jrups
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Abstract: Phase separated blends of polymers and low molecular weight liquid crystals, commonly known as polymer dispersed liquid crystals in short PDLCs, are investigated. These materials offer a realm of applications in modern technologies, including sensors, commutable windows, display devices and telecommunication systems. A particular attention is given to the effects of anisotropy of the liquid crystal on the phase behavior under equilibrium and non equilibrium conditions. The theoretical formalism used is based on the lattice model of isotropic mixing, combined with standards theories of nematic and smectic-A orders. Considering the equilibrium phase behavior, we find that the nematic order enhances the polymer / solvent phase separation, and that the osmotic pressure shows substantial changes for relatively small polymer volume fractions. We find that the anisotropy enhanced phase separation is more pronounced for a smectic-A liquid crystal, and the miscibility gap is widened. The kinetics of swelling by nematic LCs is examined using a linear solvent diffusion process, with a rate of swelling directly related to the derivative of the osmotic pressure. An abrupt swelling / de-swelling transition is found, due to overwhelming effects of the anisotropic interaction beyond the threshold LC concentration. Anisotropy enhanced phase separation is also investigated in the method of synthesis based on the polymerization induced phase separation mechanism. We find that the kinetics of separation during early stages of polymerization is faster, due to the anisotropic interaction of the low molecular weight solvent. The kinetics speed up is favored by the long range viscous flow effects due to hydrodynamic interactions. A limited selection of experimental data in the literature is chosen to validate some theoretical predictions obtained from the present formalisms. Keywords: PDLCs, anisotropy, nematic, phase separation, swelling, kinetics, PIPS. |
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Abstract: The increase in the disposal of new polymeric materials is growing considerably in recent years, causing a major environmental impact. In view of this factor, many researchers have been studying and producing biodegradable composites whose shorter time to degradation reduces the volume of waste in landfills. Reinforcements made from natural fibers, especially sisal, has been much used in these new composites due to their low density, because they are derived from renewable source, are not toxic and their low cost compared to synthetic fibers. In view of this need, this study evaluated an epoxy/sisal composite via TGA (Thermogravimetric Analysis), DSC (Differential Scanning Calorimetry), the impact resistance and the mass variation evaluation before and after exposure in simulated soil for a period of 8 weeks. Keywords: Biodegradation, epoxy resin/sisal composite, simulated soil, impact resistance, thermal analysis. |
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Abstract: Natural fibers obtained from sugarcane bagasse were used as reinforcement for high impact polystyrene (HIPS) composites. Fibers were chemically treated with an alkaline solution and then bleached with sodium chlorite and acetic acid, in order to remove amorphous constituents and improve adhesion with polimeric matrix.The alkali-treated and bleached fibers over a range of 10-30 wt% were mixed with HIPS and placed in an injector chamber in order to obtain tensile and flexural test specimens. Chemical treatment effects on composites properties were evaluated through mechanical tests and thermal and microscopy analysis. Experimental results show that composites with 30 wt% of alkali-treated fibers present an improvement in the tensile strength (17%), tensile modulus (96%) and flexural modulus (34%) with a consequent decrease in the ductility and in the thermal properties in comparisson to pure HIPS. An huge increase of 191% in the flexural modulus for composites with 30 wt% of bleached fibers was obtained compared to pure HIPS. Keywords: High impact polystyrene, Mechanical properties, Thermal properties, Sugarcane Bagasse fibers. |
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Abstract: Studies focused on generating products able to reduce environmental impact have been put in place, and those aiming at finding polymeric composites strengthened with natural fibers stand out among them. A composite was strengthened with coconut fibers in the present study, since the generation of coconut residues has increased in Brazil due to coconut water industrialization. The aim of the present study is to process a high impact polystyrene (HIPS) composite strengthened with coconut fiber and to verify its possible application in the automotive industry. Strengthened samples were prepared using coconut fibers in the proportions 10, 20 and 30% fiber to develop such HIPS composite. Analyses were performed in order to set the density, Shore A hardness and impact resistance of the processed composites. It was found that the density and Shore A hardness results in the HIPS did not change due to the addition of coconut fibers. However, it was observed that the impact resistance was reduced because of it. It was noteworthy that the decreased impact resistance did not preclude the use of the HIPS/coconut fiber composite, since it showed that such material can be used in parts free from strong mechanical stress such as those inside the automobiles. The use of coir HIPS also aimed at reducing costs related to the polymer’s consumption and at reusing the waste (coconut husk) from coconut water industries. Keywords: Composites, characterizations, coconut fiber, HIPS, Shore A hardness. |