Search

open, In my PhD project, I investigated the photophysical process of photon upconversion assisted by triplet-triplet annihilation (sTTA-UC) through spectroscopy studies in a variety of systems, profoundly different on many levels. In sTTA-UC high energy radiation is emitted from the fluorescent recombination of the excited singlet of an emitter molecule, previously populated via annihilation of the metastable triplet states of two emitters. This is a sensitized process since a sensitizer is necessary to harvest the low energy incident light and to transfer the stored energy to the emitters via Dexter energy transfer. Because its functioning relies on long-lived metastable triplets, this process can be highly efficient also under low power, noncoherent light. As such, sTTA-UC is particularly suited for solar applications as it can increase the conversion efficiency by reducing transmission losses. During my studies, I focused on addressing two crucial issues that still limit the application of upconverters in solar technologies, i.e. the limited storage ability of common organic sensitizers and the poor sTTA-UC performance in solid-state upconverters, which are intrinsically better suited than liquid solutions for technological applications. To solve the first problem, I investigated hybrid sensitizers, composed of semiconductor nanostructures decorated with conjugated organic ligands characterized by broadband absorption. CdSe nanocrystals (NCs) doped with gold cations and decorated with 9-anthracene carboxylic acid demonstrated to be efficient innovative broadband hybrid sensitizers. The doping strategy inserts into the NCs energy gap localized hole-accepting states where the holes localize on the picosecond timescale, outpacing hole transfer to the ligand HOMO. With this strategy, I achieved the UC efficiency of 12%, the record performance obtained so far for hybrid upconverters. I then discussed how the CdSe nanoplatelets surface and photophysical properties make them, Durante il dottorato ho investigato il processo fotofisico di "upconversion" assistito da annichilazione tripletto-tripletto (TTA-UC) tramite studi di spettroscopia in sistemi profondamente differenti gli uni dagli altri. In TTA-UC radiazione ad alta energia è emessa dalla ricombinazione radiativa dello stato di singoletto eccitato di una molecola emettitore, popolato precedentemente dall'annichilazione dei tripletti di due emettitori. Un sensibilizzatore immagazzina la luce incidente a bassa energia e trasferisce l'eccitazione agli emettitori tramite trasferimento di energia alla Dexter. Poiché il suo funzionamento si basa su tripletti mestastabili, TTA-UC può essere altamente efficiente anche in condizioni di luce non coerente e a bassa energia. Come tale, è particolarmente adatto per dispositivi che sfruttano l'energia solare poiché è in grado di aumentarne l'efficienza di conversione limitando le perdite per trasmissione. Mi sono concentrata su due problemi importanti che tuttora limitano l'impiego di materiali che attuano TTA-UC (upconverters), ossia la limitata capacità di immagazzinare energia dei comuni sensibilizzatori organici e le scarse prestazioni di TTA-UC in upconverters a stato solido, i quali sono più adatti per applicazioni tecnologiche rispetto a sistemi liquidi. Per risolvere il primo problema ho investigato sensibilizzatori ibridi, composti da nanostrutture a semiconduttore decorate con molecole organiche, con ampio assorbimento. Nanocristalli di CdSe drogati con cationi d'oro e decorati con acido antracenico carbossilico si sono dimostrati essere sensibilizzatori ibridi efficienti ed innovativi. Il drogante introduce nel gap energetico dei nanocristalli livelli localizzati su cui le lacune si localizzano sulla scala dei picosecondi, più velocemente dell'estrazione di lacune sul livello HOMO dei leganti. Con tale strategia ho raggiunto l'efficienza di UC del 12%, record per sistemi ibridi. Ho poi mostrato come le proprietà superficiali e fotofis, 0, open, Ronchi, A

A new fitting model for sorption by glassy polymers is suggested based on the Flory–Huggins (FH) equation with a composite formula for the FH interaction parameter, χ, which is applicable if sorption experimental data shows a single-maximum variation of the FH parameter. Namely, a power-like and a linear approximation is assumed for χ(φ1), as a function of solvent volume fraction φ1, before and after the point of its maximum. After determining the maximum point from a direct inspection of the sorption data, the three fitting parameters are evaluated by solving two independent least-square minimization problems. Several sorption studies of biopolymers taken from the literature show that the endset of the glass transition region is correlated with the position of the maximum of the FH interaction parameter. Based on this hypothesis and the Vrentas–Vrentas model for sorption of glassy polymers, a theoretical framework for the glass transition analysis is developed. In particular, the solvent-induced glass transition temperature variation can be estimated from the sorption isotherm as a function of the solvent content corresponding to temperatures above the temperature of sorption.

A new fitting model for sorption by glassy polymers is suggested based on the Flory–Huggins (FH) equation with a composite formula for the FH interaction parameter, χ, which is applicable if sorption experimental data shows a single-maximum variation of the FH parameter. Namely, a power-like and a linear approximation is assumed for χ(φ1), as a function of solvent volume fraction φ1, before and after the point of its maximum. After determining the maximum point from a direct inspection of the sorption data, the three fitting parameters are evaluated by solving two independent least-square minimization problems. Several sorption studies of biopolymers taken from the literature show that the endset of the glass transition region is correlated with the position of the maximum of the FH interaction parameter. Based on this hypothesis and the Vrentas–Vrentas model for sorption of glassy polymers, a theoretical framework for the glass transition analysis is developed. In particular, the solvent-induced glass transition temperature variation can be estimated from the sorption isotherm as a function of the solvent content corresponding to temperatures above the temperature of sorption.

The effect of plastic deformation on the molecular dynamics of atactic polystyrene (a-PS) was studied by broadband dielectric relaxation spectroscopy (BDRS), Fourier-transform infrared spectroscopy (FTIR) and polarized-light microscopy. Sheets of a-PS have been subjected to cold rolling, that is, mechanical rejuvenation, followed by a quenching step and fast heating above its glass-transition temperature, resulting in thermal rejuvenation. Cold rolling revealed, in addition to the known α- and γ(I)-relaxations, four hitherto unknown relaxation processes (II, III, IV and V). Using the framework of craze formation and multiplicity of the glass transition (E. Donth, G. H. Michler, Colloid Polym. Sci. 1989, 267, 557–567), supported by an activation-enthalpy/entropy analysis (Starkweather, W. Howard, Macromolecules 1981, 14, 1277–1281), the following physical picture emerges: (a) processes I and II represent local conformation transitions γ referring to chains of two different degrees of stretching (T/G-ratio); and (ii) processes III and IV were identified as helix-inversion processes of T2G2 helices as reported earlier for syndiotactic-rich PS—an assignment supported by FTIR results. Finally, the relaxation V could be attributed to the onset of the fibrillar glass transition (within crazes), leading to stress release by collapse of the fibrils and hence dying out of process V. Polarized-light microscopy confirmed the creation of oriented structures and internal stresses upon cold rolling, and their removal upon thermal rejuvenation.

The local and cooperative dynamics of atactic PS (a-PS) were studied by broadband dielectric relaxation spectroscopy (BDRS) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The a-PS has been subjected to thermal rejuvenation and subsequent quenching, short-term aging (6 weeks), and long-term aging (1 year) at ambient conditions. Where for the rejuvenated sample only an α- and a γ-relaxation is observed, short-term aging results in an additional β*-relaxation that merges with the α-relaxation at longer aging times. The γ-relaxation is increasing in intensity and activation energy during aging. The α-process shows no spectral changes and shift in the relaxation time upon aging. This may be attributed to a possible erasure of history of the material during the temperature-sweep mode measurement. Fourier transform infrared spectroscopy (FTIR) results suggest that the energetically favorable trans-trans (tt) conformers are increased in population with aging.

n this study, the rate‐ and temperature‐dependent strain hardening and the Bauschinger effect is studied for three glassy polymers. It appeared that for all materials, an equal distribution of elastic and viscous hardening was necessary to accurately predict the Bauschinger effect, as well as the rate‐ and temperature‐dependent strain hardening response. As for the elastic contribution, the viscous contribution appears to increase with an increase in entanglement network density. Investigating the effect of temperature on the Bauschinger effect revealed that at elevated temperatures the model predictions are not accurately enough. It is shown that this is caused by the magnitude of the elastic hardening contribution; to improve the predictions, a temperature‐dependent elastic contribution is necessary.

n this study, the rate‐ and temperature‐dependent strain hardening and the Bauschinger effect is studied for three glassy polymers. It appeared that for all materials, an equal distribution of elastic and viscous hardening was necessary to accurately predict the Bauschinger effect, as well as the rate‐ and temperature‐dependent strain hardening response. As for the elastic contribution, the viscous contribution appears to increase with an increase in entanglement network density. Investigating the effect of temperature on the Bauschinger effect revealed that at elevated temperatures the model predictions are not accurately enough. It is shown that this is caused by the magnitude of the elastic hardening contribution; to improve the predictions, a temperature‐dependent elastic contribution is necessary.

n this study, the rate‐ and temperature‐dependent strain hardening and the Bauschinger effect is studied for three glassy polymers. It appeared that for all materials, an equal distribution of elastic and viscous hardening was necessary to accurately predict the Bauschinger effect, as well as the rate‐ and temperature‐dependent strain hardening response. As for the elastic contribution, the viscous contribution appears to increase with an increase in entanglement network density. Investigating the effect of temperature on the Bauschinger effect revealed that at elevated temperatures the model predictions are not accurately enough. It is shown that this is caused by the magnitude of the elastic hardening contribution; to improve the predictions, a temperature‐dependent elastic contribution is necessary.

Polymers are increasingly used in impact and complex high rate loading applications. Generally, the mechanical response of glassy polymers under high strain rates has been determined in compression. Some research programs have studied the combined effects of temperature and strain rate, still primarily in compression, providing better understanding of the physics behind the observed response and enhancing the models for these materials. However, limited data are available in tension, and even more limited are data describing both the compressive and tensile response of the same glassy polymer. This paper investigates the compressive and tensile response of glassy polymers across a range of stain rates from quasi-static to dynamic. Experimental results from dynamic mechanical analysis, quasi-static compression and tension, and split Hopkinson tension/pressure bars on several representative glassy polymers will be presented. The pressure dependant yield in these materials will be discussed through comparison of the tensile and compressive yield stresses., Published in EPJ Web of Conferences, v26, 2012. Prepared in cooperation with the Dept of Engineering Science, University of Oxford, UK. Sponsored in part by AFOSR. The original document contains color images.

Since the early 1990s, a range of experimental data has been generated describing the response of glassy polymers to high strain rate loading in compression. More recently, research programs that study the combined effects of temperature and strain rate have made significant steps in providing better understanding of the physics behind the observed response, and also in modeling this response. However, limited data are available in tension, and even more limited are data describing both the compressive and tensile response of the same polymer. This paper investigates the compressive and tensile response of glassy polymers, using poly(vinyl chloride) as a representative material, across a range of stain rates from quasi-static to dynamic. The pressure dependant yield in glassy polymers will be discussed through comparison of the tensile and compressive yield stresses., The original document contains color images. Prepared in collaboration with Univ. of Oxford, Dept. of Engineering Science, Oxford, United Kingdom.

New experimental methods, holographic interferometry and quantitative transmission electron microscopy, have been developed to investigate the microstructure, mechanical properties and failure of crazes and deformation zones DZ's in glassy polymers. Plasticizaton by the environment is found to be the major factor in environmental crazing and cracking. Air crazing, and the transition to more ductile DZ behavior, is controlled largely by the entanglement density of the polymer glass. Low entanglement density polymer craze readily and the craze fibrils break down rapidly to form cracks. High entanglement density polymers preferentially form non-fibrillated DZ's; crazes when formed are more resistant to craze breakdown in these polymers. A comprehensive model of these entanglement effects has been developed which explain the natural extension ratios of crazes and DZ's, the craze to DZ transition and the craze fibril stability., Sponsored in part by Grant DAAG29-76-G-0103.

A study of chain dynamics in the solid state of the high impact resistant polycarbonate of bisphenol-A and structural analogues is carried out using nuclear magnetic resonance methods. The object is to quantitatively characterize both the structural geometry and the energetics of local motions occurring in these glassy polymers. It is the belief that only through a detailed knowledge of the solid state dynamics can one establish the structural origin of the bulk properties of these materials. It is important that such a study yield a direct connection to the dynamical mechanical properties which form the basis for characterization of high impact materials. One of the first quantitative demonstrations of this connection is noted in this study. The selection of NMR to probe structure and dynamics in the solid state is well established and the present study focuses on both lineshape and relaxation time analysis using both carbon-13 and deuterium as probes. The study clearly reveals the strengths and weaknesses of the various NMR approaches. Originator-supplied keywords include: Polycarbonates, Impact Strength, Molecular Structure, Plastics, Polymers, Glassy Polymers.

Molecular bheavior of two structurally different glassy polymers, polystyrene and polycarbonate has been studied under tensile stress, cyclic fatigue and in crazing conditions. This has been accomplished by the use of dynamic fourier transform IR spectroscopy and standard characterization techniques. The study of polystyrene revealed that bulky phenyl side groups on the polymer backbone dictate its mechanical behavior by partially interlocking the chains, causing distortions of phenyl groups and breakage of phenyl interlockings. Further, these interphenyl interferences have been found to depend on processing conditions particularly the orientation conditions. In cyclically fatigued PS films, better packing of polymer chains occurs even though serious molecular level damage occurs in the process. In unoriented films, bulky phenyl side groups highly restrict the chain mobility and therefore stress-relief occurs by crazing of the polymer at the surface. In polycarbonate polymer, the stresses which are distributed are a large number of polymer chains, because of its higher free volume. Fatigued PC films show a shift of alpha and beta relaxation peaks towards lower temperatures indicating an increase in free volume through molecular chain scission. Fatigue-crazing and solvent-crazing in PC occurs via different molecular mechanism, with an extensive damage occurring during the cyclic fatigue.(Author)

The local molecular mobility of polysulfone and polycarbonate was studied using a method of conformational probes. Freezing temperatures of conformational equilibria of probe molecules in these polymers were determined using FTIR spectroscopy. The obtained temperatures of relaxation transitions were assigned to the types of conformational mobility of benzene rings of the main polymer chain. © 2011 Springer Science+Business Media, Inc.

Local molecular dynamics in a set of glassy polymers has been studied by the IR-spectroscopic method of conformationally-inhomogeneous probes. This method allows one to determine those macromolecules' fragments, of which the rotation is freezing at certain secondary relaxation transition in polymers. Relaxation transitions observed were assigned to certain types of molecular mobility of the side and lateral groups of polymers.

This work is devoted to development of FTIR spectroscopic method of conformational probes in glassy polymers. A new approach is employed to model the freezing of conformational transitions in probe molecules in glassy polymers. This method is based on introducing small molecules (probes) in a polymer and following conformational dynamics in the probe molecules at different temperatures. For each polymer-probe system, freezing of conformational transitions is observed by FTIR at a certain individual temperature, Tf. A linear correlation between the freezing temperature Tf of conformational transitions of probe molecules and the size of rotational fragments of probe molecules is observed. The magnitude of Tf increases with the size of rotational fragments of the probe molecules and decreases with the free volume fraction of the studied polymers. The obtained results were interpreted in terms of existing "mobile" and "fixed" free volume entities in glassy polymers. The new representations about the processes occurring in polymeric matrices at Tf are given. Finally, relaxation transitions observed in the polymers at the temperatures Tf were assigned to certain types of molecular mobility of their lateral groups. © 2006 Elsevier B.V. All rights reserved.

A new method for studying molecular mobility in glassy polymers-the method of conformationally inhomogeneous probes-is described. A small quantity of a low-molecular-weight compound (probe) having unidentical conformations with different vibrational spectra is introduced into a polymer. The thermodynamic and activation parameters responsible for the concentrations of the conformers and the rates of conformational transitions are connected with both the intramolecular interactions and the properties of the medium (e.g. polymer). Information about the kinetics and equilibria of the conformers could be obtained by investigating the vibrational spectra. This information may be the key to understanding the molecular mobility and free-volume distribution in polymers. The following compounds were used as probes: 1,2-bromofluoroethane, methyl dichlorophosphate and trans-1,2-dichlorocyclohexane. The polymers poly-styrene, poly(vinyl chloride), poly(methyl methacrylate), poly(vinyl acetate), poly(vinylbutyral), poly-propylene and polybutadiene were investigated. The temperature dependences of the absorption-band intensities of the probes were studied. Freezing of the conformational transitions in the probe molecules was found at temperatures Tf characterizing specific polymer-probe systems. The Tf are close to the temperatures of the relaxation transitions in the pure polymers. The possibilities of the method are discussed. © 1994.

Conformational mobility of 1,2-di-(p-bromophenyl)ethane (DPBPE) introduced into glassy poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) matrices has been studied by infrared absorption spectra. For both systems under investigation the freezing of the trans-gauche conformational transitions in DPBPE has been found. The temperatures of freezing of the conformational equilibrium (Tf) have been determined: 276±8 (PVC) and 326±18 K (PMMA). The Tf values are correlated with the secondary relaxation transition temperatures of the pure polymers. In the view of the obtained data, the processes responsible for the relaxation transitions in PVC and PMMA are analysed. The volume of the conformationally mobile group of DPBPE (108.2 Å3) was found to be close to the free volume entity sizes of the polymers, measured at Tf temperatures by positron annihilation lifetime spectroscopy.

The conformational dynamics in 1,2-dichloroethane (DCE)/poly(methyl methacrylate) (PMMA) mixtures has been studied at different concentrations of DCE. The intensities of the infrared absorption bands belonging to the trans and gauche conformers have been measured in the 77-300 K temperature range. Besides, the glass transition temperatures of the mixtures T g were determined by the differential scanning calorimetry. For all the systems under investigation the freezing of the conformational transitions has been found: below a certain temperature T f the trans/gauche ratio becomes temperature independent. The T f value for the lowest volume fraction of DCE (C = 0.01) was found to be close to the temperature of one of the secondary relaxation transitions of pure PMMA (195 K). At this temperature the free volume entities in PMMA should have the volume close to that of the conformationally mobile groups of DCE (∼25 Å 3). The concentration dependence of T f consists of two branches. At low concentrations of the diluent (C < 0.6) T f slowly decreases with the concentration of DCE, reflecting the concentration behaviour of the relaxation transition temperature. At the higher concentrations the freezing of the conformational equilibrium in DCE occurs at the glass transition of the mixtures, and the T f values are close to the T g ones. Copyright © 1996 Elsevier Science Ltd.

Local molecular dynamics in a set of glassy polymers has been studied by the IR-spectroscopic method of conformationally-inhomogeneous probes. This method allows one to determine those macromolecules' fragments, of which the rotation is freezing at certain secondary relaxation transition in polymers. Relaxation transitions observed were assigned to certain types of molecular mobility of the side and lateral groups of polymers.

A new method for studying molecular mobility in glassy polymers-the method of conformationally inhomogeneous probes-is described. A small quantity of a low-molecular-weight compound (probe) having unidentical conformations with different vibrational spectra is introduced into a polymer. The thermodynamic and activation parameters responsible for the concentrations of the conformers and the rates of conformational transitions are connected with both the intramolecular interactions and the properties of the medium (e.g. polymer). Information about the kinetics and equilibria of the conformers could be obtained by investigating the vibrational spectra. This information may be the key to understanding the molecular mobility and free-volume distribution in polymers. The following compounds were used as probes: 1,2-bromofluoroethane, methyl dichlorophosphate and trans-1,2-dichlorocyclohexane. The polymers poly-styrene, poly(vinyl chloride), poly(methyl methacrylate), poly(vinyl acetate), poly(vinylbutyral), poly-propylene and polybutadiene were investigated. The temperature dependences of the absorption-band intensities of the probes were studied. Freezing of the conformational transitions in the probe molecules was found at temperatures Tf characterizing specific polymer-probe systems. The Tf are close to the temperatures of the relaxation transitions in the pure polymers. The possibilities of the method are discussed. © 1994.

Conformational mobility of 1,2-di-(p-bromophenyl)ethane (DPBPE) introduced into glassy poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) matrices has been studied by infrared absorption spectra. For both systems under investigation the freezing of the trans-gauche conformational transitions in DPBPE has been found. The temperatures of freezing of the conformational equilibrium (Tf) have been determined: 276±8 (PVC) and 326±18 K (PMMA). The Tf values are correlated with the secondary relaxation transition temperatures of the pure polymers. In the view of the obtained data, the processes responsible for the relaxation transitions in PVC and PMMA are analysed. The volume of the conformationally mobile group of DPBPE (108.2 Å3) was found to be close to the free volume entity sizes of the polymers, measured at Tf temperatures by positron annihilation lifetime spectroscopy.

The conformational dynamics in 1,2-dichloroethane (DCE)/poly(methyl methacrylate) (PMMA) mixtures has been studied at different concentrations of DCE. The intensities of the infrared absorption bands belonging to the trans and gauche conformers have been measured in the 77-300 K temperature range. Besides, the glass transition temperatures of the mixtures T g were determined by the differential scanning calorimetry. For all the systems under investigation the freezing of the conformational transitions has been found: below a certain temperature T f the trans/gauche ratio becomes temperature independent. The T f value for the lowest volume fraction of DCE (C = 0.01) was found to be close to the temperature of one of the secondary relaxation transitions of pure PMMA (195 K). At this temperature the free volume entities in PMMA should have the volume close to that of the conformationally mobile groups of DCE (∼25 Å 3). The concentration dependence of T f consists of two branches. At low concentrations of the diluent (C < 0.6) T f slowly decreases with the concentration of DCE, reflecting the concentration behaviour of the relaxation transition temperature. At the higher concentrations the freezing of the conformational equilibrium in DCE occurs at the glass transition of the mixtures, and the T f values are close to the T g ones. Copyright © 1996 Elsevier Science Ltd.

This work is devoted to development of FTIR spectroscopic method of conformational probes in glassy polymers. A new approach is employed to model the freezing of conformational transitions in probe molecules in glassy polymers. This method is based on introducing small molecules (probes) in a polymer and following conformational dynamics in the probe molecules at different temperatures. For each polymer-probe system, freezing of conformational transitions is observed by FTIR at a certain individual temperature, Tf. A linear correlation between the freezing temperature Tf of conformational transitions of probe molecules and the size of rotational fragments of probe molecules is observed. The magnitude of Tf increases with the size of rotational fragments of the probe molecules and decreases with the free volume fraction of the studied polymers. The obtained results were interpreted in terms of existing "mobile" and "fixed" free volume entities in glassy polymers. The new representations about the processes occurring in polymeric matrices at Tf are given. Finally, relaxation transitions observed in the polymers at the temperatures Tf were assigned to certain types of molecular mobility of their lateral groups. © 2006 Elsevier B.V. All rights reserved.

The local molecular mobility of polysulfone and polycarbonate was studied using a method of conformational probes. Freezing temperatures of conformational equilibria of probe molecules in these polymers were determined using FTIR spectroscopy. The obtained temperatures of relaxation transitions were assigned to the types of conformational mobility of benzene rings of the main polymer chain. © 2011 Springer Science+Business Media, Inc.

It is well known that reinforcing polymers with small amounts of nano-sized fillers is one of the most effective methods for simultaneously improving their mechanical and thermal properties. However, only a small number of studies have focused on environ-mental stress cracking (ESC), which is a major issue for premature failures of plastic products in service. Therefore, the contribution of this work focused on the influence of nano-SiO2 particles on the morphological, optical, mechanical, thermal, as well as envi-ronmental stress cracking properties of amorphous-based nanocomposites. Polycarbonate (PC), polystyrene (PS) and poly(methyl methacrylate) (PMMA) nanocom-posites containing different amounts and sizes of nano-SiO2 particles were prepared using a twin-screw extruder followed by injection molding. Adding a small amount of nano-SiO2 caused a reduction in optical properties but improved the tensile, toughness, and thermal properties of the polymer nanocomposites. The significant enhancement in mechanical and thermal properties was attributed to the adequate level of dispersion and interfacial interaction of the SiO2 nanoparticles in the polymer matrix. This situation possibly increased the efficiency of stress transfer across the nanocomposite compo-nents. Moreover, the data revealed a clear dependency on the filler size. The polymer nanocomposites filled with smaller nanofillers exhibited an outstanding enhancement in both mechanical properties and transparency compared with nanocomposites filled with larger particles. The best compromise of strength, toughness, and thermal proper-ties was achieved in PC-based nanocomposites. Therefore, special attention to the influ-ence of nanofiller on the ESC resistance was given to PC. The ESC resistance of the materials was investigated under static loading with and without the presence of stress-cracking agents. Interestingly, the incorporation of nano-SiO2 greatly enhanced the ESC resistance of PC in all investigated