David Attwood

Prediction of the glass transition temperatures for epoxy resins and blends using group interaction modelling

Application of modern simulation methods for the prediction of the engineering properties of polymeric materials may be a substitute for more time consuming experiments. The principles of molecular modelling have been combined with group interaction modelling (GIM) for the prediction of properties of thermoset resins. The glass transition temperature of the systems was predicted from the chemical structure of the resins and the effect of different hardeners on Tg was assessed. Different chemical reaction mechanisms which occur during resin cure were incorporated into the model for better predictions. A new set of expanded GIM equations include one for determination of model input parameters from the conventional modelling principles for the estimate of Tg. Differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) experimental values of degree of cure and glass transition temperature were used to provide comparative analysis for the computations. q 2001 Elsevier Science Ltd. All rights reserved.

pH-Dependent gold nanoparticle self-organization on functionalized Si/SiO2surfaces

The self-organization of citrate- and acrylate-stabilized gold nanoparticles onto SiO2/hydroxyl-, amino- and nitro-terminated surfaces was investigated as a function of pH. Bare clean Si/SiO2 substrates were used as the SiO2/hydroxyl-terminated surfaces and self-assembled monolayers (SAM) of (3-aminopropyl)trimethoxysilane (APTMS) and 3-(4-nitrophenoxy)-propyltrimethoxysilane (NPPTMS) on Si/SiO2 were employed as the amino- and nitro-terminated surfaces, respectively. All the surfaces were fully characterized by contact angle, atomic force microscopy (AFM), ellipsometry and X-ray photoelectron spectroscopy (XPS). Citrate- and acrylate-stabilized gold nanoparticle stability was also investigated as a function of pH by UV–visible absorption spectroscopy and Z-potentiometry. The gold nanoparticle surface coverage of the substrates was independently estimated by AFM and XPS. The results show that colloid deposition on bare SiO2/OH surfaces and on NPPTMS monolayers is negligible with the exception of acrylate-stabilized gold nanoparticles which were found to be immobilized on nitro-terminated surfaces at pH lower than 3.5. Nevertheless, APTMS monolayers interact strongly with citrate- and acrylate-stabilized gold nanoparticles exhibiting a dependence of the surface coverage from the pH of the colloidal solution.

Chemical manipulation by X-rays of functionalized thiolate self-assembled monolayers on Au.

The chemical modification caused by prolonged exposure to X-rays on a series of para-substituted phenyl moieties (-NO2, -CN, -CHO, -COOH, -CO2Me, and -CO2(1)Bu) at the surface of thiolate-Au self-assembled monolayers (SAMs) has been investigated by X-ray photoelectron spectroscopy (XPS). Furthermore, the influence that the phenyl group has on the chemical modification induced by the X-ray irradiation on the SAMs was investigated by comparing the XPS results obtained from irradiation on a NO2-aromatic-terminated SAM (6-(4-nitro-phenoxy)-hexane-1-thiolate (NPHT)) and NO2-aliphatic-terminated SAM (thioacetic acid S-(12-nitrododecyl) ester (TNDDE)). The NPHT and TNDDE SAMs have been shown to behave differently to X-ray exposure. The irradiation of the NPHT SAM led to the reduction of the nitro (-NO2) moiety to the amine (-NH2) moiety, as shown by the decrease in the intensity of the N 1s photoelectron peak for -NO2 (406 eV) in the XPS spectra with the concomitant increase in the N 1s photoelectron peak for -NH2 (399 eV). On the TNDDE SAM, XPS showed the -NO2 photoelectron peak again decreasing with prolonged X-ray irradiation whereas no peak was observed at 399 eV; therefore, the -NO2 moieties are selectively cleaved. No change was observed on the other functionalized monolayers apart from the -CO2(t)Bu-functionalized monolayer, where after 100 min of X-ray irradiation approximately 11% of the carbon content was lost. The S 2p and O 1s spectra remained unchanged during the irradiation suggesting the conversion of the -CO2(t)Bu to the -COOH moiety, although the conversion was not complete because the tertiary butyl moiety contributes 25% to the total carbon content of the SAM. Also, there was no evidence of the molecules desorbing from the substrate for any of the SAMs studied during the X-ray irradiation as shown by no change in the S 2p and C 1s XPS spectra taken during the X-ray irradiation.

Fabrication of a nanoparticle gradient substrate by thermochemical manipulation of an ester functionalized SAM

The hydrolysis of methyl ester (–CO2Me) and tert-butyl ester (–CO2tBu) functionalized SAMs as a function of subphase temperature and pH is described. Contact angle measurements show that the methyl ester functionalized monolayer does not hydrolyse in pH 1–13 aqueous solutions heated up to 80 °C. In contrast, the –CO2tBu functionalized monolayer hydrolysed below pH 5. The rate and the extent of the hydrolysis were dependent on the temperature and pH of the aqueous solution. Using the Cassie equation, the activation energy for the hydrolysis of CO2tBu-phenyl functionalized SAM was determined as 75 ± 7 kJ mol−1 from the contact angle measurements. Furthermore, the adhesion properties of –CO2tBu and –COOH functionalized SAMs were investigated by depositing –NR2 and –COOH functionalized polystyrene nanoparticles onto the surfaces at pH 3 and 9. By AFM, it was observed that the particles bind preferentially to the –COOH functionalized SAM and the adhesion was pH dependent, with the largest coverage being observed at pH 3. Using the acquired understanding of the hydrolysis of –CO2tBu functionalized SAM and the particle adhesion properties, a simple and facile approach towards fabricating a particle density gradient on this surface is demonstrated. An acid gradient SAM (20 mm long) was prepared by mounting one end of a –CO2tBu functionalized SAM onto the hot side of a Peltier element (80 °C) in pH 1 aqueous solution. The substrate was subsequently immersed into a colloidal solution of –NR2 functionalized polystyrene nanoparticles, removed and rinsed. By AFM, the particle density was shown to be dependent on the surface coverage of –COOH moieties of the underlying SAM. The density started at 104 particles µm−2 on the hydrolysed end down to 0 particles µm−2 on the non-hydrolysed end.

Photochemical fabrication of three-dimensional micro- and nano-structured surfaces from a C60 monoadduct

Exposure of Langmuir–Blodgett (LB) films of a C60 adduct supported on silicon wafers to UV light leads to cross-linking of the C60 moieties, which are resistant to removal by solvent exposure, whereas unexposed moieties are readily removed. This process provides a convenient and simple route for the fabrication of highly conjugated surface-attached structures, with dimensions ranging from micrometres (using masks) to a few tens of nanometres using light emitted from a scanning near-field optical microscope (SNOM). The SNOM writing velocity was found to significantly affect the lateral resolution and the height of the three-dimensional nanostructures. Increasing the writing velocity from 0.3 to 2 μm s−1 resulted in a decrease in the width of the structures from 240 nm to 70 nm (corresponding to the SNOM aperture diameter), respectively, and a reduction in the height from 8 nm (the thickness of the original film) to 3 nm, respectively. This approach provides a simple, direct route to surface-bound nanometre scale assemblies of C60.

Novel 3,4-disubstituted thiophenes for weak passivation of Au nanoparticles

Novel self-assembled monolayers of 3,4-disubstituted thiophenes (3,4-dioctylthiophene and 3,4-diheptyloxythiophene) were prepared and characterized by contact angle measurements, ellipsometry and X-ray photoelectron spectroscopy. In addition, Au nanoparticles passivated with the 3,4-disubstituted thiophenes (3,4-dioctylthiophene and 3,4-diheptyloxythiophene) and monosubstituted thiophene (3-octylthiophene) were synthesized and characterized by UV/visible spectroscopy, transmission electron microscopy and dynamic light scattering. The Au nanoparticles had diameters in the range 5–7 nm. The Au nanoparticles stabilized with the thiophene derivatives were, as expected, less stable than the Au nanoparticles passivated with decanethiol and didecylsulfide. Surprisingly, the particles passivated with the monosubstituted 3-octylthiophene were more stable than the 3,4-dioctyl and 3,4-diheptyloxythiophene passivated particles. Such lower stability Au nanoparticles may find uses as negative tone resists in the formation of nanowires by e-beam lithography, via the more readily cleavable Au–S bond