The rheological behavior of two hyperbranched polymer/silica suspensions with different dispersion states, surface chemistries, and concentrations of the silica nanoparticles was investigated in terms of viscoelastic properties, activation energy for viscous flow, and yield stress. The viscoelastic properties of both types of suspensions were reduced to a master curve that was a function of the limiting viscosity and shear modulus. A liquid-to-solid transition and correlated activation energy change were found to occur for particle volume fraction in the range of 5-10% for well-dispersed systems and 20-25% for systems where silylated particles were agglomerated. The viscosity of the suspensions was found to be considerably higher than that predicted by the classical percolation model for concentrated particle suspensions; this was argued to result from an immobilized layer of polymer on the surface of the silica particles. The percolation model was therefore modified to include such confined layer in order to predict the viscosity as a function of filler fraction. In the case of silylated particles with weak interactions with the polymer, the model based on an immobilized layer of thickness in the range of 2-5 nm reproduced the data. In the case of well-dispersed particles with strong interfacial interactions, the immobilized layer was correlated to the average distance between adjacent particles. In this case the model predicted an exponential increase of the viscosity with particle fraction and that the whole matrix gelled at particle concentrations larger than 5 vol %, corresponding to a 7.5 nm thick immobilized layer.

The photo-curing behavior of composites containing nanosized SiO 2 in an acrylated hyperbranched polymer matrix was investigated by means of photo differential scanning calorimetry. The chemical conversion data were analyzed using an autocatalytic model, paying close attention to the influence of composition and UV intensity. It was shown that the reaction order and the autocatalytic exponent were independent of UV intensity and filler fraction, whereas the rate constant showed strong intensity dependence, but weak filler dependence. Maximum conversion was independent of UV intensity, but was reduced when a filler was present. The dispersion state influenced the gel-point of the composites, but had no influence on the overall cure kinetics. Cure shrinkage reduction of $ 33% could be achieved by adding 20 vol% of filler. This was attributed to the reduced double bond conversion of the matrix due to the presence of the filler. V

The glass transition behavior of hyperbranched polymer (HBP) molecules with suspensions of silica and glass particles up to the concentrated regime (25 vol %) was analyzed by modulated differential scanning calorimetry (MDSC). The reversing and nonreversing components of the MDSC signal were measured on suspensions of untreated and silylated particle of size in the nanometer and micrometer ranges. The heat capacity step (ΔC p ) at the glass transition of the HBP was found to be independent of silica loading for microparticles, whereas it decreased with increasing particle amount in the case of nanoparticles. A similar behavior was observed for the enthalpy relaxation. These changes in chain dynamics and the progressive suppression of aging were attributed to immobilization effects of the HBP at the surface of the particles, which became detectable only in the case of a very high specific surface. The immobilized HBP fraction was assumed to form a shell of constant thickness around individual particles and was calculated from the ΔC p at the transition. In the case of untreated particles with a silanol surface, the immobilized shell was formed by HBP molecules H-bonded to the particles. The thickness of the shell was found to be equal to 1.9 nm, which corresponded to half the size of the HBP. In the case of methacrylsilane-treated silica, the immobilized shell thickness was found to be equal to 1.3 nm, which corresponded to a monolayer of covalently bound silane.

SummaryThe catalytic chemical vapour deposition (c-CVD) technique was applied in the synthesis of vertically aligned arrays of nitrogen-doped carbon nanotubes (N-CNTs). A mixture of toluene (main carbon source), pyrazine (1,4-diazine, nitrogen source) and ferrocene (catalyst precursor) was used as the injection feedstock. To optimize conditions for growing the most dense and aligned N-CNT arrays, we investigated the influence of key parameters, i.e., growth temperature (660, 760 and 860 °C), composition of the feedstock and time of growth, on morphology and properties of N-CNTs. The presence of nitrogen species in the hot zone of the quartz reactor decreased the growth rate of N-CNTs down to about one twentieth compared to the growth rate of multi-wall CNTs (MWCNTs). As revealed by electron microscopy studies (SEM, TEM), the individual N-CNTs (half as thick as MWCNTs) grown under the optimal conditions were characterized by a superior straightness of the outer walls, which translated into a high alignment of dense nanotube arrays, i.e., 5 × 108 nanotubes per mm2 (100 times more than for MWCNTs grown in the absence of nitrogen precursor). In turn, the internal crystallographic order of the N-CNTs was found to be of a ‘bamboo’-like or ‘membrane’-like (multi-compartmental structure) morphology. The nitrogen content in the nanotube products, which ranged from 0.0 to 3.0 wt %, was controlled through the concentration of pyrazine in the feedstock. Moreover, as revealed by Raman/FT-IR spectroscopy, the incorporation of nitrogen atoms into the nanotube walls was found to be proportional to the number of deviations from the sp2-hybridisation of graphene C-atoms. As studied by XRD, the temperature and the [pyrazine]/[ferrocene] ratio in the feedstock affected the composition of the catalyst particles, and hence changed the growth mechanism of individual N-CNTs into a ‘mixed base-and-tip’ (primarily of the base-type) type as compared to the purely ‘base’-type for undoped MWCNTs.

Nanocomposite materials based on a HBP and silica are produced using either a dual‐cure sol/gel and photopolymerization process or by mixing silica nanoparticles with the HBP. In both cases the conversion of the HBP is independent of composition and obeys a time‐intensity superposition with power‐law dependence on UV intensity. Optimization of the dual‐cure process leads to transparent sol/gel composites with ultrafine structures. These materials systematically outperform the particulate composites, including an increase of the glass transition temperature of 63 °C and a process‐induced internal stress as low as 2.5 MPa. Nano‐sized gratings are produced from the sol/gel composites by low‐pressure UV nanoimprint lithography.magnified image

Summary: Nano-scale patterns were produced with UV-curable acrylated hyperbranched polymer nanocomposites using nanoimprint lithography with a glass master in a rapid, low-pressure process. The pattern of the glass master was replicated with composites containing up to 25 vol% SiO 2 with a shape fidelity better than 98%. Photo-rheology, interferometry and atomic force microscopy were used to analyze the material behavior. Attention was paid to the relationship between composition, nanoparticle dispersion, kinetics of photo-polymerisation, shrinkage, pressure and shape fidelity of nano-gratings. It was shown that the gel-point of the nanocomposite was an important factor that determined the stability as well as the dimensions of the imprinted structure. Dimensional accuracy also strongly depended on the level of internal stress, which in fact increased with the amount of silica. A resin rich layer on the surface of the composite accounted for the good surface quality of the nano-pattern.

a b s t r a c tThe validity of the time-intensity superposition principle for the photoinitiated polymerization of nanocomposites based on a monofunctional fluorinated acrylate and on a multifunctional hyperbranched polyether acrylate was investigated in this work. Master curves were obtained for the conversion as a function of time, measured by photo differential scanning calorimetry, by shifting on the time axis the curves obtained at different intensities. A power-law dependence of the shift factor on the intensity was found for all materials, with exponents equal to 0.45 AE 0.03 for the fluorinated acrylates and to 0.71 AE 0.05 for the hyperbranched polyether acrylates. Consequently it is inferred that the radiant exposure reciprocity law, implying linear dependence of the shift factor on intensity, does not apply to the studied compositions. The kinetics of the photopolymerization of materials based on the fluorinated acrylate was analyzed with the autocatalytic model. The final conversion was independent on intensity and filler content. The rate constants showed for all materials a power-law dependency on intensity, with exponents similar to those of the corresponding shift factors.

Polymer nanocomposite gratings with a 363 nm period and a 12 nm step height were replicated using a glass master in a rapid, low-pressure imprint process. The composite materials were based on a UV-curable acrylated hyperbranched polymer and nanosized SiO2 particles. The influence of particle fraction up to 25 vol%, process pressure and UV intensity on the grating geometry was analyzed using atomic force microscopy. The period of the grating was found to be identical to that of the glass master for all investigated conditions. It was shown that the gel point of the nanocomposite was an important factor that determined the stability as well as the dimensions of the imprinted structure. However, a distortion of the grating was observed with increasing fraction of SiO2, which was correlated to the increased internal stress of the composite. Wavelength interrogated optical sensors were produced by depositing a high refractive index TiO2 layer on the composite gratings. The laser signal strength of the polymer sensors was equal to that of the reference high precision glass sensor with 10-12 g/mm2 sensitivity. The strength was lower for the nanocomposites due to propagation losses argued to result from residual porosity.