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.

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.

Nylon 11/organoclay nanocomposites have been successfully prepared by melt-compounding.X-ray diffraction and transmission electron microscopy indicate the formation of the exfoliated nanocomposites at low clay concentrations (less than 4 wt%) and a mixture of exfoliated and intercalated nanocomposites at higher clay contents.Thermogravimetric and dynamic mechanical analyses as well as tensile tests show that the degree of dispersion of nanoclay within polymer matrix plays a vital role in property improvement.The thermal stability and mechanical properties of the exfoliated nylon 11/clay nanocomposites (containing lower clay concentrations) are superior to those of the intercalated ones (with higher clay contents),due to the finer dispersion of organoclay among the matrix.

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.