Structure–property relationships for film‐forming copoly(amide imide)s

Thin films of copoly(amide imide)s (coPAIs) from dichloro‐dianhydride of trimellitimide‐N‐acetic acid and mixtures of diphenylmethane diamine (DPA) and cardo 9,9′‐bis‐phenylfluorene diamine (CDA) cast from solutions in dimethylacetamide (DMAA) were characterized by wide‐angle and small‐angle X‐ray scattering (WAXS and SAXS), dynamic mechanical thermal analysis (DMTA) (temperature interval: 293–703 K, frequency range: 1–100 Hz), and thermogravimetric analysis (TGA) (nitrogen flux, temperature interval: 303–973 K). The mean interchain spacings (WAXS) smoothly increased with the CDA/DPA molar ratio from 0.55 nm for CDA/DPA = 0/1 up to 0.60 nm for CDA/DPA = 1/0. The smooth patterns of the SAXS curves for all coPAIs were explained by the smearing‐out of electron density differences between densely‐packed and loosely‐packed microregions of coPAIs due to the wide dispersion of their sizes. The step‐like patterns of the TGA traces in the temperature intervals below and above 600 K were associated with successive weight losses due to the evaporation of residual water and of DMAA, and to the thermal degradation of diamine and dianhydride chain fragments, respectively. As could be inferred from the TGA data, the loosely‐packed regions comprise about 25–35% of the total volume of studied coPAIs. The mechanical relaxations observed in all coPAIs at T_β < T_α′ < T_α (DMA) were attributed to the onset of non‐cooperative segment motion in loosely‐packed regions, of cooperative segment motion in loosely‐packed regions, and of cooperative segment motion in densely‐packed regions, respectively. At constant frequency, the sub‐glass relaxations were roughly composition‐independent, while chain‐stiffening effect was assumed to be responsible for the smooth increase of T_α′ and T_α, as well as of the corresponding apparent activation energies with the CDA/DPA ratio. Copyright © 2003 John Wiley & Sons, Ltd.     

Reentrant Structural and Optical Properties and Large Positive Thermal Expansion in Perovskite Formamidinium Lead Iodide

The structure of the hybrid perovskite HC(NH₂)₂PbI₃ (formamidinium lead iodide) reflects competing interactions associated with molecular motion, hydrogen bonding tendencies, thermally activated soft octahedral rotations, and the propensity for the Pb²⁺ lone pair to express its stereochemistry. High‐resolution synchrotron X‐ray powder diffraction reveals a continuous transition from the cubic α‐phase (Pm m, #221) to a tetragonal β‐phase (P4/mbm, #127) at around 285 K, followed by a first‐order transition to a tetragonal γ‐phase (retaining P4/mbm, #127) at 140 K. An unusual reentrant pseudosymmetry in the β‐to‐γ phase transition is seen that is also reflected in the photoluminescence. Around room temperature, the coefficient of volumetric thermal expansion is among the largest for any extended crystalline solid.     

Dielectric studies of chain dynamics in homogeneous semi‐interpenetrating polymer networks

Semi‐interpenetrating polymer networks (semi‐IPNs) were prepared from linear polyurethane (PUR) and polycyanurate (PCN) networks. Wide‐angle X‐ray scattering measurements showed that the IPNs were amorphous, and differential scanning calorimetry and small‐angle X‐ray scattering measurements suggested that they were macroscopically homogeneous. Here we report the results of detailed studies of the molecular mobility in IPNs with PUR contents greater than or equal to 50% via broadband dielectric relaxation spectroscopy (10⁻²–10⁹ Hz, 210–420 K) and thermally stimulated depolarization current techniques (77–320 K). Both techniques gave a single α relaxation in the IPNs, shifting to higher temperatures in isochronal plots with increasing PCN content, and provided measures for the glass‐transition temperature (T_g) close to and following the calorimetric T_g. The dielectric response in the IPNs was dominated by PUR. The segmental α relaxation, associated with the glass transition and, to a lesser extent, the local secondary β and γ relaxations were analyzed in detail with respect to the timescale, the shape of the response, and the relaxation strength. The α relaxation became broader with increasing PCN content, the broadening being attributed to concentration fluctuations. Fragility decreased in the IPNs in comparison with PUR, the kinetic free volume at T_g increased, and the relaxation strength of the α relaxation, normalized to the same PUR content, increased. The results are discussed in terms of the formation of chemical bonds between the components, as confirmed by IR, and the reduced packing density of PUR chains in the IPNs. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3070–3087, 2000     

Water sorption and diffusion studies in an epoxy resin system

The kinetics of water absorption in epoxide materials was studied by the aid of a diglycidyl ether of bisphenol‐A–triethylenetetramine (DGEBA–TETA) epoxy–resin system containing various amounts of the plasticizer THIOCOL (0–40 phr). The presence of plasticizer permits the formation of products with different crosslinking densities and hydrophilic characters. Dynamic water absorption experiments were carried out at 15, 40, and 70°C. For the fitting of the experimental results, a new model was used, based on a model proposed earlier by Jacobs and Jones. This model considers epoxide product as a two‐phase system consisting of a master phase (where the major part of the water is absorbed), which is homogeneous and nonpolar (phase 1), and of a second phase with different density and/or hydrophilic character (phase 2). By making the assumption that water diffusion can take place independently in the different phases of the material in accordance with Fick's second law, we can calculate the diffusion coefficient D and the water content at saturation M_∞ for each phase separately. Equilibrium water sorption measurements were performed at 40°C, and the data were analyzed and discussed based on the Guggenheim–Anderson–de Boer (GAB) equation, the results being in support of the two‐phase model used in the analysis of absorption kinetics. The linear expansion coefficient and the glass transition temperature of the materials, employed in the discussion of the results, were measured by thermomechanical analysis. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1165–1182, 1999     

Water sorption and thermally stimulated depolarization currents in nylon-6

The influence of water on the low temperature dielectric relaxations of nylon-6 has been investigated in the range from dry to water-saturated polymer by means of sorption experiments and thermally stimulated depolarization currents (TSDC). Three different ranges of water sorption are observed and their reflection in the TSDC spectra is examined. It is found that position and intensity of the TSDC maxima exhibit characteristic and different behavior in each of these sorption ranges. The critical concentrations are 1.5 ± 0.5% and 6.0 ± 0.5% water content. An interpretation of the experimental results is given with respect to the molecular origins of the low-temperature dielectric relaxations and the mechanisms of water sorption in nylons. The most important idea is that water exists as firmly bound, loosely bound, and clustered water, depending on its concentration. An attempt is made to determine relations between these different states of water in nylon-6 and some characteristic features of the low-temperature TSDC spectrum. © 1996 John Wiley & Sons, Inc.     

Synthesis,characterization and crystal structure of the cis-[RhL2Cl2]Cl complex with the bifunctional ligand (L) 2-(2′-pyridyl)quinoxaline. Biological activity towards PAF (Platelet Activating Factor) induced platelet aggregation

The reaction of RhCl₃·3H₂O with the ligand L = 2-(2′-pyridyl)quinoxaline (pqo) in a 1:2 molar ratio formed the mononuclear complex cis-[RhL₂Cl₂]Cl (1), which has been characterized by elemental analysis, FT-IR, FT-Raman, ¹H, ¹³C NMR, electronic absorption spectroscopy and by electrospray mass spectrometry. The molecular structure of 1 (needle like and prismatic polymorphs) in the crystal has been elucidated by single-crystal X-ray diffraction, revealing a bidentate behavior of L, while the geometry around the Rh(III) atom is that of a distorted octahedron.^. Preliminary biological tests revealed that this compound inhibited PAF-induced rabbit platelet aggregation.     

Molecular mobility in biodegradable poly( $ \varepsilon$ -caprolactone)/poly(hydroxyethyl acrylate) networks

Poly( e \varepsilon -caprolactone)/poly(hydroxyethyl acrylate) networks have been investigated by thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC). The introduction of hydrophilic units (HEA) in the system aiming at tailoring the hydrophilicity of the system results in a series of copolymer networks with microphase separation into hydrophobic/hydrophilic domains. Polycaprolactone (PCL) crystallization is prevented by the topological constraints HEA units imposed in such heterogeneous domains. Moreover, the mobility of the amorphous PCL chains is enhanced as revealed by the main relaxation process which becomes faster. The glass transition of PHEA-rich domains shifts to lower temperatures, as the total amount of PCL in the copolymer increases, due to the presence of PCL units within the same region. The behaviour of the copolymer networks swollen with different content of water has been investigated to analyze the interaction between water molecules and hydrophobic/hydrophilic domains and provide further insights into the molecular structure of the system.     

Polymer segmental dynamics and solvent thermal transitions in poly(ethyl acrylate)/p‐xylene mixtures

A poly(ethyl acrylate) polymer network was swollen with different concentrations of the nonpolar solvent p‐xylene, c_px, from xerogel until saturation (0 ≤ c_px ≤ 0.85). Differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) techniques were employed to study the polymer segmental dynamics and the solvent thermal transitions in homogeneous (c_px < 0.20) and partially crystallized (c_px ≥ 0.20) PEA/p‐xylene mixtures. Our DSC measurements indicate that p‐xylene undergoes cold crystallization for intermediate solvent concentrations, 0.20 ≤ c_px ≤ 0.30 while for higher c_px values crystallization takes place during cooling. The results show that for c_px ≤ 0.30 the T_g decreases with increasing c_px (plasticization effect) obeying the respective Fox equation. For the same c_px range we found that both the dielectric strength and the heat capacity increment of the segmental (α) relaxation process increase gradually with c_px whereas the distribution of relaxation times for the underlying molecular relaxations does not change. For c_px > 0.30 the partially crystallized mixtures exhibit a constant T_g corresponding to the gel phase of PEA with an amount of p‐xylene which is not able to crystallize under any conditions. The concentration of this noncrystallized p‐xylene, c_UCpx, has been estimated to be between 0.12 and 0.15, independent of the total p‐xylene concentration in the mixtures. When a separate p‐xylene crystal phase is formed (for c_px > 0.30) the segmental dielectric strength and heat capacity increment decrease significantly exhibiting values significantly lower than those measured for the homogeneous gels. In addition, we found that the presence of p‐xylene crystals may induce marginal spatial heterogeneity of polymer (or p‐xylene) concentration within the gel phase affecting thus slightly the breath of the segmental relaxation of PEA. We attribute these results to restrictions of polymer segmental configurations due to constraints imposed by the p‐xylene crystals and/or to the immobilization of a part of the polymer chains. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Dielectric and thermally stimulated discharge current studies of rubber-modified epoxy resins

Comparison is drawn between data obtained from dielectric relaxation and thermally stimulated discharge current (TSDC) measurements on blends of carboxyl-terminated butadiene acrylonitrile (CTBN) with an epoxy resin (ER) obtained by reaction of the diglycidyl ether of bisphenol A (DGEBA) with either triethylenetetramine (TETA) or diamino-diphenylmethane (DDM). The effect of varying the composition of CTBN on the nature of the phase separation was examined using electron microscopy and was also evident from changes in the dielectric spectrum and the calculated activation energy for the molecular relaxation process. The morphology consists of spherical rubber particles dispersed in an epoxy matrix at low levels of CTBN, and at higher concentrations epoxy resin is observed to be precipitated as spheres within the rubbery phase. Detailed analysis of the dipolar spectra indicates that at low concentrations there appears to be some cosolution of the epoxy in the CTBN.     

A Family of Potent Ru(II) Photosensitizers with Enhanced DNA Intercalation: Bimodal Photokillers

A new family of Ru(II)‐based photosensitizers was synthesized and systematically characterized. The ligands employed to coordinate the ruthenium metal center were the commercially available 2,2′‐bipyridine and a pyridine‐quinoline hybrid bearing an anthracene moiety. The complexes obtained carry either or Cl^? counterions. These counterions determine the complexes' hydrophobic or hydrophilic character, respectively, therefore dictating their solubility in biologically related media. All photosensitizers exhibit characteristic, relatively strong and wide UV–Vis absorption spectral profiles. Their high efficiency in generating cytotoxic singlet oxygen was established (up to Φ_Δ ~0.8). Moreover, the interaction of these photosensitizers with double‐stranded DNA was studied fluoro‐ and photospectroscopically and their binding affinities were found to be of the order of 3 × 10⁷ M^?1. All complexes are photocytotoxic to DU145 human prostate cancer cells. The highest light‐induced toxicity was conferred by the photosensitizers bearing Cl^? counterions, probably due to the looser ionic “chaperoning” of Cl^?, in comparison to , leading to higher cell internalization.