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Herein, we demonstrate that blending an organic dye (guest/filler), with a vinyl polymer (host template), is an inexpensive and simple approach for the fabrication of multifunctional photonic materials which could display an enhancement in the desirable properties of the constituent materials and, at the same time provide novel synergistic properties for the guest-host system. A new guest-host nanocomposite system comprising Phenol Red dye and poly (vinyl alcohol) as guest and host template, respectively, which exhibits tunable optical characteristics and saturable absorption behavior, is introduced. The dependence of local electronic environment provided by the polymer template and the interactions of the polymer molecules with the encapsulated guest molecules on the observed optical/nonlinear absorption behavior is discussed. An understanding of the tunability of the optical/ photophysical processes, with respect to the filler content, as discussed herein could help in the design of improved optical materials for several photonic device applications like organic light emitting diodes and saturable absorbers. [ABSTRACT FROM AUTHOR]

The dissociation of polaron pairs into free charge carriers in organic bulk heterojunction solar cells is a fundamental step in generating photocurrent and is still in debate. In this study, we propose two simple criteria that can be used to test the validity of any polaron pair dissociation model for polymer-fullerene bulk heterojunction solar cells. The first criterion states that the ratio of the bimolecular recombination current density to the maximum photocurrent density should increase as a function of applied voltage. The second criterion states that the ratio of the bimolecular recombination current density to the maximum photocurrent density at short circuit should not be larger than 1. We apply these criteria to test the validity of the widely used Onsager-Braun model by using the experimental current-voltage data of poly[2-methoxy-5-(3'-7'-dimethyloctyloxy)-p-phenylene vinylene] (OC1C10-PPV) and [6,6]-phenyl C61-butyric acid methylester (PCBM) based solar cells. We find that our numerical analysis is not suitable to employ these criteria. Our analytical analysis, on the other hand, clearly demonstrates that the Onsager-Braun model simply cannot fulfill the first criteria. The reason is because the polaron pair dissociation given by the Onsager- Braun model is too strongly influenced by the electric field (i.e., decreases too rapidly as the electric field decreases). The analysis provides a further evidence against the widely used Onsager- Braun model. The proposed criteria can help us to determine the correct model for polaron pair dissociation by serving as a guideline on how strongly the electric field is allowed to influence the polaron pair dissociation. [ABSTRACT FROM AUTHOR]

In this report, an attempt has been made to investigate the morphological evolution of nanoscale surface ripples on aliphatic (polypropylene, PP) and aromatic (polyethylene terephthalate, PET) polymeric substrates irradiated with 50 keV Ar+ ions. The specimens were sputtered at off normal incidence of 30° with 5 x 1016 Ar+ cm-2. The topographical features and structural behavior of the specimens were studied using Atomic Force Microscopy (AFM) and UV-Visible spectroscopy techniques, respectively. The Stopping and Range of Ions in Matter simulations were performed to calculate sputtering yield of irradiated PP and PET polymers. Sputtering yield of carbon atoms has been found to be smaller for PP (0.40) as compared to PET (0.73), which is attributed to the different structures of two polymers. AFM analysis demonstrates the evolution of ripple like features with amplitude (2.50 nm) and wavelength (690 nm) on PET while that of lower amplitude (1.50 nm) and higher wavelength (980 nm) on PP specimen. The disorder parameter (Urbach energy) has been found to increase significantly from 0.30 eV to 1.67 eV in case of PP as compared to a lesser increase from 0.35 eV to 0.72 eV in case of PET as revealed by UV-Visible characterization. A mutual correlation between ion beam sputtering induced topographical variations with that of enhancement in the disorder parameter of the specimens has been discussed. [ABSTRACT FROM AUTHOR]

The forced translocation of homopolymers through αβα sandwich-like compound channels was investigated by Monte Carlo simulation. The interaction between polymer and part α is strongly attractive, whereas that between polymer and part β is purely repulsive. Simulation results show that the translocation is influenced obviously by the length of part β (Lβ) and the starting position of part β (Lα1). For small Lβ, the translocation is mainly governed by the escaping process, and polymer is trapped near the exit of the channel. However, the translocation time can be tuned by varying Lα1 and the fastest translocation can be achieved at relatively large Lα1. Whereas for large Lβ and small Lα1, the translocation is mainly controlled by the filling process. It is difficult for polymer to enter the channel, and polymer is trapped at the first αβ interface. Finally, the dynamics for the filling process and the escaping process are discussed from the view of free-energy landscape, respectively. [ABSTRACT FROM AUTHOR]

This article aims to describe the implementation, evaluation, and outcomes associated with a science, technology, engineering, and mathematics (STEM) research fellow experience at an historically Black college and university (HBCU) funded by the National Science Foundation. This experience, Polymer Research, was part of a multipronged approach for infusing polymers into the General and Physical Chemistry curricula. After briefly reviewing the literature on broadening participation in STEM and undergraduate research experiences, the authors provide a description of the context, evaluation, and project outcomes and conclude with a discussion on project impact and the role of HBCUs in training and graduating underrepresented students in STEM. [ABSTRACT FROM AUTHOR]

Reconfigurable metamaterials have great potential to alleviate complications involved in using passive metamaterials to realize emerging electromagnetic functions, such as dynamical filtering, sensing, and cloaking. This paper presents a new type of tunable meta-atoms in the X-band frequency range (8-12 GHz) toward reconfigurable metamaterials. The meta-atom is made of all flexible materials compliant to the surface of an interaction object. It uses a liquid metal-based split-ring resonator as its core constituent embedded in a highly flexible elastomer. We demonstrate that simple mechanical stretching of the meta-atom can lead to the great flexibility in reconfiguring its resonance frequency continuously over more than 70% of the X-band frequency range. The presented meta-atom technique provides a simple approach to dynamically tune response characteristics of metamaterials over a broad frequency range. [ABSTRACT FROM AUTHOR]

Using dissipative particle dynamics simulations the aggregation of nanoparticles on single and two-component bilayers is investigated. For a uniform bilayer the aggregation of nanoparticles depends strongly on the location of the particles in the bilayer; particles residing on the bilayer exterior cluster strongly under the influence of bilayer-mediated interactions, whereas the interaction between the particles in the bilayer interior is significantly weaker leading to more loosely bound, dynamic aggregates. The aggregation of nanoparticles on two-component bilayers composed of immiscible components changes due to competition between nanoparticle clustering and their adsorption on the boundary between the bilayer components. This reduces the size of the nanoparticle clusters formed on the bilayer exterior, with the clusters adhering onto the boundary between the bilayer components. Due to their weaker attraction nanoparticles in the interior of a mixed bilayer no longer aggregate and instead form strings along the boundary between the two bilayer components. Nanoparticles with an affinity to one bilayer component nucleate small domains of their favoured component around themselves. For asymmetric mixtures this leads to a notable change in the aggregation behaviour of the nanoparticles. [ABSTRACT FROM AUTHOR]

Bidisperse melts of linear, entangled polymer chains were studied using dissipative particle dynamics. The entanglement constraints were mimicked with our newly developed slip-spring approach. The compositions cover blends with short matrix chains, slightly above the molecular entanglement weight as well as blends were both chain lengths exhibit distinct entangled dynamics at various weight fractions. The Struglinsky-Graessley parameter Gr, which is the ratio between the relaxation time of the long chains due to pure reptation and the relaxation time of the tube caused by constraint release, ranges between values high above and below unity. We compare our slip-spring model with simulations that use conventional generic polymer models where bond crossings are prevented by excluded-volume interactions and find fairly good agreement in terms of the mean squared displacement. However, the slip-spring approach requires only a fraction of the computational time, making large scale systems feasible. The dynamical interference of the two different chain lengths is discussed in terms of reptation and constraint release dynamics. For bidisperse melt compositions with Gr < 1.0 the relaxation time of the long chain component is not affected by constraint release. However, for compositions where constraint release is supposed to contribute significantly to the relaxation mechanism (Gr > 1.0), we find strong evidence that the long chains reptate inside a dilated tube whose diameter increases with an exponent of 1/2 towards lower weight fraction of the long chains. Furthermore we observe a linear relation between the relaxation time and weight fraction. Therefore, based on the relaxation times, our results support the validity of the tube dilation model as proposed by Doi et al. [Macromolecules 20, 1900-1906 (1987)]. [ABSTRACT FROM AUTHOR]

In this work, a graphene based strain sensor has been reported for explosive vapour detection applications by exploiting the piezoresistive property of graphene. Instead of silicon based cantilevers, a low cost polymeric micro-cantilever platform has been used to fabricate this strain sensor by embedding the graphene nanoplatelet layer inside the beam. The fabricated devices were characterized for their mechanical and electromechanical behaviour. This device shows a very high gauge factor which is around ~144. Also the resonant frequency of these cantilevers is high enough such that the measurements are not affected by environmental noise. These devices have been used in this work for reliable detection of explosive vapours such as 2,4,6-Trinitrotoluene down to parts-per-billion concentrations in ambient conditions. [ABSTRACT FROM AUTHOR]

tandard models explaining the spin coating of polymer solutions generally fail to describe the early stages of film formation, when hydrodynamic forces control the solution behavior. Using in situ light scattering alongside theoretical and semi-empirical models, it is shown that inertial forces (which initially cause a vertical gradient in the radial solvent velocity within the film) play a significant role in the rate of thinning of the solution. The development of thickness as a function of time of a solute-free liquid (toluene) and a blend of polystyrene and poly(methyl methacrylate) cast from toluene were fitted to different models as a function of toluene partial pressure. In the case of the formation of the polymer blend film, a concentration-dependent (Huggins) viscosity formula was used to account for changes in viscosity during spin coating. A semi-empirical model is introduced, which permits calculation of the solvent evaporation rate and the temporal evolution of the solute volume fraction and solution viscosity. [ABSTRACT FROM AUTHOR]

Mechanical characterization is important for understanding small-scale systems and developing devices, particularly at the interface of biology, medicine, and nanotechnology. Yet, monitoring sub-surface forces is challenging with current technologies like atomic force microscopes (AFMs) or optical tweezers due to their probe sizes and sophisticated feedback mechanisms. An alternative transducer design relying on the indentation mechanics of a compressible thin polymer would be an ideal system for more compact and versatile probes, facilitating measurements in situ or in vivo. However, application-specific tuning of a polymer's mechanical properties can be burdensome via experimental optimization. Therefore, efficient transducer design requires a fundamental understanding of how synthetic parameters such as the molecular weight and grafting density influence the bulk material properties that determine the force response. In this work, we apply molecular-level polymer scaling laws to a first order elastic foundation model, relating the conformational state of individual polymer chains to the macroscopic compression of thin film systems. A parameter sweep analysis was conducted to observe predicted model trends under various system conditions and to understand how nano-structural elements influence the material stiffness. We validate the model by comparing predicted force profiles to experimental AFM curves for a real polymer system and show that it has reasonable predictive power for initial estimates of the force response, displaying excellent agreement with experimental force curves. We also present an analysis of the force sensitivity of an example transducer system to demonstrate identification of synthetic protocols based on desired mechanical properties. These results highlight the usefulness of this simple model as an aid for the design of a new class of compact and tunable nanomechanical force transducers. [ABSTRACT FROM AUTHOR]

This work formulates, at a molecular level, a phenomenological theoretical description of the brittle-ductile transition (BDT) in tensile extension, exhibited by all polymeric glasses of high molecular weight (MW). The starting point is our perception of a polymer glass (under large deformation) as a structural hybrid, consisting of a primary structure due to the van der Waals bonding and a chain network whose junctions are made of pairs of hairpins and function like chemical crosslinks due to the intermolecular uncrossability. During extension, load-earing strands (LBSs) emerge between the junctions in the affinely strained chain network. Above the BDT, i.e., at "warmer" temperatures where the glass is less vitreous, the influence of the chain network reaches out everywhere by activating all segments populated transversely between LBSs, starting from those adjacent to LBSs. It is the chain network that drives the primary structure to undergo yielding and plastic flow. Below the BDT, the glassy state is too vitreous to yield before the chain network suffers a structural breakdown. Thus, brittle failure becomes inevitable. For any given polymer glass of high MW, there is one temperature TBD or a very narrow range of temperature where the yielding of the glass barely takes place as the chain network also reaches the point of a structural failure. This is the point of the BDT. A theoretical analysis of the available experimental data reveals that (a) chain pullout occurs at the BDT when the chain tension builds up to reach a critical value fcp during tensile extension; (b) the limiting value of fcp, extrapolated to far below the glass transition temperature Tg, is of a universal magnitude around 0.2-0.3 nN, for all eight polymers examined in this work; (c) pressurization, which is known [K. Matsushige, S. V. Radcliffe, and E. Baer, J. Appl. Polym. Sci. 20, 1853 (1976)] to make brittle polystyrene (PS) and poly(methyl methacrylate) (PMMA) ductile at room temperature, can cause fcp to rise above its ambient value, reaching 0.6 nN at 0.8 kbar. Our theoretical description identifies the areal density ψ of LBSs in the chain network as the key structural parameter to depict the characteristics of the BDT for all polymer glasses made of flexible (Gaussian) linear chains. In particular, it explains the surprising linear correlation between the tensile stress σBD at the BDT and ψ. Moreover, the theoretical picture elucidates how and why each of the following four factors can change the coordinates (σBD,TBD) of the BDT: (i) mechanical "rejuvenation" (i.e., large deformation below Tg), (ii) physical aging, (iii) melt stretching, and (iv) pressurization. Finally, two methods are put forward to delineate the degree of vitrification among various polymer glasses. First, we plot the distance of the BDT from Tg, i.e., Tg/TBD as a function of ψ to demonstrate that different classes of polymer glasses with varying degree of vitrification show different functional dependence of Tg/TBD on ψ. Second, we plot the tensile yield stress σY as a function Tg/T to show that bisphenol-A polycarbonate (bpA-PC) is less vitreous than PS and PMMA whose σY is considerably higher and shows much stronger dependence on Tg/T than that of bpA-PC. [ABSTRACT FROM AUTHOR]

We analyze the universal size characteristics of flexible ring polymers in solutions in presence of structural obstacles (impurities) in d dimensions. One encounters such situations when considering polymers in gels, colloidal solutions, intra- and extracellular environments. A special case of extended impurities correlated on large distances r according to a power law ~r-a is considered. Applying the direct polymer renormalization scheme, we evaluate the estimates for averaged gyraion radius and spanning radius of typical ring polymer conformation up to the first order of double ε = 4 - d, δ = 4 - a expansion. Our results quantitatively reveal an extent of the effective size and anisotropy of closed ring macromolecules in disordered environment. In particular, he size ratio of ring and open (linear) polymers of the same molecular weight grows when increasing he strength of disorder according to >/ > 1/2 (1 + 13/48 δ). [ABSTRACT FROM AUTHOR]

We studied the effect of elongational flow on structure and kinetics of phase separation in immiscible blends using molecular dynamics simulations. Two different blend systems have been investigated—binary blend of polymers and binary mixture of molecular fluids. The interaction potential parameters in both material systems were chosen to ensure complete phase-separation in equilibrium. We found that elongational flow, beyond a certain rate, significantly alters the steady state morphology in such immiscible mixtures. For the case of polymer blends, perpendicular lamellar morphology was formed under elongation rates (˙ε) from 0.05 to 0.5 MD units possibly due to the interplay of two opposing phenomena—domain deformation/rupture under elongation and aggregation of like-domains due to favorable energetic interactions. The elongation timescale at the critical rate of transition from phase-separated to the lamellar structure (˙ε = 0.05) was found to be comparable to the estimated polymer relaxation time, suggesting a cross-over to the elongation/rupture-dominant regime. Under strong elongational flow rate, ˙ε > 0.5, the formation of disordered morphology was seen in polymer blend systems. The kinetics of phase separation was monitored by calculating domain size as a function of time for various elongational flow rates. The domain growth along the vorticity-axis was shown to follow a power law, Rz(t) ∼ t α. A growth exponent, α of 1/3 for the polymer blend and 0.5–0.6 for the fluid molecular mixture was found under elongation rates from 0.005 to 0.1. The higher growth exponent in the fluid mixture is a result of its faster diffusion time scale compared to that of polymer chains. The steady state end-to-end distance of polymer chains and viscosity of the polymer blend were examined and found to depend on the steady state morphology and elongation rate. [ABSTRACT FROM AUTHOR]

Velocity relaxation of a spherically symmetric polymer, immersed in a viscous incompressible fluid, and after a sudden small impulse or a sudden twist from a state of rest, is studied on the basis of the linearized Navier-Stokes equations with an added Darcy type drag term. Explicit expressions for the translational and rotational velocity relaxation functions of the polymer and for the flow pattern of the fluid are derived for a uniform permeable sphere. Surprisingly, it is found that the added mass vanishes. For fairly large values of the ratio of sphere radius to the screening length characterizing the permeability, the velocity relaxation functions in the short and intermediate time regime differ significantly from that of a sphere with no-slip boundary condition. At long times, both relaxation functions show universal power law behavior. [ABSTRACT FROM AUTHOR]

Polymer-matrix nanocomposites based on Poly(lactic-co-glycolic) acid (PLGA) and Graphene platelets (GNPs) were studied. GNPs, nanomaterials with a 2D flat surface, were chosen with or without chemical modification in PLGA/GNP nanocomposites and their microstructure, thermal property, and their compatibility as scaffolds for cell growth were investigated. PLGA/GNP nanocomposites (0, 1, and 5wt.% of GNPs) were prepared using a solution based technique. Transmission electron microscopy, X-ray diffraction, Differential scanning calorimeter, and Thermogravimetric analyzer were used to analyze morphology and thermal properties. This work demonstrated the role of GNPs flat surface to provide a favorable platform resulting in an enhanced PLGA crystallization. Functionalized GNPs suppress both the thermal stability and the crystallization of PLGA. Finally, to determine the potential usefulness of these scaffolds for biomedical applications, mammalian cells were cultured on various PLGA/GNP nanocomposites (0, 1, and 5wt.% GNPs). 1wt.% PLGA/GNP nanocomposites showed better biocompatibility for cell growth with/without graphenes functionalization compared to pure PLGA and 5wt.% PLGA/GNP. The function of GNPs in PLGA/GNPs (1wt.%) composites is to provide a stage for PLGA crystallization where cell growth is favored. These results provide strong evidence for a new class of materials that could be important for biomedical applications. [ABSTRACT FROM AUTHOR]

To investigate the mechanism of structural changes of water and polymer networks with drying and swelling, we measured the Raman spectra of a physically cross-linked poly(vinyl alcohol) (PVA) hydrogel synthesized using the freezing–thawing method. The results show that the vibrational frequencies of the O–H and C–H stretching modes decrease with dehydration. The frequency shifts observed are attributed to reduction of free water inside the polymer network. The C–H bonds elongate as the water density decreases, and the average length of the O–H bonds increases with increasing proportion of bound water to the total amount of water. On the basis of the dependence of the frequency shifts on the PVA concentration of the original solution, it was found that the structure of the polymer network in the reswollen hydrogel becomes inhomogeneous due to shrinkage of the polymer network with drying. Furthermore, to investigate the effects of the crosslinking structure on the drying process, these results were compared with those of a chemically cross-linked PVA hydrogel synthesized using glutaraldehyde as a cross-linker. The result shows that the vibrational frequency of the O–H stretching mode for the chemically cross-linked hydrogel increases with dehydration, whereas that of the C–H stretching mode decreases. The opposite trend observed in the O–H stretching mode between the physically and chemically crosslinked hydrogels is due to the difference in the shrinkage rate of the polymer network. Because the rate of shrinking is slow compared with that of dehydration in the chemically cross-linked hydrogel, water density in the polymer network decreases. For the physically cross-linked hydrogel, the polymer network structure can be easily shrunken, and the average strength of hydrogen bonds increases with dehydration. The results show that the structures of the polymer network and water change with the gel preparation process, cross-linking method, and drying and reswelling processes. The structure of the polymer network and the behavior of water accommodated in the network are important factors governing the chemical and physical properties of gel materials. [ABSTRACT FROM AUTHOR]

Between 20 and 90 MHz frequency-dependent shear viscosities of the polystyrene-cyclohexane mixture of critical composition have been measured at polymer molar weight Mw = 30 000. The viscosity data reveal dispersion, in conformity with relaxation characteristics in the non-critical background contributions to the ultrasonic attenuation, i.e., in the longitudinal viscosity of the critical system. The dispersion behavior is discussed with a view to its effect on the critical dynamics of the liquid near its consolute point. Attention is especially given to the relaxation rates of fluctuations of that system. The data as resulting from ultrasonic attenuation spectroscopy on the one hand and from quasi-elastic light scattering and viscosity measurements on the other hand differ near the critical temperature. It is concluded that likely an additional dispersion exists in the shear viscosity at frequencies below the presently available frequency range of measurement. [ABSTRACT FROM AUTHOR]

Aimed to optimize the ratio accuracy/computational cost, in this work we study the performance of three different theoretical methodologies in the calculation of the optical bandgap for a test set made of a number of poly(aryl-ethynylene)s related polymers. Infinite, ideal polymer chains were first optimized by means of periodic calculations. Different length oligomers were afterward generated by direct replication of the corresponding periodic structure and their optical bandgaps were calculated by means of different time dependent-density functional theory (TD-DFT) methodologies. These results were fitted to an exponential function for each oligomer family in order to get a theoretical estimation of the optical bandgap for each polymer to be compared to the experimental reported values. The best result was obtained for TD-M06-2X yielding an average deviation of 3.4% with respect to the experimental values. [ABSTRACT FROM AUTHOR]

Porous coordination polymers (PCPs) with soft frameworks show a gate phenomenon consisting of an abrupt structural transition induced by adsorption of guest molecules. To understand the dependence of the gating behavior on the host framework structure, we conduct grand canonical Monte Carlo simulations and a free-energy analysis of a simplified model of a stacked-layer PCP. The interlayer width of the rigid layers composing the simplified model can be changed by guest adsorption and by varying the initial interlayer width h0, which is controlled by the length of pillars between the layers. We introduce three types of gating behavior, one-step gating, filling and gating, and double gating, which depend on three parameters: the initial interlayer width h0; the interaction parameter εss, which determines the host–guest framework interaction as well as the inter-framework interaction; and the elastic modulus of the framework, which depends on the stiffness of the pillars. We show that the one-step gating and the filling and gating behaviors depend strongly on h0 rather than on εss, and thus a transformation from filling and gating to double gating can be achieved by reducing the stiffness of the host framework. This study should be a guideline for controlling the gating pressure of PCPs by modifying their chemical components. [ABSTRACT FROM AUTHOR]

For a double-ring polymer in solution we evaluate the mean-square radius of gyration and the diffusion coefficient through simulation of off-lattice self-avoiding double polygons consisting of cylindrical segments with radius rex of unit length. Here, a self-avoiding double polygon consists of twin self-avoiding polygons which are connected by a cylindrical segment. We show numerically that several statistical and dynamical properties of double-ring polymers in solution depend on the linking number of the constituent twin ring polymers. The ratio of the mean-square radius of gyration of self-avoiding double polygons with zero linking number to that of no topological constraint is larger than 1, in particular, when the radius of cylindrical segments rex is small. However, the ratio is almost constant with respect to the number of vertices, N, and does not depend on N. The large-N behavior of topological swelling is thus quite different from the case of knotted random polygons. [ABSTRACT FROM AUTHOR]

Using fast lattice Monte Carlo (FLMC) simulations [Q. Wang, Soft Matter 5, 4564 (2009)] and the corresponding lattice self-consistent field (LSCF) calculations, we studied a model system of grafted homopolymers, in both the brush and mushroom regimes, in an explicit solvent compressed by an impenetrable surface. Direct comparisons between FLMC and LSCF results, both of which are based on the same Hamiltonian (thus without any parameter-fitting between them), unambiguously and quantitatively reveal the fluctuations/correlations neglected by the latter. We studied both the structure (including the canonical-ensemble averages of the height and the mean-square end-to-end distances of grafted polymers) and thermodynamics (including the ensemble-averaged reduced energy density and the related internal energy per chain, the differences in the Helmholtz free energy and entropy per chain from the uncompressed state, and the pressure due to compression) of the system. In particular, we generalized the method for calculating pressure in lattice Monte Carlo simulations proposed by Dickman [J. Chem. Phys. 87, 2246 (1987)], and combined it with the Wang-Landau– Optimized Ensemble sampling [S. Trebst, D. A. Huse, and M. Troyer, Phys. Rev. E 70, 046701 (2004)] to efficiently and accurately calculate the free energy difference and the pressure due to compression. While we mainly examined the effects of the degree of compression, the distance between the nearest-neighbor grafting points, the reduced number of chains grafted at each grafting point, and the system fluctuations/correlations in an athermal solvent, the θ-solvent is also considered in some cases. [ABSTRACT FROM AUTHOR]

We present results of molecular dynamics simulations for coarse-grained polymer brushes in a wide temperature range to investigate the factors that affect the glass transition in these systems. We focus on the influences of free surface, polymer-substrate interaction strength, grafting density, and chain length not only on the change of glass transition temperature Tg, but also the fragility D of the glass former. It is found that the confinement can enhance the dependence of the Tg on the cooling rate as compared to the bulk melt. Our layer-resolved analysis demonstrates that it is possible to control the glass transition temperature Tg of polymer brushes by tuning the polymer-substrate interaction strength, the grafting density, and the chain length. Moreover, we find quantitative differences in the influence range of the substrate and the free surface on the density and dynamics. This stresses the importance of long range cooperative motion in glass formers near the glass transition temperature. Furthermore, the string-like cooperative motion analysis demonstrates that there exists a close relation among glass transition temperature Tg, fragility D, and string length 〈S〉. The polymer brushes that possess larger string length 〈S〉 tend to have relatively higher Tg and smaller D. Our results suggest that confining a fragile glass former through forming polymer brushes changes not only the glass transition temperature Tg, but also the very nature of relaxation process. [ABSTRACT FROM AUTHOR]

A merged vector grating, which is holographically fabricated in an anisotropic medium by irradiation with interference light with intensity modulation and polarization modulation, was designed to detect the polarization of light. The merged vector grating is recorded by the interference of two elliptically polarized beams with equal intensities, parallel azimuths, equal ellipticities, and different directions of rotation. We clarified theoretically that the Stokes parameters of the incident light beam are described by the diffraction efficiency of the merged vector grating. Also, to apply this property to formation of a polarimeter without any moving parts or mechanisms, two merged vector gratings with different grating vectors were recorded in a photocrosslinkable polymer liquid crystal film by angle-multiplexed holography. By investigation of the diffraction properties of the gratings obtained, we demonstrated the applicability of the merged vector gratings for use in polarimetry. [ABSTRACT FROM AUTHOR]

Photorefractive grating formation dynamics in long-timescale writing and the effects of periodic writing through the control of writing beam irradiation or electric field application were investigated using typical photorefractive polymers. Both dark current and writing beam irradiation affected grating formation dynamics. Dark current in polymers changed the effective trap density over time through deep trap filling and/or detrapping and thus affected grating formation considerably. The writing beam irradiation also affected grating development in the presence of an electric field owing to the accumulation of filled deep traps. However, grating development recovered after the elimination of the electric field freed up the filled deep traps. [ABSTRACT FROM AUTHOR]

We developed a phenomenological theory to describe the aligning field of polymer networks in polymer stabilized liquid crystals where sub-micron size polymer networks are phase separated from the liquid crystal in dispersion. The polymer networks are anisotropic and anchor the liquid crystals in their longitudinal direction. They inhibit the liquid crystals reorientation when external stimuli, such as electric field and temperature, are applied and reduce the relaxation time from distorted states. We model the effects produced by the polymer networks as an effective aligning field. We calculate the effective field as a function of the polymer network volume fraction and the lateral size of the network. The theory is compared with experimental results and good agreements were obtained. It is very useful in predicting how much polymer networks change the driving voltage and response time of liquid crystal devices. [ABSTRACT FROM AUTHOR]

Tailorable anisotropic intrinsic and scale-dependent properties of carbon nanotubes (CNTs) make them attractive elements in next-generation advanced materials. However, in order to model and predict the behavior of CNTs in macroscopic architectures, mechanical constitutive relations must be evaluated. This study presents the full stiffness tensor for aligned CNT-reinforced polymers as a function of the CNT packing (up to ∼20 vol. %), revealing noticeable anisotropy. Finite element models reveal that the usually neglected CNT waviness dictates the degree of anisotropy and packing dependence of the mechanical behavior, rather than any of the usually cited aggregation or polymer interphase mechanisms. Combined with extensive morphology characterization, this work enables the evaluation of structure-property relations for such materials, enabling design of aligned CNT material architectures. [ABSTRACT FROM AUTHOR]

A large variety of composites for electrical applications are developed worldwide on a daily basis. Most of these materials are made from carbonaceous fillers dispersed in polymers. The optimization of the formulations is complex and depends on parameters that are difficult to identify ab initio. The results might also be very sensitive to the processing conditions. There is therefore a need for a fast and accurate method to measure the electrical properties of samples with unfamiliar geometric features and without altering their shape. A four point probes method is incremented that fulfills all the above mentioned requirements. An analytical model is proposed that extends Uhlirs theory and permits to quickly determine the resistivity distribution. Experimental and theoretical approaches were performed to validate the method. An example is proposed with the measurement of samples initially designed for fuel cell-bipolar plates-application. [ABSTRACT FROM AUTHOR]

A Monte Carlo (MC) study is made of the persistence length q and the binary cluster integral β (or the excluded-volume strength B) for polyelectrolytes by the use of the discrete Kratky-Porod wormlike chain with hard-core-effective Debye-Hückel electrostatic pair potentials. The quantity q is determined from the initial decay rate of the bond correlation function after preliminary confirmation of the validity of this procedure using the chain with Lennard-Jones pair potentials. The quantity B is determined from the mean-square radius of gyration along with q by the use of the quasi-two-parameter (QTP) excluded-volume theory. They are evaluated for two model cases of polyelectrolytes, sodium hyaluronate as an example of semiflexible polymers and poly(sodium 4-styrenesulfonate) as a typical example of flexible polymers, both in aqueous sodium chloride. The behavior of MC data so obtained for q and B as functions of added salt concentration c is examined in detail, comparing them with the Odijk-Skolnick-Fixman theory of q and the Fixman-Skolnick (FS) theory of B and also with literature experimental data. In particular, the MC values of B are in almost complete agreement with the FS theory for large c, although the latter still overestimates B somewhat for small c. The values of B themselves and also the validity of the QTP theory in general are discussed in comparison with the case of nonionic polymers. [ABSTRACT FROM AUTHOR]

Developing a composite material of polymers and micrometer-sized fillers with higher heat conductance is crucial to realize modular packaging of electronic components at higher densities. Enhancement mechanisms of the heat conductance of the polymer-filler interfaces by adding the surface-coupling agent in such a polymer composite material are investigated through the non-equilibrium molecular dynamics (MD) simulation. A simulation system is composed of α-alumina as the filler, bisphenol-A epoxy molecules as the polymers, and model molecules for the surface-coupling agent. The inter-atomic potential between the α-alumina and surface-coupling molecule, which is essential in the present MD simulation, is constructed to reproduce the calculated energies with the electronic density-functional theory. Through the non-equilibrium MD simulation runs, we find that the thermal resistance at the interface decreases significantly by increasing either number or lengths of the surface-coupling molecules and that the effective thermal conductivity of the system approaches to the theoretical value corresponding to zero thermal-resistance at the interface. Detailed analyses about the atomic configurations and local temperatures around the interface are performed to identify heat-transfer routes through the interface. [ABSTRACT FROM AUTHOR]

The structural and mechanical properties of imperfect entangled polymer networks with various fractions of elastically active chains are studied using a generic coarse-grained model. Network topology is analyzed at various degrees of cross-linking and correlated with the mechanical response under uniaxial deformation at various strain rates. We found excellent agreement between results obtained from the structural analysis and from fitting to stress relaxation data. The relaxation tensile modulus at various engineering strains was also calculated as a function of the fraction of active strands. Results indicate that the mechanical and viscoelastic properties of entangled polymer networks are susceptible to variation in the network structure, where defects can affect the mechanical response especially at low strain rates and the relaxation behavior at long times. [ABSTRACT FROM AUTHOR]

Chemical epitaxy based on the self-assembly of block copolymers is viewed as a promising technique to achieve ordered patterns on a large scale. Herein, we study the kinetics of lamellar formation of block copolymers under the direction of sparsely stripped patterns using cell dynamics simulations of the time-dependent Ginzburg-Landau theory. First, a scaling law is unveiled with the ordering time of lamellae, tp, with respect to the multiples between the periods of lamellae and stripe patterns, which is consistent with the power law evolution of the correlation length existing in the bulk phase of lamellae. Second, the tolerative windows of perfect order, with deviation from integer multiples, are also estimated from the aspect of kinetics. The results of the ordering time and tolerative windows are of great interest for relevant experiments or applications. Finally, a two-stage evolution is explored during the pattern formation of chemical epitaxy by probing into the evolution of defects, which is of fundamental interest for us to understand the coarsening kinetics of block copolymers under the direction of chemical patterns. [ABSTRACT FROM AUTHOR]

We investigate the influence of addition of hydrophilic and amphiphilic polymer on percolation behavior and micellar interactions in AOT-based water-in-oil droplet microemulsions. We focus on two series of samples having constant molar water to surfactant ratio W = 20 and constant droplet volume fraction [uppercase_phi_synonym] = 30%, respectively. From dielectric spectroscopy experiments, we extract the bending rigidity of the surfactant shell by percolation temperature measurements. Depending on droplet size, we find stabilization and destabilization of the surfactant shell upon addition of hydrophilic poly(ethylene glycol) (PEG) (Mn = 3100 g mol-1) and amphiphilic poly(styrene)-b-poly(ethylene glycol) copolymer with comparable length of the hydrophilic block. Complementary small angle X-ray scattering experiments corroborate the finding of stabilization for smaller droplets and destabilization of larger droplets. Subsequent analysis of dielectric spectra enables us to extract detailed information about micellar interactions and clustering by evaluating the dielectric high frequency shell relaxation. We interpret the observed results as a possible modification of the inter-droplet charge transfer efficiency by addition of PEG polymer, while the amphiphilic polymer shows a comparable, but dampened effect. [ABSTRACT FROM AUTHOR]

Single-layer blue phosphorescence organic light emitting diodes (OLEDs) with either small-molecule or polymer hosts are fabricated using solution process and the performances of devices with different hosts are investigated. The small-molecule device exhibits luminous efficiency of 14.7 cd/A and maximum power efficiency of 8.39 lm/W, which is the highest among blue phosphorescence OLEDs with single-layer solution process and small molecular hosts. Using the same solution process for all devices, comparison of light out-coupling enhancement, with brightness enhancement film (BEF), between small-molecule and polymer based OLEDs is realized. Due to different dipole orientation and anisotropic refractive index, polymer-based OLEDs would trap less light than small molecule-based OLEDs internally, about 37% better based simulation results. In spite of better electrical and spectroscopic characteristics, including ambipolar characteristics, higher carrier mobility, higher photoluminescence quantum yield, and larger triplet state energy, the overall light out-coupling efficiency of small molecule-based devices is worse than that of polymer-based devices without BEF. However, with BEF for light out-coupling enhancement, the improved ratio in luminous flux and luminous efficiency for small molecule based device is 1.64 and 1.57, respectively, which are significantly better than those of PVK (poly-9-vinylcarbazole) devices. In addition to the theoretical optical simulation, the experimental data also confirm the origins of differential light-outcoupling enhancement. The maximum luminous efficiency and power efficiency are enhanced from 14.7 cd/A and 8.39 lm/W to 23 cd/A and 13.2 lm/W, respectively, with laminated BEF, which are both the highest so far for single-layer solution-process blue phosphorescence OLEDs with small molecule hosts. [ABSTRACT FROM AUTHOR]

Polymer nanocomposites have shown to exhibit improved mechanical properties compared to their pure host polymers. These property changes have been primarily attributed to the nature of polymer/nanoparticle interactions. Molecular dynamics simulations of model polymer nanocomposites have provided new insights into the molecular origin of property-changes in these nanocomposites. It was observed that addition of nanoparticles, induced adsorption of monomer segments onto the surface of nanoparticles creating high-density regions of polymer segments in the interfacial zones. A closer look into the morphology of these regions surrounding the nanoparticles revealed that mechanical reinforcement and changes in flow properties may be attributed to the formation of a percolated network of these high-density regions. [ABSTRACT FROM AUTHOR]

In this work, we present the dielectric characterization of polyaniline/polyurethane composite. The samples consisting of 0.5%, 1%, and 5% of polyaniline were deposited on glass fiber, and the measurements were performed in a frequency range of 20 Hz to 20 GHz. The results showed a dielectric relaxation strongly dependent on the concentration of polyaniline. This phenomenon is explained by a theoretical model. In this model, we assume that the alternative conductivity of the polymer network systems is due to conducting clusters whose lengths followed a Gaussian distribution. Depending on their size and the frequency of the excitation signal, the clusters showed a resistive or capacitive effect. [ABSTRACT FROM AUTHOR]

The angular dependent reflection of a monodomain blue phase liquid crystal (BPLC) is investigated. Unlike a cholesteric liquid crystal with single twist structure, a monodomain BPLC exhibits several reflection orders from different crystal planes. As incident angle increases, the first order reflection experiences a blue shift while the second order experiences a red shift. We analyze the physical mechanism of the reflection from six (110) crystal planes. Good agreement between calculated and experimental results is obtained. Undoubtedly, this angular dependency would affect the performance of BPLC photonic devices that require an oblique incidence. [ABSTRACT FROM AUTHOR]

In this study, dissipative particle dynamics (DPD) method was employed to investigate the translational diffusion of rodlike polymer in its nematic phase. The polymer chain was modeled by a rigid rod composed of consecutive DPD particles and solvent was represented by independent DPD particles. To fully understand the translational motion of the rods in the anisotropic phase, four diffusion coefficients, D|u, D⊥u, D|n, D⊥n were obtained from the DPD simulation. By definition, D|n and D⊥n denote the diffusion coefficients parallel and perpendicular to the nematic director, while D|u and D⊥u denote the diffusion coefficients parallel and perpendicular to the long axis of a rigid rod u. In the simulation, the velocity auto-correlation functions were used to calculate the corresponding diffusion coefficients from the simulated velocity of the rods. Simulation results show that the variation of orientational order caused by concentration and temperature changes has substantial influences on D|u and D⊥u. In the nematic phase, the changes of concentration and temperature will result in a change of local environment of rods, which directly influence D|u and D⊥u. Both D|n and D⊥n can be represented as averages of D|u and D⊥u, and the weighted factors are functions of the orientational order parameter S2. The effect of concentration and temperature on D|n and D⊥n demonstrated by the DPD simulation can be rationally interpreted by considering their influences on D|u, D⊥u and the order parameter S2. [ABSTRACT FROM AUTHOR]

In this study, we investigated the interfacial adhesion strength (σint) of transparent conductive oxide (TCO) coatings on polymer substrates using a nanosecond Nd:YAG pulsed laser. We compared our results with those achieved using conventional testing methods such as bending and fragmentation tests as well as theoretical calculations. In the fragmentation and bending tests, mechanical compressive stress is induced in the film due to mismatches in Poisson's ratio and Young's modulus between the substrate and film. But, the incident laser makes the film under compression due to the mismatch in thermal expansion between the TCO and the polymer substrate. With a pulse incident to the substrate, the TCO rapidly expands by laser-induced instant heating while the transparent polymer does little, which causes the TCO to buckle and delaminate over the critical pulse energy. The critical compressive stress that scales with σint was calculated using simple equations, which agreed well with the results from previous theoretical calculations. Because the films preferentially delaminate at the defects and grain boundaries, this technique also provided useful information regarding the interface microstructures. Moreover, because the laser can scan over large areas, this method is suitable for flexible substrates that are produced by a roll-to-roll process. Nevertheless, the mechanical stress introduced by the bending and fragmentation tests causes the TCO to buckle without interfacial delamination. Hence, the stresses at the buckling disagreed with the results obtained from the laser test and the theoretical calculations. [ABSTRACT FROM AUTHOR]

Milestoning is an efficient approximation for computing long-time kinetics and thermodynamics of large molecular systems, which are inaccessible to brute-force molecular dynamics simulations. A common use of milestoning is to compute the mean first passage time (MFPT) for a conformational transition of interest. However, the MFPT is not always the experimentally observed timescale. In particular, the duration of the transition path, or the mean transit time, can be measured in single-molecule experiments, such as studies of polymers translocating through pores and fluorescence resonance energy transfer studies of protein folding. Here we show how to use milestoning to compute transit times and illustrate our approach by applying it to the translocation of a polymer through a narrow pore. [ABSTRACT FROM AUTHOR]

Ordering and alignment of π-conjugated polymer chains are highly desirable for high performance and long life organic electronic devices. We report here self assembly of ordered and aligned solution processible Poly(3,3″′-dialkylquaterthiophene) (PQT-12) polymer at macroscopic level using Floating Film Transfer Method (FTM). PQT-12 polymer film is formed over solution of ethylene glycol and glycerol at different temperatures viz. 22, 26, 33, and 38 °C. PQT-12 films formed by FTM technique are further characterized for optical and morphological properties. UV-vis absorption (for polarize and unpolarize light) and surface topography/phase imaging are carried out by using UV-vis spectrometer and atomic force microscope (AFM), respectively. UV-vis spectra show the polymer chains alignment perpendicular to the film propagation direction and it is well supported by AFM images. The effect of temperature on ordering and alignment of PQT-12 shows 33 °C as an optimum temperature for alignment of polymer chains (a little compromise in ordering). The heating of polymer films at 110 °C for 2 h in ambient causes significant changes in UV-vis absorbance spectra, optical anisotropy, and AFM topography/phase imaging. Our studies provide better understanding of ordering and alignment of PQT-12 chains and also disordering on heating. This work further provides a facile and user-friendly technique for the long range ordered self assembly of PQT-12, which shows enormous potential for various electronic applications. [ABSTRACT FROM AUTHOR]

Ferro- and piezo-electrets are non-polar polymer foams or film systems with internally charged cavities. Since their invention more than two decades ago, ferroelectrets have become a welcome addition to the range of piezo-, pyro-, and ferro-electric materials available for device applications. A polarization-versus-electric-field hysteresis is an essential feature of a ferroelectric material and may also be used for determining some of its main properties. Here, a modified Sawyer-Tower circuit and a combination of unipolar and bipolar voltage waveforms are employed to record hysteresis curves on cellular-foam polypropylene ferroelectret films and on tubular-channel fluoroethylenepropylene copolymer ferroelectret film systems. Internal dielectric barrier discharges (DBDs) are required for depositing the internal charges in ferroelectrets. The true amount of charge transferred during the internal DBDs is obtained from voltage measurements on a standard capacitor connected in series with the sample, but with a much larger capacitance than the sample. Another standard capacitor with a much smaller capacitance-which is, however, still considerably larger than the sample capacitance-is also connected in series as a high-voltage divider protecting the electrometer against destructive breakdown. It is shown how the DBDs inside the polymer cavities lead to phenomenological hysteresis curves that cannot be distinguished from the hysteresis loops found on other ferroic materials. The physical mechanisms behind the hysteresis behavior are described and discussed. [ABSTRACT FROM AUTHOR]

In this work, we perform atomistic model potential molecular dynamics simulations by means of state-of-the art force-fields to study the implantation of a single Au nanocluster on a polydimethylsiloxane substrate. All the simulations have been performed on realistic substrate models containing up to ∼4.6 × 106 of atoms having depths up to ∼90 nm and lateral dimensions up to ∼25 nm. We consider both entangled-melt and cross-linked polydimethylsiloxane amorphous structures. We show that even a single cluster impact on the polydimethylsiloxane substrate remarkably changes the polymer local temperature and pressure. Moreover, we observe the presence of craters created on the polymer surface having lateral dimensions comparable to the cluster radius and depths strongly dependent on the implantation energy. Present simulations suggest that the substrate morphology is largely affected by the cluster impact and that most-likely such modifications favour the penetration of the next impinging clusters. [ABSTRACT FROM AUTHOR]

The curled actuated shapes of ionic polymer metal composites (IPMCs) are described within a nonlinear physics-based model of IPMC actuators. A key characteristic of the model is the refined, even if black box based, modeling of the relative permittivity of the IPMCs which strongly influences the actuation performances of the IPMC, when voltages higher than 1 V are involved. A varying-along-the-thickness relative permittivity is proposed to take into account the highly heterogeneous layers resulting from electrode deposition, where a charge redistribution occurs. Moreover, the presence of the metal electrodes has been considered as hampering the IPMCs' bending deformations, so reducing the actuation performances of the IPMC. A series of numerical tests have been planned and discussed to show the characteristics of the model; in particular, the model is shown to be strong enough to catch the not monotonic behavior of IPMCs, when back relaxation is manifested. [ABSTRACT FROM AUTHOR]

A model for positive hole transport in polyethylene is developed which takes particular account of the crystalline-amorphous morphology of the polymer. The significant feature is the employment of super-exchange quantum mechanical tunneling to explain hole transport through the amorphous phase. The consequence is that the hole mobility exhibits a maximum as a function of the electric field, a manifestation of the inverse Marcus effect. It is shown that this feature accounts for the majority of the reported high-field hole transport effects in polyethylene, including packet formation. [ABSTRACT FROM AUTHOR]

We propose a device structure to lower the lasing threshold of a uniformly lying helix cholesteric liquid crystal (ChLC) laser. We place a π-conjugated polymer active layer beneath the ChLC layer to provide auxiliary gain, and demonstrate an improvement in the lasing threshold by a factor of 2.3. We also perform finite difference time domain calculations coupled with rate equations for a four-level system, and clarify the effect of the additional active layer on both the photonic density of states and the inversion population density. Although the addition of an extra layer lowers the photonic density of states, the gain provided by the auxiliary layer is sufficient to overcome the losses and decrease the lasing threshold. Our concept is useful for obtaining high-performance ChLC lasers. [ABSTRACT FROM AUTHOR]

A new method for measuring exciton diffusion length in nanoparticles (NPs) of conjugated materials is presented. Cationic acceptor dyes are used to quench the photoluminescence in NPs of the prototypical conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). Amplified quenching of MEH-PPV emission is observed with an initial Stern-Volmer constant in excess of 105 M-1. Stern-Volmer plots are nonlinear with two distinct quenching regimes, hinting saturation of NP surfaces with acceptor molecules at some point during titration experiments. Using an assumption that highly efficient quenching of excitons occurs after saturation with acceptors at the NP surfaces, the amount of maximum emission quenching can be compared with a model of exciton diffusion to determine exciton diffusion length. By assuming quenching efficiency above 80%, the measured three dimensional exciton diffusion length is estimated to be 12 ± 1 nm. This result is in the lower region of reported values ranging from 10 to 25 nm in MEH-PPV thin films. Both the derived model and the experimental methodology allow robust measurement of exciton diffusion length for any luminescent conjugated material from which NPs can be prepared. [ABSTRACT FROM AUTHOR]

We apply united-atom (UA) nonequilibrium molecular dynamics simulations to predict the thermal conductivity for amorphous polyethylene at room temperature with chain length (CL) increasing from 4 to 1260. The thermal conductivity is determined by the competition of the two mechanisms: (1) The collision among the UA beads and (2) the phonon vibration. The collision mechanism mainly dominates CL≤7, while the phonon vibration mechanism mainly determines CL ≥ 140. The competition between the two mechanisms determines 12 ≤ CL ≤ 140. In particular, the thermal conductivity for 8 ≤ CL ≤ 11 cannot be obtained accurately due to the aggregation of the beads. [ABSTRACT FROM AUTHOR]

Selecting and tuning modes are useful in ultrasonic guided wave non-destructive testing (NDT) since certain modes at various center frequencies are sensitive to specific types of defects. Ideally one should be able to select both the mode and the center frequency of the launched waves. We demonstrated that an affordable laser diode array can selectively launch either the S0 or A0 ultrasonic wave mode at a chosen center frequency into a polymer plate. A fiber-coupled diode array (4 elements) illuminated a 2 mm thick acrylic plate. A predetermined time delay matching the selected mode and frequency was employed between the output of the elements. The generated ultrasound was detected by a 215 kHz piezo receiver. Our results imply that this array permits non-contacting guided wave ultrasonic NDT. The solution is small, affordable, and robust in comparison to conventional pulsed lasers. In addition, it does not require experienced operators. [ABSTRACT FROM AUTHOR]

Systematic atomic simulations based on molecular mechanics were conducted to investigate the pull-out behavior of a capped carbon nanotube (CNT) in CNT-reinforced nanocomposites. Two common cases were studied: the pull-out of a complete CNT from a polymer matrix in a CNT/polymer nanocomposite and the pull-out of the broken outer walls of a CNT from the intact inner walls (i.e., the sword-in-sheath mode) in a CNT/alumina nanocomposite. By analyzing the obtained relationship between the energy increment (i.e., the difference in the potential energy between two consecutive pull-out steps) and the pull-out displacement, a set of simple empirical formulas based on the nanotube diameter was developed to predict the corresponding pull-out force. The predictions from these formulas are quite consistent with the experimental results. Moreover, the much higher pull-out force for a capped CNT than that of the corresponding open-ended CNT implies a significant contribution from the CNT cap to the interfacial properties of the CNT-reinforced nanocomposites. This finding provides a valuable insight for designing nanocomposites with desirable mechanical properties. [ABSTRACT FROM AUTHOR]