All issues / Volume 7 (2013) / Issue 3 (March)
This is an editorial article. It has no abstract.
Multiwall carbon nanotube (MWCNT) based nanocomposites were prepared by a two-step process. Firstly, titanium dioxide (TiO2) coated MWCNT was prepared via sol-gel technique. In the second step, the acid modified MWCNTs were dispersed in the thermoplastic polyurethane matrix by solution blending process. Characterizations of the nanocomposites were done by X-ray diffraction analysis, X-ray photoelectron spectroscopy, Scanning Electron Microscopy, Transmission Electron Microscopy and Energy-dispersive X-ray spectroscopy. Microwave absorption studies of the nanocomposites were carried out in X-band region. The microwave absorption result was discussed with the help of complex permittivity and permeability of the prepared radar absorbing material (RAM). The result showed superior microwave absorption property of the composite containing both TiO2 coated MWCNT and magnetite (Fe3O4). This result is due to the effective absorption of both electrical and magnetic components of the microwave. RAM-MW, RAM-Ti, RAM-Ti@MW and RAMTi@ MW/Fe and showed the maximum reflection loss of –16.03 dB at 10.99 GHz, –8.4 dB at 12.4 GHz, –36.44 dB at 12.05 GHz and –42.53 dB at 10.98 GHz respectively. Incorporation of MWCNT enhanced the thermal stability of the composite which has been confirmed by thermogravimetric analysis.
The paper investigates the morphology and the structural properties of blends of isotactic polypropylene (iPP) and syndiotactic polypropylene (sPP) having different compositions using different techniques. Solid-state nuclear magnetic resonance (NMR) permits via appropriate sequence to measure the composition, conformation and dynamics, and intimacy of mixing of polymeric materials. Measurement of relaxation times gives information on local structure of phases with different mobility. The morphology is directly related to the structural organization of the blends. The crystallization rate decreases as a function of the sPP content. The minor component is dispersed as a nodule in the main component of the blend and it plays the role of nucleating agent on it. Besides, morphology changes occur for the composition 50/50 (wt/wt) of the blend iPP/sPP. Different phases are identified, namely free amorphous, constrained amorphous and crystalline regions which exhibit different molecular mobilities. It is also shown at the interphase matrix-nodules, the nodules create a constrained amorphous zone.
Centrifugal spinning (C-spin) is one of the emerging techniques for the production of ultrafine fibrous web which mimics Extracellular matrix (ECM). Due to its unique characteristic features it is widely used in bio-medical applications such as tissue engineered scaffolds, wound dressing materials and drug delivery vehicles. In the present study tetracycline loaded polycaprolactone (PCL) blended polyvinyl pyrrolidone (PVP) fibers were fabricated using in-house built C-spin system. The developed ultrafine fibers were morphologically characterized by Scanning Electron Microscope (SEM) before and after drug release and the results showed that the developed webs were highly porous and the pores were evenly distributed. Fourier Transform Infrared (FTIR) spectroscopy results confirmed that the drug was incorporated on the fibers. The antibacterial activity and drug releasing strategy were examined and the results showed that the developed webs can effectively act as a drug delivery vehicle.
ARGET (activators regenerated by electron transfer) ATRP (atom transfer radical polymerization) has been successfully performed (in flasks fitted with rubber septa without the need for use of Schlenk line) in the presence of limited amount of air and with a very small (370 ppm) amount of copper catalyst together with an appropriate reducing agent Cu(0). Novelty of this work is that the poly(methyl methacrylate)-block-polyurethane-block-poly(methyl methacrylate) triblock copolymers were synthesized for the first time through ARGET ATRP, by using tertiary bromine-terminated polyurethane as a macroinitiator (MBP-PU-MBP), CuBr2 or CuCl2 as a catalyst and N,N,N',N",N"-pentamethyldiethylenetriamine (PMDETA) or 2,2'-bipyridine (Bpy) as a complexing agent. As the polymerization time increases, both the monomer conversion and ln([M]0/[M]) increased and the molecular weight of copolymer increases linearly with increasing conversion. Theoretical number-average molecular weight (Mn, th) of the tri-block copolymers was found to be comparable with number-average molecular weight determined by GPC analyses (Mn, GPC). These results indicate that the formation of the tri-block copolymers was through atom transfer radical polymerization mechanism. 1H and 13C NMR spectral methods were employed to confirm chemical structures of synthesized macroinitiator and tri-block copolymers. Mole percentage of PMMA in the tri-block copolymers was calculated using 1H NMR spectroscopy and was found to be comparable with the GPC results. Additionally, the studies of surface properties (confocal microscopy and SFE) of tri-block copolymer coatings confirmed the presence of MMA segments.
Inorganic nanofillers are often added into polymer/elastomer blends as a third component to modify their performance. This work aims to clarify the role of interface-localized spherical nanoparticles in determining the impact toughness of polymer blends. The selective distribution of titanium dioxide (TiO2) nanoparticles in poly(L-lactide)/poly(ether) urethane (PLLA/PU) blends was investigated by using scanning electron microscope. It is interesting to find that, regardless of the method of TiO2 introduction, nano-TiO2 particles are always selectively localized at the phase interface between PLLA and PU, leading to a significant improvement in notched Izod impact toughness. The moderately weakened interfacial adhesion induced by the interfacially-localized nano-TiO2 particles is believed to be the main reason for the largely enhanced impact toughness.
Glycidyl esters of epoxidized fatty acids derived from soybean oil (EGS) and linseed oil (EGL) have been synthesized to have higher oxirane content, more reactivity and lower viscosity than epoxidized soybean oil (ESO) or epoxidized linseed oil (ELO). The EGS and ESO, for comparison, were used neat and in blends with diglycidyl ether of bisphenol A (DGEBA). Thermosetting resins were fabricated with the epoxy monomers and either BF3 catalyst or anhydride. The curing behaviors, glass transition temperatures, crosslink densities and mechanical properties were tested. The results indicated that polymer glass transition temperatures were mostly a function of oxirane content with additional influence of glycidyl versus internal oxirane reactivity, pendant chain content, and chemical structure and presence of saturated components. EGS provided better compatibility with DGEBA, improved intermolecular crosslinking and glass transition temperature, and yielded mechanically stronger polymerized materials than materials obtained using ESO. Other benefits of the EGS resin blend systems were significantly reduced viscosities compared to either DGEBA or ESO-blended DGEBA counterparts. Therefore, EGS that is derived from renewable sources has improved potential for fabrication of structural and structurally complex epoxy composites, e.g., by vacuum-assisted resin transfer molding.
To reduce secondary processing such as sanding or chemical etching in the manufacture of composite structures, we present an in-mold surface preparation process using imprint lithography on carbon/epoxy composite adhesive joints. In the proposed in-mold process, microstructures are designed and fabricated on the surface of the mold to form composites. Through the formation of composites on the mold at an appropriate temperature and pressure, the shapes of the microstructures are imprinted onto the surface of the composite. Because molding and surface preparation can be performed simultaneously, the time and costs required are reduced compared to conventional surface preparations. In this paper, concavo-convex microstructures were fabricated on the surface of carbon/epoxy composites using in-mold surface preparation, which improved the apparent mode I fracture toughness of the composite/adhesive interface. From double-cantilever-beam tests, we confirmed that as the aspect ratio of the concavo-convex microstructures increased, the steady-state fracture toughness increased by up to 113% compared to structures without in-mold surface preparation, and the fracture mode changed from interfacial failure to complex cohesive adhesive failure.
One-step reactive extrusion-calendering process (REX-Calendering) was used in order to obtain sheets of 1mm from two PD,L-LA extrusion grades modified with a styrene-acrylic multifunctional oligomeric agent. In a preliminary internal mixer study, torque versus time was monitored in order to determine chain extender ratios and reaction time. Once all parameters were optimized, reactive extrusion experiments were performed. Independently of the processing method employed, under the same processing conditions, PD,L-LA with the lower D enantiomer molar content revealed a higher reactivity towards the reactive agent, induced by its higher thermal sensitivity. REXCalendering process seemed to minimize the degradations reactions during processing, although a competition between degradation and chain extension/branching reactions took place in both processes. Finally, the rheological characterization revealed a higher degree of modification in the melt rheological behaviour for REX-Calendered samples.