All issues / Volume 3 (2009) / Issue 12 (December)
*** Editorial Corner ***
In this study we report on morphological and rheological characterization of multi-walled carbon nanotube (MWNT)-polycarbonate composites produced by injection molding. The main focus is to carry out nonlinear viscoelastic experiments that allow following the structural rearrangements of carbon nanotubes in the polycarbonate melt. Small angle X-ray scattering reveals only a slight orientation of MWNTs in the as-received samples, i.e. after application of extremely high shear rates. Thus, the main structural effect observed during the stress growth experiment is the breakage of MWNT agglomerates. To study this effect in detail a flocculation experiment, in which the sample undergoes oscillatory deformation first at a small strain amplitude in the linear regime succeeded by higher amplitudes in the nonlinear regime, has been carried out. The agglomeration process manifests itself in an increase of the storage and loss moduli in the linear regime, whereas the deagglomeration process does vice versa. The corresponding effects can be described in the frame of a superposition approach that takes into account the stress contribution of the polycarbonate matrix, the hydrodynamic reinforcement due to embedded nanotubes and the viscoelastic stress due to the presence of a MWNT-network.
In this paper, pristine and oxidized multi-walled carbon nanotube (MWCNT)/poly(vinylidene fluoride) (PVDF) composites were prepared and the temperature dependence of some electrical properties of these composites were studied. It is found that the transition temperature (Tt), from positive temperature coefficient (PTC) to negative temperature coefficient (NTC) effect, of the oxidized MWCNT/PVDF composites shifted to a higher temperature. The shift of the Tt of the oxidized MWCNT/PVDF composites can be attributed to the chemical functionalization of the MWCNTs. The dielectric constants of these composites are enhanced remarkably, which can be understood by the interfacial polarization effect. The largest dielectric constant of 3600 is obtained in the composite with about 8 vol% oxidized MWCNTs at 1 kHz. The dielectric constants of these composites increase firstly and then decrease with increasing temperature. However, when the temperature reaches a higher value, the dielectric constants increase again with increasing temperature. The ‘wave’ phenomenon of the temperature dependence of the dielectric constants can be understood by the temperature dependence of the interfacial polarization.
Cationic, hydrophobically associating polyacrylamide (PDA) was synthesized via the inverse miniemulsion polymerization in the presence of template. Dimethyloctane(2- acrylamidopropyl)ammoniumbromide (DOAB) was synthesized via quaternization reaction and used as the hydrophobic monomer. Polyacrylic acid (PAA) was used as template for the oppositely charged DOAB. The distribution of DOAB in the inverse miniemulsion and the solution viscosity behaviors of PDA were investigated. The results showed that the complexes of DOAB and PAA were located not only at the interface of the inverse miniemulsion droplets and oil phase but also in the interior of inverse miniemulsion droplets. PDA prepared with template exhibited remarkable enhancement of solution viscosity (thickening ability). And the optimal thickening ability was obtained when the aqueous phase pH was 6.5 and the ratio of DOAB to PAA was 1. The thickening ability of PDA was improved with increasing DOAB content. PDA prepared with template showed stronger association ability than that prepared without template for its longer hydrophobic block structure, which was further supported by the plots of fluorescence spectra.
This paper describes the curing behaviours, thermal properties and flame-resistance of a novel halogen-free epoxy hybrid thermoset, prepared by the curing reaction of hexakis (methoxymethyl) melamine (HMMM), a phosphorouscontaining epoxy resin (EPN-D) with 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) group and phenol formaldehyde novolac (n-PF). The resultant thermosets showed high glass-transition temperatures (Tg, 123–147°C) as determined by thermal mechanical analysis (TMA), excellent thermal stability with high 5 wt% decomposition temperatures (Td,5% ≥308°C) and high char yields (Yc ≥39.4 wt%) from the thermogravimetric analysis (TGA). All the cured EPND/ HMMM/n-PF hybrid resins achieved the UL 94 V-0 grade with high limited oxygen indices (LOI > 45.7). It is found that phosphorous and nitrogen elements in the cured EPN-D/HMMM/n-PF hybrid resins had a positive synergistic effect on the improvement of the flame retardancy.
Thermo-oxidative degradation of graphite/epoxy composite laminates due to exposure to elevated temperatures was characterized using weight loss and short beam strength (SBS) reduction data. Test specimens obtained from 24-ply, unidirectional AS4/3501-6 graphite/epoxy laminates were subjected to 100, 150, 175, and 200°C for 5000 hours (208 days) in air. Predictive differential models for the weight loss and short beam strength reduction were developed using the isothermal degradation data only up to 2000 hours. Then, the predictive capabilities of both models were demonstrated using the longer term, 5000 hours degradation data. The proposed models were first order differential expressions that can be used to predict degradation in an arbitrary, time-dependent temperature environment. Both models were able to estimate the actual degradation levels accurately. In particular, excellent agreement was obtained when the degradation temperature was lower than 200°C.
In this paper, acoustic emission (AE) monitoring with a wavelet-based signal processing technique is developed to detect the damage types during mode I delamination on glass/polyester composite materials. Two types of specimen at different midplane layups, woven/woven (T3) and unidirectional/unidirectional (T5), leading to different levels of damage evolution, were studied. Double cantilever beam (DCB) is applied to simulate delamination process for all specimens. Firstly, the obtained AE signals are decomposed into various wavelet levels. Each level includes detail and approximation that are called components and related to a specific frequency range. Secondly, the energy distribution criterion is applied to find the more significant components each one of which is in relation to a distinct type of damage. The results show that the energy of AE signals has been concentrated in three significant components for both of the specimens. There is a difference in energy distribution of similar components of two specimens. It indicates that there is a dissimilar dominant damage mechanism for two different interfaces during the delamination process. Additionally, the microscopic observation (SEM) is used to determine how the different fracture mechanisms are related to the dominant corresponding wavelet components.
Acrylic polymers are widely used for fabricating pressure-sensitive adhesives (PSAs) with the inherent unique advantages of transparency and superior intrinsic adhesive properties over other polymer-based adhesives. In this study, we have developed and evaluated a method of obtaining by radical copolymerization PSAs for liquid crystalline (LCD) applications. Various factors including the amount of monomers, amount of cross-linker, coating weight, dwell time and thermal treatment are investigated for further optimizing the properties of acrylic polymer based PSAs to meet the emerging strict requirements for practical uses related mainly to holding powder and peel strength. The results illustrate that novel crosslinking reagents coupled with the thermal treatment at 70°C can make the resultant PSAs with the improved adhesive properties. The coating weight variation from 10 to 40 g/m2 can significantly enhance the peel strength from 4.0 g/25 mm to 12.5 g/25 mm with about 310% increment. If the dwell time of PSAs with cross-linking reagent is more than 10 hrs, the peel strength can be reduced down to a suitable value to meet the criterion for use. Therefore, acrylic PSAs with peel strength less than 20 g/25 mm and holding power above 120 hrs were successfully synthesized by elaborately designing the reaction system, which are practically applicable for advanced industrial applications.
The membranes of sulfonated polyetheretherketone(SPEEK) doped with rare earth metal oxide nanometer cerium oxide (CeO2) were prepared for direct methanol fuel cell (DMFC) application, which was treated by parallel or perpendicular high magnetic field of 6 Teslas (T) at 100°C. The proton conductivity of membrane specimens increased with temperature raised from 20 to 60°C and decreased with increasing CeO2 contents. The proton conductivity of membrane specimens under treatment with high magnetic field was better than that without treatment. The membrane specimens treated with perpendicular magnetic field demonstrated better proton conductivity than those treated with parallel magnetic field. The methanol permeation coefficient of membrane specimens decreased with increasing CeO2 contents and furthermore reduced by about 20% after treated with perpendicular high magnetic field. The water uptake of membrane specimens decreased with CeO2 doping, but would not be influenced by the magnetic field. Fourier transform infrared spectroscopy (FTIR) and small-angle X-ray scattering (SAXS) revealed certain reaction between oxygen anion in sulfonic groups and cerium cation in the CeO2 which dispersed evenly in the membranes but formed small conglomerates as shown by the atomic force microscopy (AFM) images. X-ray diffraction (XRD) proved the stability of the crystal structure of the nanometer CeO2 in polymer membranes, indicating that the reaction occurred only at the interface between SPEEK resin and CeO2 particles.