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.

The nanoindentation behavior and morphology of the injection-molded specimens of nylon-6 (PA6)/clay nanocomposites prepared by meltcompounding have been studied in present study.The elastic and plastic properties as well as creep behavior of PA6 and its nanocomposites are comparatively evaluated as the function of clay loading by using nanoindentation technique.The anisotropic characteristics in mechanical properties are studied by indenting the injection-molded specimens in two different directions (i.e. parallel and perpendicular to the injection direction).The uneven distribution of both the clay nanofiller and the crystallinity of the polymeric matrix induced by melt-processing leads to the variation of the mechanical property of the nanocomposites in certain directions and locations within the molded specimens.The microstructural and morphological changes of PA6 upon incorporating with clay nanofiller are evidenced by transmission electron microscopy and small-angle Xray scattering, which are closely correlated with the anisotropy of the mechanical properties observed by nanoindentation.

Strain rate effects on surface deformation behavior of exfoliated nylon 66 (PA66)/organoclay nanocomposites have been explored by nanoindentation in present study.Sharp indenter (Berkovich) has been used to indent on the surfaces of polymer/clay nanocomposite with different strain rates.Significant strain-rate hardening has been found consistently existing in both neat PA66 and its nanocomposite systems from surface to subsurface (a few micron deep into the bulk).However,strain rate shows almost no effect on the elastic moduli of the neat system and the nanocomposites.The elastic modulus and hardness increase with the indentation depth due to inhomogeneous distributions of the crystalline morphology as well as clay concentration for the case of the nanocomposites along the indentation direction.The mechanical properties observed are correlated with the inhomogeneous microstructures of the studied systems.The plastic index of PA66 and the nanocomposites have been evaluated as a function of strain rate.

Carbon nanotubes (CNTS) have been grown on MCM-41 supported Fe nanoparticles and the as-prepared (no further purification) CNT-silica hybrid was directly incorporated into nylon-6 (PA6) by simple melt-compounding.The urchin-shaped CNT-silica hybrid filler was observed to be homogeneously dispersed throughout the matrix by scanning electron and transmission electron microscopy.Compared with neat PA6,the tensile modulus and strength of the composite are greatly improved by about 110%,with incorporation of only 1 wt.-% CNT-silica filler.

Multiwalled carbon nanotubes (MWNTS)/nylon-6 (PA6) nanocomposites with different MWNTS loadings have been prepared by the simple melt-compounding approach.A fine and homogeneous dispersion of MWNTS throughout PA6 matrix is observed by transmission electron microscopy.Scanning electron microscopy observation on the fracture surfaces of the composites shows not only a uniform dispersion of MWNTS but also a 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.Beadlike morphology is also observed along the stretched CNTS and their bundles,probably indicating the anchoring locations of the CNTS defects (within the beads) along the tubes where the PA6 matrix has strong interfacial interactions with the CNTS,thus being favorable to stress transfer from polymer to CNTS.Mechanical testing (by tensile and nanoindentation tests as well as dynamic mechanical analysis) shows that,compared with neat PA6, the elastic modulus and the yield strength of the composite are greatly improved by about 214% and 162%,respectively,with incorporating only 2 wt % MWNTS.In addition,a unique crystallization and melting behavior of MWNTS/PA6 composites are observed and discussed by combining differential scanning calorimetry and X-ray diffraction;that is,only the R-form crystals are observed in MWNTS/PA6 composites,which is quite different from the case observed in PA6/clay nanocomposites.

Nanoindentation technique has been used to investigate the mechanical properties of exfoliated nylon 66 (PA66)/clay nanocomposites in present study.The hardness, elastic modulus and creep behavior of the nanocomposites have been evaluated as a function of clay concentration.It indicates that incorporation of clay nanofiller enhances the hardness and elastic modulus of the matrix.The elastic modulus data calculated from indentation load-displacement experiments are comparable with those obtained from dynamic mechanical analysis and the tensile tests.However,the creep behavior of the nanocomposites shows an unexpected increasing trend as the clay loading increases (up to 5 wt%).The lowered creep resistance with increasing clay content is mainly due to the decrease of crystal size and degree of crystallinity as a result of clay addition into PA66 matrix,as evidenced by optical microscopy and X-ray diffraction. At lower clay concentration (here ≤5 wt%),morphological changes due to addition of clay plays the dominant role in creep behavior compared with the reinforcement effect from nanoclay.