Preparation Of Silica Colloidal Crystal/PDMS Composites And Study Of Their Transparancy And Cracks On The Metal Films Supported By Them

Functional flexible materials has been a research focus in recent years,and it is also a disruptive progress in the field of materials.Flexible optical materials have unique external stimuli responsive optical properties,thus achieving hiding or discoloration of the object.Flexible conductive materials can be used for fabrication of a variety of flexible electronic devices,such as wearable electronic products.However,flexible materials with single function can hardly meet various needs.Take the individual camouflage equipment as an example,if one type of material could possess all the camouflage,electronic communication,charging and lighting functions,the number of equipment that one soldier carries could be greatly reduced.To solve this problem,a multifunctional flexible material,which is color changeable and conductive is proposed.In this work,a transparent flexible composite was prepared by embedding inorganic silica colloidal crystals into elastomer polydimethylsiloxane(PDMS).Due to presence of silica colloidal crystals,the transmittance of the prepared composite changed reversibly with the external stress.Moreover,the introduction of inorganic silica could significantly reduce the cracks on the supported metal films and retain the conductivity of the metal films at the same time.Monodisperse silica nanoparticles(NPs)were synthesized via the modified St?ber method,and then self-assembled into silica colloidal crystals through the vertical deposition method.Glasses were used as substrates in the vertical deposition method to study the effects of solvent and temperature on the self-assembly process.Results showed that ethanol at 50℃ worked more favorably.Three other polymer substrates were used to study the influence of the property of substrates on the selfassembly process.Three polymeric substrates are hydrophilic polyvinyl alcohol film(PVA)spin-coated on glass,hydrophilic polydopamine film deposited on glass through in-situ polymerization,and hydrophobic polydimethylsiloxane sheet(PDMS)cast coated on glass,among which,PDMS is a kind of elastomer with lowest Young’s modulus.Three materials together with glass were used as substrate s to support selfassembled colloidal crystal via vertical deposition using silica alcosol with different concentrations.Results showed that at low silica deposition concentrations,films of silica colloidal crystals could form uniformly on glass,PDA and PVA,but only form partially on PDMS.While at high silica deposition concentrations,films of colloidal crystals formed on all substrates;however,the colloidal crystals delaminated from glass,PDA and PVA surfaces;in contrast,they remained intact on PDMS.It can be seen,particles deposition is favored on hydrophilic substrates,while thick silica colloidal crystals is favored on soft substrates.A PDA modified PDMS was prepared and used as the substrate to hold silica colloidal crystals,and the results confirmed the former speculation.Then,a silica colloidal crystal/PDMS composite was prepared by irrigating premixed PDMS solution into the voids in the ordered silica colloidal crystal.Thanks to close effective refractive indexes of silica and PDMS,the prepared silica colloidal crystal/PDMS composite was clear with a transmittance above 91 %.The transmittance reduction was only less than 3 % comparing with that of the pure PDMS,indicating that the introduction of silica colloidal crystals into PDMS had only minor effects on the transmittance.Moreover,the transparency of this composite was susceptible to stress.When the composite deformed under stress,the transparency became lower.As the stress was released,both the shape and the transparent recovered,and such process could be repeated more than ten times.It was found that transparency change was related to the surface morphology of the composite.When it was stretched,the composite elongated along the drawing direction,but shriked perpendicularly.Because the silica colloidal crystal could not be compressed,large wrinkles formed on the surface of the composite follow the shrinkage deformation.The large wrinkles led to strong diffuse reflection,and thus,resulting a low transparency.As the stress was released,the large wrinkles disappeared,and then,the transmittance of the composite recovered by above 90 %.A vacuum sputtering coater was used to sputter a thin gold film on silica colloidal crystal/PDMS composite.Then an external force was applied on the film to induce cracks.Compared with the cracks formed on gold film supported by pure PDMS,the cracks on gold film supported by the composite were clearly suppressed both in length and density.Silver coatings were prepared on pure PDMS and composite.After acid treatment,silver coatings on composites became conductive while silver coatings on PDMS sheets kept unconductive,when they were tested by a four proble system.This result verified that the composite surface containing both organic PDMS and inorganic silica has a strong suppression effects on the cracks of supported metal films.This work could contribute to the conductive polymer supported metal films,pave a way for application in flexible conductive devices.