Design And Fabrication Of The Structural Colored Materials Via RAFT Emulsion Polymerization

Structural colored materials have received extensive attentions.Compared with traditional pigments and dyes,the production and application process of the structural colored materials is much more environment-friendly.Also,the structural colored materials could show unique optical responsivity to external stimulus.The application of the structural colored materials has been hindered by the following problems:firstly,the structural colored materials especially colloidal crystals have always been criticized for poor mechanical strength due to the porous structure.As a matter of fact,tensile properties of colloidal crystal materials have scarcely been reported yet.Secondly,the large-scale fabrication of structural colored materials in a facial and versatile way remains a challenge.To solve the above-mentioned problems,we designed two classes of soft structured latex particles of copolymer,i.e.soft shell/hard core two-layer structured and corona/hard shell/soft core three-layer structured latex particles at a molecular level according to light scattering and interfering theories.Well-defined structured particles were synthesized in a RAFT emulsion polymerization.The as-prepared particles were then used to fabricate two classes of high performance structural colored materials with dense physical structures.The relationship between particle structures(chemical composition,molecular weight,physical structure,and particle size)and performance of these materials was systematically investigated.Through facial and scalable-up latex casting method,two types of structural colored materials with good mechanical properties and high color qualities were successfully fabricated.And the colloidal crystals even exhibited multiple responsive properties.The main achievements are as follows:(1)With the proper design of amphiphilic macro-RAFT agent,a series of mono-dispersed nano-particles with tunable particle size and composition,i.e.soft shell/hard core particles of poly(n-butyl acrylate)-b-polystyrene(PnBAS)and poly(methyl acrylate)-b-polystyrene(PMAS)and hard shell/soft core nano-particles of polystyrene-b-poly(n-butyl acrylate)(PSnBA)were prepared via RAFT emulsion polymerization.Through the post-addition of macro-RAFT agent in the synthesis of nano-particles of PSnBA,mono-dispersed three-layer structured nano-particles with a corona of carboxyl groups were successfully synthesized.(2)Casting of soft shell/hard core PnBAS and PMAS latex particles was found to be a facile and scalable-up method to fabricate structural colored materials based on Rayleigh scattering.With water evaporation,the soft shells would fuse together and form dense continuous phase to achieve high mechanical performance,and the hard cores would be the well-dispersed phase to selectively scatter short-wavelength lights and generate blue color.When the dispersed phase was too small(<80 nm),the light scattering intensity was too low and the surface reflection of films dominated,which resulted in pale colors.Increasing the size of the dispersed microphase by increasing molecular weight and adding plasticizer was able to increase the scattering intensity of blue light,thus leading to high color quality.The additional amount of plasticizer mattered a lot.For PnBAS(60k-40k)(the content of PnBA was 60%),adding 40 wt%plasticizer would result in a desirable dispersed phase morphology to generate the high-quality blue color.On the other hand,a strong phase separation system(PMAS)could led to sharp phase interface,which resulted in higher scattering selectivity and better structural color.When the film was too thick(>0.35 mm),the possibility of scattering long-wavelength lights would increase,lowering color saturation.The adverse effect of the film thickness could be partly eliminated by adding proper amount of carbon black(<0.01 wt%).The as-prepared structural colored films exhibited high mechanical properties.For PnBAS(60k-40k)film,the modulus,tensile strength and strain at break were 9 MPa,0.9 MPa and 116%,respectively.(3)A new route to high qualified colloidal crystals was demonstrated from casting of mono-dispersed three-layer structured latex particles.The fabrication process was facile and scalable-up.The latex particles boasted a responsive corona(PAA),a hard shell(PS)to avoid possible coalescence during the fabrication of colloidal crystals and a soft core(PnBA)to modulate modulus to form a dense and carck-free film with good tensile properties.When the PnBA content was too low,the modulus of latex particles was so high that the film was full of cracks.When the PnBA content was too high,the latex particles were so soft that they would coalesce during crystallization,which was adverse to the regularity of colloidal crystals.When the PnBA content was 52.1%～59.0%,the as-prepared colloidal crystal films with the modulus around 5～21 MPa showed brilliant colors and importantly were crack-free.Full color colloidal crystal films were able to be fabricated by modulating the particle size.The as-prepared structural colored films exhibited high mechanical properties and good stability during bending.The modulus,tensile strength and strain at break of the colloidal crystal film of 54.6%PnBA were 2.4 MPa,0.26 MPa,and 40.8%,respectively.The tensile performance of colloidal films little changed after the filtration of water.It was found that the additional amount of carbon black and silica particles and film thickness would affect the color quality.The highest performance of the colloidal crystal films was obtained with 0.02 wt%carbon black and 10 wt%nano-silica with film thickness 0.3 mm.Very interestingly,the influence of substrates on color quality was negligible.(4)The as-prepared colloidal crystal films were responsive to hydrophilic liquids such as water,ethylene glycol and ionic liquid because of the presence of the carboxyl groups.Using the hydrophilic liquids as "inks" and the colloidal films as "paper",well-designed patterns could be drawn.Based on the different volatilities of liquids,patterns with different decaying rates could be fabricated.Using volatile "ink",erasable colloidal paper was made.Using ionic liquid as the "ink",permanent color pattern was made.And with the combination of several "inks" with varied volatilities,smart patterns that could change "meanings" as time went on could also be achieved.Besides,the as-prepared colloidal crystal films were responsive to pH values.The higher pH value is,the more water is absorbed in,inducing the red shifting of the diffraction peaks and the change of structural colors.