Relaxations IN amorphous and semi-crystalline polyesters

Thermally stimulated depolarization currents and differential scanning calorimetry are performed on thermoplastic polyesters to characterize both a and b relaxations. The influence on the different relaxations phenomena of the chemical structure (size of the naphthalene groups, presence of cyclohexane, length of the aliphatic group, ...) as well as the influence of the crystallinity are discussed. The three phases model with a crystalline part, a rigid amorphous part unable to relax and an amorphous phase able to relax at various temperatures depending on the distribution of the relaxation times is used to explain the evolution of the main α relaxation while the standard two-phases model is sufficient to explain the variations of the β relaxation mode. Elementary analysis of both α and β relaxations show that the β relaxation characterized by a continuous variation of activation energies as a function of temperature follows the activated state equation with a zero activation entropy while the cooperative a relaxation exhibits a prominent maximum of the activation energies at the glass transition temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

A donor-acceptor molecular dyad showing multiple electronic energy-transfer processes in crystalline and amorphous states

Singlet-singlet, singlet-triplet, and triplet-triplet energy transfer takes place within single crystals and amorphous solid-state solutions of a molecular dyad comprising boron dipyrromethene and oligo-thiophene subunits. The crystal and sublimed thin-films are strongly fluorescent.     

A comparative molecular dynamics study of crystalline,paracrystalline and amorphous states of cellulose

The quintessential form of cellulose in wood consists of microfibrils that have high aspect ratio crystalline domains embedded within an amorphous cellulose domain. In this study, we apply united-atom molecular dynamics simulations to quantify changes in different morphologies of cellulose. We compare the structure of crystalline cellulose with paracrystalline and amorphous phases that are both obtained by high temperature equilibration followed by quenching at room temperature. Our study reveals that the paracrystalline phase may be an intermediate, kinetically arrested phase formed upon amorphisation of crystalline cellulose. The quenched structures yield isotropic amorphous polymer domains consistent with experimental results, thereby validating a new computational protocol for achieving amorphous cellulose structure. The non-crystalline cellulose compared to crystalline structure is characterized by a dramatic decrease in elastic modulus, thermal expansion coefficient, bond energies, and number of hydrogen bonds. Analysis of the lattice parameters shows that Iβ cellulose undergoes a phase transition into high-temperature phase in the range of 450–550 K. The mechanisms of the phase transition elucidated here present an atomistic view of the temperature dependent dynamic structure and mechanical properties of cellulose. The paracrystalline state of cellulose exhibits intermediate mechanical properties, between crystalline and amorphous phases, that can be assigned to the physical properties of the interphase regions between crystalline and amorphous cellulose in wood microfibrils. Our results suggest an atomistic structural view of amorphous cellulose which is consistent with experimental data available up to date and provide a basis for future multi-scale models for wood microfibrils and all-cellulose nanocomposites.     

Non-isothermal crystallization in amorphous selenium films

The usual determination of kinetic parameters of crystallization of amorphous products is based on isothermal measurements. In general the crystallization of amorphous selenium thin films is studied by non-isothermal experiments (DTA). The adaptation to non-isothermal crystallization of the Avrami transformation rate equation allows us to determine different types of crystallization. These different regions enable one to determine the variations of the growth rate and of the nucleation rate versus temperature. The influence of the wavelength of illumination during the crystallization time on these parameters is also investigated.

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Zusammenfassung Die übliche Bestimmung kinetischer Parameter der Kristallisation amorpher Produkte basiert auf isothermen Messungen. Die Kristallisation von amorphem Selen in dünnen Schichten wird im allgemeinen in nicht-isothermen Experimenten (DTA) untersucht. Die Adaptation der Transformationsgeschwindigkeitsgleichung nach Avrami an die nicht-isotherme Kristallisation ermöglicht die Bestimmung verschiedener Kristallisationtypen. Dadurch wird es möglich, die Veränderungen der Wachstumsgeschwindigkeit und der Geschwindigkeit der Nukleation in Abhängigkeit von der Temperatur zu bestimmen. Der Einfluß der Wellenlänge bei der Bestrahlung während der Kristallisation auf diese Parameter wird ebenfalls untersucht.

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Relaxations in liquid crystalline poly(tetraethylene oxide terephthaloyl-bis-4-oxybenzoate)

The relaxation behavior of poly(tetra ethylene oxide terephthaloyl-bis-4-oxybenzoate), PTETOB, was analyzed by thermally stimulated depolarization currents, TSDC, and dynamic mechanical techniques, DMTA, and the results compared with those obtained by differential scanning calcrimetry, thermo-optical analysis, and x-ray diffraction. In the low temperature region, ?173–30°C, three main transitions were observed and assigned to the γ relaxation, the glass transition of the mesophase and the glass transition temperature of the amorphous material. The complex behavior observed in the range 110–160°C was as signed to a crystal-crystal transition which competed with the formation of a mesophase and afterward the formation of a smectic A mesophase. At higher temperatures, was observed the transition from the smectic A mesophase to a nematic one, prior to the isotropization temperature. In the TSDC experiments the formation of a permanent electret was detected and the charges trapped in the mesophase were canceled only at the isotropization temperature. © 1995 John Wiley & Sons, Inc.     

Photoresponsive liquid crystalline and amorphous polymers

Isomerization processes of azobenzene dyes dissolved in a glassy polymeric matrix or attached in glassy amorphous or liquid crystalline polymers to the backbone as side groups are induced by light. The isomerization process, in turn, causes the dye to reorient provided that polarized light is used: the long axis of the dye is oriented perpendicular to the polarization direction in the stationary case. Such a reorientation gives rise to strong modifications of the optical properties. This contribution is concerned with the analysis of the correlation between the nature of the azobenzene dyes, the isomerization, reorientation and modulations discussed above and with possible applications in the optical holographic storage. Considered are, in particular, dye/matrix combinations giving rise to nonlinear holographic responses, two photon holography, transient holographic modes applicable for holographic displays and the optical switching of other than optical properties.     

Surface hydration of crystalline and amorphous silicas

The surface properties of three SiO₂ samples, one crystalline (quartz) and two amorphous, with a large difference in particle size, have been investigated by thermal analysis, adsorption calorimetry and infrared spectroscopy. The variation in the silanol group population upon thermal treatment has been followed via the evolution of the i.r. bands at 3745–3750 cm^–1 (free hydroxyl groups) and 3650-3550 cm^–1 (adjacent pairs of SiOH) and the evolution of the heat of adsorption of water with coverage on the outgassed samples.The adsorption capacities increase in the sequence high surface area amorphous sample T=673 K) brings about dehydroxylation leaving only isolated silanols on the high surface area amorphous silica, and partial dehydroxylation of the low surface area material. Quartz is totally hydrophilic, as its regular structure probably stabilizes the hydroxyl layers at the surface.

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Zusammenfassung Die Oberflächeneigenschaften von einer kristallinen (Quarz) und zwei amorphen SiO₂-Proben mit großen Unterschieden in der Partikelgröße wurden durch thermische Analyse, Adsorptionskalorimetrie und Infrarotspektroskopie untersucht. Bei thermischer Behandlung eintretende Veränderungen in der Silanolgruppenpopulation wurden durch Messung der IR-Banden bei 3745–3750 cm^–1 (freie Hydroxylgruppen) und 3650–3550 cm^–1 (benachbarte SiOH-Paare) und der Adsorptionswärme von Wasser an ausgeheitzten Proben untersucht. Die Adsorptionskapazitäten nehmen in folgender Reihenfolge ab: amorphe Probe mit großer Oberfläche > amorphe Probe mit geringer Oberfläche > Quarz. Dehydroxylierung der Oberfläche hat zunehmenden hydrophoben Charakter (Gestalt der Isothermen, Adsorptionswärme kleiner als Kondensationswärme von Wasser) zur Folge. Bei der gleichen thermischen Behandlung (673 K) werden bei amorphen Siliciumdioxid mit großer Oberfläche eine nur isolierte Hydroxylgruppen zurücklassende Dehydroxylierung und bei Material mit kleiner Oberfläche eine teilweise Dehydroxylierung beobachtet. Quarz ist vollkommen hydrophyl, da die reguläre Struktur wahrscheinlich die Hydroxylschichten an der Oberfläche stabilisiert.

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, , (), — . 3745–3750 ^–1 ( ) 3650–3550 ^–1 ( SiOH), . < < . ( , ). (T=673 K) . , , , .

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This research was supported by the Ministry of Public Education within a national programme relating to the structure and reactivity of surfaces.     

Volume photodeposition processes in amorphous selenium hydrosols

A photodeposition process adapted to the preparation of thin films of amorphous Selenium (a-Se) has been investigated previously. This communication gives the results of kinetic experiments performed on the a-Se-hydrosol under photoexcitation. The investigation reveals two distinct effects occurring in the bulk of the irradiated hydrosol i. e., photoprecipitation and photocoagulation. This result allows us to enhance our understanding of the photodeposition process.     

Crystallization and shape evolution of single crystalline selenium nanorods at liquid-liquid interface: from monodisperse amorphous Se nanospheres toward Se nanorods

The reduction of selenious acid solution with hydrazine hydrate in the presence of poly(vinylpyrrolidone) (PVP) can produce a stable dispersion of uniform and amorphous selenium particles capped with PVP with a size of 100 nm. Further addition of a solvent with low polarity such as n-butyl alcohol into this aqueous solution and mild stirring result in the transportation of amorphous selenium particles onto a liquid-liquid interface between water and n-butyl alcohol. Subsequent crystallization and shape evolution on this interface occurred and finally resulted in the formation of single crystalline selenium nanorods. The results demonstrated that the enrichment of nanoparticles with amphiphilic property at a liquid-liquid interface between a polar solvent and another solvent of low polarity can result in crystallization and phase transformation for the formation of nanostructures.     

Polarized Raman studies of crystalline and amorphous orientation in polyethylene

Polarized Raman intensities have been obtained from thin films of oriented low-density polyethylene (PE) immersed in silicone oil to minimize surface scattering. Studies were made using the 1170 cm^?1 crystalline band and the 1081 cm^?1 amorphous band, and from these the orientation averages 〈cos² θ〉 and 〈cos⁴ θ〉 were calculated. These were found to compare favorably with the values of 〈cos² θ〉 for the polymer chain in the crystalline and amorphous phases obtained from measurements of infrared dichroism. Both orientation averages could be theoretically fitted by using reasonable parameters.     

Internal structure rebuilding reactions of crystalline and amorphous solids

Structural mechanism of the internal thermal reactions of dehydration and dehydroxylation as of solids well as crystallization of their amorphous products and glasses are considered. They are multi-stage processes, realized through small translative displacements of polymerized anion network elements and diffusional shift of cations. It is the way to equilibrium state of the lowest energy consumption and the least energetic barrier to overcome.

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Zusammenfassung Es werden Strukturmechanismen von internen thermischen Reaktionen von Dehydratation und Dehydroxylierung sowie der Kristallisierung der entstehenden amorphen Reaktionsprodukte betrachtet. Dabei handelt es sich um mehrstufige Prozesse, die über kleine translative Verschiebungen von polimerisierten Elementen des Anionennetzwerkes und durch Diffusionsverschiebung von Kationen ablaufen. Dies ist der Weg zum Gleichgewichtszustand mit dem geringsten Energieverbrauch und der niedrigsten, zu überwindenden Energiebarriere.

     

Sticking of CO to crystalline and amorphous ice surfaces

We present results of classical trajectory calculations on the sticking of hyperthermal CO to the basal plane (0001) face of crystalline ice Ih and to the surface of amorphous ice Ia. The calculations were performed for normal incidence at a surface temperature Ts = 90 K for ice Ia, and at Ts = 90 and 150 K for ice Ih. For both surfaces, the sticking probability can be fitted to a simple exponentially decaying function of the incidence energy, Ei: Ps = 1.0e(-Ei(kJ/mol)/90(kJ/mol)) at Ts = 90 K. The energy transfer from the impinging molecule to the crystalline and the amorphous surface is found to be quite efficient, in agreement with the results of molecular beam experiments on the scattering of the similar molecule, N2, from crystalline and amorphous ice. However, the energy transfer is less efficient for amorphous than for crystalline ice. Our calculations predict that the sticking probability decreases with Ts for CO scattering from crystalline ice, as the energy transfer from the impinging molecule to the warmer surfaces becomes less efficient. At high Ei (up to 193 kJ/mol), no surface penetration occurs in the case of crystalline ice. However, for CO colliding with the amorphous surface, a penetrating trajectory was observed to occur into a large water pore. The molecular dynamics calculations predict that the average potential energy of CO adsorbed to ice Ih is -10.1 +/- 0.2 and -8.4 +/- 0.2 kJ/mol for CO adsorbed to ice Ia. These values are in agreement with previous experimental and theoretical data. The distribution of the potential energy of CO adsorbed to ice Ia was found to be wider (with a standard deviation sigma of 2.4 kJ/mol) than that of CO interacting with ice Ih (sigma = 2.0 kJ/mol). In collisions with ice Ia, the CO molecules scatter at larger angles and over a wider distribution of angles than in collisions with ice Ih.     

Self-assembling behavior of amphiphilic dendron coils in the bulk crystalline and liquid crystalline states

We prepared a series of amphiphilic dendron coils (1-3) containing aliphatic polyether dendrons with octadecyl peripheries and a poly(ethylene oxide) (PEO) coil (DP = 44). The molecular design in this study is focused on the variation of dendron generation (from first to third) with a fixed linear coil, upon which the thermal and self-assembling behavior of the dendron coils was investigated in the bulk. All the dendron coils exhibit two crystalline phases designated as k1 (both crystalline octadecyl chains and PEO) and k2 states (crystalline octadecyl chains and molten PEO). Crystallinities for both octadecyl peripheries and the PEO decrease as generation increases. In particular, the dendron coil (3) containing third generation shows a drastic reduction of the PEO crystallinity, which is attributed to the considerable chain folding and plasticization effects by the largest hydrophilic dendritic core segment. All the crystalline phases are bilayered lamellar morphologies. On going from k1 to k2, the periodic lamellar thickness decreases in the dendron coil (1) with first generation, but interestingly increases in 3. After melting of octadecyl peripheries, 1 shows no mesophase (i.e., liquid crystalline phase). Additionally, dendron coil 2 (3) displays a network cubic mesophase with Ia3d symmetry (micellar cubic with Pm3n) which is transformed into a lamellar (hexagonal columnar) mesophase upon heating. Remarkably, the temperature-dependent mesomorphic behavior in 2 and 3 is a completely reverse pattern in comparison with conventional linear-linear block copolymers. The unusual bulk morphological phenomena in the crystalline and liquid crystalline phases can be elucidated by the dendron coil architecture and the associated coil conformational energy.     

Application of thin-film DTA to amorphous selenium layers

A system of evaporated thin-film Ag-Bi thermocouples was used as a differential temperature sensor for DTA measurements. The crystallization of thin amorphous selenium layers was observed by means of this method. The presented method may be useful in studying the thermal transformations of thin films that have different compositions and various applications.

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Zusammenfassung Das System von aufgedampften Ag-Bi Dünnschicht-Thermoelementen wurde als Differentialtemperatursensor zu DTA-Messungen eingesetzt. Die Kristallisation dünner amorpher Selenschichten wurde durch diese Methode verfolgt. Die beschriebene Methode kann bei der Untersuchung thermischer Umwandlungen der dünnen Schichten verschiedener Zusammensetzungen Anwendung finden.

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Résumé Des thermocouples en films minces évaporés Ag-Bi ont été utilisés pour la détection de la différence de température en ATD. On a observé, par cette méthode, la cristallisation des couches minces de sélénium amorphe. La méthode présentée peut être utile dans l'étude des transformations thermiques des pellicules minces de compositions différentes et d'applications diverses.

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Ag-Bi , . . .

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The work was financially supported by the Ministry of Science, Higher Education and Technology, according to Contract No. MR-I-21.     

Interaction of amorphous and crystalline nickel nanoparticles with hydrogen

Nickel nanoparticles were deposited on the surface of highly oriented pyrolytical graphite by laser electrodispersion and precipitation from a solution of reverse micelles. The particles were studied using scanning tunneling microscopy and spectroscopy as well as Auger spectroscopy. The elemental composition of the nanoparticles and specific features of the electronic structure of their surface were determined. Adsorption of molecular hydrogen was also studied.     

ESCA investigations of amorphous and crystalline zirconium acid phosphates

Amorphous and micro-crystalline α- and γ-zirconium acid phosphates have been investigated by the ESCA technique. α-zirconium phosphate has been also studied as a single crystal. Both the B.E. of zirconium and phosphorous electrons in zirconium phosphates are slightly higher than those reported for zirconium derivatives and trivalent metal phosphates, indicating a stronger polarization of the ZrO and PO bonds. Furthermore, it seems that some ESCA features of the zirconium phosphates investigated are structure dependent. However, no definitive conclusions have been drawn because of the lack of knowledge of the “charging-up” phenomena in protonic conductors such as zirconium phosphates. Satellite structures have been observed in the P_2s, P_2p, Zr_3d and Zr_4p regions. The position and the intensity of these “shake-up” effects are again dependent on the nature of the examined phase and they seem to be due to the presence of electron defect structures induced by X-rays or already present in the original samples.     

Metastable states in antiferroelectric liquid crystalline mixtures

We report on the observation of relaxation phenomena with extremely long relaxation times, amounting to several hours. These effects take place in some liquid crystal mixtures exhibiting ferroelectric and antiferroelectric dipole order. The observed phenomena are connected with the transformation from the supercooled ferroelectric state to another, metastable state. This transition may be described using a Debye type relaxation formula. At low temperatures, a second slow transition takes place: from the metastable intermediate state to the antiferroelectric phase. This transition is characterized by unidimensional growth of the antiferroelectric domains with a constant velocity. Close to the lower temperature limit of existence of the ferroelectric phase, a direct transition from the ferroelectric to the antiferroelectric phase takes place. This transition is described by an Avrami model, hence it is governed by the creation and growth of nuclei of the antiferroelectric phase.     

Molecular-dynamics study of photodissociation of water in crystalline and amorphous ices

We present the results of classical dynamics calculations performed to study the photodissociation of water in crystalline and amorphous ice surfaces at a surface temperature of 10 K. A modified form of a recently developed potential model for the photodissociation of a water molecule in ice [S. Andersson et al., Chem. Phys. Lett. 408, 415 (2005)] is used. Dissociation in the top six monolayers is considered. Desorption of H(2)O has a low probability (less than 0.5% yield per absorbed photon) for both types of ice. The final outcome strongly depends on the original position of the photodissociated molecule. For molecules in the first bilayer of crystalline ice and the corresponding layers in amorphous ice, desorption of H atoms dominates. In the second bilayer H atom desorption, trapping of the H and OH fragments in the ice, and recombination of H and OH are of roughly equal importance. Deeper into the ice H atom desorption becomes less important and trapping and recombination dominate. Motion of the photofragments is somewhat more restricted in amorphous ice. The distribution of distances traveled by H atoms in the ice peaks at 6-7 Angstroms with a tail going to about 60 Angstroms for both types of ice. The mobility of OH radicals is low within the ice with most probable distances traveled of 2 and 1 Angstrom for crystalline and amorphous ices, respectively. OH is, however, quite mobile on top of the surface, where it has been found to travel more than 80 Angstroms. Simulated absorption spectra of crystalline ice, amorphous ice, and liquid water are found to be in very good agreement with the experiments. The outcomes of photodissociation in crystalline and amorphous ices are overall similar, but with some intriguing differences in detail. The probability of H atoms desorbing is 40% higher from amorphous than from crystalline ice and the kinetic-energy distribution of the H atoms is on average 30% hotter for amorphous ice. In contrast, the probability of desorption of OH radicals from crystalline ice is much higher than that from amorphous ice.