Local piezoelectric activity of single poly(L-lactic acid) (PLLA) microfibers

Local piezoelectric properties of the poly(L-lactic acid) individual electrospun fibers have been studied by Piezoresponse Force Microscopy. Piezoelectric response, polarization switching, and nanoscale patterning of the fibers have been demonstrated in this important biomaterial, thus opening interesting possibilities for tissue engineering and sensing/actuating applications.     

On the morphology and structure of poly(p-phenylene terephthalamide) single crystals and spherulites

Poly(p-phenylene terephthalamide) (PPTA) single crystals and negative spherulites obtained from concentrated sulfuric acid solution were studied by polarizing microscopy, transmission electron microscopy, electron diffraction, and infrared spectroscopy techniques. Electron microscopy shows that the single crystals with nearly pyramidal shape are composed of rectangular platelets which are packed in parallel with the same orientation. The long edge of the platelet is parallel to [avec]?, while the macromolecular chains are located in the plane of the platelet and perpendicular to the long edge. The width of the platelet is dependent on the molecular weight. The radius direction of PPTA negative spherulites is [avec]?. Electron diffraction patterns indicate that the unit cell parameters of PPTA single crystals and spherulites are not the same as those of Kevlar fibers. For the former c = 11.5 Å, while for the latter c = 12.9 Å. This indicates that the molecular conformation of PPTA molecules in the single crystals and spherulites is different from the conformation of PPTA molecules in the fibers. Infrared absorption spectra show large shifts of the amide I and amide II bands between the two conformations.     

The anomalous RHEED patterns observed from (111)Ag-(111)Cu bilayers

RHEED patterns obtained from thin expitaxial bilayers consisting of Ag grown on smooth (111)Cu surfaces have been shown by Gradmann and Krause to contain anomalous reflections. Previously, these reflections were interpreted as due to a regular arrangement of interfacial dislocations or to multiple diffraction effects. In the present work a thin film technique was used to form smooth, easily detachable (111)Ag/Cu bilayers from which RHEED patterns containing these extra reflections were obtained. These films were grown, annealed, and examined in situ by UHV-RHEED techniques. They were then removed from the vacuum system, detached from their substrates and examined by TEM and TED. Thus, contrary to previous work, the present observations consisted of a series of experiments using RHEED, TED, and TEM carried out on the same specimen. No evidence was obtained to indicate that the anomalous reflections were due to interfacial dislocations. Rather, all the observed phenomena could be explained entirely by multiple diffraction effects.     

Structure and mechanical properties of Al-Si(Ge) alloys upon melt centrifugation and quenching

The structures of eutectic binary Al-12.7 at. % Si and Al-29.7 at. % Ge alloys and a ternary Al-10 at. % Si-10 at. % Ge alloy produced by quenching levitated melts or through solidification either in the presence or in the absence of a centrifugal acceleration of 7g are studied. Centrifugation is found to cause an increase in the silicon content in the Al-Si alloy in the direction opposite to the direction of centrifugal acceleration and an increase in the germanium content in the Al-Ge alloy in the direction of centrifugal acceleration. These differences are explained by the fact that the densities of silicon and germanium clusters and solidification centers differ from the liquid-phase density at temperatures of solidification. The related changes in the values of the Young’s modulus and in the stress-strain curves can be due to sedimentation-induced changes in the composition of samples cut from the middle part of an ingot. The processes of decomposition and recovery are shown to have a substantial effect on the elastic moduli of these alloys.     

Superior performance characteristics for the poly(2,5-dimethoxyaniline)–poly(styrene sulfonic acid)-based electrochromic device

We have fabricated an electrochromic (EC) device with poly(2,5-dimethoxyaniline), PDMA, entrapped in poly(styrene sulfonic acid) (PSS) as an electrochromic layer. The device showed improved performances like stability, optical contrast, etc., over the device with a PDMA layer doped by H₂SO₄. In the process of fabrication of the EC device with a sandwich configuration, indium tin oxide (ITO)/PDMA–PSS||poly(ethyleneimine) (PEI)/orthophosphoric acid (H₃PO₄)/WO₃/ITO, electrochemical polymerization of 2,5-dimethoxyaniline (DMA) was performed with PSS as electrolyte and ITO coated glass as working electrode. The performance characteristics of EC device, like optical contrast, stability, switching time, etc., were followed by cyclic voltammetry, double potential step chronoamperometry and in-situ spectroelectrochemistry. The device was operated in between − 1 V and + 1 V, and absorption characteristics were followed by in-situ UV–visible spectroscopy. A visible contrast in color upon switching the potential from − 1 V to + 1 V was noticed for the device. The device was pale yellow at − 1 V and dark green at + 1 V. Incorporation of PSS into PDMA resulted enhancement in the performance of the complementary electrochromic device. The optical contrast of the device was improved by incorporating PSS into PDMA matrix. The device retained nearly 50% of their optical contrast after 10,000 double steps informing the superior performance of PDMA–PSS in the EC device.     

Effects of poly(ethylene glycol) on electrical conductivity of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonic acid) film

A series of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonic acid) composite thin films with prescribed concentrations of poly(ethylene glycol) were prepared. The PEDOT–PSS pristine film and PEDOT–PSS/PEG films were studied using four-probe method, photoelectron spectroscopy and atomic force microscopy. The electrical conductivity of PEDOT–PSS/PEG hybrid films was found to be enhanced compared to the PEDOT–PSS pristine film, depending on the PEG concentration and molecular weight. XPS analysis and AFM results showed that PEG induces the phase separation between the PEDOT–PSS conducting particles and the excessive PSSNa shell. Simultaneously PEG may form hydrogen bond with sulfonic groups of PSSH, and hence weaken the electrostatic interactions between PEDOT cationic chains and PSS anionic chains. These resulted in the creation of a better conduction pathway among PEDOT–PSS particles, attributed to the improvement of conductivity.     

Alkaline poly(vinyl alcohol)/poly(acrylic acid) polymer electrolyte membrane for Ni-MH battery application

Ionics - Alkaline polymer electrolyte membrane with high ionic conductivity of 0.019 S cm−1 at room temperature was prepared from poly(vinyl alcohol) and acrylic acid...     

Preparation of Poly (L-lactic Acid) Microsphere

Poly (L-lactic acid) (PLLA) microspheres were prepared by a solvent evaporation method based on an oil/water emulsion. The effect of the mass ratio of PLLA and poly(vinyl alcohol) (PVA) on the formation of the microspheres was discussed, and the influence of extraction speed of dichloromethane on the microsphere morphology was also studied. Moreover, the influences of the PLLA concentration and the volume ratio of water phase to dichloromethane phase were investigated. The results showed that stable microspheres can be obtained under the conditions that the mass ratio of PLLA to PVA is 20:1. Porous microspheres were obtained under faster evaporating speed of dichloromethane. The microsphere size increased with increasing PLLA concentration. The microsphere size also increased with the increase of the volume ratio of water phase to dichloromethane phase.     

Interactions between poly(diallyldimethylammonium chloride) and poly(acrylic acid) in dependence on polymer concentration

Summary The phase diagram of the ternary system,i.e. poly(diallyldimethylammonium chloride), poly(acrylic acid, sodium salt), and water was investigated by varying the molecular weight of the components. In general, a range of phase separation is surrounded by a one-phase system. The range of phase separation can be subdivided into a region of flocculation and/or coacervation in contrast to stable symplex dispersions. By increasing the molecular weight of the components the range of phase separation is increased and phenomena of flocculation followed by coacervation were observed predominantly. Above a molecular weight of 10 000 g/mol the transition to concentrated macroscopic homogeneous one-phase systems was observed at a constant ionic strength of 2 mol/l coinciding with the ?critical ionic strength? for Coulombic interactions in this system. That means the transition to concentrated one-phase systems can be understood by the disappearance of Coulombic forces above a system-specific ionic strength resulting in a quite additive behaviour of the mixtures with regard to the components. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Proton-conducting properties of the membranes based on poly(vinyl phosphonic acid) grafted poly(glycidyl methacrylate)

Proton-conducting properties of the graft copolymer electrolytes were examined throughout this work. The homopolymers poly(glycidyl methacrylate), PGMA and poly(vinyl phosphonic acid), PVPA were synthesized by free-radical polymerizations of the monomers glycidyl methacrylate, GMA, and vinyl phosphonic acid, VPA, respectively. The graft copolymers were produced by grafting of PVPA onto PGMA via ring opening of ethylene oxide groups. To examine the influence of the concentration of VPA on the proton conductivity, several graft copolymers were produced at various stoichiometric ratios with respect to monomer repeat units. The materials were characterized by FT-IR and ¹H NMR spectroscopy and the thermal properties were studied by thermogravimetry (TG) and differential scanning calorimetry (DSC). The TGA results demonstrated that the samples are thermally stable up to at least 150 °C. The proton conductivities of humidified and dry samples were studied via impedance spectroscopy. In the anhydrous state, the proton conductivity of P(GMA)-graft-P(VPA)₁₀ was 5 × 10^? 5 S/cm at 150 °C. The proton conductivity of the same material increased with the humidity content and reached to 0.03 S/cm at 80 °C under 50% of RH, which approached to that of Nafion^® at the same humidification level.     

PEG crosslinked poly(vinylbenzene boronic acid) polymer electrolytes for Li-ion batteries

Poly(4-vinylbenzeneboronic acid), PVBBA was synthesized via free-radical polymerization of 4-vinylbenzeneboronic acid (4-VBBA) and followed by crosslinking with polyethylene glycol (PEG) with different molecular weights to produce boron containing crosslinked polymers. Prior to crosslinking, the materials were doped with CF₃SO₃Li at several stoichiometric ratios to get PVBBAPEGX-Y where X is the molecular weight of PEG and Y is the EO/Li ratio. The materials were characterized by using Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC). The ionic conductivity of these novel crosslinked electrolytes was studied by dielectric-impedance spectroscopy. Li-ion conductivity of these polymer electrolytes depends on the length of the side units as well as the doping ratio. PVBBAPEG200-10 illustrated a satisfactory ionic conductivity of 3.1 × 10^?5 S/cm at 20 °C and 1.8 × 10^?3 S/cm at 100 °C.     

Preparation and Characterization of Poly(L-lactic acid)/Polybutadiene Block Copolymers

A two-step process to prepare poly(L-lactic acid) (PLLA)/polybutadiene (PB) block copolymers was studied. Melt polymerization was used to prepare poly(L-lactic acid) terminated with hydroxy groups, at the same time hydroxy-terminated polybutadiene and 1,6-hexamethylene diisocyanate were employed to synthesize isocyanate-terminated polybutadiene (ITPB). Then PLLA and ITPB were reacted with different PB/PLLA weight ratios. Consequently, a series of PLLA/PB copolymers with linear or crosslinked chains was obtained. Viscosity of the linear copolymer was measured by an Ubbelohde viscometer. Swelling characteristics and crosslink density of the crosslink copolymer were investigated. Fourier transform infrared and proton nuclear magnetic resonance were used to characterize the structure of the copolymer, and dynamic mechanical analysis (DMA) was applied to characterize its thermal properties. Mechanical property measurements showed that a toughened PLLA polymer was synthesized. Atomic force microscope was utilized to characterize its micro-morphology.     

Environmentally friendly films based on poly(3-hydroxybutyrate) and poly(lactic acid): A review

A review of data on the thermo, photo-, and biodegradation of compositions of synthetic polymers with poly-3-hydroxybutyrate and poly(lactic acid) is presented. The influence of these polymers on the thermal, microstructural, and rheological properties of mixtures is examined. The destruction of pure biopolymers, as well as compositions thereof with polyethylene, poly-(3-hydroxybutyrate-co-3-hydroxyvalerate), and polycaprolactone is studied.     

New interpretation of local dynamics of poly(dimethyl siloxane) observed by quasielastic neutron scattering

Synthetic and natural polymers have complex dynamic behavior with distinct motions taking place on a wide range of time and length scales. For poly(dimethyl siloxane) we show that, at temperatures above the melting point, the reorientation of the CH3 groups provides a non-negligible contribution to the incoherent dynamic structure factor. Analysis of the quasielastic neutron scattering data is carried out using a model function that includes fast rotational motion of the CH3 groups and local conformational transitions between isomeric states. By using this model, detailed comparison between experimental data and theoretical predictions at distances where deviations from the traditional Rouse model are expected becomes possible.     

Analysis of X-ray and electron-beam diffraction patterns from poly(dipropyl siloxane)

The crystalline polysiloxane (—R2SiO—)n. where R= n-propyl, has been characterized through high-resolution X-ray diffraction patterns of the polycrystalline material and selected-area electron-beam diffraction patterns of single crystals. The smallest unit cell consistent with the diffraction data is tetragonal, with dimensions a = 9.52 ? 0.01 and c = 9.40 ± 0.05 . The probable space group is P41 (or P43).     

Conformational studies of poly(methacrylic acid). I. Conformational energy calculations on lsotactic and syndiotactic poly(methacrylic acid)

The conformational energies of isotactic and syndiotactic poly (methacrytic acid) were calculated for isolated chains in order to determine the most probable helical conformations. It was concluded that the most probable backbone conformation of the isotactic polymer is either a 5₂ or 8₃ helix, where the internal rotations of the two helices are nearly equal. The left-handed and right-handed conformations were also essentially equienergy conformations. Two lowenergy helical conformations were found for the syndiotactic polymer. One of these conformations had backbone internal rotations nearly identical to those of the 5₂ and 8₃ helices of isotactic poly(methacry1ic acid). The second conformation was a nearly planar structure containing two monomer units in the axial repeat unit, with backbone rotation angles nearly identical to those of the 5₂ and 8₃ helices or their negatives.

Deformation of the backbone C─C─C bond angles was considered in the conformational energy calculations. The optimum bond angles for both isotactic and syndiotactic polymer chains were considerably larger than the normal tetrahedral bond angle. The relative energies calculated for helical conformations were influenced by the backbone bond angles. It was demonstrated that the failure to consider backbone bond-angle deformation may lead to erroneous conformational assignments in disubstituted vinyl polymers.     

Characterization of Nano-Structured Poly(D,L-lactic acid) Nonwoven Mats Obtained from Different Solutions by Electrospinning

Two different solvent mixtures, chloroform/dimethylformamide (DMF) and chloroform/ acetone, in 60/40 v/v concentrations, were used to electrospin poly(D,L-lactic acid) (PDLLA). The influence of solvent type, solution concentration, and processing conditions on the morphology and properties of the electrospun mats was studied. The nanofibers characterization was done by scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The smallest nanofibers’ diameters from both mixtures were obtained from solutions with 5 wt%/v PDLLA concentration using a 1.0 kV/cm electrical field. In general the nanofibers from the chloroform/DMF mixture had smaller diameters than the nanofibers from the chloroform/acetone mixture. However, the latter ones were porous, while the nanofibers from the chloroform/DMF mixtures were not. All the PDLLA nanofibers, independent of solvent mixture, had a very low amount of crystallinity and were composed of very small and imperfect α and β crystals.     

Sorption of water vapor into polyelectrolyte complex of poly(styrenesulfonic acid)/poly(4-vinyl-N-ethylpyridinium bromide)

Successive differential and integral sorptions of water vapor into the polyelectrolyte complex of poly (sty renesulfonic acid) / poly(4-vinyl-N-ethylpyridinum bromide) have been measured; films of stoichiometric (1/1 molar ratio) and nonstoichiometric (2/1 molar ratio) complex were employed in the experiments. Sorption isotherms have been also determined by using the values obtained from integral sorption experiments. The sorption data have suggested the following. (i) The rearrangement of polymer chains plays a relatively unimportant role in sorption of water. (ii) The relaxation motion of network chains is suppressed. (iii) The change of morphological structure caused by vapor sorption is obscure. (iv) The ionic character of the 1/1 complex is higher than that of 2/1 complex. (v) A small amount of water can be dispersed on specific polymer sites, but the sorption of a large amount of water is accompanied by an expansion of network structure. Points (i)-(v) have been consistently interpreted on the basis of the network structure and ionic character of this complex.     

Studies of fractal growth patterns in poly (ethylene oxide) and chitosan membranes

Polymer electrolyte membranes containing various weight percent of salt have been prepared in the authors’ laboratory for the study of their potential applications in solid state electrochemical devices; however, the polymer electrolyte membranes were found to become sort of media for fractal growth, months after they were prepared. Fractal growth patterns in polymer membranes of poly (ethylene oxide) and chitosan membranes have been analyzed, and their fractal dimensions were determined. A diffusion-limited aggregate model which is based on the Brownian motion theory is applied to simulate the experimentally obtained fractal patterns. The approach was extended by performing computer simulations with computing resources available. The fractal dimension values of the simulated and the experimentally obtained original fractal patterns were marginally close. This indicates that the simulation work has successfully produced fractal patterns that are in fairly acceptable conformity with the fractal patterns observed in the polymer membranes.     

pH effects in the formation of interpolymer complexes between poly(N-vinylpyrrolidone) and poly(acrylic acid) in aqueous solutions

The critical pH values for hydrogen-bonded interpolymer complexes of poly(N-vinylpyrrolidone) and poly(acrylic acid) in aqueous solutions were determined. The effect of polymer concentrations, molecular weights and addition of inorganic salts (NaCl and KCl) on the complexation was studied. Received 29 April 2002 and Received in final form 24 June 2002 Published online: 21 January 2003