Polymer Physics

Knots appear in a wide variety of biophysical systems, ranging from biopolymers, such as DNA and proteins, to macroscopic objects, such as umbilical cords and catheters. Although significant advancements have been made in the mathematical theory of knots and some progress has been made in the statistical mechanics of knots in idealized chains, the mechanisms and dynamics of knotting in biophysical systems remain far from fully understood. We report on recent progress in the biophysics of knotting—the formation, characterization, and dynamics of knots in various biophysical contexts.

The Sandorfy C model of alternating transfer integrals t( 1 f 6) for u conjugation in ( SiRR' ). polymers is extended to include Coulomb interactions. The resulting Parker-Parr-Pople (PPP) models are solved exactly for oligomers with nd 6 and extrapolated to polymers. Increasing alternation 6 reverses the position of the one-photon excitation to 1 'B and two-photon excitation to 2 'A for PPP parameters appropriate to either polyenes with 6~0. IO or polysilanes (PS) with 6s l/3. The PS excitation to 1 'B is about 1 eV below the 2 'A level due to exciton formation. The corresponding states are essentially degenerate in Hubbard models restricted to on-site interactions U= 2t for 6> 0.10. The dimer limit (6% 1) relates PPP or Hubbard models and their microscopic parameters to bond, charge-transfer, and triplet excitons and rationalizes the numerical results for arbitrary 6.

Die Anforderungen an elastomere Dichtungen im Automobilbereich hinsichtlich Temperatur- und Medienbeständigkeit sind je nach Einsatzbereich sehr unterschiedlich. Insbesondere die Beständigkeit gegen ein bestimmtes Medium in Kombination mit ausgezeichneten Tieftemperatureigenschaften und Hochtemperaturstabilitäten ist eine große Herausforderung an Kautschukhersteller und Elastomerverarbeiter. Zudem werden die Anforderungen an Maßtoleranzen und Oberflächengüten der Bauteile immer anspruchsvoller. Viele Neuanwendungen basieren auf individuellen Dichtungslösungen, und den spezifischen Anforderungsprofilen der Bauteile können nur hochwertige und
auf die jeweilige Applikation hin maßgeschneiderte Werkstoffe gerecht werden.

We have investigated the reduction of the glass transition temperature, Tg, for thin supported films of particularly small molecular weight (MW = 2 kg/mol) polystyrene, and found good agreement with earlier studies on larger molecules. By combining the eigenmodes of the films with mode coupling theory, we arrive at a simple and predictive model which is in quantitative agreement with the data. Its only fitting parameter is the shear modulus governing the relevant fluctuations. The weak dependence of the latter upon the molecular weight of the polymer is in accordance with the general observation that the reduction of Tg is largely independent of chain length at sufficiently low molecular weight.

Hill -Significant tension on the order of 1 nN is self-generated along the backbone of bottlebrush macromolecules due to steric repulsion between densely grafted side chains. The intrinsic tension is amplified upon adsorption of bottlebrush molecules onto a substrate and increases with grafting density, side chain length, and strength of adhesion of the substrate. This allows us to employ these molecular bottlebrushes as miniature tensile machines to probe the mechanochemistry of specific bonds. For this purpose, bottlebrush macromolecules with a disulfide linker in the middle of the backbone were synthesized by atom transfer radical polymerization (ATRP). Two processes, (i) homolytic cleavage of disulfide and (ii) scission of disulfide due to reduction by dithiothreitol were monitored through molecular imaging by atomic force microscope (AFM). In both cases, the corresponding rate constants increase exponentially with mechanical tension along the disulfide bond. Moreover, the reduction rate at zero force is found to be significantly lower than that in bulk solution, which suggests an acidic composition of the water surface with pH=3.7. This opens a new application of brush-like macromolecules as surface pH sensors.

Since Robert Brown's first observations of random walks by pollen particles suspended in solution, the concept of diffusion has been subject to countless theoretical and experimental studies in diverse fields from finance and social sciences, to physics and biology. Diffusive transport of macromolecules in cells is intimately linked to essential cellular functions including nutrient uptake, signal transduction, gene expression, as well as DNA replication and repair. Advancement in experimental techniques has allowed precise measurements of these diffusion processes. Mathematical and physical descriptions and computer simulations have been applied to model complicated biological systems in which anomalous diffusion, in addition to simple Brownian motion, was observed. The purpose of this review is to provide an overview of the major physical models of anomalous diffusion and corresponding experimental evidence on the target search problem faced by DNA-binding proteins, with an emphasis on DNA repair proteins and the role of anomalous diffusion in DNA target recognition.

Since Robert Brown's first observations of random walks by pollen particles suspended in solution, the concept of diffusion has been subject to countless theoretical and experimental studies in diverse fields from finance and social sciences, to physics and biology. Diffusive transport of macromolecules in cells is intimately linked to essential cellular functions including nutrient uptake, signal transduction, gene expression, as well as DNA replication and repair. Advancement in experimental techniques has allowed precise measurements of these diffusion processes. Mathematical and physical descriptions and computer simulations have been applied to model complicated biological systems in which anomalous diffusion, in addition to simple Brownian motion, was observed. The purpose of this review is to provide an overview of the major physical models of anomalous diffusion and corresponding experimental evidence on the target search problem faced by DNA-binding proteins, with an em...

This paper addresses the author's current understanding of the physics of interactions in polymers under a voltage field excitation. The effect of a voltage field coupled with temperature to induce space charges and dipolar activity in dielectric materials can be measured by very sensitive electrometers. The resulting characterization methods, thermally stimulated depolarization (TSD) and thermal-windowing deconvolution (TWD), provide a powerful way to study local and cooperative relaxations in the amorphous state of matter that are, arguably, essential to understanding the glass transition, molecular motions in the rubbery and molten states and even the processes leading to crystallization. Specifically, this paper describes and tries to explain 'interactive coupling' between molecular motions in polymers by their dielectric relaxation characteristics when polymeric samples have been submitted to thermally induced polarization by a voltage field followed by depolarization at a constant heating rate. Interactive coupling results from the modulation of the local interactions by the collective aspect of those interactions, a recursive process pursuant to the dynamics of the interplay between the free volume and the conformation of dualconformers, two fundamental basic units of the macromolecules introduced by this author in the "dual-phase" model of interactions. This model reconsiders the fundamentals of the TSD and TWD results in a different way: the origin of the dipoles formation, induced or permanent dipoles; the origin of the Wagner space charges and the T g,ρ transition; the origin of the T LL manifestation; the origin of the Debye elementary relaxations' compensation or parallelism in a relaxation map; and finally, the dual-phase origin of their super-compensations. In other words, this paper is an attempt to link the fundamentals of TSD and TWD activation and deactivation of dipoles that produce a current signal with the statistical parameters of the "dual-phase" model of interactions underlying the Grain-Field Statistics.

Addition of nanoparticles to polymers leads to enhancement of Tg when attractive interactions (e.g., hydrogen bonding) are present at the nanoparticle-polymer interface. Nanoparticle concentration and dispersion play major roles in determining the Tg enhancement. Unfortunately, characterization of dispersion by transmission electron microscopy is difficult and tedious. Here we show by determination of the Tg-confinement effect in ``model'' polymer-silica nanocomposites (NCs), i.e., a polymer film of known thickness with two silica substrates supporting both sides of the film, that it is possible to characterize the effect of interparticle spacing on Tg and the approximate interparticle spacing in real polymer-silica NCs. Studies of model poly(2-vinyl pyridine) (P2VP)-silica NCs with 200-900 nm interlayer spacing reveal that a significant Tg enhancement is observed at a 500-nm interlayer spacing and that the Tg enhancement exceeds 20 K at a 200-nm interlayer spacing. Studies ...

We discuss the driven translocation of semi-flexible polymer under strong driving force. In the course of the driven translocation, the chain shape is dynamically deformed with the propagation of the tensile force along the chain. We obtain the process time of the driven translocation under the strong force, taking account of the dynamical deformation of the shape, the finite extensibility and the bending elasticity.

In this paper we studied the way in which the surface morphology and the bulk properties are modified when some porous polymer membranes, obtained from poly (hydroxy-urethane)(PHU) and poly (vinyl alcohol)(PVA) were treated with gamma radiation. When ...

Fluorescent Polystyrene Sulfonate for Polyelectrolyte Studies WAYNE HUBERTY, XIAOWEI TONG, SREELATHA BALAMURUGAN, DONGHUI ZHANG, PAUL RUSSO, Louisiana State University -The slow-mode decay found by dynamic light scattering for polyelectrolytes in low-salt conditions has perplexed investigators since its first observation. Many characterization methods have suggested temporary or transient aggregation, although there is still no consensus on the cause. Many different polyelectrolytes demonstrate the slow-mode decay, but the sodium salt of polystyrene sulfonate (NaPSS) is the most popular choice for study. Commercially available NaPSS may have hydrophobic patches due to incomplete sulfonation leading to associations apart from any putative ionic mechanisms. Therefore, essentially full sulfonation, or "patchless", NaPSS should be synthesized. To facilitate fluorescence measurements, which can provide new insights to the slow-mode phenomenon, the material must be rendered fluorescent (F-NaPSS). Several approaches to F-NaPSS have appeared; some labeled a previously synthesized NaPSS without concern for its hydrophobic patches. Other strategies include a free radical copolymerization of styrene sulfonate and a vinyl amine to provide side chains viable for labeling. This method is successful, but yields only small amounts of nearly monodisperse polymer after fractionation. In this presentation, a high-yield synthesis of fully sulfonated, low-polydispersity, fluorescently tagged polymer will be discussed.

The constant development of sophisticated technologies is allowing to dissect three-dimensional chromatin structure at high resolution level. The tremendous amount of quantitative experimental data available today requires a conceptual framework able to make sense of them. In this perspective, polymer physics offers a key tool to interpret chromatin architecture data and to unveil the basic mechanisms shaping its structure. In the very last years, several polymer models have been proposed and have allowed to capture complex features emerging from the data. The major peculiarity distinguishing the different models is represented by the more or less complicated physical mechanism used to explain chromatin folding. Here, we review very popular models which have been recently developed and which represent brilliant examples from this interdisciplinary research field. In order to highlight the wide range of practical applications they have, we discuss the cases of the murine Pitx1 and the human EPHA4 loci, showing that polymer physics allows to effectively study chromatin structure in different cell lines and to predict the impact of pathogenic structural variants on the genome three-dimensional architecture.

The constant development of sophisticated technologies is allowing to dissect three‐dimensional chromatin structure at high resolution level. The tremendous amount of quantitative experimental data available today requires a conceptual framework able to make sense of them. In this perspective, polymer physics offers a key tool to interpret chromatin architecture data and to unveil the basic mechanisms shaping its structure. In the very last years, several polymer models have been proposed and have allowed to capture complex features emerging from the data. The major peculiarity distinguishing the different models is represented by the more or less complicated physical mechanism used to explain chromatin folding. Here, we review very popular models which have been recently developed and which represent brilliant examples from this interdisciplinary research field. In order to highlight the wide range of practical applications they have, we discuss the cases of the murine Pitx1 and th...

This work describes the first detailed model of meltblowing process which allows prediction of such integral laydown properties as thickness, porosity and permeability. Also, such laydown properties as the detailed three-dimensional micro-structure, fiber-size distribution and polymer mass distribution are predicted. The effects of the governing meltblowing parameters on the variation of all these laydown properties are accounted for, with the influence of the collector screen velocity being in focus. For this aim numerical solutions of the system of quasi-one-dimensional equations of the dynamics of free liquid polymer jets moving, cooling and solidifying when driven by surrounding air jet are constructed. Multiple polymer jets are considered simultaneously when they are deposited on a moving screen and forming a nonwoven laydown. The results reveal the three-dimensional configuration of the laydown and, in particular, its porosity and permeability, as well as elucidate the dependence of the laydown structure on the forming conditions, in particular, on the velocity of the screen motion. It is shown and explained how an increase in the velocity of the collector screen increases porosity and permeability of the meltblown nonwoven laydown.

The dynamic model of polymer fibers in hydroentanglement process developed in Part 1 (G. Li, C. Staszel, A.L. Yarin, B. Pourdeyhimi. Hydroentanglement of Polymer Nonwovens. 1: Experimental and Theoretical/Numerical Framework) is generalized here to simulate simultaneous evolution and entanglement of multiple fibers. The fiber-fiber entanglement morphologies are established and the number of entanglements predicted as a function of time. The results of the experiments of Part 1 are compared with the numerical predictions based on the present model. It is shown that the model can successfully be used to describe hydroentanglement of nonwovens. The thickness reduction and the increase in the number of entanglements are predicted for different velocities of water jets.

We explore the mechanical response of a single polyelectrolyte chain under tension in good and poor solvents using a combination of simulation and theory. In poor solvents, where the equilibrium state of the chain is a collapsed globule, we find that the chain undergoes a globule-coil transition, as the magnitude of the force is increased beyond a critical value. This transition, where the polymer size changes discontinuously from a small to a large value, is accompanied by release of bound counterions from the chain. We explain these results by adhering to a statistical mechanical theory of counter-ion condensation on flexible polyelectrolytes.

Small-angle neutron scattering (SANS) experiments are used to study fully charged poly-(styrenesulfonate) (PSS) polyions in the semidilute regime, in the presence of multivalent cations. PSSNa solutions with added Na + , Ca 2+ , or La 3+ ions and PSSCa solutions with added Ca 2+ ions are studied. The chain conformation of the single chain is directly measured by the zero-averaged contrast method. Fitting the scattered intensity with the wormlike chain model leads to the determination of the weight per unit length 1/a and of the persistence length lp. The results can be separated in two regimes. For low ionic strength in PSSNa solutions, the chain remains wormlike, and lp roughly varies as I -1/3 , I being the ionic strength calculated without counterion condensation. The conformation seems to be ruled mostly by I for both multivalent and monovalent cations. For higher ionic strength in PSSNa solutions and for PSSCa solutions, the chain is no longer wormlike if multivalent ions are added, the chain appears "thicker", and I is not the only parameter that controls the conformation. The phenomena are qualitatively identical for both Ca 2+ and La 3+ although a phase separation occurs for La 3+ contrary to Ca 2+ . The conformation changes may result from bridging phenomena between monomers or from counterions condensation, which leads to a more compact chain.

The collapse kinetics of strongly charged polyelectrolytes in poor solvents is investigated by Langevin simulations and scaling arguments. The rate of collapse increases sharply as the valence of counterions, z, increases from one to four. The combined system of the collapsed chain and the condensed counterions forms a Wigner crystal when the solvent quality is not too poor provided z ≥ 2. For very poor solvents the morphology of the collapsed structure resembles a Wigner glass. For a fixed z and quality of the solvent the efficiency of collapse decreases dramatically as the size of the counterion increases. A valence dependent diagram of states in poor solvents is derived.

We review the picture of chromatin large-scale 3D organization emerging from the analysis of Hi-C data and polymer modeling. In higher mammals, Hi-C contact maps reveal a complex higher-order organization, extending from the sub-Mb to chromosomal scales, hierarchically folded in a structure of domains-within-domains (metaTADs). The domain folding hierarchy is partially conserved throughout differentiation, and deeply correlated to epigenomic features. Rearrangements in the metaTAD topology relate to gene expression modifications: in particular, in neuronal differentiation models, topologically associated domains (TADs) tend to have coherent expression changes within architecturally conserved metaTAD niches. To identify the nature of architectural domains and their molecular determinants within a principled approach, we discuss models based on polymer physics. We show that basic concepts of interacting polymer physics explain chromatin spatial organization across chromosomal scales and cell types. The 3D structure of genomic loci can be derived with high accuracy and its molecular determinants identified by crossing information with epigenomic databases. In particular, we illustrate the case of the Sox9 locus, linked to human congenital disorders. The model in-silico predictions on the effects of genomic rearrangements are confirmed by available 5C data. That can help establishing new diagnostic tools for diseases linked to chromatin mis-folding, such as congenital disorders and cancer.

We have studied the structure formation in compositionally nearly symmetric poly(4octylstyrene-b-butylmethacrylate) diblock copolymer films during exposure to saturated vapor of n-hexane (a poor solvent for both blocks) and subsequent drying using real-time, in situ grazing-incidence small-angle X-ray scattering (GISAXS). Previous bulk studies revealed a lamellar structure after thermo-annealing; and surface studies on hexane treated samples revealed a lateral surface structure (Č ernoch et al., 2007) [14]. We focus here on the effect of film thickness which is varied between 1.3 and 2.0 times the bulk lamellar thickness. We report on the lateral repeat distance as well as on the correlation length in the film plane. Complex, non-monotonous behavior is observed for both parameters. Upon drying, the lateral structure created during vapor treatment is stable only for the thick film, not for the intermediate and the thin film. The kinetics depends strongly on the film thickness; especially for the thinnest film, it is very slow. For all film thicknesses, we attempt to identify the most stable morphology, occurring when the inner film structure is close to the one in the bulk.

We study an ensemble of branched polymers which are embedded on other branched polymers. This is a toy model which allows us to study explicitly and in detail the reaction of a statistical system on an underlying geometrical structure, a problem of interest in the study of the interaction of matter and quantized gravity. We nd a phase transition at which the embedded polymers begin to cover the basis polymers. At the transition point the susceptibility exponent takes the value 3=4 and the two-point function develops an anomalous dimension 1=2.

Viscosity, the stress divided by the strain rate, is the physical variable to follow to describe the effect of the molecular weight of the macromolecules, M, and of the temperature, T, on the ability of the polymer melts to flow in molds, extruders and injection molding machines. Fundamentally, the current paradigm of polymer flow relies on assumptions made at the onset of the investigations in rheology that have rarely been questioned, if they ever were: 1st: is the separation of the M and T variables in the expression of the viscosity truly validated?; 2nd :is the molecular weight for entanglement, Mc, truly independent of T, and is the determination of Mc by the break in the log viscosity curve against log M, unequivocally  assigned to characterize “entanglements” ? This paper provides the experimental and mathematical evidence to be able to answer to both of these questions : No!
We use reliable rheological data on monodispersed polystyrene samples from very low molecular weight (M/Mc = 0.015) to relatively high molecular weight (M/Mc=25) to test the separation of M and T in the expression of the viscosity; we reveal that an overall illusion of the validity of the separation of T and M is mathematically comprehensible, especially at high temperature and for M > 2Mc, but that, strictly speaking, the separation of M and T is not valid, except for certain periodic values of M equal to Mc, 2Mc, 4Mc, 8Mc , 16Mc etc.(period doubling) organized around a “pole reference” value MR= 4Mc. We also reveal, for M < Mc, the existence of a lower molecular weight limit, M’c= Mc /8 for the onset of the macromolecular behavior (macro-coil).  The discrete and periodic values of M that validate the separation of the effect of M and T on the viscosity generate the fragmentation of the molecular range into 3 rheological ranges for M > Mc. Likewise, we show that the effect of temperature is also fragmented into 3 rheological ranges for T > Tg: Tg < T< (Tg+23o),  (Tg+23 o) <T< TLL  and T > TLL, where TLL is the liquid-liquid temperature.  Our conclusion is that the classical formulation of the viscosity of polymer melts so overly simplified that it is missing out important experimental facts, such as period doubling for the separation of T and M, TLL, M’c and Mc, resulting in its inability to understand the true nature of entanglements. We present in the discussion of the paper the alternative approach to the viscoelastic behavior, “the duality and cross-duality” of the Dual-conformers showing how this model formalism was used to test mathematically and invalidate the separation of T and M in the classical formulation of viscosity.

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Staudinger taught us that macromolecules were made up of covalently bonded monomer repeat units chaining up as polymer chains. This paradigm is not challenged in this paper. The main question raised in polymer physics remains: how do these long chains interact and move as
a group when submitted to shear deformation at high temperature when they are viscous liquids?
The current consensus is that we need to distinguish two cases: the deformation of “un-entangled chains” for macromolecules with molecular weight, M, smaller thanMe, “the entanglement molecular weight”, and the deformation of “entangled” chains for M > Me. The current paradigm stipulates that the properties of polymers derive from the statistical characteristics of the macromolecule itself, the designated statistical system that defines the thermodynamic state of the polymer. The current
paradigm claims that the viscoelasticity of un-entangled melts is well described by the Rouse model and that the entanglement issues raised when M > Me, are well understood by the reptation model introduced by de Gennes and colleagues. Both models can be classified in the category of “chain dynamics statistics”. In this paper, we examine in detail the failures and the current challenges facing the current paradigm of polymer rheology: the Rouse model for un-entangled melts, the reptation model for entangled melts, the time–temperature superposition principle, the strain-induced time dependence of viscosity, shear-refinement and sustained-orientation. The basic failure of the current paradigm and its inherent inability to fully describe the experimental reality is documented in this
paper. In the discussion and conclusion sections of the paper, we suggest that a different solution to explain the viscoelasticity of polymer chains and of their “entanglement” is needed. This requires a change in paradigm to describe the dynamics of the interactions within the chains and across the chains. A brief description of our currently proposed open dissipative statistical approach, “the Grain-Field Statistics”, is presented.



Keywords: viscoelasticity; polymer physics; paradigm of polymer rheology; entanglements; Rouse model; reptation model; dual-phase model; grain-field statistics; sustained-orientation; shear-refinement.

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Multiscale modeling of physical systems often requires the use of multiple types of simulations to bridge the various length scales that. need to be considered: for example, a density-functional theory at the electronic scale will be combined with a molecular-dynamics simulation at the atomistic level, and with a finite-element method at the macroscopic level. An improvement to this scheme would be a method which is capable of consistently simulating a system at multiple levels of resolution without passing from one simulation type to another, so that different simulations can be studied at a common length scale by appropriate coarse-graining or refinement of a given model. We introduce the wavelet transform as the basis for a new coarse-graining framework. A family of orthonormal basis, the wavelet transform separates data sets, such as spatial coordinates or signal strengths, into subsets representing local averages and local differences. The wavelet transform has several desirable properties for coarse-graining: it is hierarchical, 7 Finally, the unflagging, unstinting, and unending support of my parents has sustained me through this long, exciting, bewildering, perplexing, illuminating, and fascinating period of my life and career. Without their kindness and example, I wouldn't be who I am today. For that, they deserve above all my greatest love and admiration.

Through coarse-grained molecular dynamics simulation, we construct a novel kind of end-linked polymer network by employing dual end-functionalized polymer chains that chemically attach to the surface of nanoparticles (NPs), so that the NPs act as large cross-linkers. We examine the effects of the length and flexibility of polymer chains on the dispersion of NPs, and the effect of the chain length on the stress-strain behavior and the segment orientation during the deformation process. We find that the stress

We report chain self-diffusion and viscosity data for sodium polystyrene sulfonate (NaPSS) in semidilute saltfree aqueous solutions measured by pulsed field gradient NMR and rotational rheometry respectively. The observed concentration dependence of η and D are consistent with the Rouse-Zimm scaling model with a concentration dependent monomeric friction coefficient. The concentration dependence of the monomeric friction coefficient exceeds that expected from free-volume models of diffusion, and its origin remains unclear. Correlation blobs and dilute chains with equivalent end-to-end distances exhibit nearly equal friction coefficients, in agreement with scaling. The viscosity and diffusion data are combined using the Rouse model to calculate the chain dimensions of NaPSS in salt-free solution, these agree quantitatively with SANS measurements.

Particle based methods for the description of hydrodynamic or fluid phenomena have attracted much interest during the last decade. Methods like the Multi-Particle Collision Dynamics method [1] (MPCD) or the Direct Simulation Monte Carlo method [2] (DSMC) represent schemes which close the gap between a purely microscopic description of matter, characterised by force fields, and a macroscopic continuum description. The intermediate level, often referred to as mesoscopic scale, is thereby described on a coarser level. Methods are based on a pseudo-particle description where fluid properties within small volumes are described by discrete particles, thereby allowing for local fluctuations of mass, momentum, kinetic energy and particle density. Globally, conservation laws for mass, momentum, energy and particle number are fulfilled and effective interactions, modelled by stochastic collisions, between particles, are prescribed in such a way that even locally properties are conserved before and after an interaction step. Interactions are thereby described as binary-(DSMC) or multi-particle (MPCD) collisions which are performed in so called collision-cells, into which particles are sorted in each simulation step and which have a volume to enclose O(10) particles. It has been demonstrated in various works that in the limit these methods can be reduced to the Boltzmann equation (for the case of DSMC) or to the Navier-Stokes equations (for the case of MPCD) and various hydrodynamic and fluid flow phenomena, have been reproduced with these methods. Hydrodynamic flows, like pressure driven Poiseuille flow or shear flow between parallel plates require proper boundary conditions in order to fulfil prescribed no-slip, partial-slip or slip conditions. Often, so called bounce back boundary conditions are applied in order to model no-slip conditions. It has been

Deuteration effects on the vibronic structure of the emission and excitation spectra of triangular [4]phenylene (D 3h [4]phenylene) were studied using laser-excited Shpol'skii spectroscopy (LESS) in an octane matrix at 4.2 K. For correct assignment of the vibrational modes, the experimental results were compared with calculated frequencies (B3LYP/6-31G*). CH vibrations were identified by their characteristic isotopic shift in the spectra of deuterated triangular [4]phenylenes. Two CC stretching modes, at 100 cm-1 and 1176 cm-1 , suitable as probes for bond strength changes in the excited state, were identified. The isotope effect on the internal conversion rates of triangular [4]phenylene was evaluated from temperature dependent lifetime measurements. Isotope dependency and the magnitude of the internal conversion rates indicate that internal conversion in triangular [4]phenylene is most likely induced by CH vibrations. The results obtained by LESS and lifetime measurements were compared with PM3 PECI calculations of the excited state structure. The theoretical results and the relation between ground and excited state vibration energies of the 1176 cm-1 probe vibration indicate a reduction of bond alternation of the central cyclohexatriene ring in the excited state.

We performed annealing experiments on monolamellar polyethyleneoxid crystals. Even well below the melting point, local relaxations of metastable folded states led to pronounced liquidlike morphological changes on a micron scale of shape and perimeter of the crystalline domains, similar to dewetting of liquids. First, a rim of less folded molecules was formed around the lamellae, making relaxations of the interior difficult. On longer time scales, hole formation within these domains was observed. Our experiments showed that, due to such spatial anisotropy in relaxations, polymer crystal morphology depends on the thermal pathway taken, implying that a time-temperature superposition principle does not apply.

Broadband Dielectric Spectroscopy (BDS) in combination with a nanostructured electrode arrangementwhich circumvents the conventional need to evaporate metal electrodes onto soft matteris used to study the molecular dynamics of several glass forming materials confined in nanometric ( 5 nm) layers. Other complementary experimental tools employed in this work include spectroscopic vis-Ellipsometry (SE), AC-chip calorimetry (ACC), X-ray reflectrometry (XRR), Differential Scanning Calorimetry (DSC) and Atomic Force Microscopy (AFM). The latter is used to characterize the topography of the samples and to determine their thicknesses. Under the conditions of annealing samples (K) in high oil-free vacuum (10-6 mbars) for at least 12 h and carrying out measurements in inert (dry nitrogen or argon) atmosphere, it is found for all studied thin layers that the structural relaxation, and hence the dynamic glass transitionin its mean relaxation times remains within a margin ±3 K from the respective bulk behaviour. It is revealed, inter alia, that the one-dimensional confinement of thin films introduces restrictions on other (slower) molecular relaxation processes which manifest, depending on the specific system under investigation, as (i) an interruption of the end-to-end (normal mode) fluctuation of the chains, or (ii) a slowing down of the -relaxation when the system is cooled towards glass-formation. Furthermore, (iii) evidence is provided to show that the dimensionality of confinement plays a significant role in determining the resulting dynamics. A molecular understanding of these findings is given, and the discussion presented with respect to the ongoing international debate about dynamics in confinement. 1 …S. (Seitenzahl insgesamt) 2 …Lit. (Anzahl der im Literaturverzeichnis ausgewiesenen Literaturangaben) 3 …Abb. (Abbildungen insgesamt) 4 …Tab. (Gesamtzahl der Tabellen)

There is ample evidence to show that the essential physics governing yielding of entangled polymers is the same, independent of the mode of deformation, e.g., shear versus extension. In either of these two most commonly studied forms of deformation, the elastic retraction force associated with the chain deformation cannot grow without bound during continuous deformation. In practice, a transition from the initial dominantly elastic deformation to flow (irreversible deformation) inevitably takes place. Such yielding can produce strain localization in large deformation of well entangled polymer melts. Apart from the superficial difference related to the confusion about the "strain hardening" behavior, a true difference in the respective responses of entangled melts to shear and extension arises when the strain rate is sufficiently high. The entanglement network can still yield on its path to the eventual flow state upon startup shear. However, startup extension could cause the entanglements to lock in, and the melt undergoes rubber-like rupture instead of yielding. This presentation raises the question of whether shear is an intrinsically different deformation from uniaxial extension in the extremely high rate limit.

When polymer chains are grafted to solid surfaces at sufficiently high density, they form brushes that can modify the surface properties. In particular, polymer brushes are increasingly being used to reduce friction in water-lubricated systems close to the very low levels found in natural systems, such as synovial joints. New types of polymer brush are continually being developed to improve with lower friction and adhesion, as well as higher load-bearing capacities. To complement experimental studies, molecular simulations are increasingly being used to help to understand how polymer brushes reduce friction. In this paper, we review how molecular simulations of polymer brush friction have progressed from very simple coarse-grained models toward more detailed models that can capture the effects of brush topology and chemistry as well as electrostatic interactions for polyelectrolyte brushes. We pay particular attention to studies that have attempted to match experimental friction dat...

Softness and firmness are opposing traits that synergistically define the elastic response of biological systems. Currently, no single class of synthetic materials including elastomers and gels provides independent control of these mechanical characteristics, particularly without altering chemical composition. To address this challenge, we explore a hierarchical bottom-up approach via architectural modulation of bottlebrush mesoblocks followed by their self-assembly into linearbrush-linear triblock copolymer networks. By judiciously incorporating side chains of different lengths, we seamlessly demonstrate full control over elastomer firmness at a fixed Young's modulus thus bypassing the infinitely laborious synthesis of targeted side chain lengths. This industrially scalable iteration upon the design-by-architecture approach to network construction delivers thermoplastic elastomers with unprecedented softness-firmness combinations desired in soft robotics, flexible electronics, and biomedical devices. Synthetic procedures with 1 H-NMR and sample summaries, atomic force microscopy and mechanical characterization with stress-elongation curves of triblock and pentablock plastomers with mixed side chains.

There are limited theoretically predicted phase diagrams for polymer nanocomposites (PNCs) because conventional modeling techniques are largely unable to predict the macroscale phase behavior of PNCs. Here, we show that recent field-based methods, including PNC field theory (PNC-FT) and theoretically informed Langevin dynamics, can be used to calculate phase diagrams for polymer-grafted nanoparticles (gNPs) incorporated into a polymer matrix. We calculate binodals for the transition from the miscible, dispersed phase to the macrophase separated state as functions of important experimental parameters, including the ratio of the matrix chain degree of polymerization (P) to the grafted chain degree of polymerization (N), the enthalpic repulsion between the matrix and grafted chains, the grafting density (σ), the size of the NPs, and the NP volume fraction. We demonstrate that thermal and polymer conformational fluctuations enhance the degree of phase separation in gNP-PNCs, a result of depletion interaction effects. We support this conclusion by experimentally investigating the phase separation of poly(methyl methacrylate)-grafted silica NPs in a polystyrene matrix as a function of P/N. The simulations only agree with experiments when fluctuations are included because fluctuations are needed to properly capture the depletion interactions between the gNPs. We clarify the role of conformational entropy in driving depletion interactions in PNCs and suggest that inconsistencies in the literature may be due to polymer chain length effects since conformational entropy increases with increasing chain length.

There are limited theoretically predicted phase diagrams for polymer nanocomposites (PNCs) because conventional modeling techniques are largely unable to predict the macroscale phase behavior of PNCs. Here, we show that recent field-based methods, including PNC field theory (PNC-FT) and theoretically informed Langevin dynamics, can be used to calculate phase diagrams for polymer-grafted nanoparticles (gNPs) incorporated into a polymer matrix. We calculate binodals for the transition from the miscible, dispersed phase to the macrophase separated state as functions of important experimental parameters, including the ratio of the matrix chain degree of polymerization (P) to the grafted chain degree of polymerization (N), the enthalpic repulsion between the matrix and grafted chains, the grafting density (σ), the size of the NPs, and the NP volume fraction. We demonstrate that thermal and polymer conformational fluctuations enhance the degree of phase separation in gNP-PNCs, a result of depletion interaction effects. We support this conclusion by experimentally investigating the phase separation of poly(methyl methacrylate)-grafted silica NPs in a polystyrene matrix as a function of P/N. The simulations only agree with experiments when fluctuations are included because fluctuations are needed to properly capture the depletion interactions between the gNPs. We clarify the role of conformational entropy in driving depletion interactions in PNCs and suggest that inconsistencies in the literature may be due to polymer chain length effects since conformational entropy increases with increasing chain length.

Polymer diffusion in polymer nanocomposites (PNCs) is significantly modified relative to the neat state. While it is suspected that nanoparticle-induced confinement plays a key role in the diffusion process, a detailed understanding of this process remains nonetheless elusive. We present recent studies of the temperature dependent polymer center-of-mass tracer diffusion coefficient in an athermal PNC comprising polystyrene and phenyl-capped, spherical silica NPs using elastic recoil detection. We find that the polymer tracer diffusion coefficient in the PNC relative to the bulk decreases with increasing nanoparticle concentration and is unexpectedly more strongly reduced at higher temperatures. This unusual temperature dependence of polymer diffusion in PNCs cannot be explained by the reptation model or a modified version incorporating an effective tube diameter, but instead it is the direct result of entropic free energy barriers imposed on polymer chains under confinement.

Polymer diffusion in polymer nanocomposites (PNCs) is significantly modified relative to the neat state. While it is suspected that nanoparticle-induced confinement plays a key role in the diffusion process, a detailed understanding of this process remains nonetheless elusive. We present recent studies of the temperature dependent polymer center-of-mass tracer diffusion coefficient in an athermal PNC comprising polystyrene and phenyl-capped, spherical silica NPs using elastic recoil detection. We find that the polymer tracer diffusion coefficient in the PNC relative to the bulk decreases with increasing nanoparticle concentration and is unexpectedly more strongly reduced at higher temperatures. This unusual temperature dependence of polymer diffusion in PNCs cannot be explained by the reptation model or a modified version incorporating an effective tube diameter, but instead it is the direct result of entropic free energy barriers imposed on polymer chains under confinement.

Polymer structure and dynamics are perturbed under confinement, especially when the dimension of the confinement is smaller than the polymer's radius of gyration (R g). While most studies focus on thin film confinement, we study cylindrical confinement using experiments, simulations and theory. Our MD simulations study the change in R g , local dynamics, entanglement molecular weight (N e), and diffusion coefficient (D) for polymers in cylindrical confinement with different diameters (d/2R g ∼ 0.4-6). We found increased N e and D in cylindrical confinement for d/2R g <1.5. Moreover, R g decreases in the direction of confinement and increases along the cylinder axis. We developed an analytical theory to relate the transformation of chain conformations to the preferential orientation of primitive path steps, and further predicted the increase of N e. Experimentally, we infiltrated 400 kg/mol polystyrene (2R g ∼ 35nm) into anodized aluminum oxide (AAO) membranes (d ∼ 18-150nm) to confine polymers into cylindrical nanopores (d/R g ∼ 0.5-4). We use SANS to probe R g , QENS to probe the local dynamics, and elastic recoil detection to measure D of deuterated PS inside PS-filled AAO nanopores. Values obtained from our experiments and literature are quantitatively compared with our simulation results.

We use single-particle tracking (SPT) to explore the role of nanoparticles/polymer interactions and polymer molecular weight on nanoparticle (NP) diffusion in unentangled polymer melts. The very dilute NP concentrations (∼10 −7 wt %) in SPT measurements enable tuning NP/polymer interactions so that the systems with unfavorable or neutral NP/polymer interactions in polymer melts can be studied without nanoparticle aggregation. Here, the diffusion coefficients of weakly interacting (methyl-capped, CH 3 QDs) and strongly interacting (carboxylic acid-capped, COOH QDs) nanoparticles (radius = 6.6 nm) in poly(propylene glycol) (PPG) melts were measured via SPT. Mean-squared displacements and van Hove distributions of nanoparticle motion are consistent with Brownian motion of single nanoparticles in the long-time diffusion regime. The effective COOH QD size increases with PPG molecular weight as M w 0.5 , indicating a long-lived bound layer. However, for weakly interacting CH 3 QDs, the effective nanoparticle radius is independent of PPG M w due to the absence of a bound layer. In contrast to ensemble average methods (i.e., X-ray photon correlation spectroscopy), SPT methods directly detect spatial and temporal diffusion behavior of individual nanoparticles and provide previously inaccessible information about nanoparticle diffusion in polymer melts.

We use single-particle tracking (SPT) to explore the role of nanoparticles/polymer interactions and polymer molecular weight on nanoparticle (NP) diffusion in unentangled polymer melts. The very dilute NP concentrations (∼10 −7 wt %) in SPT measurements enable tuning NP/polymer interactions so that the systems with unfavorable or neutral NP/polymer interactions in polymer melts can be studied without nanoparticle aggregation. Here, the diffusion coefficients of weakly interacting (methyl-capped, CH 3 QDs) and strongly interacting (carboxylic acid-capped, COOH QDs) nanoparticles (radius = 6.6 nm) in poly(propylene glycol) (PPG) melts were measured via SPT. Mean-squared displacements and van Hove distributions of nanoparticle motion are consistent with Brownian motion of single nanoparticles in the long-time diffusion regime. The effective COOH QD size increases with PPG molecular weight as M w 0.5 , indicating a long-lived bound layer. However, for weakly interacting CH 3 QDs, the effective nanoparticle radius is independent of PPG M w due to the absence of a bound layer. In contrast to ensemble average methods (i.e., X-ray photon correlation spectroscopy), SPT methods directly detect spatial and temporal diffusion behavior of individual nanoparticles and provide previously inaccessible information about nanoparticle diffusion in polymer melts.

Abtract.We investigate the effect of nanoparticles on polymer structure, nanoparticle dynamics and topological constraints (entanglements) in polymer melts for nanoparticle loading above percolation threshold as high as 40.9% using stochastic molecular dynamics (MD) simulations. An increase in the number of entanglements (decrease of N e with 40.9% volume fraction of nanoparticles dispersed in the polymer matrix) in the nanocomposites is observed as evidenced by larger contour lengths of the primitive paths. Attraction between polymers and nanoparticles affects the entanglements in the nanocomposites and alters the primitive path. The diffusivity of small sized nanoparticles deviates significantly from the Stokes-Einstein relation.

both the zero shear viscosity and the polymer diffusion coefficient in homopolymer melts. This talk will present the temperature-dependence of polymer dynamics in polymer nanocomposites comprised of polystyrene and phenyl-capped silica nanoparticles (0-50 vol%). The WLF equation fits the temperature dependence of the tracer polymer diffusion coefficient and the fitting parameter (B/fo) decreases smoothly with nanoparticle concentration suggesting an increase in the thermal expansion coefficient for the free volume. The WLF equation also fits the temperature dependence of the zero shear viscosity from oscillatory shear experiments, although the fitting parameter (B/fo) increases substantially with nanoparticle concentration. This discrepancy between the diffusion and rheology will be discussed with respect to the reptation model, which predicts that the temperature dependence of polymer diffusion depends predominately on the temperature dependence of local viscosity, and the elastic response in nanocomposites.

In this paper we studied the way in which the surface morphology and the bulk properties are modified when some porous polymer membranes, obtained from poly (hydroxy-urethane)(PHU) and poly (vinyl alcohol)(PVA) were treated with gamma radiation. When ...