Volume 38, Issue 10

Developing the shear thickening fluids (STF) which can be used for soft body armours requires an in depth study of various parameters related to its constituents so that a high critical viscosity along with high critical shear rate can be obtained. Shape of the constituting particles is one such important parameter. Elongated and nanosize particles provide high critical viscosity to the fluid, whereas spherical particles show high critical shear rates. STF were prepared using halloysite (Hal) nanotubes of different concentrations with fumed silica (spheres) and their rheological properties were studied. A better non-flocculated structure was obtained at 1% Hal in 20% fumed silica composition, exhibiting a critical viscosity of 25 Pas at a critical shear rate 160 s −1 as compared to that of only spherical particle STF (10 Pas and 200 s −1 ). The oscillatory tests revealed that this composition, with a better consistent reproducible behaviour and better stability than the STF without Hal, would be suitable as a high impact resistant material. Gel formation does not take place, rather the fluid behaves like a dispersed sol, making it a better choice for using with protective fabrics. The rheology was studied at different temperatures ranging from 0°C to 55°C.

Diffusion of penetrants in polyethylene below the melt temperature is heavily dependent on the crystallinity of the polyethylene, the temperature of the experiment, and the concentration of solvent in the polymer. As the crystallinity of the polyethylene increases, there is an increase in the path that the solvent must travel as the solvent cannot penetrate the tightly packed chains in the crystalline domain. This effect is typically accounted for by a tortuosity factor. In this work, a simple and effective characterization of the tortuosity factor based simply on the crystal weight fraction has been developed. Data have been collected for six polyethylenes having densities ranging from 0.912 to 0.961 g/cm 3 and for three solvents – isopentane, cyclohexane, and 1-hexene. Diffusivity predictions have been obtained using the free-volume theory of Vrentas and Duda in conjunction with the new tortuosity factor. The polyethylenes had crystallinities varying from 40% to 82% effecting an approximately 60% change in the diffusivity. The decrease resulting from ignoring the crystallinity altogether was in some cases essentially a factor of 5. The error in the predicted diffusivities over all the systems was 25%. For cyclohexane, it is shown that the same model parameters characterize data below the melt temperature (in the semi-crystalline region) as well as above the melt temperature (in the amorphous region).

The mechanical properties of polybenzoxazine (PBa) resins were improved by adding polyamide 12 (PA12) and modified montmorillonite (OMMT) as additives. The mechanical properties and thermal stability of PBa and resulting PBa composites were investigated using an Instron universal material testing instrument and dynamic mechanical analysis. The morphologies of the fracture surfaces were characterized by scanning electron microscopy. The results of morphological studies showed that PBa/PA12/OMMT composites exhibit significantly improved mechanical properties and thermal stability compared with that of the pristine PBa. When the OMMT content increased to 1 wt%, the fracture toughness (1.36 MPa·m 1/2 ) and the fracture energy ( G IC , 315.76 J·m −2 ) of PBa/PA12/OMMT-1 composites increased by 67.9% and 181.4%, respectively, compared with those of the pristine PBa. The thermal stability properties demonstrated that the storage modulus and glass transition temperature ( T g ) of PBa/PA12/OMMT composites gradually increased with the addition of OMMT particles. The scanning electron microscopy results indicated that PBa/PA12/OMMT composites possess a toughening mechanism of crack deflection, with a large bulk of voids and debonding induced by PA12 and OMMT clay particles. Moreover, the OMMT might provide microvoid nucleating sites at its surface to release constrains for shear yielding.

The aim of this study was an investigation on the proliferation rate of normal human epidermal keratinocytes (NHEK) on the cross-linked and uncross-linked fibrinogen/polycaprolactone (Fbg/PCL) nanofibers to determine a suitable scaffold for skin tissue engineering. Nanofibrous scaffolds were prepared by electrospinning of different weight ratios of Fbg to PCL and were analyzed as morphology, surface chemical properties and cytocompatibility by scanning electron microscopy (SEM), attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay, respectively. The diameters of the blended uncross-linked scaffolds were in the range of 124±43 nm–209±155 nm. Cross-linking of scaffolds with glutaraldehyde did not make a significant change in the diameter of blended scaffolds in 16 h. Cross-linking also improved the tensile strength and weight loss rate of scaffolds. However, cross-linking demonstrated an unfavorable effect on the attachment and proliferation of NHEK cells. The proliferation study revealed that uncross-linked scaffolds containing 50% and 70% Fbg provide a better environment for the growth of NHEK cells, and can be considered promising scaffolds in tissue engineering applications.

Electrical impedance characteristics of polyaniline (PANI)-dodecylbenzenesulfonic acid (DBSA)/divinylbenzene (DVB) composite were evaluated as a function of PANI content in the frequency range of 10 2 –10 8 Hz. Polymer composites were prepared by a one-step thermal process where doping of the PANI and curing of the polymer matrix DVB have occurred simultaneously in the presence of a strong protonic acid, i.e. DBSA. The alternating current (AC) conductivity value with respect to frequency of the PANI-DBSA/DVB composite shows a direct current (DC) plateau up to an extensive frequency range. Almost metal-like behavior is obtained with such highly conductive plastic material. In all frequency regions, composites with higher PANI concentration showed a frequency independent behavior, while in the case of neat composites without PANI, a frequency dependent behavior is observed. A plot of the real part of complex impedance vs. frequency and a plot of AC conductivity of the composites indicate that the high electrical conductivity in high PANI concentration composites is because of the direct contacts between filler PANI particles. In this work, AC conductivity behavior and dielectric properties of very highly conductive thermoset composites are presented, and it is shown that conductivity is frequency independent for a very high frequency range (up to 10 8 Hz).

Soybean selenoprotein/carboxymethyl chitosan (SSP/CMCS) composite hydrogel obtained by the crosslinking of genipin was evaluated for caffeine release. The gelation process of the hydrogel was investigated by resonance Rayleigh scattering spectra and viscosity methods. The hydrogels presented a compact network structure, which was observed by positive fluorescence microscopy (PFM). The structural properties of the hydrogel were revealed by fluorescence and FT-IR. The swelling characteristic of the hydrogel and its application in the slow release of caffeine were also studied. These results indicate that there is obvious interaction between SSP and CMCS by the addition of genipin, and the CMCS/SSP solution experiences a significant sol-gel phase transition process upon polymerization. The swelling ratio and release of caffeine slow down obviously at pH 1.2. However, larger swelling and more drug release can be observed at pH 7.4. The experimental values of the empiric diffusional exponent show that the release profiles abide by the non-Fickian diffusion process under both investigated pH conditions. The hydrogel, which is pale transparent with light yellow color at room temperature, can be formulated to be a suitable carrier for site-specific drug delivery.

Interaction of multi-walled carbon nanotubes and graphene nanoplatelets within injection-molded thermoplastic-based nanocomposites is investigated. The research shows how the nanofillers’ combination ratio and content influence the properties in the semicrystalline thermoplastic-based composites. Three main groups of polyamide-based composite specimens containing either or both of the nanofillers are prepared. Results show that the single inclusion of the nanomaterials improves the mechanical and thermal properties significantly. However, the combined incorporation of both the nanofillers in the polymeric matrix does not lead to the linear combination of the observed behaviors. While the mechanical properties and the degrees of crystallinity improve, the thermal conductivities decrease in the hybrid composites for similar nanofiller contents. Rheological and crystallization investigations also showed the dominant influence of the nanotubes in the structure of the hybrid composites. The underlying mechanisms of modulation in the properties were also investigated in detail.

The objective of this study was to build a model that can predict the yellowing of styrene- stat -acrylonitrile (SAN) and acrylonitrile-butadiene-styrene (ABS) during processing in an internal mixer. Considering the change of color of SAN and ABS as they degrade, we correlate the yellowing of SAN and ABS to process parameters. We propose a method to represent the energy of mixing of polymer blends in an internal mixer to understand the complex mixing process of materials under different mixing conditions. Indeed, during processing, the properties of polymers could be affected by both thermal and mechanical degradation leading to several mechanisms of degradation. We presented a simplified model “ h ” taking into account the contribution of each process parameters applied in the internal mixer. Three process parameters were studied: the temperature T , the rotation speed of the blades V , and the residence time t . Thanks to this method of approach, a good correlation between the yellowing of styrenics and the mixing energy in the internal mixer was described. We validated this equation with two polymers: SAN and ABS, and this model can now predict the yellowing of SAN and ABS as a function of the process parameters in an internal mixer.

In this research, the influences of milling parameters on the surface quality and coat adhesion of rigid polyurethane (PU) foam are highlighted. Several surface texture parameters were correlated with the milling parameters. The correlation between the coat adhesion strength, as determined by the pull-off test, and the milling parameters was also established. The investigation revealed that traditional height distribution roughness parameters, such as Ra , Sa , Rz , and Sz , do not offer sufficient information for a proper surface adhesion evaluation. Shaping and bearing surface parameters, on the contrary, provide more information for the surface quality assessment, although the structure of the PU was found to be inhomogeneous. The evaluation of milling process effects on surface texture and coat adhesion and the determination of optimal machining conditions were derived based on response surface methodology. The goal was an adequate surface texture that provides the best coat adhesion strength.

We investigate the effects of non-Newtonian parameters on the exiting sheet thickness in the calendering of Oldroyd 4-constant fluid. The governing equations are first converted into dimensionless form and then simplified under lubrication approximation theory. A complete numerical approach is developed based on Matlab built-in routine “bvp4c” in order to obtain stream function and pressure gradient. The pressure is computed using Runge-Kutta method. The effects of involved material parameters on various quantities of interest are highlighted through graphs. The results indicate that Oldroyd 4-constant model predicts lower pressure in the nip region than that of the Newtonian model. The force and power for Oldroyd 4-constant model are also lower than their counterparts for Newtonian model. Moreover, the leave-off distance is nearly independent of the material parameters of the Oldroyd 4-constant model for larger value of the entering sheet thickness.