41. Polymer Nanoparticle Sizes from Dynamic Light Scattering and Size Exclusion Chromatography: The Case Study of Polysilanes.Artem Badasyan, Andraž Mavrič, Irena Kralj Cigić, Tim Bencik, Matjaž Valant, 2018, original scientific article Abstract: Dynamic Light Scattering (DLS) and Size Exclusion Chromatography (SEC) are among the most popular methods for determining polymer sizes in solution. Taking dendritic and network polysilanes as a group of least soluble polymer substances, we critically compare and discuss the difference between nanoparticle sizes, obtained by DLS and SEC. Polymer nanoparticles are typically in poor solution conditions below the theta point and are in globular conformation therefore. Determination of particle sizes in presence of attractive interactions is not a trivial task. The only possibility to measure aggregation-free, a true molecular size of polymer nanoparticles in such regime of solution, is to operate with the dilute solution of globules (below theta point and above the miscibility line). Basing on results of our polysilane measurements, we come to a conclusion that DLS provides more reliable results than SEC for the dilute solution of globules. General implications for the size measurements of polymer nanoparticles in solutions are discussed.
Keywords: Polymer Nanoparticle, Dynamic Light Scattering, Size Exclusion Chromatography, Polysilanes
Published in RUNG: 16.05.2018; Views: 4231; Downloads: 16
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42. Unusual magnetodielectric effects in La2CuMnO6 induced by a dynamic crossover in dielectric relaxation at TCJasnamol Pezhumkattil Palakkal, Cheriyedath Raj Sankar, Ajeesh Parayancheri Paulose, Matjaž Valant, Artem Badasyan, Manoj Raama Varma, 2018, original scientific article Abstract: A series of fixed frequency dielectric measurements shows dielectric relaxation in La2CuMnO6, with a dynamic
(Arrhenius to Arrhenius) crossover at TC. The external magnetic field alters the relaxation parameters in the
vicinity of crossover and induces an unusual trend in the magnetodielectric coupling around TC. A large magnetodielectric
coupling of 55% (at 68 K, 4 kHz) and 61% (at 105 K, 285.8 kHz) under a small magnetic field of
5 kOe is discovered. Presence of ferromagnetic short-range correlations above TC and a sign reversal of magnetoresistance
around TC are observed. Specific heat analysis revealed the presence of ferromagnetic, variable
range hopping active charge localized state. The presence of ferromagnetic short-range correlations and the
influence of a core-grain dominated magnetoresistance on the Maxwell-Wagner interfacial polarization are responsible
for the observed large magnetodielectric effect. Both the magnetic ordering and external magnetic
field control the electric dipole relaxation in the material.
Keywords: Double perovskite
Griffiths-like phase
Dielectric
Magnetoresistance
Magnetodielectric
Published in RUNG: 03.01.2018; Views: 3750; Downloads: 0
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43. FROM SPINODAL DECOMPOSITION AND MOLECULAR DISPERSION OF POLYSILANES TO SUPER-HARD NANOCOATINGArtem Badasyan, Boštjan Mavrič, Matjaž Valant, published scientific conference contribution abstract Abstract: Conformations of polymer molecules in solution crucially depend on the sign of the
effective potential energy of interaction between the monomers, also known as the
quality of solvent. Therefore in “poor” solvent regime, when effective attraction
overwhelms, the experimental measurements of polymer sizes are complicated by the
agglomeration of macromolecules, followed by precipitation. This phenomenon, also
known as spinodal decomposition, causes serious problems when the goal is to
determine properties of individual macromolecules. Interestingly, while in the case of
carbon-based polymers the precipitation-related problems can be easily avoided with
dilution, this is not the case for polysilanes, i.e. polymeric chains on basis of silicon.
Although the linear polysilanes were first synthesized in early 1920’s, the aggregationrelated
problems have hampered their studies and applicability until recently.
In the Materials Research Laboratory of University of Nova Gorica we have developed
a technology to strengthen the scratch-resistance nanocoating for glass on the basis of
polysilane dendritic polymers we synthesized. Through the prism of the Flory-Huggins
theory, that provides a miscibility phase diagram in temperature-volume fraction
variables, the quality of polymer solution can be manipulated by changing the
temperature. Using Dynamic Light Scattering (DLS) and Differential Scanning
Calorimetry (DSC) we have managed to show, that at temperatures in the range of 40-
50 C the deagglomeration of the dendritic polysilane takes place in tetrahydrofuran
(THF) [1], and the system becomes a true molecular dispersion with particles 20 nm in
size [2]. Introducing such molecular dispersion into the alumina precursor solution
yields an amorphous nanocomposite stabilized by a high level of strain. This resulted in
an extraordinary increase of hardness and scratch resistance of the alumina – polymer
nanocomposite coating that can be used for glass protection [3].
Keywords: Polysilane, dendrimer, solubility
Published in RUNG: 12.09.2017; Views: 4314; Downloads: 0
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46. Growth mechanism and structure of electrochemically synthesized dendritic polymethylsilane moleculesAndraž Mavrič, Artem Badasyan, Gregor Mali, Matjaž Valant, 2017, original scientific article Abstract: The study of an electrochemical synthesis of polymethylsilane from trifunctional trichloro-
methylsilane monomers identified a single polymerization pathway involving reduction of the
monomer to silyl anions and their addition to the growing polymer. The sizes of the synthesized
macromolecules, measured with dynamic light scattering, are much larger than the theoretical
size estimated for an ideal dendrimer. The reason for this, found by NMR analysis, is in a large
number of branching irregularities (defects) contained in the molecular structure, which can be
described as a hyperbranched dendritic polymer. Theoretical estimates of sizes for an ideal
dendrimer were corrected assuming a branching defect is a cavity with the volume of one
monomer. Appropriateness of the theoretical and experimental models was confirmed with a
good quantitative agreement between the experimental densities and the theoretically calculated
values.
Keywords: Polymethylsilane
Dendritic polymer
Electropolymerization
Published in RUNG: 20.03.2017; Views: 4301; Downloads: 14
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47. Physics behind the Conformational Transitions in Biopolymers. Demystification of DNA melting and Protein FoldingArtem Badasyan, invited lecture at foreign university Abstract: Biophysics is the area of research, devoted to the studies of physical problems related to living systems. Animal cell is the smallest unit of an organism and mainly contains water solutions of structurally inhomogeneous polymers of biological origin: polypeptides (proteins) and polynucleotides (DNA, RNA). Statistical physics of macromolecules allows to describe the conformations of both synthetic and bio-polymers and constitutes the basis of Biophysics. During the talk I will report on the biophysical problems I have solved with numerical simulations (Langevin-based Molecular Dynamics of Go-like protein folding model and Monte Carlo with Wang-Landau sampling) and analytical studies of spin models (formula evaluation by hand, enforced with computer algebra systems). The direct connections with the theory of phase transitions, algebra of non-commutative operators and decorated spin models will be elucidated.
Keywords: Biophysics, protein folding, helix-coil transition, spin models
Published in RUNG: 13.12.2016; Views: 5731; Downloads: 0
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49. WATER-POLYPEPTIDE INTERACTION IN CLASSICAL MODELS OF HELIX-COIL TRANSITIONKnarik Yeritsyan, 2016, undergraduate thesis Abstract: Zimm-Bragg model is the simplest to describe the conformational transitions in biopolymers and is regularly used for preocessing the experimental data. We review the model and its Hamiltonian definition with the goal to introduce the interaction with water into the picture. We show how modified ZB model with the account of water-polypeptide interactions allows to describe both cold denaturation and helix-coil transition and derive such the formula explicitly. The obtained theoretical expression for the helicity degree contains two independent parameters that can be fitted with the experimental data to determine the parameters of cold denaturation and helix-coil transition from a single fit and for a single set of experimental data.
Keywords: Zimm-Bragg model, conformational transitions in biopolymers, cold denaturation
Published in RUNG: 28.09.2016; Views: 7302; Downloads: 268
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50. Entropic cost of folding and phase diagrams of polypeptides: Why are IDPs unfolded at room temperature?Artem Badasyan, invited lecture at foreign university Abstract: In spin models, that are applied to describe the conformational transitions in polymers, the number of spin orientations, that correspond to the disordered conformation, can be estimated using fundamental definitions of Statistical Physics. For instance, when considering alpha-helix to coil transition in polypeptides, the role of generalized coordinates is played by pairs of torsional angle, and the repeating unit populates different regions of that 2D contour map, depending on conformation. By scanning over all possible torsional angles, that do not violate the obvious limitations due to the excluded volume, the so-called Ramachandran map can be plotted, which is actually the phase space visualization for the helix-coil transition problem. The region of phase space, corresponding to the ordered, helical conformations, is much more limited, than the one, corresponding to all other (allowed) conformations. We can calculate the areas of these regions as Γhelix and Γcoil , and construct the ratio Q = Γcoil . Naturally, it can be interpreted as log(Q) = Scoil − Shelix = ΔS, the entropic cost of helix with respect to coil. To illustrate the importance of the entropic price of ordered conformation we report our recent results, that allowed to explain the peculiarity of phase diagrams of Intrinsically Disordered Proteins (IDP) out of larger Q-values, as compared to globular counterparts. In particular, it has been shown, that due to larger Q, the phase diagram of IDP is shifted towards higher temperatures.
Keywords: IDP, protein folding, phase diagram
Published in RUNG: 23.06.2016; Views: 4596; Downloads: 0
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