The structural evolution and properties of poly(9,9-dihexylfluorene-alt-2,5-dialkoxybenzene) with different lengths of alkoxy side chains on phenylene have been systematically investigated by means of thermogravimetric analysis (TGA),X-ray diffraction (XRD),differential scanning calorimetry (DSC),polarizing light microscopy (PLM),atomic force microscopy (AFM),and cyclic voltammetry (CV) techniques.The polymer self-organizes into a lamellar structure consisting of both two- and one-layer packing,and the two-layer packing style is the dominant structure.In addition,the two-layer and one-layer packing structures also accompany the presence of planar stacking and/or crystalline and noncrystalline structures,thus maintaining the stability of the packing. PF6OC6 shows three ordered phases (two crystalline phases and one nematic phase) during the heating process.With further increase of the length of alkoxy side chains, only two ordered phases (one crystalline phase and one nematic phase) are observed and the polymers show a melting-recrystallization phenomenon,which is steadily inhibited as the length of the alkoxy side chains increases.The optical and electrochemical properties of the polymers do not exhibit noticeable dependence on the length of the alkoxy side chains.However,the thermal stability,the vibronic structures, and the full width at half-maximum (fwhm) in photoluminescence spectra of the films gradually decrease,and the oxidation onset potentials and the corresponding HOMO energy levels slightly increase with increasing length of alkoxy side chains on phenylene.These results indicate that the length variation of alkoxy side chains does not change the electronic structure of the polymer backbones,but remarkably affects the microphase separation between the flexible side chains and the conjugated backbones.

Multi-walled carbon nanotube (MWCNT)/polyetherimide (PEI) nanocomposite films have been prepared by casting and imidization.A homogeneous dispersion of MWCNTs throughout the PEI matrix is observed by scanning electron microscopy of fracture surfaces,which shows not only a fine dispersion of MWCNTS but also strong interfacial adhesion with the matrix, as evidenced by the presence of many broken but strongly embedded carbon nanotubes (CNTS) in the matrix and by the absence of debonding of CNTS from the matrix.Differential scanning calorimetry and dynamic mechanical analysis show that the glass transition temperature of PEI increases by about 10℃ by the addition of 1 wt% MWCNTS.Mechanical testing shows that for the addition of 1 wt% MWCNTS,the elastic moduli of the nanocomposites are significantly improved by about 250% while the tensile strength is comparable to that of the matrix.This improvement is due to the strong interfacial interaction between the MWCNTs and the PEI matrix which favors stress transfer from the polymer to the CNTS.

The origin of the low-energy emission of fluorene-based homo- and copolymers still remains controversial.In this work,the effect of thermal treatment on the emission properties of poly[(9,9-dihexylfluorene)-alt-co-(1,4-phenylene)] (PF6P) and its four derivatives modified by attaching different lengths of alkoxy side chains on the phenylene rings has been systematically investigated.By comparing the photoluminescence (PL) spectra of PF6P and the modified polymers,Fourier-transform infrared (FTIR) spectroscopy,X-ray diffraction (XRD) and PL lifetime measurements have revealed that the long wavelength emission could be attributed to the formation of fluorenone-based excimers rather than to the localized fluorenone π–π* transition,the energy transfer from fluorene segments to the fluorenone moieties,or the fluorenone defects generated by thermal oxidation during thermal treatment.Compared with PF6P, the attachment of alkoxy side chains on the phenylene rings effectively inhibits the aggregation of backbone chains,thus restrains the formation of fluorenone-based excimers and remarkably improves thermal stability of the spectra.

A series of well-defined conjugated-liquid crystalline (LC) block copolymers containing oligofluorene and side-chain liquid crystalline polymers with cyanobiphenyl moieties were successfully synthesized by atom transfer radical polymerization.The block copolymers were prepared with number-averaged molecular weights (Mn) ranging from 8000 to 16 000 and narrow molecular weight distribution less than 1.20.The chemical structures of these block copolymers were confirmed by 1H NMR,13C NMR,and FTIR studies. All of the block copolymers exhibited the smectic mesophase as illustrated by differential scanning calorimetry,polarized optical microscopy, and wide-angle X-ray diffraction.A bilayer structure of mesogens was formed in the smectic layer of block copolymers with a thickness of 3.5 nm.The isotropization of the smectic phase increased with the molecular weight and leveled off at Mn ) 14 000.The optical properties of these block copolymers in solution and solid-films were investigated comparatively by UV spectroscopy,photoluminescence,and photoluminescent excitation characterization.The results suggest that energy transfer from the LC mesogens to the conjugated oligomer occurs both in dilute solution and in the solid state,which was more efficient in solid state due to higher local chromophore density.