Design And Synthesis Of Discotic Liquid Crystals And Probing Into Hierarchical Structures Of Side-Chain Discotic Liquid Crystalline Block Copolymers

Discotic liquid crystalline(DLC)materials have attracted great attention due to their capability of self-assembling into well-ordered supramolecular structures as a new generation of organic semiconductors with various mesophases and unique physical properties,which can be applied in various optoelectronic devices.Among them,triphenylene(TP)derivatives are probably the most widely studied and particularly promising because of their thermal and chemical stability,relatively easy availability,self-assembly ability and high one-dimensional charge transport performance.Liquid crystalline block copolymers(LCBCPs)combining the molecular level LC order(around 1～10 nm)with long range ordered microphase-separated domain morphologies(around 10～100 nm)provide such a platform for studying the influence of morphology of block copolymers on the phase behavior of the LC segments and elucidating the orientation of mesogens with respect to the morphologies and mesophase transitions under confinement.However,most LCBCPs investigated are based on rod-like mesogens,there are surprisingly few reports of LCBCPs bearing discotic mesogens,which may mainly originate from the synthesis difficulties in obtaining suitable polymerizable discotic monomers and also their LC polymers.The synthesis of discotic LCBCPs with good control on molecular weight and distribution,as well as chemical structure and composition still remains a challenge.Whereas the better understanding of the interrelation between discotic mesogenic orders and overall morphological structures is especially demanding,which requires the acquisition of a series of well-defined discotic LCBCPs.In this dissertation,since the TP-based discotic LCBCPs can not be prepared directly through Atom Transfer Radical Polymerization(ATRP),following the synthesis of reactive precursor block copolymers poly(ethylene glycol)-b-poly(2-hydroxyethyl acrylate)(PEG-PHEA)by ATRP,a series of well-defined TP-based side chain diseotic liquid crystalline diblock eopolymers PEG-poly(TPm)(m=6 or 10)with DLC block weight fraction(fw,DLC)ranging from 37%to 90%have been successfully prepared through a polymer analogous reaction,which allows a systematic study of microphase-separated structures and their interplay with discotic mesogenic orders as a function of fw,DLC and temperature.The important intermediate compounds and polymer products were fully characterized by 1H NMR,FT-IR,GPC and so on.An intriguing microphase-separated superstructure evolution and the correlation between overall morphologies and discotic mesogenic orders as a function of fw,DLC and temperature have been demonstrated by combination of DSC,POM,and variable temperature small/wide angle X-ray scattering(SAXS/WAXS).Those copolymers with lower DLC contents(fw,DLC=37%and 43%)and at lower temperatures formed lamellar structures of variant periods and underwent order-order transitions upon PEG region crystallization at 45℃ and different discotic mesophases of ND or Ncol transition at about 25 ℃.For the copolymer with intermediate fw,DLC=62%,a high temperature hexagonal packed cylinder(HPC)structure of amorphous PEG nanocylinders in the matrix of DLC was formed above 35℃,while upon cooling below 35℃ it turned into a mixed lamellar structure with PEG region crystallization.The higher fw,DLC(67%～80%)copolymers exhibited HPC structures with the DLC matrix showing Ncol or ND mesophases.For copolymers with the highest fw,DLC around 90%,an overall ND phase was developed in sharp contrast to the ordered columnar phase formed by their corresponding DLC homopolymers,which was quite inspiring and might suggest another pathway of attaining this important nematic discotic phase through introducing a suitable copolymerized block.To investigate the influence of flexible spacer length on the phase behavior and the correlation between overall morphologies and discotic mesogenic orders in such kind of discotic LCBCPs,another series of TP-based side chain discotic liquid crystalline diblock copolymers with relative short flexible spacers PEG-poly(TP6C3)were synthesized by the above-mentioned two-step route.The bulk phase behavior and morphology of the short spacer series discotic SCLCBCs were investigated in detail by POM,DSC and variable temperature SAXS/WAXS.Comparing to the rich phase transitions and ordered superstructures of the long spacer series,the phase transitions and superstructures for the short spacer series discotic side chain LCBCPs appeared to be much simple.The shortening of the spacer length strongly enhanced the interaction between side-chain discotic mesogens and the whole polymer backbones,as a result,the self-assembly of the side-chain TP mesogens was hindered by the polymer backbones.The combining effect of steric hindrance and crowding by the side-chain TP mesogens made the whole polymer backbones lose chain mobility and strongly confined.The better understanding of the interrelation of microstructures and discotic mesogenic orders constitutes the key basis for utilizing such type of organic semiconductor materials and could help to guide the design of complex DLC polymer materials with hierarchical structures for variant applications.Microphase separation was enhanced by doping with lithium trifluoromethanesulfonate(LiCF3SO3)in the short spacer series discotic LCBCPs.The effect of lithium salt content(the molar ratio to EO units,fLi=0,0.01,0.025,0.05,0.1,0.2,1)on the phase transitions and mesophase structures of PEG127-poly(TP6C3)10 was systematically investigated by combining DSC and SAXS/WAXS.The adding of lithium salts enhanced the immiscibility between the two blocks,thus remarkably improved the microphase separation.As fLi increasing,the crystallization of PEG block was further confined due to Li:EO coordination.The complex of L1CF3SO3/PEG formed when fLi=0.1,which interrupted the microphase separation of block copolymers.As fLi reached 1,excess amount of LiCF3SO3 disturbed the crystallization of the LiCF3SO3/PEG complex.Among the discotic molecules,porphyrins are a class of important compounds which are widely present in the nature organisms and play an crucial role in the biochemical reaction for life sustaining.The unique optoelectronic properties of porphyrin LC materials have attracted great attention.Most of the reported meso-substituted porphyrin LCs so far showed discotic lamellar structure,with narrow LC temperature range and high crystallization temperature.In this dissertation,a series of novel meso-substituted tetraphenylporphyrin LCs based on porphryin octaesters(R=OOCCnH2n+1,n=7,11,15)was designed and synthesized.The LC properties of these porphyrins were modulated by varying the length of alky chains of fatty acid and introducing metal ions to form metalloporphyrins.The important intermediate compounds and porphyrin products were fully characterized by H NMR,FT-IR,elemental analyses and so on.The bulk phase behavior and structure of the free base porphyrins and metalloporphyrins were investigated in detail by POM,DSC and variable temperature SAXS/WAXS.The eight-carbons fatty acid ester(C8,n=7)series porphyrin in free base form can not form LC phases because of too short alky chains,while their metalloporphyrins formed highly ordered columnar phases with some crystal characteristics.The laurel acid ester(C12,n=11)and sixteen-carbons fatty acid ester(C16,n=15)series porphyrins showed hexagonal columnar mesophase(Colh).The magnesium porphyrin did not show LC phase due to the out-of-plane coordination.After complexing with metal ions,the LC temperature range of porphyrins was significantly widened,and the Colh became more ordered,as a result,the LC phases were stabilized.Among the newly synthesized metal porphyrin LC materials,C12-TPPZn showed the widest LC temperature range 123 C and C12-TPPNi showed the lowest crystallization temperature 25.8 C,which are quite rare in the reported symmetrical meso-substituted porphyrin LCs.The spectroscopic properties of thus synthesized porphyrin derivatives were characterized by UV-vis and fluorescence emission spectra.In UV-vis spectra,after complexing with different metal ions,the number of Q bands of metelloporphyrins decreased and the Soret bands shifted.The Q band emission of the porphyrins can be observed in the fluorescence emission spectra,while no emission for nickel porphyrins and copper prophyrins were observed.