Kyriakos Viras

Electrical and dielectric behavior in blends of polyurethane-based ionomers

Abstract In the present work, the electrical and dielectric behaviors in ionomer blends of an anion-containing polyurethane (PU 1 ) and polyaminourethane (PU 2 ) have been investigated by using ac Dielectric Relaxation Spectroscopy (DRS), Differential Scanning Calorimetry (DSC) and Thermally Stimulated Depolarization Currents (TSDC) methods. The ionomer blends are characterized from microphase separation of soft-rich and hard microregions. Two conductivity mechanisms contribute to the dc conductivity of the ionomer blends. That of the shorter relaxation time is correlated to the soft-rich microregions and the other with the longer relaxation time is correlated to the hard microregions. From the comparison between ionomers of different composition, it is found that a faster conductivity relaxation mechanism of the soft-rich microregions implies a faster conductivity relaxation mechanism of the hard microregions. This behavior can be understood in terms of concept of the dynamic energy barriers. From the comparison between the ionomer blends, a smaller temperature difference, Δ T 1 = T MWS − T α , between the temperatures of the current maximum of the Maxwell–Wagner–Sillars (MWS) and α-relaxation mechanisms, corresponds to a greater dc conductivity. The formalisms of the dielectric function e *, electric modulus M *, and complex impedance Z * of the ac dielectric spectroscopy reveal the existence, with different weights, of the various mechanisms of dipolar and conductivity relaxation. The combined use of these formalisms, and especially their imaginary parts, gives the possibility to extract conclusions about the origin and the characteristics of the various relaxation mechanisms, as well as about their correlation with the physical processes which take place in the bulk of the materials.

Interactions at the bilayer interface and receptor site induced by the novel synthetic pyrrolidinone analog MMK3

This work presents a thorough investigation of the interaction of the novel synthetic pyrrolidinone analog MMK3 with the model membrane system of dipalmitoylphosphatidylcholine (DPPC) and the receptor active site. MMK3 has been designed to exert antihypertensive activity by functioning as an antagonist of the angiotensin II receptor of subtype 1 (AT(1)). Its low energy conformers were characterized by 2D rotating-frame Overhauser effect spectroscopy (ROESY) in combination with molecular dynamics (MD) simulations. Docking study of MMK3 shows that it fits to the AT(1) receptor as SARTANs, however, its biological activity appears to be lower. Thus, differential scanning calorimetry (DSC), Raman spectroscopy and small angle X-ray scattering (SAXS) experiments on the interaction of MMK3 with DPPC bilayers were carried out and results demonstrate that the drug is well incorporated into the membrane leaflets and furthermore causes partial bilayer interdigitation, although less effective than SARTANs. Thus, it appears that the nature of the bilayer matrix and the stereoelectronic active site requirements of the receptor are responsible for the low bioactivity of MMK3.

Low-molar-mass cyclic poly(oxyethylene)s studied by Raman spectroscopy, X-ray scattering and differential scanning calorimetry

Abstract Crystalline cyclic poly(oxyethylene)s of number-average molar mass 1000, 1500, 2000 and 3000 g mol−1 were studied by high-frequency and low-frequency laser-Raman spectroscopy, wide-angle (WAXS) and small-angle (SAXS) X-ray scattering, and differential scanning calorimetry (d.s.c.). Comparison was made with the properties of corresponding linear poly(oxyethylene) dimethyl ethers. The crystal structures of the linear and cyclic polymers were essentially the same as that of high-molar-mass poly(oxyethylene). The cyclic polymers crystallized in the twice-folded conformation and the linear polymers in the extended conformation, as confirmed by their lamellar spacings (SAXS) and the frequencies of their single-node longitudinal acoustic modes (LAM-1). Compared at identical lamellar spacing, the cyclic polymers had identical enthalpies of fusion (per mol of chain units) as the linear polymers but higher melting temperatures, consistent with lower entropies of fusion (per mol of chain units) for the cyclic polymers.

Interactions of an anionic surfactant with poly(oxyalkylene) copolymers in aqueous solution.

The interactions of sodium dodecyl sulfate (SDS) with poly(ethylene oxide)/poly(alkylene oxide) (E/A) block copolymers are explored in this study. With respect to the specific compositional characteristics of the copolymer, introduction of SDS can induce fundamentally different effects to the self-assembly behavior of E/A copolymer solutions. In the case of the E(18)B(10)-SDS system (E = poly(ethylene oxide) and B = poly(butylene oxide)) development of large surfactant-polymer aggregates was observed. In the case of B(20)E(610)-SDS, B(12)E(227)B(12)-SDS, E(40)B(10)E(40)-SDS, E(19)P(43)E(19)-SDS (P = poly(propylene oxide)), the formation of smaller particles compared to pure polymeric micelles points to micellar suppression induced by the ionic surfactant. This effect can be ascribed to a physical binding between the hydrophobic block of unassociated macromolecules and the non-polar tail of the surfactant. Analysis of critical micelle concentrations (cmc(*)) of polymer-surfactant aqueous solutions within the framework of regular solution theory for binary surfactants revealed negative deviations from ideal behavior for E(40)B(10)E(40)-SDS and E(19)P(43)E(19)-SDS, but positive deviations for E(18)B(10)-SDS. Ultrasonic studies performed for the E(19)P(43)E(19)-SDS system enabled the identification of three distinct regions, corresponding to three main steps of the complexation; SDS absorption to the hydrophobic backbone of polymer, development of polymer-surfactant complexes and gradual breakdown of the mixed aggregates.

Differential scanning calorimetry (DSC) and Raman spectroscopy study of Poly(dimethylsiloxane)

Poly(dimethylsiloxane) was studied by laser Raman spectroscopy and differential scanning calorimetry. The Si-O-Si skeletal mode at 489 cm -1 and the C-Si-C deformation bands at 188 cm -1 and 158 cm -1 were studied as functions of temperature from ambient to - 130°C, and effects of temperature interpreted in accordance with results from thermal analysis.

Investigation of the interactions of silibinin with 2-hydroxypropyl-β-cyclodextrin through biophysical techniques and computational methods.

Cyclodextrins (CDs) are a well-known class of supermolecules that have been widely used to protect drugs against conjugation and metabolic inactivation as well as to enhance the aqueous solubility and hence to ameliorate the oral bioavailability of sparingly soluble drug molecules. The hepatoprotectant drug silibinin can be incorporated into CDs, and here we elucidate the interaction between the drug and the host at the molecular level. The complexation product of silibinin with 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) is characterized by Differential Scanning Calorimetry, mass spectrometry, solid and liquid high-resolution NMR spectroscopy. The chemical shift changes using (13)C CP/MAS on the complexing of the guest with the host provided significant information on the molecular interactions, and they were in agreement with the 2D NOESY results. These results point out that in both solid and liquid forms, the drug is engulfed and interacts with HP-β-CD in identical manner. Molecular dynamics calculations have been performed to examine the thermodynamic characteristics associated with the silibinin-HP-β-CD interactions and to study the stability of the complex. To approximate the physiological conditions, the aqueous solubility and dissolution characteristics of the complex at pH states simulating those of the upper gastrointestinal tract have been applied. To evaluate the antiproliferative activity of silibinin-HP-β-CD complex comparatively to silibinin in MCF-7 human cancer cells, MTT assays have been performed.

Interactions of Bovine Serum Albumin with Ethylene Oxide/Butylene Oxide Copolymers in Aqueous Solution

The interactions of bovine serum albumin (BSA) with three ethylene oxide/butylene oxide (E/B) copolymers having different block lengths and varying molecular architectures is examined in this study in aqueous solutions. Dynamic light scattering (DLS) indicates the absence of BSA-polymer binding in micellar systems of copolymers with lengthy hydrophilic blocks. On the contrary, stable protein-polymer aggregates were observed in the case of E 18B 10 block copolymer. Results from DLS and SAXS suggest the dissociation of E/B copolymer micelles in the presence of protein and the absorption of polymer chains to BSA surface. At high protein loadings, bound BSA adopts a more compact conformation in solution. The secondary structure of the protein remains essentially unaffected even at high polymer concentrations. Raman spectroscopy was used to give insight to the configurations of the bound molecules in concentrated solutions. In the vicinity of the critical gel concentration of E 18B 10 introduction of BSA can dramatically modify the phase diagram, inducing a gel-sol-gel transition. The overall picture of the interaction diagram of the E 18B 10-BSA reflects the shrinkage of the suspended particles due to destabilization of micelles induced by BSA and the gelator nature of the globular protein. SAXS and rheology were used to further characterize the structure and flow behavior of the polymer-protein hybrid gels and sols.

Solid−Solid Phase Transitions in dl-Norvaline Studied by Differential Scanning Calorimetry and Raman Spectroscopy

The structural modifications of the amino acid DL-Norvaline have been studied using differential scanning calorimetry (DSC) and Raman spectroscopy. DSC results showed that this amino acid undergoes two solid-solid phase transitions at -116.9 and -76.1 degrees C in the temperature range -130 to +300 degrees C. Raman spectroscopy was applied to complement DSC results. The combination of the two methodologies point out that the observed phase transitions correspond to an increment of disordering in the aliphatic side chain of amino acid, an augmentation of the rotational motion of the amino group and a decrease of the strength of the intramolecular hydrogen bonding of the initial dimers at low temperatures. The observed phase transitions of DL-norvaline are compared with those found in DL-norleucine.

Morphological, Thermal, and Electrical Characterization of Syndiotactic Polypropylene/Multiwalled Carbon Nanotube Composites

In this work, syndiotactic polypropylene/multiwalled carbon nanotubes (MWCNT) nanocomposites, in various concentrations, were produced using melt mixing. The influence of the addition of MWCNT on the morphology, crystalline form, and the thermal and electrical properties of the polymer matrix was studied. To that aim, scanning electron microscopy, Raman spectroscopy, X-ray diffraction, differential scanning calorimetry, and dielectric relaxation spectroscopy were employed. Significant alterations of both the crystallization behavior and the thermal properties of the matrix were found on addition of the carbon nanotubes: conversion of the disordered crystalline form I to the ordered one, increase of the crystallization temperature and the degree of crystallinity, and decrease of the glass transition temperature and the heat capacity jump. Finally, the electrical percolation threshold was found between 2.5–3.0 wt.% MWCNT. For comparison purposes, the results of the system studied here are also correlated with the findings from a previous work on the isotactic polypropylene/MWCNT system.

Polymer-surfactant vesicular complexes in aqueous medium.

The introduction of ionic single-tailed surfactants to aqueous solutions of EO(18)BO(10) [EO = poly(ethylene oxide), BO = poly(1,2-butylene oxide), subscripts denote the number of repeating units] leads to the formation of vesicles, as probed by laser scanning confocal microscopy. Dynamic light scattering showed that the dimensions of these aggregates at early stages of development do not depend on the sign of the surfactant head group charge. Small-angle X-ray scattering (SAXS) analysis indicated the coexistence of smaller micelles of different sizes and varying polymer content in solution. In strong contrast to the dramatic increase of size of dispersed particles induced by surfactants in dilute solution, the d-spacing of corresponding mesophases reduces monotonically upon increasing surfactant loading. This effect points to the suppression of vesicles as a consequence of increasing ionic strength in concentrated solutions. Maximum enhancements of storage modulus and thermal stability of hybrid gels take place at different compositions, indicating a delicate balance between the number and size of polymer-poor aggregates (population increases with surfactant loading) and the number and size of polymer-surfactant complexes (number and size decrease in high surfactant concentrations).