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All issues / Volume 7 (2013) / Issue 1 (January)

Preface – The critical seventh year
T. Czigany
Vol. 7., No.1., Pages 1-1, 2013
DOI: 10.3144/expresspolymlett.2013.1
This is an editorial article. It has no abstract.
Nanospheres and nanocapsules of amphiphilic copolymers constituted by methoxypolyethylene glycol cyanoacrylate and hexadecyl cyanoacrylate units
I. Valente, L. J. del Valle, M. T. Casas, L. Franco, A. Rodriguez-Galan, J. Puiggali, D. Marchisio
Vol. 7., No.1., Pages 2-20, 2013
DOI: 10.3144/expresspolymlett.2013.2
Nanospheres and nanocapsules of an amphiphilic copolymer having methylated polyethylene glycol and hexadecyl lateral groups were prepared by the solvent displacement method and using confined impinging jet mixers. Degradation, thermal properties and crystalline structure were investigated. Interestingly, pegylated chains hydrolyzed through ester bond cleavage, whereas the more hydrophobic hexadecyl ester groups were resistant to degradation in aqueous media. The copolymer crystallized from the melt, giving rise to spherulites with a negative birefringence and domains corresponding to crystallization of the different lateral groups. Size distribution and morphology of nanoparticles were mainly evaluated by electron microscopy. Nanocapsules were characterized by a stable membrane with a thickness close to 5 nm that allowed efficient encapsulation of a triglyceride oil. Triclosan was selected as an example of a hydrophobic drug to be loaded in both nanospheres and nanocapsules. The release behavior of these dosage forms was clearly different. Thus, the Burst effect was practically suppressed when using nanocapsules; in addition, these showed a sustained, controlled release over a greater time period. Antimicrobial activity of triclosan loaded nanospheres and nanocapsules was evaluated using Gram-negative and Gram-positive bacteria. The former were highly sensitive to the released triclosan whereas the latter strongly depended on the number of particles in the culture medium.
Tuning the processability, morphology and biodegradability of clay incorporated PLA/LLDPE blends via selective localization of nanoclay induced by melt mixing sequence
L. As'habi, S. H. Jafari, H. A. Khonakdar, R. Boldt, U. Wagenknecht, G. Heinrich
Vol. 7., No.1., Pages 21-39, 2013
DOI: 10.3144/expresspolymlett.2013.3
Polylactic acid (PLA)/linear low density polyethylene (LLDPE) blend nanocomposites based on two different commercial-grade nanoclays, Cloisite® 30B and Cloisite® 15A, were produced via different melt mixing procedures in a counter-rotating twin screw extruder. The effects of mixing sequence and clay type on morphological and rheological behaviors as well as degradation properties of the blends were investigated. The X-ray diffraction (XRD) results showed that generally the level of exfoliation in 30B based nanocomposites was better than 15A based nanocomposites. In addition, due to difference in hydrophilicity and kind of modifiers in these two clays, the effect of 30B on refinement of dispersed phase and enhancement of biodegradability of PLA/LLDPE blend was much more remarkable than that of 15A nanoclay. Unlike the one step mixing process, preparation of nanocomposites via a two steps mixing process improved the morphology. Based on the XRD and TEM (transmission electron microscopic) results, it is found that the mixing sequence has a remarkable influence on dispersion and localization of the major part of 30B nanoclay in the PLA matrix. Owing to the induced selective localization of nanoclays in PLA phase, the nanocomposites prepared through a two steps mixing sequence exhibited extraordinary biodegradability, refiner morphology and better melt elasticity.
Sensing strain and damage in polyurethane/MWCNT nano-composite foams using electrical measurements
A. Baltopoulos, N. Athanasopoulos, I. Fotiou, A. Vavouliotis, V. Kostopoulos
Vol. 7., No.1., Pages 40-54, 2013
DOI: 10.3144/expresspolymlett.2013.4
This work deals with the damage identification in polymeric foams through the monitoring of the electrical resistance of the system. To assess this idea electrically conductive rigid Poly-Urethane (PUR) foams at various densities were prepared. Multi-Wall Carbon Nanotubes (MWCNT) were dispersed in the host polymer at various concentrations through high shear mixing to provide electrical conductivity to the system. The PUR/MWCNT foams exhibited varying electrical conductivity on a wide range of densities and nano-filler contents. The prepared foams were subject to compression tests. Electrical resistance was recorded online during the tests to monitor the change of the bulk property of the materials. A structural-electrical cross-property relation was exhibited. The distinctive phases of foam compression were successfully identified from the electrical resistance profile recorded during the tests. A characteristic master curve of the change of electrical resistivity with respect to load and damage is proposed and analyzed. It was shown that the found electrical resistance profile is a characteristic of all the MWCNT contents and depends on density and conductivity. MWCNT content contributes mainly to the sensitivity of electrical sensing in the initial stage of compression. Later compression stages are dominated by foam microstructural damage which mask any effect of CNT dispersion. Micro-structural observations were employed to verify the experimental findings and curves.
Fabrication and formation mechanism of poly (L-lactic acid) ultrafine multi-porous hollow fiber by electrospinning
Q. Z. Yu, Y. M. Qin
Vol. 7., No.1., Pages 55-62, 2013
DOI: 10.3144/expresspolymlett.2013.5
Poly(L-lactic acid) (PLLA) ultrafine multi-porous hollow fibers are fabricated by electrospinning with methylene dichloride as solvent. The Kirkendall effect has been widely applied for the fabrication of hollow structure in metals and inorganic materials. In this study, a conceptual extension is proposed for the formation mechanism: the development of porous hollow fiber undergoes three stages. The initial stage is the generation of small voids or pits on the surface of the fiber via surface diffusion and phase separation; the second stage is the formation of multi-pores penetrating the core of the fiber through the interaction of Kirkendall effect, surface diffusion and phase separation; the third stage is dominated by surface diffusion of the core material along the pore surface. To explore the formation conditions, the factors including ambient temperature, relativity humidity (R. H.), molecular weight and fiber diameter are studied. The longitudinal and cross sectional morphologies of these fibers are examined by scanning electron micrograph (SEM). The results show that the prerequisite for the formation of uniform porous hollow PLLA fibers include moderate ambient temperature (10~20°C) and appropriate molecular weight for the PLLA, as well as the diameter of the fiber in the range of several micrometers to about 100 nanometers.
Photovoltaic properties and annealing effects of a low bandgap copolymer containing dithienothiophene and benzothiadiazole units
T. L. Wang, Y. T. Shieh, C. H. Yang, T. H. Ho, C. H. Chen
Vol. 7., No.1., Pages 63-75, 2013
DOI: 10.3144/expresspolymlett.2013.6
A conjugated alternating copolymer as the donor material of the active layer in polymer solar cells has been designed and synthesized via Stille coupling reaction. The alternating structure consisted of 3,5-didecanyldithieno[3,2-b:2',3'-d]thiophene (DDTT) donor unit and 5,6-bis(tetradecyloxy)benzo-2,1,3-thiadiazole (BT) acceptor unit. Since both units have been attached pendant chains, the polymer was soluble in common organic solvents. UV-vis spectrum exhibited a broad absorption band in the range of 270–780 nm and a low bandgap of 1.83 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymer were estimated to be –5.10 and –3.27 eV, respectively. Based on the ITO/PEDOT:PSS/PDDTTBT:PCBM/Al device structure, the power conversion efficiency (PCE) under the illumination of AM 1.5 (100 mW/cm2) was 0.127%. The effects of annealing temperature (50–150°C) for 30 min on the device performance were studied. It was found that PCE of 0.292% could be acquired under the annealing condition at 50°C for 30 min. The improved device efficiency under the optimal condition was confirmed by the higher light harvest in UV-vis spectra, the enhanced quenching of photoluminescence (PL) emission, and the improved nanoscale morphology by atomic force microscopy (AFM) examination.
Polyamide from lactams by reactive rotational molding via anionic ring-opening polymerization: Optimization of processing parameters
N. Barhoumi, A. Maazouz, M. Jaziri, R. Abdelhedi
Vol. 7., No.1., Pages 76-87, 2013
DOI: 10.3144/expresspolymlett.2013.7
A reactive rotational molding (RRM) process was developed to obtain a PA6 by activated anionic ring-opening polymerization of epsilon-caprolactam (APA6). Sodium caprolactamate (C10) and caprolactam magnesium bromide (C1) were employed as catalysts, and difunctional hexamethylene-1,6-dicarbamoylcaprolactam (C20) was used as an activator. The kinetics of the anionic polymerization of !-caprolactam into polyamide 6 was monitored through dynamic rheology and differential scanning calorimetry measurements. The effect of the processing parameters, such as the polymerization temperature, different catalyst/activator combinations and concentrations, on the kinetics of polymerization is discussed. A temperature of 150°C was demonstrated to be the most appropriate. It was also found that crystallization may occur during PA6 polymerization and that the combination C1/C20 was well suited as it permitted a suitable induction time. Isoviscosity curves were drawn in order to determine the available processing window for RRM. The properties of the obtained APA6 were compared with those of a conventionally rotomolded PA6. Results pointed at lower cycle times and increased tensile properties at weak deformation.
Microencapsulation of self-healing agents containing a fluorescent dye
H. H. Noh, J. K. Lee
Vol. 7., No.1., Pages 88-94, 2013
DOI: 10.3144/expresspolymlett.2013.8
Two different self-healing agent candidates, endo-dicyclopentadiene (endo-DCPD) and 5-ethylidene-2-norbornene (ENB), containing a fluorescent dye surrounded by a melamine–urea–formaldehyde (MUF) shell were microencapsulated by in-situ polymerization and the resulting microcapsules were characterized in this work. The microcapsules showed a narrow size distribution with a spherical shape and rough outer and smooth inner surfaces for both healing agent systems. Shell thicknesses of the microcapsules were ~880±80 nm for endo-DCPD and ~620±60 nm for ENB. The incorporation of a fluorescent dye as tracer into self-healing agents did not disturb the formation of microcapsules. The release of self-healing liquid into the induced crack from ruptured microcapsules in an epoxy coating layer was observed using a fluorescence microscopy. The use of a fluorescent dye is very effective in the observation of a damage site.
A versatile characterization of poly(N-isopropylacrylamide-co-N,N'-methylene-bis-acrylamide) hydrogels for composition, mechanical strength, and rheology
A. Chetty, J. Kovacs, Zs. Sulyok, A. Meszaros, J. Fekete, A. Domjan, A. Szilagyi, V. Vargha
Vol. 7., No.1., Pages 95-105, 2013
DOI: 10.3144/expresspolymlett.2013.9
Poly(N-isopropylacrylamide-co-N,N'-methylene-bisacrylamide) (P(NIPAAm-co-MBA)) hydrogels were prepared in water using redox initiator. The copolymer composition at high conversion (> 95%) was determined indirectly by HPLC (high performance liquid chromatography) analysis of the leaching water and directly by solid state 13C CP MAS NMR (cross polarization magic angle spinning nuclear magnetic resonance) spectroscopy of the dried gels, and was found to be close to that of the feed. The effect of cross-linker (MBA) content in the copolymer was investigated in the concentration range of 1.1–9.1 mol% (R:90–10; R = mol NIPAAm/mol MBA) on the rheological behaviour and mechanical strength of the hydrogels. Both storage and loss modulus decreased with decreasing cross-linker content as revealed by dynamic rheometry. Gels R70 and R90 with very low cross-linker content (1.2–1.5 mol% MBA) have a very loose network structure, which is significantly different from those with higher cross-linker content manifesting in higher difference in storage modulus. The temperature dependence of the damping factor served the most accurate determination of the volume phase transition temperature, which was not affected by the cross-link density in the investigated range of MBA concentration. Gel R10 with highest cross-linker content (9.1 mol% MBA) behaves anomalously due to heterogeneity and the hindered conformation of the side chains of PNIPAAm.
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Budapest University of Technology and Economics,
Faculty of Mechanical Engineering, Department of Polymer Engineering