| Polyimide?PI?has widely been used in various high-tech fields such as microelectronics,aerospace,separation membranes and sensors due to the excellent thermal stability,chemical resistance,good mechanical strength and film-forming capacity.However,most of the conventional PI has the poor solubility and transparency and relatively high dielectric constant?k,?3.5?,which limit the applications in some fields.The development of functional PI materials with special properties,such as good solubility,high transparency and low k is still one of the research hotspots.In addition,polymer-based chemosensors have attracted more and more attentions by virtue of their additional functions,such as the good thermal and light stability,enhanced binding efficiency and sensing signal amplification.Compared with other polymers,PI materials show the higher thermal stability and mechanical properties,and excellent film-forming capacity.PI materials can be directly used for fabricating film-based sensor devices without any carriers,which has the important practicability.Therefore,as the excellent potential sensing materials,PI materials have received considerable attention in the application of chemosensors.Two kinds of fluorene-based Cardo monomers containing hydroxyl groups and their corresponding series of fluorene-based Cardo polyimides containing hydroxyl groups?PI-OH?were successfully synthesized.And a series of multifunctional PIs with special side groups were conveniently synthesized by the directly chemical modification on PI-OH.The properties of all the obtained PIs were systematically investigated and discussed.Finally,the obtained PIs and their films were applied as the chemosensors for the detection of F-and water in organic solvents.In this paper,the design strategy,synthesis method and sensing application research of novel multifunctional PI materials are the main issue as follows.?1?Two kinds of fluorene-based cardo diamine monomers containing hydroxyl groups were synthesized and then polymerized with commercial dianhydride monomers through one-step high-temperature polycondensation procedure to obtain a series of fluorene-based Cardo polyimides containing hydroxyl groups?PI-OH?.The products of PI-OH show high molecular weights with a number-average molecular weight of 6.87×104 g/mol-14.28×104 g/mol.Due to the synergistic effect of fluorene-based Cardo structure and polar hydroxyl groups in polymer chains,the products of PI-OH show the good solubility,high thermal stability and excellent mechanical properties.The glass transition temperature?Tg?and the 5%thermal weight loss(T5%)in N2 are 342.7?-455.9??DMA?,and 481?-527°C,respectively.The tensile strength?TS?of PI-OH films is 78.2 MPa-101.5 MPa.Owing to the fluorene-based Cardo structure in polymer chains,the hydroxyl-containing PI films still exhibit relatively high transparency with the cut-off wavelength of 343-366 nm and the transmittance at 500 nm is higher than 80%.?2?A series of fluorene-based Cardo PIs with silyl ether side groups were successfully synthesized by a direct silyl ether reaction of PI-OH with chlorosilane reagent.Fluorene-based Cardo PIs with silyl ether side groups remain relatively high thermal stability with a Tg of201.4?-338.9?and a T5%of 470?-509°C in N2 and good mechanical properties with a TSof 56.7 MPa-80.3 MPa.Due to the reduced charge transfer?CT?interaction in PI chains chemically bonded by bulky non-polar silyl ether side groups,the solubility,optical,hydrophobic and dielectric properties of the PI films are significantly improved compared with those of the precursor PI-OH.These PIs exhibit the high solubility at room temperature not only in high-boiling-point solvents,but also in low-boiling-point solvents,such as acetone,CH2Cl2,and THF.In addition,the PI films presented the bright fluorescence and high transparency with the cut-off wavelength of 337-352 nm and the transmittance at 450 nm is higher than 80%.Moreover,these PI films show low refractive indices?1.552-1.631 at 633 nm?,low water uptakes?0.19%-1.05%?and low dielectric constant?2.44-2.73,1 MHz?.?3?A series of fluorene-based Cardo PIs with the acyloxy groups were successfully synthesized by a direct esterification reaction of PI-OH with anhydride.The obtained PIs with the acyloxy groups show excellent thermal properties with a Tg of 281.1°C-390.9°C?DSC?,a T5%of 413°C-453°C.The TS of the PI films is 71.3 MPa-83.0 MPa.The fluorene-based Cardo units with the acyloxy groups in the polymer chain are able to weaken inter-or intra-molecular CTC effects and to hinder the close packing of the polymer chains,which are conducive to the solubility,optical and dielectric properties of the obtained PIs.PIs show the excellent solubility not only in polar solvents,but also in low polar solvents at room temperature,such as CHCl3,CH2Cl2,and THF.In addition,these PI films display the bright fluorescence and the high optical transparency with the low cut-off wavelength of 325 nm-357 nm and high transmittances at 400nm and 450 nm of 65.4%-83.5%and 83.1%-86.9%,respectively.Furthermore,the introduction of acyloxy groups can improve the hydrophobic properties and the free volume of PI films to endow PIs with low k value of 2.58-2.96 at 1 MHz.?4?In consideration of the fact that the fluoride ion?F-?can form a strong H-bonding with hydroxyl groups,the solution and film of PI-OH are applied as chemosensors via H-bond interactions for detection of F-with high selectivity and sensitivity.These chemosensors are able to quantitatively detect the wide concentration range of F-with a detection limit?LOD?as low as 10-6 M.It is confirmed that the F-sensing mechanism involves H-bond interactions and subsequent deprotonation in the chemosensor,resulting the enhancement of the intramolecular charger transfer?ICT?process.Moreover,PI-OH in DMSO solution can be applied to the visual and reversible detection of F-.Besides,hydroxyl groups on the surface of PI-OH films are bonded with F-and the bonded F-can be removed quantitatively.?5?Based on the knowledge that the fluoride ion-induced deprotonated species are easily to be reprotonated by even trace water,hydroxyl-containing polyimide-fluoride complexes?PI-OH·F?are exploited as ratiometric and colorimetric chemosensors for detecting trace water in organic solvents with high sensitivity and fast respone time.Chemosensors PI-OH·F exhibit a good naked-eye visual detection as well as an excellent linear relationship between the UV-vis absorbance ratio and the low water content in DMSO and DMF.Among these chemosensors,PI-OH-2F·F shows the higher sensitivity for water detection with extremely low LODs of0.00084%and 0.0015%?v/v?in DMSO and DMF,respectively.It is confirmed that the deprotonation of hydroxyl-containing PIs by F-and reprotonation followed by water are the working principle of these chemosensors for water.Furthermore,PI-OH films are able to be directly used for the visual and reversible sensing of water in some low-boiling-point water-miscible solvents like acetonitrile,acetone and ethanol with high sensitivity.?6?PI with tert-butyldimethylsiloxy side groups?PI-DM-2B?is applied as a reactive chemosensor for the detection of F-with fast respone time and high selectivity and sensitivity,in consideration of the cleavage of Si-O bond triggered by F-.The chemosensor PI-DM-2B in DMF solution is able to quantitatively detect the wide concentration range of F-with a LOD of0.31?M.Similarly,the chemosensor PI-DM-2B film quantitatively detects F-in acetonitrile solution by fluorescence spectroscopy.And it exhibits high selectivity response to F-with a LOD of 0.63?M.The fluorescence of the PI-DM-2B film is quenched after immersing into the solution of F-.And other anions have no interference.The related experiments confirm that the quenching mechanism involves the fluoride-triggered Si-O bond cleavage resulting the enhancement of the ICT in polymer chains.Moreover,the PI-DM-2B film is directly used to fast detect F-by colorimetric mode.This work provides a novel PI film-based fluorescent sensor device for the detection of F-,which showes a good practical application prospect and extendes the application field. |
PDF Full Text Request