Preparation And Properties Of PDMS-SiO₂ Photonic Elastomers

Photonic elastomers with mechanochromic properties show a wide range of application prospects in the fields of anti-counterfeiting,smart wearing and visual flexible sensing.At present,the preparation of mechanochromic photonic elastomers is mainly based on long-range ordered photonic crystals as the template,which has the problems of long preparation period,difficult to prepare large area,and angle-dependent structural color,which is not conducive to practical application.In contrast,amorphous photonic crystals can be prepared with high efficiency and angle-independent of structural color,but due to their low structural order,the prepared photonic elastomers have low structural color saturation and even no mechanochromic properties.To solve the above problems,in this thesis,we first prepared monodisperse SiO₂microspheres with controllable particle size,and explored the self-assembly method and conditions to rapidly obtain SiO₂amorphous photonic crystal films with high order;Then the mechanochromic polydimethylsiloxane（PDMS）-based photonic elastomer was prepared using SiO₂amorphous photonic crystal film as a template,and the structural color saturation of the photonic elastomer was improved by adding black polydopamine（PDA）;on this basis,the feasibility of preparing photonic elastomer by directly assembling SiO₂amorphous photonic crystal film on the surface of hydrophilic modified PDMS film was also explored.The main contents of the paper are as follows:（1）Preparation of SiO₂amorphous photonic crystal films.SiO₂microspheres with sixs different particle sizes were synthesized by controlling the amount of ammonia using the St（?）ber method.The results of SEM and laser particle size analyzer tests showed that SiO₂microspheres had good monodispersity with the average particle size ranging from 208 nm to 368 nm.FTIR and XRD confirmed that the SiO₂microspheres with abundant surface hydroxyl groups were successfully synthesized and were amorphous structures.SiO₂microspheres were used as the construction units to prepare SiO₂structural color films by vertical deposition method,thermal assisted evaporation method and fast lifting method.Compared with vertical deposition method and thermal assisted evaporation method,the films prepared by fast lifting method have uniform color distribution,no obvious defects,and high preparation efficiency.The influence of SiO₂concentration,lifting speed and times on the structure color of the films was further explored.The spectrophotometer test showed:With the increase of SiO₂concentration from 1wt%to 10wt%,or the increase of lifting times from 10 to 40,the reflection peaks of the films remained unchanged,the intensity increased,and the structure color saturation decreased.As the increase of lifting speed from 1mm/s to 15 mm/s,the intensity of the reflection peak increased first and then remained basically unchanged.Considering the preparation efficiency,structural color brightness and saturation,SiO₂concentration of 10wt%was selected during the preparation of the template,the times of lifting was 30times,and the speed was 10 mm/s.The microscopic morphology of the structured color films prepared by SiO₂microspheres with the six different particle sizes was characterized by SEM,and the reflection spectra and transmission spectra at different angles were characterized by spectrophotometer,which showed that the SiO₂microspheres in the films were arranged in long-range disorder and short-range order,and the surface was flat,which is an amorphous photonic crystal film with high order degree.With the SiO₂particle size increases,the photonic lattice spacing increases and the structural color redshifts,which is in accordance with Bragg’s law;the position of the valley of the transmission spectrum at different angles did not change and the structural color was isotropy.However,due to the high degree of short-range structure order,the structure color has angle-dependence under diffused light,but has low angle-dependence under direct light.（2）PDMS-based photonic elastomers were prepared using SiO₂amorphous photonic crystal films as templates.Amorphous photonic crystal films of SiO₂,SiO₂@PDA（core-shell structure）,and SiO₂-PDA（PDA-doped SiO₂）were prepared by the fast lifting method.Color analysis showed that the structural color saturation of SiO₂-PDA films was the highest,therefore,SiO₂and SiO₂-PDA amorphous photonic crystals were selected to prepare PDMS/SiO₂and PDMS/SiO₂-PDA photonic elastomers.However,PDMS/SiO₂-PDA film is difficult to demold due to strong interface adhesion between PDA and substrate.PDMS/SiO₂film has low structural color saturation,making it difficult to observe the mechanochromic properties,so PDMS/SiO₂/PDMS-PDA films were prepared by coating PDA doped PDMS（PDMS-PDA）on their surfaces,and the PDMS-PDA layer effectively improved the saturation of the structural color of the photonic elastomer film during the stretching process.Fiber optic spectrometer test and naked eye observation showed that when the tensile strain was 20%,the film showed a distinct structural color and blue-shift with increasing strain.The films have good mechanical properties with an elongation at break of 140%.（3）Hydrophilic modification of PDMS surface and preparation of its photonic elastomer.Hydrophilic modification of PDMS films was carried out by surface coating of PDA and plasma treatment.When the DA concentration was 0.15%（w/v）,the PDMS/PDA film had the best hydrophilicity with the water contact angle of 67°.However,the surface of the films had poor hydrophilic uniformity and were not suitable for constructing photonic structures as substrates;while the plasma treatment showed better hydrophilic modification of PDMS and PDMS-PDA films with the water contact angle of26°for PDMS films and 31°for PDMS-PDA films one hour after modification.The modified films were used as substrates to prepare SiO₂amorphous photonic crystals by fast lifting method,and bright structural colors were obtained,but both the PDMS/SiO₂/PDMS and PDMS-PDA/SiO₂/PDMS films prepared by this template do not did not have mechanochromic properties,and the interface disadhesion occurred during the stretching process,the adhesion force of the interface needs to be improved.