Preparation Of Nanocellulose/Polyimide Composite And Its Application In Flexible Electronics

Polyimide?PI?is a well-known high-performance polymer material and is considered as the most suitable candidate material for flexible electronic device substrates.Since the traditional PI film is often yellow or brownish-yellow,it seriously affects the transparency requirements of flexible electronic device fabrication.For this reason,the method of introducing fluorine-containing groups such as-CF₃ is often used to improve transparency to obtain colorless polyimide?CPI?,and the introduction of groups such as hydroxyl groups is used to make up for the defect that the existing CPI film is not easy to decompose,which solves the problem that the existing flexible substrate is not easy to decompose or has poor thermal stability.This will open up a new way for the research of flexible green electronic substrate.Further,the high thermal stability cellulose nanocrystals?CNC?is used to modify and enhance the CPI,and a CNC/CPI composite substrate with improved performance can be prepared,which has a balanced high mechanical and thermal stability as well as optical performance,coupled with excellent OLED performance,it may bring new opportunities for manufacturing high-quality and low-cost flexible OLEDs.Using cotton fiber as the raw material,two different types of nanocellulose were prepared by TEMPO oxidation method and oxalic acid hydrolysis method.According to the different fiber morphology and size,they were respectively named as cellulose nanocrystals?CNC?and nanometer microfibrillated cellulose?CNF?.Among them,the CNF prepared by TEMPO oxidation is mainly fibrillar,with a longer fiber length,interwoven into a network structure,while the CNC prepared by oxalic acid hydrolysis has an elliptical rod structure with a shorter length.At the same time,the CNF film is more transparent?the light transmittance can reach 92%?,and the mechanical properties are slightly higher than the CNC,but the thermal performance is obviously poor;and the thermal stability of the CNC film is very prominent,its thermal decomposition temperature is higher than that of the CNF film90?,can be used as high thermal stability material,and because of its smaller size,larger specific surface area,light transmittance and mechanical properties can also meet the requirements of use,we can consider using it as a good filler in the subsequent in application to improve the performance of composite materials.At the same time,in view of the problem that the current flexible electronic substrates are difficult to decompose green or have poor thermal stability,after molecular structure design,BAHPFP is used as the diamine monomer,6FDA is used as the dianhydride monomer,and a new CPI with a hydroxyl group is synthesized as the substrate of the flexible electronic.The substrate has excellent thermal and mechanical properties,and maintains excellent optical property.Among them,the thermal decomposition temperature reaches 430?,the maximum working temperature reaches 300?,the tensile strength and elastic modulus are 103MPa and2.49GPa,respectively.At the same time,the substrate has a light transmittance of 91%at 600nm.And the substrate has good surface flatness,and its roughness is only 0.38nm,which meets the requirement of flexible electronic substrate roughness less than 5nm.The flexible printed circuit board?FPC?prepared on this substrate exhibits excellent decomposability in green solvent ethanol,and has excellent bending resistance and good electrical conductivity.Furthermore,in view of the problem of loss of thermal and mechanical properties after the traditional brown PI is transparentized to CPI,the aforementioned high thermal stability CNC is introduced into the CPI matrix as a reinforcing filler to prepare a CNC/CPI composite substrate.Due to the excellent mechanical and thermal properties of CNC and their strong interfacial interaction with the CPI matrix,the resulting substrate material obtains significantly improved mechanical and thermal properties while maintaining good optical transparency.Compared with pure CPI,when the content of CNC is 4%,the thermal decomposition temperature of the composite film is increased by 13?,the elastic modulus and tensile strength are increased by 49%and 24%,respectively,and the folding resistance is significantly enhanced.What's more,the surface of the composite film is flat,the roughness is0.74nm,the light transmittance is 86%@600nm,and the maximum operating temperature of the composite film substrate is 320?.In addition,compared with OLED@glass,the OLED@CNC/CPI device made with 4%CNC/CPI composite film not only can stably emit light in a curved state,but also has better flexibility and more high current efficiency,up to72.7cd/A at 20,000cd/m²,41% higher than glass substrate.