Nucleating and retarding effects of nanohydroxyapatite on the crystallization of poly(butylene terephthalate-co-alkylene dicarboxylate)s with different lengths

New biodegradable and biocompatible composites are continuously developed for biomedical applications (e.g., from drug delivery devices to tissue engineering scaffolds). Properties of such systems may depend on their morphology and structure, which are attained after their processing, and therefore, the study of the crystallization kinetics has a particular relevance. The crystallization kinetics of hydroxyapatite-filled poly(butylene terephthalate-co-alkylene dicarboxylate)s has been studied under non-isothermal conditions, using a wide range of cooling rates and different kinetic models. Based on our results, nanohydroxyapatite (nHAp) particles were found to effectively act as additional nucleation sites for poly(butylene terephthalate-co-succinate) (PBST), giving rise to an increased crystallization rate with respect to pure PBST. However, the overall growth rate of HAp nanocomposites decreased compared to the corresponding homopolymers with longer aliphatic dicarboxylic acids (i.e., adipic and sebacic acid derivatives). In order to clarify this point, the activation energy for non-isothermal crystallization was evaluated using the Friedman method and significant differences were observed, suggesting a disturbing effect of nanoparticles on the motion of molecular chains that hindered their capability to reach the growing crystallization front. Isoconversional methods provided a good understanding of the kinetics of the crystallization process and significant information regarding the activation energy, relative crystallinity, and global and local Avrami exponents.Peer ReviewedPostprint (published version

The enhancement effect of lithium ions on actuation performance of ionic liquid-based IPMC soft actuators

Abstract Soft actuators are of great technological interest and one class of these is made from ionic polymer-metal composites (IPMCs). It has been established that replacement of water with an ionic liquid (IL) in IPMCs results in larger actuation response and considerably longer operating life. However, the rate of displacement of IL-based IPMCs is very low. In the current work, IPMC actuators were fabricated using Nafion membrane and an imidazolium-based IL. The effects of incorporating the IL with and without Li+ ions were followed using electromechanical and electrochemical measurements and were compared with the corresponding behavior of water-based Li+-exchanged and un-exchanged IPMC actuators. The addition of Li+ ions to the IL-based system resulted in dramatic increases in the capacitance, ionic conduction, operating life and in the displacement rate of the actuator. This strategy is of considerable interest for enabling the use of IPMC-based soft actuators in medicine and robotics.PostprintPeer reviewe

Effect of hydroxyapatite nanoparticles on the degradability of random poly(butylene terephthalate-co-aliphatic dicarboxylate)s having a high content of terephthalic units

Copolyesters derived from 1,4-butanediol and constituted also of aliphatic and aromatic dicarboxylate units in a molar ratio of 3:7 were synthesized by a two-step polycondensation procedure. Succinic, adipic, and sebacic acids were specifically selected as the aliphatic component whereas terephthalic acid was chosen as the aromatic moiety. The second synthesis step was a thermal transesterification between the corresponding homopolymers, always attaining a random distribution as verified by NMR spectroscopy. Hybrid polymer composites containing 2.5 wt % of hydroxyapatite (HAp) were also prepared by in situ polymerization. Hydroxyl groups on the nanoparticle surface allowed the grafting of polymer chains in such a way that composites were mostly insoluble in the typical solvents of the parent copolyesters. HAp had some influence on crystallization from the melt, thermal stability, and mechanical properties. HAp also improved the biocompatibility of samples due to the presence of Ca2+ cations and the damping effect of phosphate groups. Interestingly, HAp resulted in a significant increase in the hydrophilicity of samples, which considerably affected both enzymatic and hydrolytic degradability. Slight differences were also found in the function of the dicarboxylic component, as the lowest degradation rates was found for the sample constituted of the most hydrophobic sebacic acid units. View Full-TextPeer ReviewedPostprint (published version

Biodegradability and biocompatibility of copoly(butylene sebacate-co-terephthalate)s

In the present study poly (butylene sebacate-co-terephthalate)s having different compositions were synthesized with a high yield and a random distribution by thermal transesterification of poly (butylene sebacate) and poly (butylene terephthalate) homopolymers. The copolymer with the highest comonomer ratio was the least crystalline sample, although the melting peaks corresponding to both, sebacate and terephthalate-rich phases were still observable in calorimetric heating runs. This copolymer was associated with interesting thermal and mechanical properties, as the maximum melting point was higher than 100 °C and the storage modulus was also high (i.e. 1.1 × 109 N/m2 and 1.7 108 N/m2 were determined just before and after the main glass transition temperature of -12 °C). As all studied samples were thermally stable up to temperatures clearly higher than the fusion temperature, they could be easily processed. Increasing the terephthalate content of the copolymers resulted in higher hydrophobicity, which had a minor influence on cell adhesion and proliferation of both fibroblast-like and epithelial-like cells. Hydrolytic and enzymatic degradability were assessed and the effect of composition and crystallinity on the degradation rate was investigated. Molecular weight measurements during exposure to a hydrolytic media indicated a first order kinetic mechanism during the initial stages of degradation before reaching a limiting molecular size, which was indicative of solubilization. The most amorphous sample appears as a highly promising biodegradable material since it showed a significant weight loss during exposure to all selected degradation media, but also exhibited good performance and properties that were comparable to those characteristic of polyethylenePeer ReviewedPostprint (author's final draft

Thermal degradation of random copolyesters based on 1,4-butanediol, terepthalic acid and different aliphatic dicarboxylic acids

Thermal stability and degradation kinetics have been studied for a series of aliphatic-aromatic copolyesters where the terephthalate content was varied between 30 mol-% and 70 mol-%. Succinate, adipate and sebacate were considered as the aliphatic dicarboxylate unit. All copolyesters were synthesized with a perfect random distribution by a thermal transesterification process from the corresponding homopolyesters. A complex degradation was deduced for all copolymers taking into account the increment of the activation energy with conversion. In fact, thermogravimetric curves showed a minor decomposition process in the low conversion region that was more significant for the succinate derivative and specifically for that having the lowest aromatic content. The sebacate derivative was characterized by the presence of an additional and minor decomposition process that took place at the highest conversion. All copolyesters were defined by a major decomposition process, which has similar values of activation energy regardless of the method used to calculate them (e.g. Kissinger, KAS or Friedman methodologies). This decomposition reaction followed a A4 Avrami-Erofeev mechanism when Coats-Redfern and Criado methodologies were applied. In summary, all the studied copolymers thermally decompose following a complex process but in all cases the main degradation step corresponds to a similar degradation mechanism.Postprint (author's final draft

Influence of electrolytes of Li salts, EMIMBF4 and mixed phases on electrochemical and physical properties of Nafion membrane

Electrolyte-soaked Nafion is commonly used as an ionic polymer in soft actuators. Here, a multi-technique investigation was applied to correlate the electrochemical behavior of Nafion membranes with their microstructures and nanostructures as a function of electrolyte type. The influence of electrolytes of Li salts with different counteranions on the Nafion membranes was investigated in terms of hydration level, structure (using X-ray diffraction and small angle X-ray scattering), stress–strain characteristics, and electrochemical behavior (by cyclic voltammetery and electrochemical impedance spectroscopy). The effects of using ionic liquid (IL), as the electrolyte, addition of different supporting solvent and the addition of Li+ ions to water-free IL-soaked membranes on the structural and electrochemical properties of Nafion were examined. The nano- and microstructure of the Nafion changed considerably as a function of the identity of the electrolyte solution. The electrochemical behavior of the IL-soaked samples was compared with that of the water-soaked Li+-exchanged Nafion. It was seen that the ionic conductivity of the Nafion membranes was reduced significantly when water was replaced by pure IL. Using the supporting solvents increased the conductivity of IL-soaked Nafion membranes dramatically. The presence of a small amount of Li+ ions together with the IL ions caused a significant decrease in charge transfer resistance and increases in double layer capacitance and in ionic conductivity over that of the water-free sample and also over water-soaked Li+-exchanged Nafion. These findings can be useful to improve the knowledge on Nafion's microstructure and also to improve the electromechanical behavior of Nafion-based ionic polymer–metal composites actuators.PostprintPeer reviewe

Mechanically stable solution-processed transparent conductive electrodes for optoelectronic applications

The bilayer structure of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) coating on silver nanowires (AgNWs) film is a promising structure for replacing indium tin oxide (ITO) as a flexible transparent conductive electrode. Pristine PEDOT:PSS film due to its hydrophilicity and high permeability cannot fully protect AgNWs from mechanical stress and oxidation. Here, we present a composite approach that improves mechanical properties and lifespan of the AgNWs/PEDOT:PSS electrode by adding polyvinyl alcohol (PVA) as a polymer-surfactant. It is shown that addition of PVA improves the conductivity as well as the stability of hybrid electrode under demanding mechanical stress conditions. The drop in conductivity of the hybrid electrode is only 17% after 2000 repeated bending cycles whereas the reference electrode has shown a dramatic drop of 180% in the conductivity. We speculate that generation of hydrogen bonds between PEDOT:PSS and PVA increases adhesivity and cohesivity of the conductive polymer film to the sublayer. So PEDOT:PSS-PVA film not only fixes the arrangement of AgNWs but also improves the welding on cross junction points. By addition of PVA, optoelectronic performance (Figure-of-merit (ΦTC)) of the electrode is improved from ΦTC = 2.646 × 10-3 Ω-1 for AgNWs/PEDOT:PSS to ΦTC = 3.819 × 10-3 Ω-1 for AgNWs/PEDOT:PSS-PVA electrode and power conversion efficiency (PCE) of the polymer solar cell (PSC) is increased by over 17%.</p

Visible Light-Assisted Photoreduction of Graphene Oxide Using CdS Nanoparticles and Gas Sensing Properties

Graphene oxide sheets suspended in ethanol interact with excited CdS nanoparticles and contributed to photocatalytic reduction by accepting electron from nanoparticle. The UV-Vis measurement showed that electrical absorbance of the CdS/graphene oxide sheets increased by decreasing the irradiation time and after 2 h it remained constant which indicates the optimum reduction time. Furthermore, the direct interaction between CdS nanoparticles and graphene sheets hinders the collapse of exfoliated sheets of graphene. The 4-point probe measurement of nanocomposite with different ratios of graphene oxide in CdS solution after irradiation shows that the conductivity of them increased by increasing the amount of GO, but further increasing causes incomplete photo reduction process due to exorbitance increasing GO sheets which contribute to decreasing the conductivity. The CdS/RGO composite material can be used as a gas sensor for CO2 based on its electrocatalytic behavior. The low-cost and easy fabrication sensor shows rapid response and high sensitivity. By varying the amount of GO the optimum concentration which shows high sensitivity is found and its good performance compared with other is attributed to its higher conductivity due to complete reduction. Moreover, the effects of thermal annealing on the conductivity of CdS/RGO film and the performance of devices are researched

(E)-2-tert-Butyl-6-[(naphthalen-1-yl)imino­meth­yl]phenol

The asymmetric unit of the title Schiff base compound, C21H21NO, contains two crystallographicaly independent mol­ecules. The dihedral angles between the naphthalene mean plane and the benzene ring are 29.28 (8) and 26.92.(8)° in the two mol­ecules. An intra­molecular O—H⋯N hydrogen bond and weak intra­molecular C—H⋯O hydrogen bonds stabilize the structure of each independent mol­ecule