Optical Diffuser Film Constructed By Hydrogen-Bonded Interfacial Polymer Complexation

Optical diffusion film is a type of optical element that scatters point or line light sources to form a uniform illumination when light propagating through the film,and is an essential component in various optical devices such as displays,solar cells and light-emitting diodes.Due to the rapid development of optical technology and display industry as well as the emergence of human-computer interaction,optical diffuser films prepared by traditional approaches gradually cannot meet the requirements of emerging optical devices.Through optimization,suitable selection and modification,well-performed and multifunctional integrated optical devices can be achieved benefiting from substrate-independent interfacial complexation.In response to the trend of optical components,excellent-performed optical diffusers have been designed and constructed through interfacial polymer complexation.Hydrogen bonding polymer complexes is one of the common types of polymer complexes and are abundantly distributed in living bodies and natural organisms.Polymer complexes can rapidly assembly at the interface and precisely control the thickness of the film through hydrogen bonding,a common secondary interaction.Therefore,hydrogen-bonded polymer complexes have considerable potential for applications in coatings or optical thin films.Our group’s preliminary work found out that the hydrogenbonded complex film of poly（vinylpyrrolidone）/poly（acrylic acid）（PVPON/PAA）behave high scattering ability.The quick complexation of the hydrogen bonding polymer complexes inclines to form small particles of chain entanglement,which endows the film with optical diffusion capacity.However,the scattering ability of the complex films is relatively limited to meet the requirement of the optical diffuser.Therefore,we provide a new strategy that pre-mixing polymer complexes in aqueous solution to form colloidal particles then assembling on the substrate,and investigate the assembly process and its contribution to the optical diffusion effect.（1）In order to prepare complex film with excellent optical diffusion performance,we construct films on hard quartz substrates in direct layer-by-layer（Lb L）assembly and polymer complex nanoparticles（PCNP）system,and investigate the differences in assembly processes and optical diffusion properties.PVPON and PAA can Lb L assemble on the quartz substrate to fabricate PVPON/PAA film under p H 2.0 acidic environment.Due to the hydrogen bonding interaction and rapid complexation,PVPON and PAA are able to form particle structure.The film constructed with more assembly cycles will obtain a more pronounced particle structure.The micro-and nano-particle hierarchical structure is favorable for optical diffusion.PVPON and PAA can form PCNP particles when they are mixed in aqueous solution.The outer surface of PCNP carries a large number of carboxyl groups when PAA is excessive.When PCNP Lb L assembles with PVPON instead of PAA,the obtained film with different complexing ratio eventually has different optical properties,and their scattering capacity exhibits a process of first increasing and then decreasing.This is mainly related to the particles size and the stability of the colloidal solution.Closer stoichiometry of PVPON to PAA will enlarge the dimension of the particle but makes the colloidal incline to precipitate,leading to reduction in the assembly ability and diffusion performance.Film shows the most excellent scattering efficiency when the repeating unit ratio of PVPON:PAA is about 0.2:1.The optical diffusion performance of PVPON/PCNP is greatly improved as well as the assembly efficiency compared to that of PVPON/PAA film.The surface of the assembled film exhibits obvious "strawberry-like" particle structure,and the PCNP can partially coalesce to form a percolation structure as the assembly cycles goes.These small particles with a diameter ranging hundred nanometers（around visible light wavelength）are deposited on large particles in micron range.The light will refract several times when propagating through the hierarchical structure.Since the dimension of the particles conforms to the distribution of Mie scattering,they are favorable for optical diffusion.The optimized PVPON/PCNP exhibits excellent performance,showing comparable haze value（94 %）,higher transmittance value（86 %）and diffusing efficiency（81 %）,which is better than commercially available optical diffuser film.（2）In order to enhance the flexibility and bending-resistance capacity of the polymer complex optical diffuser film,we introduced elastic hydrogen bonding complexes and constructed a flexible optical diffuser through hydrogen bonding interface complexation on the flexible poly（ethylene terephthalate）（PET）substrate.Mix solution are obtained by introducing high elastic poly（ethylene oxide）（PEO）to the PVPON solution and then assembles with PCNP to prevent the coating from fracture on PET substrate during dehydration.PEO chain is flexible and PEO/PAA complexes show a rubber-like behavior,and its crystallization ability is restricted when forming hydrogen bonding with PCNP.Therefore,PEO can toughen PVPON/PCNP film.When PEO content is high,the flexibility of the film increases,but the optical diffusion performance drops accordingly.When the content of PEO is low,the optical diffusion performance remains relatively high but the film inclines to produce defeats or breakage.It is demonstrated that the high mobility of PEO chains tends to diffuse in and out the film,which damages the morphology of the deposited PCNP during the assembly procedure,resulting in a reduction in scattering capacity.After optimization,a compromise between optical diffusion and mechanical properties was obtained by assembling the Mix solution with a 50:50 PVPON:PEO ratio.The prepared film owns a transmittance of 85 % and a haze of 93 %.The complex film maintains stable after more than 10000 bending fatigue tests,confirming that toughened Mix50/PCNP owns bettering bendingresistance ability than commercially available optical diffuser film.（3）In order to enhance the solvent resistance of the polymer complex optical diffuser,we constructed films through alternating of interfacial complexation and thermal crosslinking of cellulose ether（CE）and PCNP.Cellulose and its derivates are natural hydrogen bonding acceptors attributed to the large amount of hydroxyl groups on the chains.The use of sustainable cellulose and its derivates is an economical and environment-friendly approaches to prepare optical diffuser films.CE and PCNP can deposit on the soft and transparent PET substrate.Due to the high mobility of polymer complex chains,it undergoes in and out diffusion in the solution and inside the film during the assembly procedure.Therefore,PCNP tends to physically merge with each other after several depositions,thus forming a flat morphology with only sporadically particles distribution on the surface.This results in only 90 % of transmittance,81 % of haze and 73 % of diffusing efficiency of CE/PCNP even after 80 assembly cycles,and it is hard to break the threshold value.CEs have abundant hydroxyl groups,which can esterify with carboxyl group on PAA.Therefore,the complex film can be further crosslinked through thermal treatment,which protects the PCNP scattering particles from physical fusion.However,the optical diffusion performance doesn’t exhibit obvious improvement when the thermal crosslinking is performed to cap the obtained film compared to the film without treatment.However,after applying of the procedure that alternate of assembling and crosslinking,the optical property is significantly enhanced.Crosslinking restricts the mobility of the PCNP which allows the particles to exist more stably and exempts from the inter-diffusion during the assembly process.The physical fusion in therefore avoided and the optical diffusion performance and durability of the film are greatly promoted.The film with proper crosslinking treatment CE/PCNP achieves 84 % of transmittance,95 % of value and 80 % of diffusing efficiency.Without crosslinking,the films will dissolve in high p H value solution.With crosslinking,the optimized crosslinking treatment,films show resistance to high p H value solutions.The exceptional adhesion to the substrate and the matching of mechanical properties account for the excellent resistance to bending fatigues.The assembled and crosslinked CE film provides a new approach for the fabrication of highperformance optical device and offers a broader perspective for the application of cellulose derivates.（4）To improve the stretchability and wearability of the films,we constructed stretchable optical diffuser on polydimethylsiloxane（PDMS）substrates by introducing highly elastic hydrogen bonding complexes with proper thermal crosslinking on polydimethylsiloxane（PDMS）substrates.MC/PCNP film are brittle and its break elongation is much lower than that of PDMS,which are easily fracture during stretching and need to be toughened.PEO/PAA complexes have exceptional elasticity,and the addition of PEO into the MC/PCNP system will enrich the coating with better tensile mechanical property.However,the physical merging between the PCNPs cannot be avoided during the assembly.Then the crosslinking is introduced to form the ester bonds between MC and PCNP to restrict mobility and diffusion of the chains,thus improving the optical diffusion performance.Optimized crosslinked Blend40/PCNP film obtains92 % of transmittance and 93 % of haze value,and possesses 73 % break elongation.During the tensile cycles,Blend40/PAA complex owns nearly 90 % of instant tensile recovery,however,the coating fabricated on the PDMS doesn’t show comparable and is much lower than Blend40/PAA complex.It is extrapolated that the particles in the film are not as easily stretchable as the curled PAA chains and the particles are difficult to re-form once it is break.However,at a stretching rate of 25 %,crosslinked Blend40/PCNP are able to recovery to its original state after 12 h.The deviance of optical diffusion performance of the film is about 2 % during stretching process.In addition,crosslinked Blend40/PCNP exhibits better stretchable optical properties which maintains 90 % of transmittance and 90 % of haze after 10000 tensile cycles.