Browsing by Author "Block, H."

Now showing 1 - 4 of 4
  • No Thumbnail Available
    ItemOpen Access
    Dielectric monitoring during the cure of epoxy resin blends
    (Cranfield University, 1991-10) Maistros, G. M.; Partridge, Ivana K.; Block, H.
    Dielectric monitoring and supporting techniques (differential Scanning calorimetry, infra-red spectrosoopy, viscometry, dynamic mechanical thermal analysis and light transmittance) were used to study the isothermal cure reaction of the CTBN rubber modified DGEBA resin/amine hardener blends. The neat system was also examined for the required knowledge of the matrix properties. The complexity of the cure kinetics was demonstrated by the use of a rapid technique for kinetic parameters evaluation. The utility of the dielectric cure monitoring is focused at the observation of evidence o phase separation, gelation and vitrification. The phase separatlon which the blends underwent during the cure was detected by the dielectric »monitoring through a permittivity increase at the low frequency response. The onset of the rapid viscosity increase leading to gelation was also indicated by the sharp decrease o the dielectric constant atlhigh frequencies. The frequency dependence of the times reach the dielectric loss peaks was used to predict successfully the vitrification times during the isothermal reactions o the blends. The in-situ nature o the technique and the basic understanding o the features appearing in the dielectric signal during the cure reaction provide the basis for the use of dielectric monitoring in the process of composite materials, manufacture.
  • Thumbnail Image
    ItemOpen Access
    Electrochemical and chemical methods of metallizing plastic films
    (Cranfield University, 1991-09) Chapples, J.; Block, H.
    This thesis describes two novel techniques for the metallization of non-electroactive polymer films and thicker sectioned polyethylene and nylon substrates. In the first approach, non-electroactive polymer substrates were impregnated with surface layers of polypyrrole and polyaniline, using electrochemical and chemical methods of polymerization. The relative merits of both these approaches are discussed and compared with other methods in the literature. The resultant composite materials exhibited sufficient conductivity to permit the direct electrodeposition of metal surface coats. Polypyrrole coated substrates were also metallized using chemical methods. Cyclic voltammetry studies and scanning electron microscopy of metal coated polypyrrole, showed that metal deposition occurred mainly at the polymer surface by a mechanism of instantaneous nucleation and growth. Using optimized deposition conditions, both electrochemical and chemical metal deposition methods were used to deposit highly reflecting and coherent metal layers onto conducting polymer coated materials. The second approach of metallizing polymers, was the metallization of non-electroactive polymer films by the electroreduction of silver from non-aqueous based silver plating solutions. The effects of the electrode substrate, the deposition potential, and the concentration of metal ions in solution were investigated to determine suitable metal salt/solvent, and polymer film/solvent combinations. The resultant metallized polymer films were evaluated using optical and scanning electron microscopy, ac impedance, and reflectance measurements. These studies enabled the optimum deposition conditions to be determined, and these were subsequently used for the preparation of high quality, uniform, and reflective metal coated films. The results for the electrodeposition of silver into polymer films using the latter approach are compared with those obtained from alternative electrochemical and chemical methods of metallizing polymer films.
  • Thumbnail Image
    ItemOpen Access
    Polymeric materials for piezoelectricity and second harmonic generation
    (Cranfield University, 1991-09) Shaw, Christopher P.; Block, H.
    Studies have been undertaken to utilise the possible potential of the rodlike, helical structures associated with polyglutamates and polyisocyanates for piezoelectric (PE) and nonlinear optical (second harmonic generation (SHG)) applications. Various techniques have been employed to form samples/films of these polymers containing oriented helices whose bulk structures are non-centrosymmetric, (an important criterium for the aforementioned applications). Owing to the poor yields obtained for certain intermediates in the synthetic stages of these polymers, only poly('S-benzyl-L-glutamate) (PBzLG) and poly(n- hexylisocyanate) (PHIC) were available for subsequent studies. Piezoelectric work was limited to hydrostatic measurements (d3h) made on electrically poled, composite (guest host), polymer samples. Phase separation was common in all the samples, but no piezoelectric response was observed. However, low concentration (not greater than 10% w/w) of active polymer (PBzLG or PHIC) and high conductivity during the poling stage, may have accounted for the lack of response. Electrically poled, homopolymer samples of PBzLG and PHIC were achieved using a solvent evaporation technique. Nonlinear optical studies of these poled samples revealed threshold fields above which detectable levels of SHG could be observed, (i. e. PBzLG > 40 V/mm, PHIC > 150 V/mm) - The variation of SHG signal versus. the angle of polarisation of the incident laser light (1064 nm) relative to the poling direction of the film,, implied biaxial symmetry was present in the case of PBzLG, whereas the more conventional uniaxial symmetry was observed for PHIC. The variation in SHG signal observed for a fixed thickness was attributed to varying degrees of alignment, resulting from uneven poling. Although PHIC and PBzLG showed low SHG activity (less than value for urea), little ageing of this activity occurred over the 6 month testing period. Langmuir Blodgett studies carried out on chemically modified, low molecular weight PBzLG molecules showed poor transference of the monolayer to a quartz substrate once ten layers had been deposited. In addition, areas per molecule (or per residue) calculated from the pressure-area isotherms, proved inconclusive when deducing possible orientations for the PBzLG helices. No SHG signal was observed for the L. B. films.
  • No Thumbnail Available
    ItemOpen Access
    Pulsed flow and time-resolved dielectric spectroscopy of electrorheological fluids
    (Cranfield University, 1994-03) Rattray, P.; Block, H.
    Research has been undertaken into the dielectric and rheological properties of electrorheological (ER) fluids. The fluids studied were based on acene-quinone radical polymers made within the department dispersed in silicone oil. A commercial poly(1ithium methacrylate) dispersion was also examined. As a means of probing the underlying mechanisms of the E phenomenon, the permittivity of the fluids was measured from 12 Hz to 100 kHz under both static and dynamic conditions. Results indicated that a interfacial polarization process was taking place. A series of visual observations were made of fluids under different fielding patterns. A series of photographs were taken that illustrated the structure formation with elapsed time in a dilute fluid. Also photographs were taken of the final structure formed under different field conditions. .. To perform permittivity measurements of the fluid when a electric field was applied, a high voltage biasing unit was designed, built and proved. This allowed the application of a continuous DC electric field of up to 3 kVmm" and the permittivity to be measured from 150 H t 100 kl-Iz. Through a series of experiments it was found that the low frequency permittivity increased with increasing electric field. This result was partially explained by the Sillars model. The fluids were also subjected to shear rates from 1500 to 60 s". Flow modified permittivity resonances were found at the predicted frequencies. However, the resonant frequency did not move significantly under the application of a electric field. The structuring process was time resolved and a model was made to predict the sealing of the characteristic structuring time. The rheological response of the fluids when subjected t pulsed DC fields was examined and found to be dominated by a instrumentational effect. Al experimental procedures are given along with a comprehensive examination of the equipment. The results are discussed as they occur in terms of the models appropriate to that particular event.