Free volume in polymer systems

Slobodni volumen u polimerima ima važnu ulogu u razumijevanju mnogih svojstava kao što su, primjerice, mehanička i električna svojstva te difuzija malih molekula i plinova. Razvoj novih metoda posljednjih je godina poboljšao određivanje mikrostrukture slobodnog volumena, veličinu i oblik šupljina te njihovu raspodjelu. U radu su opisane najčešće primjenjivane teorije slobodnog volumena i dan je pregled novijih rezultata primjene metoda probe: elektronske spinske rezonancije, ESR, metoda spinske probe/oznake, metode fluorescencije te najnovije metode spektroskopije pozitronske anihilacije, PALS. Razmotrene su prednosti i ograničenja opisanih metoda u određivanju slobodnog volumena. Analiza slobodnog volumena povezana sa svojstvima polimernih materijala omogućuje da se tijekom priprave polimera poboljšaju ili mijenjaju određena svojstva.The free volume of polymers plays an important role in the understanding of many characteristic properties such as mechanical and electrical properties and transport of small molecules and gases. The development of new methods in recent years has improved polymer characterization of free volume: the size and the shape of holes and free volume distribution. The paper reviews some of the most frequently used free volume theories and recent results of the applications of probe methods: electron spin resonance, ESR, - spin probe/label method, fluorescence method and positron annihilation spectroscopy, PALS. The scope and limitations of the reviewed methods in the study of free volume are described. The analysis of free volume correlated with the polymer properties offers a possibility to control and improve polymer characteristics during the polymer preparation and processing

Unraveling the weak hydrogen bonds of ethynylpyridines and ethynylbenzene with trimethylphosphate - A combined FT-Raman spectroscopic and quantum-chemical study

The interplay of secondary hydrogen bonds of 2- and 3-ethynylpyridine or ethynylbenzene with trimethylphosphate in tetrachloroethene was elucidated using FT-Raman spectroscopy and MP2/6-311 + G(d,p) calculations. The direct participation of C C moiety in the complex formation was demonstrated by the change in the shape of the C C stretching band and further characterized in terms of vibrational dephasing of C C stretching. With this aim, the complex band pattern in frequency domain was decomposed using analytical function, introduced by Egelstaff and Schofield and further disseminated by Kirillov, with analytical counterpart in the time domain. The amplitude of frequency fluctuations (M-2), frequency modulation time (T-omega) and vibrational dephasing time (T-v) were determined for both unassociated (C C) and associated (C C center dot center dot center dot) ethynyl moieties of 2- and 3-ethynylpyridine and ethynylbenzene. The differences in the dynamical parameters indicate broader distribution of the frequency fluctuations for C C center dot center dot center dot moiety (M-2 similar to 1-2 ps(-2)) than for C C moiety (M-2 approximate to 0.5 ps(-2)), while the average time between perturbative events, as well as the time needed for the phase being completely lost, were shorter for C C center dot center dot center dot (T-omega similar to 0.2-0.7 ps, T-nu approximate to 1ps) than for C C moiety (T-omega similar to.4-1.7 ps, T-v approximate to ps). The shorter T-omega, for moiety of 2-ethynylpyridine (T-omega approximate to .23 ps), in comparison with analogous quantity of 3-ethynylpyridine and ethynylbenzene (T-omega approximate to 0.6 and 0.68 ps), is attributed to more frequent hindering of the C C center dot center dot center dot HCH2 hydrogen bond by spatially close N-atom which competes for H-atom of CH3 group thus making the N center dot center dot center dot HCH2 hydrogen bond, as predicted by MP2 calculations. Additionally, a hydrogen bond between ortho H-atom of 3-ethynylpyridine and P-O(CH3) group of trimethylphosphate is suggested from experimental FT-Raman spectra as well and also computationally verified. (C) 2016 Elsevier B.V. All rights reserved

Reaction Mechanism of Covalent Modification of Phosphatidylethanolamine Lipids by Reactive Aldehydes 4‑Hydroxy-2-nonenal and 4‑Oxo-2-nonenal

4-Hydroxy-2-nonenal (HNE) and 4-oxo-2-nonenal (ONE) are biologically important reactive aldehydes formed during oxidative stress in phospholipid bilayers. They are highly reactive species due to presence of several reaction centers and can react with amino acids in peptides and proteins, as well as phosphoethanolamine (PE) lipids, thus modifying their biological activity. The aim of this work is to study in a molecular detail the reactivity of HNE and ONE toward PE lipids in a simplified system containing only lipids and reactive aldehydes in dichloromethane as an inert solvent. We use a combination of quantum chemical calculations, ¹H NMR measurements, FT-IR spectroscopy, and mass spectrometry experiments and show that for both reactive aldehydes two types of chemical reactions are possible: formation of Michael adducts and Schiff bases. In the case of HNE, an initially formed Michael adduct can also undergo an additional cyclization step to a hemiacetal derivative, whereas no cyclization occurs in the case of ONE and a Michael adduct is identified. A Schiff base product initially formed when HNE is added to PE lipid can also further cyclize to a pyrrole derivative in contrast to ONE, where only a Schiff base product is isolated. The suggested reaction mechanism by quantum-chemical calculations is in a qualitative agreement with experimental yields of isolated products and is also additionally investigated by ¹H NMR measurements, FT-IR spectroscopy, and mass spectrometry experiments

Ligand-Dependent Nanoparticle Clustering within Lipid Membranes Induced by Surrounding Medium

The interactions between hydrophobic or semihydrophobic gold and silver nanoparticles (NPs) and a dimyristoyl­phosphatidyl­choline (DMPC) bilayer as a model cell membrane in two ionic solutions result in the structural reorganization within the bilayer manifested as locally increased nanomechanical compaction in the vicinity of NP clusters as well as changed overall thermotropic properties. The effects of NP surface charge and hydrophobicity were examined using AFM imaging, force spectroscopy and IR spectroscopy. The NP clustering occurred during hydration process of dry films containing both the DMPC molecules and the NPs by the mechanism in which the number of bilayer deformations was reduced by NP clustering. The force spectroscopy showed increased bilayer density around (semi)­hydrophobic NP clusters and thus locally increased lateral compaction of the bilayer. The strengthening effect was observed for both the silver and the gold NPs in a high ionic strength solution such as seawater, while it was absent under physiological conditions. The local lipid rearrangement induces the long-range lipid reorganization resulting in the bilayer phase transition shifting toward lower or higher temperatures depending on the solution ionic strength (at the most by −1.0 °C in phosphate buffered saline and at the most by +0.5 °C in seawater)

High Influence of Potassium Bromide on Thermal Decomposition of Ammonia Borane^†

The present paper presents a thorough experimental investigation of mechanistic pathways of thermal decomposition of ammonia borane (AB) and its mixture with KBr. A comparative detection and temperature-dependent <i>in situ</i> monitoring of the decomposition products was done by use of temperature-dependent infrared (IR) spectroscopy of both solid (in transmission through KBr pellets and ATR mode) and gaseous products, thermogravimetry (TG) and evolved gas analysis mass spectroscopy (EGA–MS). This enables discrimination of the processes occurring in the bulk from those in the near-surface level. For the first time, a high influence of the KBr matrix on AB decomposition was found and thoroughly investigated. Although KBr does not change the chemical and physical identity of AB at ambient conditions, it dramatically affects its thermal decomposition pathway. It is found that the presence of KBr not only favors the production of diammoniate of diborane in the induction phase, but also enables an efficient catalysis of AB decomposition by NH₄⁺ ions, present at the KBr–AB interface, which leads to suppression of emission of unwanted gaseous side products other than NH₃. IR spectroscopy was also used to shed a light on the molecular background of the frequently observed, but never investigated increase of the mass of decomposition products