2,7-bis(4-aminophenylenesulfanyl) thianthrene(APTT). As a typical example,PI-1 was prepared according to the procedure described below.3SDA(8.6524 g,20 mmol) and freshly-distilled NMP(30.0 g) were added to a 100 mL three-necked round-bottom flask equipped with a high-power electromagnetic stirrer,nitrogen inlet and cold-water bath.A clear diamine solution formed after stirring for 30 min under a nitrogen flow. Then,TCAAH(4.4834 g,20 mmol) was added immediately,followed by additional NMP(22.5 g) to adjust the solid content of the mixture to be 20 wt%.The mixture was stirred at room temperature for 24 h to afford an almost colorless,highly-viscous solution,which was diluted to 15 wt% by adding additional NMP.The mixture was purified by filtration through a 0.45 μm teflon syringe filter to obtain the poly(amic acid)(PAA) solution(PAA-1).The PAA-1 solution was spin-coated on a silicon wafer or quartz substrate.The thickness was controlled by adjusting the spinning rate.PI-1 film was obtained by thermally curing the PAA solution under nitrogen in an oven at 80℃ for 3 h,120℃ for 1 h,180℃ for 1 h,250℃ for 1 h,and 300℃ for 0.5 h,then,the film was obtained by immersing the substrate in warm water.PI-2 film(TCAAH-APTT) was synthesized with a similar route. The structure of the PI films was confirmed by FT-IR measurements,in which the characteristic absorptions of imide moiety were clearly observed.The thermal properties of the PI films were evaluated by TGA,DSC,and DMA measurements.The results indicated that the PI films exhibited good thermal stability up to 480℃ and glass transition temperatures higher than 250℃. The PI films showed good optical transparency in the visible light region(400～700 nm) with the transmittance higher than 85% at 400 nm.The refractive indices of the PI films were higher than 1.68 at 632.8 nm,and the birefringence values were lower than 0.006.The refractive index of the PI could further be increased by mixing with silica-modified titanium dioxide (TiO_2) nanoparticles. For example,PI-I/TiO_2 nanocomposite film was successfully prepared by the mixing of PAA-1 and TiO_2 with the ratio of PAA-1∶TiO_2=55∶45(wt%).The nanocomposite film showed transmittance of 88% at 400 nm,slightly lower than that of PI-I(96%).The film had refractive index of 1.76 at 632.8 nm.The preliminary results indicated that the refractive indiex of the nanocomposite film increased with increasing the TiO_2 content.However,how to maintain the high optical transparency of the films is a challenging topic.