Mahdy M. Elmahdy | Mansoura University (original) (raw)
Papers by Mahdy M. Elmahdy
Journal of Physics: Condensed Matter, May 29, 2008
The molecular dynamics of discotic liquid crystalline hexa-peri-hexabenzocoronenes (HBC-C10,6 and... more The molecular dynamics of discotic liquid crystalline hexa-peri-hexabenzocoronenes (HBC-C10,6 and HBC-C14,10) with long alkyl side chains branched in close vicinity to the HBC aromatic core were studied by dielectric spectroscopy. The two compounds display a weak dipolar relaxation (alpha-process) manifested by a Vogel-Fulcher-Tammann (VFT) dependence and have almost the same glass temperature (~215 K) in excellent agreement with the value
Journal of Applied Polymer Science, Jun 11, 2010
ABSTRACT Graft copolymerization of different vinyl monomers onto chitosan (Cs) and its derivative... more ABSTRACT Graft copolymerization of different vinyl monomers onto chitosan (Cs) and its derivatives can alter their properties and consequently expands their potential applications. In this study, carboxymethyl chitosan (CMCs) was prepared and characterized. Graft copolymerization of acrylic acid sodium salt (AAs) onto CMCs was accomplished by using ammonium persulphate (APS)-induced free radical polymerization in aqueous medium under nitrogen atmosphere. Occurrence of grafting was confirmed and the effects of [AAs], [APS], reaction time, and temperature on the extent of grafting were studied. The influence of grafting yield on the structure and dynamics was also investigated by using differential scanning calorimetry (DSC), two-dimensional wide-angle X-ray scattering (2D-WAXS), scanning electron microscopy (SEM), and dielectric spectroscopy. Preliminary study, using dielectric spectroscopy, was carried out to the CMCs-g-AAs copolymer with grafting percent ∼ 2900%, as an example, in a wide range of temperature (from 299 to 473 K) and frequency (from 1 × 10−2 to 1 × 105 Hz). One relaxation process with Arrhenius type was detected. This process has the characteristics of a secondary relaxation process related to local chain dynamics with activation energy of 53 ± 3 kJ/mol. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
Journal of Chemical Physics, Sep 18, 2009
The kinetics of phase transformation from the liquid crystalline mesophase to the crystalline pha... more The kinetics of phase transformation from the liquid crystalline mesophase to the crystalline phase has been studied in a model dipole-functionalized hexabenzocoronene derivative by x-ray diffraction, solid state NMR recoupling techniques, and dielectric spectroscopy. These probes revealed long-lived metastability and an intermediate state involving a change in the unit cell prior to crystallization. The barrier properties were dominant and amount to 1 and 2.5 eV, respectively, for the unit cell transformation and crystallization processes. These effects should be considered in the design of discotic liquid crystals for electronic applications.
Journal of Materials Science: Materials in Electronics, Oct 22, 2019
Electrodeposition technique is employed to prepare cuprous oxide (Cu2O) thin film on fluorine-dop... more Electrodeposition technique is employed to prepare cuprous oxide (Cu2O) thin film on fluorine-doped tin oxide (FTO) conducting glass substrate through the reduction of copper lactate in alkaline solution at pH = 12.25. Structural, optical and dielectric properties of the prepared film is investigated by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), UV–Visible absorbance, photoluminescence (PL) and broadband dielectric spectroscopy (BDS). The structural means (XRD, SEM and EDS) revealed the formation of self-assembled cubic microstructure of Cu2O with average grain size of around 1.5 μm. The UV–Vis absorbance spectrum gives optical band gap of 2.05 eV. The PL spectrums confirmed the presence of defect centers ascribed to various forms of oxygen (VO1+,VO2+)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(V_{O}^{1 + } ,\,V_{O}^{2 + } )$$\end{document} and copper (VCu1+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$V_{Cu}^{ 1 + }$$\end{document}) vacancies which are responsible for the conduction in the Cu2O film. The conduction mechanism in the Cu2O film is successfully described by the correlated barrier hopping (CBH) model in which bipolaron hopping become prominent. The density of defect states N, the effective barrier height W and the hopping distance Rω are also calculated based on the CBH model. Two dielectric relaxation processes (β1 and β2) with Arrhenius temperature dependence and activation energies of 0.31 and 0.48 eV are observed. The fast β2-relaxation process with activation energy of 0.48 eV is attributed to the Maxwell–Wagner-Sillars (MWS) polarization while the slow β1-relaxation process with activation energy of 0.31 eV is due to the hopping of the oxygen and copper vacancies.
Physical Review Letters, Mar 14, 2008
The molecular dynamics of three dipole functionalized hexa-peri-hexabenzocoronenes have been stud... more The molecular dynamics of three dipole functionalized hexa-peri-hexabenzocoronenes have been studied using site-specific NMR techniques and dielectric spectroscopy as a function of temperature and pressure. These probes (i) suggest that the thermodynamic state completely controls the dynamic response, (ii) clarify the origin of two dynamic processes associated with the presence of two glass temperatures, and (iii) provide the first phase diagram for substances of this kind.
Journal of the American Chemical Society, Mar 7, 2008
The self-assembly, the molecular dynamics, and the kinetics of structure formation are studied in... more The self-assembly, the molecular dynamics, and the kinetics of structure formation are studied in dipole-functionalized hexabenzocoronene (HBC) derivatives. Dipole substitution destabilizes the columnar crystalline phase except for the dimethoxy- and monoethynyl-substituted HBCs that undergo a reversible transformation to the crystalline phase. The disk dynamics are studied by dielectric spectroscopy and site-specific NMR techniques that provide both the time-scale and geometry of motion. Application of pressure results in the thermodynamic phase diagram that shows increasing stability of the crystalline phase at elevated pressures. Long-lived metastability was found during the transformation between the two phases. These results suggest new thermodynamic and kinetic pathways that favor the phase with the highest charge carrier mobility.
Journal of Physical Chemistry B, Nov 9, 2012
Biphasic fluorocarbon/hydrocarbon amphiphiles tethered to cores at distances commensurate with th... more Biphasic fluorocarbon/hydrocarbon amphiphiles tethered to cores at distances commensurate with their packing requirement can provide thermodynamic pathways toward equilibrium. This contrasts with the analogous semifluorinated alkanes. The dynamics of a fluorous biphasic hexa(3,5substituted-phenyl)benzene (HPB) is studied with dielectric spectroscopy as a function of temperature and pressure in comparison to the parent biphasic diphenylacetylene (DPA). Dielectric spectroscopy is a sensitive probe of the fluorocarbon environment through the end C−F dipole. Four dielectrically active processes were observed that associate with the CF 3 environment within the different phases (isotropic, liquid-like lamellar, solid lamellar, glassy state). Pressure facilitates the construction of the equilibrium phase diagram. The kinetic pathways to fluorocarbon organization are explored by pressure-jump experiments. A highly cooperative process was found that is atypical of a nucleation and growth process expected for first-order transitions.
The Journal of Physical Chemistry, 2012
Biphasic fluorocarbon/hydrocarbon amphiphiles tethered to cores at distances commensurate with th... more Biphasic fluorocarbon/hydrocarbon amphiphiles tethered to cores at distances commensurate with their packing requirement can provide thermodynamic pathways toward equilibrium. This contrasts with the analogous semifluorinated alkanes. The dynamics of a fluorous biphasic hexa(3,5substituted-phenyl)benzene (HPB) is studied with dielectric spectroscopy as a function of temperature and pressure in comparison to the parent biphasic diphenylacetylene (DPA). Dielectric spectroscopy is a sensitive probe of the fluorocarbon environment through the end C−F dipole. Four dielectrically active processes were observed that associate with the CF 3 environment within the different phases (isotropic, liquid-like lamellar, solid lamellar, glassy state). Pressure facilitates the construction of the equilibrium phase diagram. The kinetic pathways to fluorocarbon organization are explored by pressure-jump experiments. A highly cooperative process was found that is atypical of a nucleation and growth process expected for first-order transitions.
Στην εργασία αυτή μελετήθηκε η αυτο-οργάνωση και η δυναμική μιας σειράς δισκόμορφων και ραβδόμορφ... more Στην εργασία αυτή μελετήθηκε η αυτο-οργάνωση και η δυναμική μιας σειράς δισκόμορφων και ραβδόμορφων υγρών κρυστάλλων, με εφαρμογές στα ηλεκτρονικά και σε εύκαμπτες οθόνες, με σκοπό να απαντηθούν ορισμένα ανοικτά επιστημονικά ερωτήματα. Για το σκοπό αυτό έγινε η σύνθεση μιας σειράς κατάλληλα τροποποιημένων (Mr. X. Dou/Prof. Müllen) με ηλεκτρικά δίπολα δισκόμορφων υγρών κρυστάλλων και μελετήθηκαν ως προς τη δομή (με χρήση σκέδασης ακτινών X, πυρηνικού μαγνητικού συντονισμού, διαφορικής θερμιδομετρίας σάρωσης, πολωτικής οπτικής μικροσκοπίας) και τη δυναμική (με χρήση διηλεκτρικής φασματοσκοπίας και πυρηνικού μαγνητικού συντονισμού). Τα ερωτήματα που απαντήθηκαν στην παρούσα εργασία είναι: (i) η επίδραση του δίπολου στη θερμοδυναμική σταθερότητα της κρυσταλλικής φάσης, (ii) η δυναμική των δίσκων στην κιονική φάση, (iii) το θερμοδυναμικό διάγραμμα ισορροπίας φάσεων καθώς και (iv) η κινητική της δημιουργίας της κρυσταλλικής φάσης μέσα από συγκεκριμένα «μονοπάτια» του διαγράμματος ισορροπί...
Macromolecules, Jul 6, 2006
Langmuir, May 26, 2016
In previous studies, the authors found that endgrafted layers of the weak polybase poly(2-vinylpy... more In previous studies, the authors found that endgrafted layers of the weak polybase poly(2-vinylpyridine) (P2VP) in aqueous solutions do not only swell and collapse if the pH value and salt concentration are varied but also exhibit a pH-and salinity-dependent adhesion to microsized silica spheres. For a better understanding of these effects, in situ force measurements using the AFM colloidal probe technique were applied to end-grafted P2VP layers of different grafting densities in NaCl solutions at pH 2.5. Although a mushroomto-brush transition could be seen in the dry state, the layers were in the brush regime in aqueous solutions at all NaCl concentrations and grafting densities. We observed an increase of the brush height with increasing grafting density and a salinity-dependent collapse and reswelling of the brushes. The adhesion between the P2VP layer and a silica sphere depended on both grafting density and salinity. At low salt concentrations, the adhesion reached its highest value at the intermediate grafting density and disappeared with denser brushes. Maximum adhesion was obtained for high NaCl concentrations and the lowest grafting density. From a detailed analysis of the experiments, we gained insight into chain stretching and density profiles under complex ionic conditions and into the mechanism of adhesion of polyelectrolytes to solid surfaces.
Journal of Materials Science: Materials in Electronics
Polymers for Advanced Technologies
Physical Review Letters, 2008
The molecular dynamics of three dipole functionalized hexa-peri-hexabenzocoronenes have been stud... more The molecular dynamics of three dipole functionalized hexa-peri-hexabenzocoronenes have been studied using site-specific NMR techniques and dielectric spectroscopy as a function of temperature and pressure. These probes (i) suggest that the thermodynamic state completely controls the dynamic response, (ii) clarify the origin of two dynamic processes associated with the presence of two glass temperatures, and (iii) provide the first phase diagram for substances of this kind.
Materials Chemistry and Physics, 2021
different anions (tetrafluoroborate [BF4] or trifluoromethanesulfonate [TfO] ) have been invest... more different anions (tetrafluoroborate [BF4] or trifluoromethanesulfonate [TfO] ) have been investigated by ATRFTIR,
NMR, TGA/DTA, UV–vis spectroscopy, agar-disk diffusion, minimum inhibitory concentration (MIC)
and minimum bactericidal concentration (MBC) assay. Structural investigations by ATR-FTIR and NMR spectroscopy
demonstrated that the [BMIM]+ cation exhibits hydrogen bonding with the [BF4] and [TfO] anions via
the C (2)-H and C (4, 5)-H of the imidazolium cation ring in addition to the protons of the alkyl and methyl
groups. UV–vis spectroscopy revealed an energy gaps of 3.79 ± 0.13 and 3.37 ± 0.04 eV for [BMIM][BF4] and
[BMIM][TfO], respectively. Single term Sellmeier oscillator model and the Drude-Lorentz model were employed
for the first time to investigate the optical and dielectric properties of ionic liquids. Applying the former model,
enables us to extract the average inter-band oscillator wavelength (λ0), the average oscillator strength (S0), single
oscillator energy (E0), dispersion energy (Ed) while the latter gives the plasmon frequency (ωp), reduced resonance
frequency (ω0) and the electron lifetime at femtosecond level. Antibacterial assay by MIC and MBC
demonstrated that both ionic liquids have antibacterial activity against gram-positive and gram-negative bacterial
strains.
Journal of Materials Science: Materials in Electronics
The European Physical Journal Special Topics, 2007
In this work, the effect of confinement on the local, the segmental and the ionic processes of po... more In this work, the effect of confinement on the local, the segmental and the ionic processes of poly(ethylene oxide)/montmorillonite (PEO/MMT) intercalates is investigated. X-ray diffraction and DSC data show that the intercalated PEO chains remain amorphous and it is only the excess PEO chains outside the galleries that crystallize. This is also evident in the behavior of the elastic intensity of quasi-elastic neutron scattering (QENS) experiments on the same hybrids. Dielectric relaxation spectroscopy reveals that confinement results in an acceleration of the PEO segmental dynamics that display an Arrhenius temperature dependence with low activation energy, whereas the local beta-process remains unaffected.
ChemPhysChem, 2006
The optical properties, self-assembly mechanism, thermal properties, and the associated molecular... more The optical properties, self-assembly mechanism, thermal properties, and the associated molecular dynamics of a series of stable blue-emitting oligoindenofluorenes up to the polymer were investigated by photoluminescence, wide-angle X-ray scattering, polarizing optical microscopy, differential scanning calorimetry, and dielectric spectroscopy. Oligomers of indenofluorenes possess high structural order and form smectic mesophases, as opposed to the nematic mesophases formed in the corresponding oligofluorenes. The alpha process associated with the liquid-to-glass transition displays an asymmetric broadening of relaxation times (non-Debye process), with a non-Arrhenius temperature dependence of relaxation times and a strong molecular weight dependence. The low dielectric loss and low ionic conductivity revealed the absence of ketone structural defects, which is consistent with the observed stable blue emission.
Journal of Physics: Condensed Matter, May 29, 2008
The molecular dynamics of discotic liquid crystalline hexa-peri-hexabenzocoronenes (HBC-C10,6 and... more The molecular dynamics of discotic liquid crystalline hexa-peri-hexabenzocoronenes (HBC-C10,6 and HBC-C14,10) with long alkyl side chains branched in close vicinity to the HBC aromatic core were studied by dielectric spectroscopy. The two compounds display a weak dipolar relaxation (alpha-process) manifested by a Vogel-Fulcher-Tammann (VFT) dependence and have almost the same glass temperature (~215 K) in excellent agreement with the value
Journal of Applied Polymer Science, Jun 11, 2010
ABSTRACT Graft copolymerization of different vinyl monomers onto chitosan (Cs) and its derivative... more ABSTRACT Graft copolymerization of different vinyl monomers onto chitosan (Cs) and its derivatives can alter their properties and consequently expands their potential applications. In this study, carboxymethyl chitosan (CMCs) was prepared and characterized. Graft copolymerization of acrylic acid sodium salt (AAs) onto CMCs was accomplished by using ammonium persulphate (APS)-induced free radical polymerization in aqueous medium under nitrogen atmosphere. Occurrence of grafting was confirmed and the effects of [AAs], [APS], reaction time, and temperature on the extent of grafting were studied. The influence of grafting yield on the structure and dynamics was also investigated by using differential scanning calorimetry (DSC), two-dimensional wide-angle X-ray scattering (2D-WAXS), scanning electron microscopy (SEM), and dielectric spectroscopy. Preliminary study, using dielectric spectroscopy, was carried out to the CMCs-g-AAs copolymer with grafting percent ∼ 2900%, as an example, in a wide range of temperature (from 299 to 473 K) and frequency (from 1 × 10−2 to 1 × 105 Hz). One relaxation process with Arrhenius type was detected. This process has the characteristics of a secondary relaxation process related to local chain dynamics with activation energy of 53 ± 3 kJ/mol. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
Journal of Chemical Physics, Sep 18, 2009
The kinetics of phase transformation from the liquid crystalline mesophase to the crystalline pha... more The kinetics of phase transformation from the liquid crystalline mesophase to the crystalline phase has been studied in a model dipole-functionalized hexabenzocoronene derivative by x-ray diffraction, solid state NMR recoupling techniques, and dielectric spectroscopy. These probes revealed long-lived metastability and an intermediate state involving a change in the unit cell prior to crystallization. The barrier properties were dominant and amount to 1 and 2.5 eV, respectively, for the unit cell transformation and crystallization processes. These effects should be considered in the design of discotic liquid crystals for electronic applications.
Journal of Materials Science: Materials in Electronics, Oct 22, 2019
Electrodeposition technique is employed to prepare cuprous oxide (Cu2O) thin film on fluorine-dop... more Electrodeposition technique is employed to prepare cuprous oxide (Cu2O) thin film on fluorine-doped tin oxide (FTO) conducting glass substrate through the reduction of copper lactate in alkaline solution at pH = 12.25. Structural, optical and dielectric properties of the prepared film is investigated by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), UV–Visible absorbance, photoluminescence (PL) and broadband dielectric spectroscopy (BDS). The structural means (XRD, SEM and EDS) revealed the formation of self-assembled cubic microstructure of Cu2O with average grain size of around 1.5 μm. The UV–Vis absorbance spectrum gives optical band gap of 2.05 eV. The PL spectrums confirmed the presence of defect centers ascribed to various forms of oxygen (VO1+,VO2+)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(V_{O}^{1 + } ,\,V_{O}^{2 + } )$$\end{document} and copper (VCu1+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$V_{Cu}^{ 1 + }$$\end{document}) vacancies which are responsible for the conduction in the Cu2O film. The conduction mechanism in the Cu2O film is successfully described by the correlated barrier hopping (CBH) model in which bipolaron hopping become prominent. The density of defect states N, the effective barrier height W and the hopping distance Rω are also calculated based on the CBH model. Two dielectric relaxation processes (β1 and β2) with Arrhenius temperature dependence and activation energies of 0.31 and 0.48 eV are observed. The fast β2-relaxation process with activation energy of 0.48 eV is attributed to the Maxwell–Wagner-Sillars (MWS) polarization while the slow β1-relaxation process with activation energy of 0.31 eV is due to the hopping of the oxygen and copper vacancies.
Physical Review Letters, Mar 14, 2008
The molecular dynamics of three dipole functionalized hexa-peri-hexabenzocoronenes have been stud... more The molecular dynamics of three dipole functionalized hexa-peri-hexabenzocoronenes have been studied using site-specific NMR techniques and dielectric spectroscopy as a function of temperature and pressure. These probes (i) suggest that the thermodynamic state completely controls the dynamic response, (ii) clarify the origin of two dynamic processes associated with the presence of two glass temperatures, and (iii) provide the first phase diagram for substances of this kind.
Journal of the American Chemical Society, Mar 7, 2008
The self-assembly, the molecular dynamics, and the kinetics of structure formation are studied in... more The self-assembly, the molecular dynamics, and the kinetics of structure formation are studied in dipole-functionalized hexabenzocoronene (HBC) derivatives. Dipole substitution destabilizes the columnar crystalline phase except for the dimethoxy- and monoethynyl-substituted HBCs that undergo a reversible transformation to the crystalline phase. The disk dynamics are studied by dielectric spectroscopy and site-specific NMR techniques that provide both the time-scale and geometry of motion. Application of pressure results in the thermodynamic phase diagram that shows increasing stability of the crystalline phase at elevated pressures. Long-lived metastability was found during the transformation between the two phases. These results suggest new thermodynamic and kinetic pathways that favor the phase with the highest charge carrier mobility.
Journal of Physical Chemistry B, Nov 9, 2012
Biphasic fluorocarbon/hydrocarbon amphiphiles tethered to cores at distances commensurate with th... more Biphasic fluorocarbon/hydrocarbon amphiphiles tethered to cores at distances commensurate with their packing requirement can provide thermodynamic pathways toward equilibrium. This contrasts with the analogous semifluorinated alkanes. The dynamics of a fluorous biphasic hexa(3,5substituted-phenyl)benzene (HPB) is studied with dielectric spectroscopy as a function of temperature and pressure in comparison to the parent biphasic diphenylacetylene (DPA). Dielectric spectroscopy is a sensitive probe of the fluorocarbon environment through the end C−F dipole. Four dielectrically active processes were observed that associate with the CF 3 environment within the different phases (isotropic, liquid-like lamellar, solid lamellar, glassy state). Pressure facilitates the construction of the equilibrium phase diagram. The kinetic pathways to fluorocarbon organization are explored by pressure-jump experiments. A highly cooperative process was found that is atypical of a nucleation and growth process expected for first-order transitions.
The Journal of Physical Chemistry, 2012
Biphasic fluorocarbon/hydrocarbon amphiphiles tethered to cores at distances commensurate with th... more Biphasic fluorocarbon/hydrocarbon amphiphiles tethered to cores at distances commensurate with their packing requirement can provide thermodynamic pathways toward equilibrium. This contrasts with the analogous semifluorinated alkanes. The dynamics of a fluorous biphasic hexa(3,5substituted-phenyl)benzene (HPB) is studied with dielectric spectroscopy as a function of temperature and pressure in comparison to the parent biphasic diphenylacetylene (DPA). Dielectric spectroscopy is a sensitive probe of the fluorocarbon environment through the end C−F dipole. Four dielectrically active processes were observed that associate with the CF 3 environment within the different phases (isotropic, liquid-like lamellar, solid lamellar, glassy state). Pressure facilitates the construction of the equilibrium phase diagram. The kinetic pathways to fluorocarbon organization are explored by pressure-jump experiments. A highly cooperative process was found that is atypical of a nucleation and growth process expected for first-order transitions.
Στην εργασία αυτή μελετήθηκε η αυτο-οργάνωση και η δυναμική μιας σειράς δισκόμορφων και ραβδόμορφ... more Στην εργασία αυτή μελετήθηκε η αυτο-οργάνωση και η δυναμική μιας σειράς δισκόμορφων και ραβδόμορφων υγρών κρυστάλλων, με εφαρμογές στα ηλεκτρονικά και σε εύκαμπτες οθόνες, με σκοπό να απαντηθούν ορισμένα ανοικτά επιστημονικά ερωτήματα. Για το σκοπό αυτό έγινε η σύνθεση μιας σειράς κατάλληλα τροποποιημένων (Mr. X. Dou/Prof. Müllen) με ηλεκτρικά δίπολα δισκόμορφων υγρών κρυστάλλων και μελετήθηκαν ως προς τη δομή (με χρήση σκέδασης ακτινών X, πυρηνικού μαγνητικού συντονισμού, διαφορικής θερμιδομετρίας σάρωσης, πολωτικής οπτικής μικροσκοπίας) και τη δυναμική (με χρήση διηλεκτρικής φασματοσκοπίας και πυρηνικού μαγνητικού συντονισμού). Τα ερωτήματα που απαντήθηκαν στην παρούσα εργασία είναι: (i) η επίδραση του δίπολου στη θερμοδυναμική σταθερότητα της κρυσταλλικής φάσης, (ii) η δυναμική των δίσκων στην κιονική φάση, (iii) το θερμοδυναμικό διάγραμμα ισορροπίας φάσεων καθώς και (iv) η κινητική της δημιουργίας της κρυσταλλικής φάσης μέσα από συγκεκριμένα «μονοπάτια» του διαγράμματος ισορροπί...
Macromolecules, Jul 6, 2006
Langmuir, May 26, 2016
In previous studies, the authors found that endgrafted layers of the weak polybase poly(2-vinylpy... more In previous studies, the authors found that endgrafted layers of the weak polybase poly(2-vinylpyridine) (P2VP) in aqueous solutions do not only swell and collapse if the pH value and salt concentration are varied but also exhibit a pH-and salinity-dependent adhesion to microsized silica spheres. For a better understanding of these effects, in situ force measurements using the AFM colloidal probe technique were applied to end-grafted P2VP layers of different grafting densities in NaCl solutions at pH 2.5. Although a mushroomto-brush transition could be seen in the dry state, the layers were in the brush regime in aqueous solutions at all NaCl concentrations and grafting densities. We observed an increase of the brush height with increasing grafting density and a salinity-dependent collapse and reswelling of the brushes. The adhesion between the P2VP layer and a silica sphere depended on both grafting density and salinity. At low salt concentrations, the adhesion reached its highest value at the intermediate grafting density and disappeared with denser brushes. Maximum adhesion was obtained for high NaCl concentrations and the lowest grafting density. From a detailed analysis of the experiments, we gained insight into chain stretching and density profiles under complex ionic conditions and into the mechanism of adhesion of polyelectrolytes to solid surfaces.
Journal of Materials Science: Materials in Electronics
Polymers for Advanced Technologies
Physical Review Letters, 2008
The molecular dynamics of three dipole functionalized hexa-peri-hexabenzocoronenes have been stud... more The molecular dynamics of three dipole functionalized hexa-peri-hexabenzocoronenes have been studied using site-specific NMR techniques and dielectric spectroscopy as a function of temperature and pressure. These probes (i) suggest that the thermodynamic state completely controls the dynamic response, (ii) clarify the origin of two dynamic processes associated with the presence of two glass temperatures, and (iii) provide the first phase diagram for substances of this kind.
Materials Chemistry and Physics, 2021
different anions (tetrafluoroborate [BF4] or trifluoromethanesulfonate [TfO] ) have been invest... more different anions (tetrafluoroborate [BF4] or trifluoromethanesulfonate [TfO] ) have been investigated by ATRFTIR,
NMR, TGA/DTA, UV–vis spectroscopy, agar-disk diffusion, minimum inhibitory concentration (MIC)
and minimum bactericidal concentration (MBC) assay. Structural investigations by ATR-FTIR and NMR spectroscopy
demonstrated that the [BMIM]+ cation exhibits hydrogen bonding with the [BF4] and [TfO] anions via
the C (2)-H and C (4, 5)-H of the imidazolium cation ring in addition to the protons of the alkyl and methyl
groups. UV–vis spectroscopy revealed an energy gaps of 3.79 ± 0.13 and 3.37 ± 0.04 eV for [BMIM][BF4] and
[BMIM][TfO], respectively. Single term Sellmeier oscillator model and the Drude-Lorentz model were employed
for the first time to investigate the optical and dielectric properties of ionic liquids. Applying the former model,
enables us to extract the average inter-band oscillator wavelength (λ0), the average oscillator strength (S0), single
oscillator energy (E0), dispersion energy (Ed) while the latter gives the plasmon frequency (ωp), reduced resonance
frequency (ω0) and the electron lifetime at femtosecond level. Antibacterial assay by MIC and MBC
demonstrated that both ionic liquids have antibacterial activity against gram-positive and gram-negative bacterial
strains.
Journal of Materials Science: Materials in Electronics
The European Physical Journal Special Topics, 2007
In this work, the effect of confinement on the local, the segmental and the ionic processes of po... more In this work, the effect of confinement on the local, the segmental and the ionic processes of poly(ethylene oxide)/montmorillonite (PEO/MMT) intercalates is investigated. X-ray diffraction and DSC data show that the intercalated PEO chains remain amorphous and it is only the excess PEO chains outside the galleries that crystallize. This is also evident in the behavior of the elastic intensity of quasi-elastic neutron scattering (QENS) experiments on the same hybrids. Dielectric relaxation spectroscopy reveals that confinement results in an acceleration of the PEO segmental dynamics that display an Arrhenius temperature dependence with low activation energy, whereas the local beta-process remains unaffected.
ChemPhysChem, 2006
The optical properties, self-assembly mechanism, thermal properties, and the associated molecular... more The optical properties, self-assembly mechanism, thermal properties, and the associated molecular dynamics of a series of stable blue-emitting oligoindenofluorenes up to the polymer were investigated by photoluminescence, wide-angle X-ray scattering, polarizing optical microscopy, differential scanning calorimetry, and dielectric spectroscopy. Oligomers of indenofluorenes possess high structural order and form smectic mesophases, as opposed to the nematic mesophases formed in the corresponding oligofluorenes. The alpha process associated with the liquid-to-glass transition displays an asymmetric broadening of relaxation times (non-Debye process), with a non-Arrhenius temperature dependence of relaxation times and a strong molecular weight dependence. The low dielectric loss and low ionic conductivity revealed the absence of ketone structural defects, which is consistent with the observed stable blue emission.