Microscopic Dynamics of Polymer Melts: Numerical Simulations with Coarse-Grained Models

Every liquid in principle can transform into a glass if it is cooled or compressed fast enough. Glass is one of the most important artificial materials utilized by man. Numerous applications of glass materials and glass transition phenomenon include obtaining of the optimal wet-skid resistance by using the tread rubber with high glass transition temperature, enhancement of the ballistic penetration resistance of armor coatings obtained by energy dissipation associated with an impact induced transition of the coating to the glassy state, the material selection for acoustic tiles on submarines allowing to undergo the glass transition at the frequency of the active sonar at seawater temperatures thus dampening the sonar, arterial walls restoring, preservation of food, highly soluble pharmaceuticals. So, the study of glassy state tends to be one of the most promising and interesting tasks in chemistry in our days. This fact was my main motivating factor to study the structural and dynamical characteristics of the polymer melt approaching to glass transition from above in group of Prof. Dino Leporini in Pisa University. The particles(in this Thesis the word “particle” signifies the most elementary rigid part of the system under investigation. In general, it can be either molecule for the system containing the rigid molecules, or atom for the case of flexible molecules. As far, as in this Thesis the polymer melt is the system that I am investigating, the word “particle” in context of my research signifies the monomer) spend increasing periods of time, rattling in cages formed by their neighbors while the liquid, consisting of these particles, approaches the glass transition. Finally, the particles are relaxed from their cages - this process is called the structure relaxation. The main aim of this thesis is the investigation of the microscopic dynamics of the particles trapped in these cages and the study of the possible ways to connect this fast dynamics in cage to slow macroscopic dynamics associated with structure relaxation. First direction of research is based on the density-temperature scaling of the one of the most important characteristics of the cage dynamics - the Debye-Waller factor(that is the amplitude of the particle’s rattling in the cage, see the section 1.2 for the details). The density-temperature scaling has become very popular in the recent years due to the improvement of the technical equipment of the experiments, that allowed to change both density and temperature of the system in simultaneous and prompt way. The density-temperature scaling of the structure relaxation time for single and multi-component liquids having different interaction potentials was introduced and investigated in this thesis from the different points of view, including the pressure-energy correlations and potential energy landscapes with citations of the correspondent sources (see the section 3.1). Recent research of the Prof. Leporini group has presented the universal scaling between the structure relaxation time and Debye-Waller factor(see the subsection 3.1.5). Nevertheless, the research of the density-temperature scaling of the Debye-Waller factor has been missing so far and I tried my best to improve this situation and to show that the density-temperature scaling is valid for the microscopic processes, too. In the terms of the second direction of investigation I studied several functions of cage correlation for the polymer melt. This study should have allowed me to better understand the processes taking place in the cages or “shells” surrounding the tagged particle and to discover the new types of connection between these microscopic processes from one side and the macroscopic processes, like e.g. the structure relaxation, from another side. Results of this study could help to develop the new ways of understanding of the universal scaling between the structure relaxation time and Debye-Waller factor. Furthermore, it is interesting to study the cage correlation functions of the polymer melt because the polymers have been never studied in this way so far. Previous researches of the correlation functions dealt with the binary mixtures, hard spheres and disks, etc. I also have to say that this study is based on introduction of different correlation functions including the self-correlation functions of displacement that have not been studied in context of the liquids state research so far. As to the meaning of the results of the thesis, I may state the following: • the density-temperature scaling of the Debye-Waller factor is evidenced(see the section 3.1 and the Chapter number 4). The values of the scaling exponent γ are consistent with the predictions from the study of Lennard-Jones liquids. Data of all the polymers in the study, that have different molecular masses and interaction potential, collapse the straight lines of Debye-Waller factor vs TV γ plot, where any straight line is uniquely defined by the molecular mass and parameters of the interaction potential. These lines cross in one “universal” point. • the cage correlation functions, describing the time evolution of the neighbor cage of the given particle(see the subsection 3.2.1 and section 5.1) and following from the immediate physical interpretation, represent the alternative but perfectly equivalent instrument for the description of the structure relaxation, compared to the more rigorous intermediate scattering function. • the analysis of the spatial correlation of displacement(see the subsection 3.2.2 and section 5.2) evidences the link between the well-known static properties of the system and the dynamic properties, represented by the direction and modulus correlation functions of displacement. Origin of this correlation is not perfectly clear and needs further investigation. • analysis of the time correlations of displacement (see the section 5.3) shows that the directionality of motion rather than the displacement modulus seems to connect the fast microscopic and slow macroscopic dynamics. The direction of the displacement of a particle at the time scales of cage regime determines in general the direction of the particle’s motion even at much longer times. As far, as the outlook of the Thesis is concerned, I have to point out that the further investigation, generalization and improvement of these results could be useful. The results of current Thesis allow to construct only several very narrow and rickety bridges between the fast microscopic and slow macroscopic dynamics. These narrow links should be united with the other existing ones and with those that will be created in future, thus forming the new theories, methods and approaches. Among the possible ways of developing of the results of this Thesis I can point to either the prospect of the study of the influence of TV γ on the behavior of the cage correlation functions, uniting the two main directions of research of this thesis, could be the most promising direction of further study, or to the aforementioned possible research of the reasons of the correlation between the static and dynamic(represented by the direction and modulus correlation functions of displacement) properties of the system. Results and outlook of the Thesis are discussed in the Conclusions in more detailed way. My thesis consists of five chapters. First chapter of my PhD thesis presents the introduction to glass transition phenomenon (section 1.1); covers the information about the static structure and relaxation in liquids and introduces the functions that describe these processes (section 1.2); and describes the glass transition in polymers with particular attention to the similarities and differences of the glass transition processes in simple liquids and polymers(section 1.3), thus explaining why the polymers usually are the good glass-formers. Then, in the second chapter, covering the methods of the computer simulations of polymers, there follow: • the review of numerous methods of molecular dynamics and (a bit) of monte-carlo simulations of the glass-forming polymers(section 2.1), allowing to better understand the future ways of development of the simulation studies of the polymers and to enrich the arsenal of a modern scientist with new powerful computational methods enabling the simulations to become more quick and effective • separate section (2.2) describing the numerical methods and MD model (developed by Cristiano De Michele, a former member of the Leporini group) used in the MD simulations runs during the work over the fourth and fifth chapters Third chapter contains the theoretical background for the fourth and fifth chapters. • the theoretical introduction to the popular aspect of density-temperature scaling of the relaxation time in numerous classes of simple liquids and polymers (section 3.1). Theoretical basis of the scaling and its connection to the inverse power law is discussed in the subsection 3.1.1, the numerous ways of approximation of the scaling exponent γ are presented in the subsection 3.1.2. The speculations upon the relation between the density-temperature scaling and the pressure-energy correlations in liquids are presented in the subsection 3.1.3; the connection of the temperaturedensity scaling to the fragility and potential energy landscapes is discussed in the subsection 3.1.4. Section ends with results of Leporini group (subsection 3.1.5) obtained just before beginning of my work over the PhD thesis. These results allowed to ascertain the universal scaling between the fast microscopic and the slow macroscopic dynamics and also urged me to explore the possibility of temperature-density scaling not only of the slow macroscopic dynamics reflected in such macroscopic parameter as the relaxation time, but also of the microscopic dynamics closely connected to the Debye-Waller factor. • section 3.2, consisting of the introduction to the functions regarding the cages surrounding the tagged atom (subsection 3.2.1) and the basic theoretical information about the spatial correlation functions (subsection 3.2.2) The original results of the research of temperature-density scaling of the Debye-Waller factor are presented into the fourth chapter. The fifth chapter reports an original investigation of the correlation functions of supercooled polymeric melt. The peculiarities of motion of the atoms in the supercooled liquid, especially when it approaches the glass transition, are always of great interest and importance, because the understanding of the laws of this motion could be crucial for prediction of various properties of materials close to their glass transition. The correlation functions give us a rather detailed picture of the motion of the atoms. In the section 5.1 I explored the correlation functions regarding the cages surrounding the tagged atom. The description of the program, calculating the neighbor list and cage correlation functions (already introduced in subsection 5.4.1), follows in subsection 5.1.1. The results of the run of this program using the input data from the simulations of our MD model, already described in section 2.2, are presented in subsection 5.1.2. In section 5.2 there follows the research of the spatial correlation functions. The structure of this subsection is similar to the previous one with the introductory part of the corresponding functions being presented in subsection 3.2.2, program description in subsection 5.2.1 and the results of the run in subsection 5.2.2. In the section 5.3 I introduced the original self-correlation functions of displacement in subsection 5.3.1. The program, calculating these functions, is described in the subsection 5.3.2, the results of run - in the subsection 5.3.3. Finally there follow the conclusions, appendix and bibliography

Thermodynamic scaling of vibrational dynamics and relaxation

We investigate by thorough molecular dynamics simulations the thermodynamic scaling (TS) of a polymer melt. Two distinct models, with strong and weak virial-energy correlations, are considered. Both evidence the joint TS with the same characteristic exponent γts of the fast mobility - the mean square amplitude of the picosecond rattling motion inside the cage - and the much slower structural relaxation and chain reorientation. If the cage effect is appreciable, the TS master curves of the fast mobility are nearly linear, grouping in a bundle of approximately concurrent lines for different fragilities. An expression of the TS master curve of the structural relaxation with one adjustable parameter less than the available three-parameter alternatives is derived. The novel expression fits well with the experimental TS master curves of thirty-four glassformers and, in particular, their slope at the glass transition, i.e., the isochoric fragility. For the glassformer OTP, the isochoric fragility allows to satisfactorily predict the TS master curve of the fast mobility with no adjustments

Impact of TiO2 nanostructures on dye-sensitized solar cells performance

The effect of TiO2 nanostructures such as nanoparticles, nanowires, nanotubes on photoanode properties, and dye-sensitized solar cells photovoltaic parameters were studied. The series of dye-sensitized solar cells based on two dyes, that is, commercially N719 and synthesized 3,70- bis(2-cyano-1-acrylic acid)-10-ethyl-phenothiazine were tested. Additionally, the devices containing a mixture of this sensitizer and chenodeoxycholic acid as co-adsorbent were fabricated. The amount of adsorbed dye molecules to TiO2 was evaluated. The prepared photoanodes with different TiO2 nanostructures were investigated using UV-Vis spectroscopy, optical, atomic force, and scanning electron microscopes. Photovoltaic response of constructed devices was examined based on currentvoltage characteristics and electrochemical impedance spectroscopy measurements. It was found that the highest UV-Vis absorption exhibited the photoanode with nanotubes addition. This indicates the highest number of sensitizer molecules anchored to the titanium dioxide photoanode, which was subsequently confirmed by dye-loading tests. The highest power conversion efficiency was (6.97%) for solar cell containing nanotubes and a mixture of the dyes with a co-adsorbent

New benzo[h]quinolin-10-ol derivatives as co-sensitizers for DSSCs

New benzo[h]quinolin-10-ol derivatives with one or two 2-cyanoacrylic acid units were synthesized with a good yield in a one-step condensation reaction. Chemical structure and purity were confirmed using NMR spectroscopy and elemental analysis, respectively. The investigation of their thermal, electrochemical and optical properties was carried out based on differential scanning calorimetry, cyclic voltammetry, electronic absorption and photoluminescence measurements. The analysis of the optical, electrochemical and properties was supported by density functional theory studies. The synthesized molecules were applied in dye-sensitized solar cells as sensitizers and cosensitizers with commercial N719. The thickness and surface morphology of prepared photoanodes was studied using optical, scanning electron and atomic force microscopes. Due to the utilization of benzo[h]quinolin-10-ol derivatives as co-sensitizers, the better photovoltaic performance of fabricated devices compared to a reference cell based on a neat N719 was demonstrated. Additionally, the effect of co-adsorbent chemical structure (cholic acid, deoxycholic acid and chenodeoxycholic acid) on DSSC efficiency was explained based on the density functional theory

Evolutionary Relationships Between the Laccase Genes of Polyporales: Orthology-Based Classification of Laccase Isozymes and Functional Insight From Trametes hirsuta

Laccase is one of the oldest known and intensively studied fungal enzymes capable of oxidizing recalcitrant lignin-resembling phenolic compounds. It is currently well established that fungal genomes almost always contain several non-allelic copies of laccase genes (laccase multigene families); nevertheless, many aspects of laccase multigenicity, for example, their precise biological functions or evolutionary relationships, are mostly unknown. Here, we present a detailed evolutionary analysis of the sensu stricto laccase genes (CAZy – AA1_1) from fungi of the Polyporales order. The conducted analysis provides a better understanding of the Polyporales laccase multigenicity and allows for the systemization of the individual features of different laccase isozymes. In addition, we provide a comparison of the biochemical and catalytic properties of the four laccase isozymes from Trametes hirsuta and suggest their functional diversification within the multigene family

In-Situ Characterisation of Charge Transport in Organic Light-Emitting Diode by Impedance Spectroscopy

The article demonstrates an original, non-destructive technique that could be used to in situ monitor charge transport in organic light-emitting diodes. Impedance spectroscopy was successfully applied to determine an OLED’s charge carrier mobility and average charge density in the hole- and electron-transport layer in a range of applied voltages. The fabricated devices were composed of two commercially available materials: NPB (N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine) and TPBi (2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) as hole- and electron-transport layers, respectively. By varying the thicknesses of the hole-transport layer (HTL) and the electron-transport layer (ETL), correlations between layer thickness and both charge carrier mobility and charge density were observed. A possibility of using the revealed dependencies to predict diode current–voltage characteristics in a wide range of applied voltage has been demonstrated. The technique based on a detailed analysis of charge carrier mobilities and densities is useful for choosing the appropriate transport layer thicknesses based on an investigation of a reference set of samples. An important feature of the work is its impact on the development of fundamental research methods that involve AC frequency response analysis by providing essential methodology on data processing.</jats:p