The Self-assembly Behavior Of Block Copolymer And Dye Molecule In Selective CO₂-expanded Liquids

Amphiphilic block copolymers have good structural tunability and can be self-assembled into various nanostructures such as spheres,rods,vesicles and large compound micelles in selective solvent,which has a wide application in drug loading,gene therapy,nanoreactor and other fields.CO2-expanded liquids(CXLs)are usually regarded as a new type of environmentally friendly solvent media,and many of their physicochemical properties such as polarity,viscosity and solubility can be regulated via the pressure tuning of the system under mild conditions.So we can design some properties of the gas-expanded liquids to satisfy our needs.In this work,the amphiphilic block copolymer polystyrene-b-poly(2-vinylpyridine)(PS-b-P2VP)was synthesized and was further self-assembled with dye molecule in selective CO2-expanded solvent.The morphology and luminescent properties of the self-assembled composites(SAC)were investigated under different conditions.Reversible addition-fragmentation chain transfer(RAFT)polymerization is a living controlled radical polymerization method with mild reaction conditions and wide monomer applicability.The as-prepared polymer has a narrow molecular weight distribution and reactive group at the end.We used benzyl dithiobenzoate(BDB)prepared as a RAFT reagent and then successfully obtained the amphiphilic block copolymer PS-b-P2VP with definite structure and different ratios of the segments(i.e.,PS44-b-P2VP58 and PS44-b-P2VP93).The polymer was characterized by NMR and gel permeation chromatography(GPC)to confirm its structure and molecular composition.In selective CO2-expanded liquids,the behavior of SAC between dye molecules Disperse Red 1(DR1)and copolymer PS-b-P2VP was examined under different CXLs pressure and composition,copolymer composition,as well as concentration of dye molecule and copolymer.The morphology and properties of SAC were then characterized by the fourier transform infrared(FT-IR),fluorescence spectroscopy and transmission electron microscopy(TEM).Firstly,it is found that hydrogen bonds were formed between the pyridine ring of P2VP and the-OH group of dye molecule,which restrains the trans-to-cis transition of the dye unit and therefore enhances its fluorescence intensity significantly.Secondly,in selective CO2-expanded liquids,increasing CXLs pressure leads to weakened solvent polarity and poorer copolymer solubility and mobility,which could offer effective control of the SAC morphology,along with the emission performance of the dye unit through hydrogen bonding with P2VP block.In other words,the CO2 pressure in the system can regulate the morphology and luminescent properties of the SAC effectively.At 353.2 K and within the pressure range of 0.1 to 9.0 MPa,it is found that in polar CXLs(i.e.CO2-expanded ethanol),the fluorescence intensity of SAC is enhanced as the CXLs pressure increases.However,in CO2-expanded toluene,the SAC emission increases with increasing pressure until a maximum and then becomes attenuated even though the pressure keeps rising.Finally,the CXLs pressure dependent morphological changes and infrared data further help us to explain and better understand the emission mechanism of the SAC.